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Drainage Guidelines

Walter J. Ochs and Bishay G. Bishay

The World Bank Washington, D.C. Copyright © 1992 The International Bank for Reconstruction and Development/THE WORLD BANK 1818 H Street, N.W. Washington, D.C. 20433,U.S.A.

Al rights reserved Manufactured in the United States of America First printing December 1992

Technical Papers are published to communicate the results of the Bank's work to the development community with the least possible delay. The typescript of this paper therefore has not been prepared in accordance with the procedures appropriate to formal printed texts, and the World Bank accepts no responsibility for errors. The findings, interpretations, and conclusions expressed in this paper are entirely those of the author(s) and should not be attributed in any manner to the World Bank, to its affiliated organizations, or to members of its Board of Executive Directors or the countries they represent. The World Bank does not guarantee the accuracy of the data included in this publication and accepts no responsibility whatsoever for any consequence of their use. Any maps that accompany the text have been prepared solely for the convenience of readers; the designations and presentation of material in them do not imply the expression of any opinion whatsoever on the part of the World Bank, its affiliates, or its Board or member countries concerning the legal status of any country, territory, city, or area or of the authorities thereof or concerning the delimitation of its boundaries or its national affiliation. The material in this publication is copyrighted. Requests for permission to reproduce portions of it should be sent to the Office of the Publisher at the address shown in the copyright notice above. The World Bank encourages dissemination of its work and will normally give permission promptly and, when the reproduction is for noncommercial purposes, without asking a fee. Permission to copy portions for classroom use is granted through the Copyright Clearance Center, 27 Congress Street, Salem, Massachusetts 01970,U.S.A. The complete backlist of publications from the World Bank is shown in the annual Index of Publications, wlhich contains an alphabetical title list (with full ordering information) and indexes of subjects, authors, and countries and regions. The latest eclition is available free of charge from the Distribution Unit, Office of the Publisher, Department F, The World Bank, 1818 H Street, N.W., Washington, D.C. 20433,U.S.A., or from Publications, The World Bank, 66, avenue d'Iena, 75116Paris, France.

ISSN: 0253-7494

Walter J. Ochs is drainage adviser in the Agriculture and Rural Development Department of the World Bank. Bishay G. Bishay is an agriculturad engineer and Vice President of Vittetoe, Bishay and Associates, Inc. in Alexandria, Virginia.

Library of Congress Cataloging-in-PublicationData

Ochs, Walter J., 1934- Drainage guidelines / Walter J. Ochs and Bishay G. Bishay. p. cm. - (World Bank technical paper, ISSN 0253-7494; no. 195) Includes bibliographical references (p. ). ISBN 0-8213-2312-1 1. Drainage. I. Bishay, Bishay G. II. Title. m. Series. TC970.026 1992 631.6'2-dc2O 92-40966 CI1 Foreword

Drainage is an integral componentof agriculturalsystems, playing an elementalrole in sustained, efficient, and productive agriculture. It can and should be properly utilized to help strengthen the economic, social, and environmentalconditions of the Bank's borrowing countries. The objective must be to use drainagemeasures to protect existingagricultural lands or promote sound developmentthat is in balance with natural resource conditionsand human needs.

This paper provides research results for and experiencewith agriculturaldrainage and related subjects. It has been developedto guide Bank staff, consultants, and borrowing-countrytechnicians as they work through the project cycle, seekingto assist planners and designers, as well as those responsiblefor implementationand follow-up,when projects involve drainage measures.

The guidelineswere designed to help improve the quality of drainage measures for both irrigated and rainfed agriculture under a wide range of climatic conditions,with the core objective of providing the proper inputs to improve the sustainabiity of agriculturallands and to protect the enviromnent. The relationshipbetween water managementand agriculturalproduction is crucial; thus, sound drainage investmentsmust be consideredwhen planning and developingprojects, and drainage measures must be balancedwith proper irrigation and agronomicpractices to optimize agriculturalproductivity and economicbenefits.

Michel Petit Director Agricultureand Rural DevelopmentDepartment

iii Acknowledgments

The authors appreciatethe guidancereceived from Guy Le Moigne and Shawki Barghouti during the preparation of this publication. Technicalideas and some inputs were provided by the peer reviewers. Gene Vittetoe, for example, prepared a significant section on installation,and this help was most valuable. Lambert Smedemaprovided significantinputs to Appendix5 on costs for drainageprojects. The peer reviewers were:

* Gene Vittetoe, president, Vittetoe, Bishay & Associates,Inc., International Consultants,Temple, Texas

* Robert Broughton,professor and director, Centre for Drainage Studies, McGill University, Montreal

-- Lyman Willardson,professor, Biologicaland Irrigation EngineeringDepartment, Utah State University, Logan

* Ezra Henkin, former head, Soil Conservationand Drainage Division, Ministry of Agriculture, Tel Aviv

* Lambert Smedema, Senior Consulant, Euroconsult, Arnhem, Netherlands, and currently theme manager, InternationalProgram for TechnologyResearch in Irrigation and Drainage

Several Bank staff reviewed the paper, and their commentsare greatly appreciated. These individualswere:

* Daniel Gunaratnam

* Brian Albinson

* Jorge Caparas

Lisa Garbus and Lyn Tisdale edited the paper; Catherine Kocak and S.A.D. Subasinghe created the figures; and Shelly Thorpe, Denise Duggin, Allison Richards, and Maria Mogol typed the manuscript.

The support and assistance received while preparing this paper are deeply appreciated;the sole responsibilityfor the paper, however, rests with the authors.

iv Contents

Introduction 1

I Drainage Project Planning 3

IdentificationStage 3 FeasibilityStudies Stage 8

2 Drainage System Design 36

Drainage for Control of GroundwaterTable Levels 36 CommonSystems 36 Special Systems 40

Drainage for Control of Surface Water 58 Drainage for Flood Protection 58 On-Farm Water Managementin Lowland Rice Fields 60 Land Surface Drainage 66

Mole Drainage as an IntermediateSystem 74

Main Drains and Their Structures 82

3 Project Preparation, Installation, and Maintenance 96

Final Project Preparation and Specifications 96

Installation 106

Maintenance 117

Appendixes 121

Appendix 1: Identificationof Soil Drainability 121 Appendix2: CalculatingDrain Spacingby Drainage Design Formulas 127 Appendix3: EstablishingPilot Demonstration(Testing/Research) Drainage Fields and VerifyingDrainage Parameters 140 Appendix4: EstimatingDesign Discharge from Peak Surface Runoff 159 Appendix5: Cost Estimates and Actual Costs for Some Drainage Projects 164 Appendix6: Crop Tolerance Tables 175

References 180

v

Introduction

Modem agriculture is increasinglybeing rationalizedtoward intensification,multiplication, and diversification. Drainage, a critical element of sustained, efficient, and productive agriculture, shouldproceed in the same direction in a series of related functions. In some areas, natural drainage may be adequateto permit the sustainablecropping intensity and productivitydesired. Often, however, it must be supplementedwith artificial facilitiesto remove and dispose of excess water from agriculturalland. Excess water sources includeprecipitation, snowmelt, excess irrigation water, seepage from canals or reservoirs, artesian flow from aquifers, floodwater, undergroundseepage from adjacent areas, and water applied to leach salts from crop root zones.

In planning and developingprojects, sound drainage investmentsmust be consideredand drainagemeasures balanced with proper irrigation and agronomicpractices to optimize productivity and economicbenefits. For example, appropriatewater conservationmeasures and good irrigation water managementtend to minimizethe developmentof high water tables, which lead to salinization problems in many arid and semiarid areas. Some World Bank-financedprojects have experienced problems related to insufficientconsideration of drainage; these include unanticipated,premature water table buildup; excess salinity levels in crop root zones; lack of timely planning of on-farm drainagemeasures that would have achievedproject drainage goals; improper system design; poor maintenance;and inadequateconstruction of drainage systems.

To improve the quality of drainage measures for both rainfed and irrigated agriculture, the Bank has prepared these guidelinesfor drainage system planning, design, and implementation. The guidelines are aimed at providingBank staff, consultants,and borrowing-countrytechnicians with appropriatedrainage informationthat will facilitate proper and timely considerationof important factors in Bank-financedprojects. The classificationused in the preparationof these guidelines is based primarily on the Bank's project developmentcycle. Drainage systemsare often necessary componentsof agriculturaldevelopment and irrigation projects, and numerousfactors must be evaluatedto determine whether and when drainagesystems will be needed. The guidelineshave therefore been prepared to assist in project developmentefforts by providing information on (1) drainage principles, (2) investigationmethods related to drainage, (3) analysis of data and information,(4) selectionof appropriatemeasures to control drainage problems, (5) design parameters for drainage systems, (6) installationguidance, (7) operation and maintenancefactors, (8) monitoring and evaluation, and (9) several vital environmentalconsiderations.

Land drainage is an artificial intervention,stemming from the interrelationshipsamong several variables and depending largely on the micro- and macroconditionsof the area involved. These guidelines,therefore, have been prepared consideringdrainage systemsworldwide, examiningtheir specific localitiesand physical interrelationships. This comparisonof systems seeks to enable planners and designers, as well as those responsiblefor implementationand follow-up, to benefit from global experience in drainage.

1

Drainage Project Planning

There are numeroustypes of land drainage problems, and an early diagnosisof the nature and cause of these is essentialto establishinga sound program of investigations. The project objectives, remedial measures to be considered, and time schedulesfor study and implementationalso have to be taken into account. Minimumrequirements for data and informationmust be established,and suitable means for collectingadditional informationmust be determined. Planningthe investigationsrequired for drainage projects normally proceeds in stages in which the intensitylevel of the investigations increases progressively,and in which increasinglymore refined and optimizedplans are made. These two stages of project preparation are often distinguished:(1) the identificationstage and (2) the feasibility studies stage.

Identification Stage

AgroclimatologicalInvestigations

Drainage problems result from different extreme variables, such as when rainfall exceeds evapotranspirationduring short or long periods in humid areas (or vice versa in dry climates). Climaticinvestigations, therefore, are able to contribute a great deal to a better understandingof these problems. Climaticanalysis can contributeto an assessmentof the scope of the excess water problems and to the identificationof appropriatedrainage methods.

For most drainage projects, daily rainfall data with an observationperiod of 20 years are adequate. The World Health Organization(1965) defined minimumrainfall network densities for different regions:

* small mountainousislands with irregular rainfall: 25 km2 per station;

* mountainousregions of temperate, Mediterranean,and tropical zones: 100 to 250 km2 per station;

* flat regions of temperate, Mediterranean,and tropical zones: 600 to 900 km2 per station; and

* arid and polar zones: 1,500 to 10,000 km2 per station.

Evapotranspirationdata generally vary less in space and in time than do rainfall data, and lower coverage and shorter observationperiods are generally acceptable. Data from an area wider than the project area may be used to establishrelationships among rainfall depth of different duration and frequency, order of magnitudeof extreme events, and regional patterns of variation. Rainfall analyses are especiallyuseful in determiningdrainage coefficientsand designing discharges. Rainfall data should be compiled into depth-duration-frequencycurves and/or tables. Such compilationsmay be made for different seasons when drainage conditionsshow significantdifferences over the year. The consideredfrequencies normally range from 1 x 1 year to 1 x 25 years. Rainfall durations of 1

- 3 - - 4 - to 3 days are most relevant for surface drainageprojects, while critical durations are usually around 3 to 5 days for groundwaterdrainage projects. For large drainage basins, rainfall distributionpatterns shouldbe studied to arrive at area reductionfactors that can be appliedto convert point rainfall data to basin data.

Another useful type of analysisinvolves soil moisture balance calculationsbased on the availablerainfall and evapotranspirationdata. The calculationswill show the periods during which there is an excess of water in the soil and the periods during which there is a deficit. This type of calculationmay be done with average monthlyrainfall and evapotranspirationdata to identify the main critical drainage periods. A more refined picture of the occurrence of excessesand deficits can be obtainedwhen these calculationsare made for shorter periods (such as 10 days) instead of for monthlyperiods and when the calculationscover several years to provide insight into the frequency of excess and deficit periods. Such calculationswill help in assessingcurrent drainage problems and in determininghow these problems can be alleviatedby adaptingcropping pattern and farming calendars. Althoughprimarily intendedfor use in qualitativeanalysis of drainageproblems, these soil moisture calculationscan also provide useful quantitativeinformation for drainage design. Computer models have now been developedto simulatesoil moisture conditionsand can provide informationfor drainage diagnosisand design. In addition,particular climaticconditions that cause critical drainage situations, such as snowmelt and cyclonicstorms, should be noted.

Geologicaland GeohydrologicalInvestigations

An understandingof the geologicalhistory of the project area and of its wide surrounding area is almost always helpful in diagnosingthe causesof drainageproblems and in conceiving possible solutions. The extent, depth, and characteristicsof the main geological formations of the area should be known. This requirenient applies mainly to surface formations, but deeper formations constitutingaquifers or barriers to groundwatermovement in the project area are also important. The soil materials in the drainageproject area are most often alluvial deposits. Soil patterns and characteristicsare often directly related to the processes by which the land has accreted from the water. This relationshipapplies in particular to river and coastal deposits. Less common are glacial till, residual, and wind-depositedsoils. An understandingof differencesin the geological origin of the soils usually contributesgreatly to an understandingof the particular drainage problems and the soil characteristics.

Barriers restricting the movementof groundwaterin the area (impermeablelayers) are importantbecause they often lead to groundwaterdrainage problems or limit the scope for finding a feasible solution to these problems. Horizontalbarriers commonlyrestrict the deep percolation flow of excess rain or irrigation water, leading to a buildup of perched groundwatertables. The identificationof aquifers in the substrata is equally important. Aquifers may be the cause of seepage drainage problems in the area, though.they may also offer scope for feasible solutionsto high water table problems, either by horizontal or vertical groundwaterdrainage methods.

The auger hole method provides a compositevalue of hydraulic conductivityfor the soil layers between the water table and a level a few centimetersbelow the bottom of the hole. For drainage design, however, it is necessaryto know the hydraulicconductivity for a greater part of the profile. It is particularly importantto determine whether there is any relatively impermeablelayer that shows a marked decrease in hydraulicconductivity with depth. The terms permeable and impermeableare relative. The impermeablelayer may be viewed as that having a hydraulic conductivity1/5 to 1/10 or less of the hydraulicconductivity of the overlying layers. The - 5 - impermeablelayer is not necessarilya more fine-texturedsubsoil. It may be a product of other soil formationfactors, such as a compressionby the weight of glaciers during the Ice Age or a stratificationof alluvial sedimentsthat have been depositedfrom streams in the irrigated areas. The influenceof the impermeablelayer depends on its depth below the field drain levels and also on the drain spacings. Waterloggingabove the impermeablelayer can easily be found if the rainifallrate or the rate at which water is added to the soil exceeds the permeabilityof this layer. The flow pattern of the water moving toward the drains will be altered drastically by the impermeablelayer if the water table depth above the drain level is less than 1/4 of the spacingbetween drains. The drains will have to be placed close together to achievethe effect they would have in a deep permeable soil. Hooghoudt (1936), however, shows that if the water table depth above the drain level is more than or equal to 1/4 of the spacingbetween drains, the flow system can be treated as if an impermeablelayer were entirely absent. Thus, it becomes less important to know the precise location of the impermeablelayer as the drain spacing decreases, that is, as the averagehydraulic conductivityof the soil decreases. It then becomes sufficientto predict the hydraulicconductivity stratification from what is known of the soil stratification.

The satisfactorydiagnosis of and solution to groundwaterdrainage problems often require informationon the substrata beyond the depth investigatedin normal soil survey work. As a rule, these investigationsdo not extend beyond some 2 to 5 m, whereas groundwaterflow to drains may extend below the drainagebase to depths of 1/6 to 1/4 of the drain spacing. Where widely spaced drains are used, though also in many other groundwaterdrainage situations, the normal soil investigationsshould be supplementedby geohydrologicalinvestigations of:

* the overall groundwaterflow pattern in the area;

* the hydraulic characteristicsof the substrata (for example, to some 10 m below the drainage base for a drain spacing S = 50 m when no shallower impermeablelayer occurs);

D piezometriclevels and gradients at different depths and sites;

- patterns and rates of natural drainage and seepage flow; and

* groundwatersalinity at different depths.

Deep geohydrologicalinvestigations (beyond 5 to 10 m) require special equipmentand experienceand are often contracted to specializedfirms.

Land Use Studies

Investigationsrelated to land use should:

* establish land use alternativesfor the area, taking into accountthe present drainage conditionsand the technical/economicfeasibility of improved drainage conditions;

* aid in the formulationof drainage requirementson the basis of these land use alternatives and the prevailing soil and climaticconditions; and

* aid in the assessmentof anticipatedbenefits from drainage. Investigationsnormally include a survey of present land use and farm practices (including irrigation practices), a survey of damage caused by excess water, considerationof various crop rotations, and farm managementstucdies. The relationshipbetween drainage and farming is complicated,and even experimentalresults are of limitedvalue because actual conditionsmay differ in innumerableways from experimentalconditions. A deep understandingof the local agricultural conditions,combined with a sound analysisof the prevailing soil and climatic conditions,offers the best basis for arriving at the correct drainage requirementsand a sound drainage plan.

TopographicStudies

Topographicmaps are essentialin any detailed drainage investigation. These maps show land slopes, length of slope, location and direction of natural drainage, potential outlets, and other special conditionsthat affect drainage. In addition,topographic maps often reveal clues as to the type of drainageneeded and, to a degree, to the practicabilityof that drainage. The map scale to be used dependsupon the size of the area being studied and the purposes of the investigation. For feasibility studies, maps with a scale of 1:10,000 to 1:25,000 showing interval contour lines of 0.5 to 1.0 m will generally suffice for planning the main system; sample areas may be mapped in more detail for use in assessingfield drainage aspects (includingcost estimates).

Final planning and design require more detailed maps with a map scale usually of 1:5,000 to 1:10,000 and with contour line intervalsof 0.25 to 0.50 m. Contour lines at an interval of 0.5 m will generally suffice for slopingland. For regular flat land, an interval of not more than 0.25 m is normally required. Map scales should match the contour line interval so that the contour lines (measuredon the map) are not more than 5 to 10 cm nor less than 0.5 to 1.0 cm apart. Thus, a map used for sloping land requires a scale smaller than that used for flat land.

Detailed topographicmaps are essentialto designingsurface drainage systems for flat land. Small differencesin elevation are important, and contour lines shouldbe based on an adequate number of points to provide a good picture of the meso-microtopography.For the design of groundwaterdrainage systems, somewhatlower standardsfor the mapping of the in-field topographic situation apply. However, the mapping should be adequateto establish alignmentsand grades. For use in planning and designingrunoff control systemsfor slopingland, the maps should show such features as slope pattern, length and degree of slope, and uniformity. The topographicmaps should also show the main elements of any existingdrainage system and all relevant infrastructuralfeatures such as roads, power lines, and settlements. To assess whether existing drains can be used, longitudinalprofiles (1:5,000 to 1:10,000)with cross-sections(every 100 to 200 m, scale 1:100) are needed, as well as data on the characteristicdimensions and levels of all structures in these systems (such as bridges and culverts). To assess outlet conditions,it may be necessaryto extend the topographicmapping well outside the project area.

Soil Survey and Land ClasstficationStudies

Standard soil survey and land classificationmethods shouldbe appliedto establish a base for assessingthe agriculturalvalue of the soil and land resources in the project area. This section lists the informationalrequirements for soil and land with respect to drainage. Some of this information will be availablefrom standard soil maps, but some of the following informationmay have to be collectedusing additionalor special studies to satisfy the drainage study needs. - 7 - Signs of Wetness. These signs include smell, indicatorplants (sedges, rushes, etc.), poor decompositionof organic matter, water on the land after rain, water in the tracks, conditionof crops, and springs/springlines and seepage water (often occurs with emergenceof rock strata onto the surface and is sometimesassociated with iron ocher deposits).

Slope Features. These includeoverall relief, length, and degree and regularity of slope (also partly derived from topographicmaps) to assess overland flow conditions.

Retention/DetentionFeatures. These features include microrelief, surface roughness, type of cover, and presence of terraces or bunds.

Erodibilityof the Top Soil Layer. Erodibilitycan be expressedby the erodibilityindex of the U.S. Departmentof Agriculture(USDA) universal soil losses equationor by some other integrated erodibilityparameter, or it can be deduced from related characteristicssuch as soil texture and structure, rainfall intensity, and length and degree of slope.

Infiltrabilityof the TopsoilLayer. Infiltrabilitycan be expressedby measured infiltration rates/curves, or can be deduced from soil texture or structure, liabilityof the soil to capping, or swelling and cracking characteristics.

Workability. Workabilitycan be expressedby the field capacity (FC) or lower plastic limit (LPL) relationshipor by some other criterion.

Soil-Water-AirRelationships of the Different Profile Layers. The moistureretention curve can be studied to assess availablemoisture and aeration problems.

GeneralStratification and Propertiesof DifferentLayers. These studies assess limitationson root zone depth, availablesoil moisture (includingcapillary rise), limitationson earth movement, state of ripening of the subsoil, and expected subsidence;the studies also assess drainage installation conditionsfor pipe drains, mole drains, and ditches (suitableexcavation methods, suitablepipe materials, required envelopes/filters,ocher problems, soil stability, soil consistency, sloughing problems, suitable side slope, bank protection requirement).

Soil Salinity and Alkalinity. The occurrence of high concentrationsof salts in the soil is most prevalent in hot arid and semiarid climates and in coastal land. Much contemporarysoil salinity is due to the introductionof irrigation. Soil and drainage conditionsare also a major influence, however, because they determine the physical possibilityof leaching and removing excess salt from the soil profile and from the land. The investigationsshould establishthe current salinity and alkalinityof the project land, the future soil salinizationhazard, and the basis for a program of salinity control and reclamationof salt-affectedsoils. An assessmentof the characteristicsof salt- affected soils should be based on measurementsof the electrical conductivity(EC) of the soil water extract and the exchangeablesodium percentage(ESP) values. Soil salinity is a highly dynamicsoil feature. Details such as time of the year, crop, irrigation schedule, and soil managementare all influential. Therefore, considerableeffort should be made to collect representativesamples. A suitableseason should be selected; for example, the end of the dry season, when soil salinityvalues would normallybe at their highest level. Samplingcan be done at standard depths unless there is reason to assume a particular vertical salt distribution(e.g., based on soil conditionsor irrigation regime) or unless interest is focusedon special depths (e.g., based on the crops). Because of the special interest in soil salinity values in the root zone and because of the wider variation in soil salinity in the upper soil layers, samplingdensity usually decreases with depth. The depth of soil samplingshould extend to the groundwatertable. Suitablestandard samplingdepths are 10, 30, 50, 100, and 150 cm. Salt crusts shouldbe sampled separately. The samplingdensity required below 150 cm should be based on the obseried variation. Usually the salt compositionis closely related to the geologicalorigin and formationof the area or to the salt compositionof the irrigation water. Where these factors do not vary much over the project area, the soil salinity compositionalso does not vary much. In such cases, it will suffice to analyze only a limited number of samples. Of most interest are the ratios of Cl to SO4 andlof Na+ to (Ca++ + Mg++). Upon concentrationof a soil solution, CaSO4 will precipitate, whereassodium salts and chlorides remain in solution;therefore, these ratios tend to increaseas the salinity levels in an area increase. If the area is affectedby saline seepage or residual salinity, differencasin salt compositionmay help to untangle complicatedflow or soil patterns.

GroundwaterSalinity. The salt concentrationand compositionof deeper groundwater are largely a reflection of the geologicalhistory and origin of the differentaquifer formations, whereas for shallow (phreatic)groundwater, salt concentrationand compositionreflect the balance between the different sources of recharge and discharge and the interactionbetween groundwater and soil salinity. The quality of the deeper groundwatershould be investigatedwhen it is considered for irrigation purposes (to determine whether it is a source of seepage water or is located within the flow zone of a groundwaterdrainage system, in which case it will influence effluent quality). The quality of the phreatic groundwateris of major interest because it is a current source or may become a future source of soil salinitydue to capillary salinizationduring dry periods. The quality of this water also largely determinesthe drainage effluentquality of horizontalgroundwater drainage systems.

Soil Reaction. Drainage of certain soils, such as continuouslyinundated areas and grassland, sometimesleads to unpleasantresults. In these cases, the soils may become very acidic after the entry of air into the soil althoughthey may show only a weak acid reaction before drainage. It is of primary importanceto know in advanceif such a phenomenonis likely to occur. The pH value as the negative logarithm of the hydrogen-ionactivity must be determinedpotentiometrically in soil samples. Opinion varies as to the proper method for determiningthe pH values-a soil-water system versus a soil-potassiumchloride systerm. The importantpoint is to select a procedure and follow it closely, so that the readings will have the maximumdiagnostic value. The method used should be described accuratelyto aid others in the interpretationof results.

Soil Organic Matter Content and Soil Texture. Drainage of certain soils, such as organic soils and polder lands, leads to decreases in the land level (subsidence)and to increases in the crack formation (increase of hydraulic conductivity). The results will affect the groundwatertable levels and the design of drainage systems and practices. Therefore, it is important in such cases to predict the magnitude of subsidence,which can be calculatedfrom diagrams after consideringthe original thickness of soil layers and their clay and organic matter content.

Feasibility Studies Stage

The informationcollected during the identificationstage will delineatethe soil classes suspectedto have drainageproblems. Additionaldrainage investigationsand analyses must be prepared for each study category during the feasibility studies stage. Before any governmentcommits itself to investmentin a drainage plan, a concise presentationof all relevant informationis needed. This additional investigationtakes the form of a feasibility study that, for drainage projects, is -9 - undertaken by engineers in associationwith agriculturalspecialists. The findingsand conclusionsof the feasibility study are set out in a report. Relevant informationappears in appendixes,and topographicmaps, land classification,hydrological soil profiles, and aerial photographsare commonly included. Once the problems have been identified, correctivemeasures are costed, the possible benefits are evaluated, and the resultingproposal is presented to financingagencies. Drainage projects are financiallyattractive and funding is usually not difficult to obtain, especially when the following two points are considered in the report: (1) the need for investmentin serious studies and surveys and (2) the program's financial attractiveness,evidenced not only by local experience, but also corroboratedby similar undertakingsin other developingcountries.

The terms of reference of feasibility studies for drainageprojects vary considerably. If there has been no previous study of a region, a comparisonof the virtues of several areas is very useful. A prefeasibilitystudy can be carried out for the region, but a feasibility study must be prepared for each of the specificproject areas. A pattern of agriculturaldevelopment and the mechanicsof a water removal system must be worked out in sufficientdetail to allow an assessmentof the viabilityof each case. The feasibility studies of a drainage system must includean evaluationof the integrated economicand physical efficienciesof the system. System efficiency, regardless of how it is measured, must reflect the integrity, manageability,and long-termstability of the system. The feasibility of a drainageproject usually involvesthe following aspects:

* agricultural/engineering; D financial/economic; 3 social/organizational;and * environmental.

Agricultural/EngineeringAspects

The purpose of drainage investigationsis to collect in the field the relevant informationon soils and hydrologyon which the requirementsof the drainage system will be based. A descriptionof these investigationsfollows.

Surface Water Studies. As a rule, drainage systemsform part of and interact with existing surface water systems in the project area. Generally, several features of the existing systemsmust be investigated. Analyses of relationshipsbetween drainage and the surface water system of the area generally includeflood control requirementsand outlet conditions.

Flooding may be generated inside or outside the project area. Investigationsshould be undertakento establish such features as origin of flooding, seasonal periods of occurrence, frequency, depth and duration of flooding, affected areas, and resultant damage. This last item should include the damage caused by floodwater in the form of erosion of the land and depositionof sediments. The causes, nature, and extent of flooding should be establishedon the basis of such resources as available records of gauging stations, topographicmaps, hydrologicaland hydraulic calculations,and interviews. Surveys of the existingsurface drainage systemsmay reveal bottlenecksdue to inadequate design or to neglect of maintenance. Damage may be assessed from availableor collectedagricultural statistics.

Drainage systems must have an adequate outlet to perform their function. When atgravity outlet is not adequateor reliable, a pumped outlet should be considered. The investigationsnecessary to determine the suitabilityof an outlet depend on the characteristicsof the stream or area ithatis to - 10- serve as the outlet or disposal area. Where drainagesystems are to discharge into rivers, creeks, lakes, or other water bodies affectedby high water, it is necessary to determine the elevation, frequency, and duration of the high water as accuratelyas possible and to analyze the effect on the drainagesystem. High water conditionscan be obtainedfrom gauge records, observationof watermarkson the banks of streams or, lakes, and discussionswith local residents. The adequacyof natural outlets can be determinedby computingthe estimatedrunoff from the entire area they serve and by checkingtheir capacity.

There may be exceptionalcases in which the effluentfrom surface drains may be disposed of by using sumps that allow the water to percolate into the ground and join the groundwaterbody. This is possible only where the ground[waterbody itself has an outlet into a stream, has other drainage features, or has an outlet into an area where it will not be a problem. The infiltrationrate in these sumps must be high enough to support disposal of the necessary quantitiesto render the method economical. In some cases, inverted wells can be used to dispose of surface waste. Where a pumped outlet is considered, the conditionsunder which the pumpshave to operate must be established (annual volume, operation periods, required lift, water quality).

Stage-dischargerelationships (rating curves) will generally be availableor shouldbe establishedfor the proposed outlet points, covering a minimumperiod of 10 to 20 years and representingthe conditionsunder which the outlets are designedto function (proposed or foreseen upstream or downstreammeasures or works affectingthe discharge of the receiving system should be taken into account). It shouldbe shownthat the designed outlet can cope with the calculateddesign dischargesunder normal conditions,and the consequencesof dischargeunder abnormalconditions should be analyzed. The effects of the project dischargeson downstreamconditions should also be studied (such as effect on river morphology,flooding, water quality).

If a gauging station exists nearby, availabledata should be compiledinto stage-frequency- duration relationshipsfor selected calendar periods. Where such a station does not exist, data from a more remote station with a similar character may be used. The stage heights recorded at such a stationmay be transposedto the outlet point either by (a) hydraulic calculations,using the Chezy- Manningformula for canal flow (with measuredor estimatedroughness values); or by (b) a short measurementprogram, correlatingthe correspondingstage heights at the two sites.

Where data from gauging stations are unavailable,a new station may be establishedat the selected outlet point. A short measuringperiod of 1 to 2 years may yield sufficient,useful information,and extreme conditionsnot covered by this period may be assessedby correlative analysis (e.g., correlating the stage heights and/or dischargesrecorded in the measuring period of 1 to 2 years to the rainfall data for this period, the latter ideally forming part of a long series covering 10 to 20 years) or by other methods (e.g., estimatingdischarges on the basis of rainfall-runoff relationshipsand then determiningstage heights either by extrapolationof the short-periodrating curve or by hydraulic calculations). The proceduresdescribed here refer to outlets onto a river, though similar procedures apply to outlets onto a lake or the sea.

GroundwaterStudies. Water table surveysprovide valuable informationon the groundwater drainage conditionsin the area. Water table levels reflect the prevailing balance among the different groundwaterrecharge/discharge components. As the balance changes, so does the water table level. When the water table is permanentlyor seasonallytoo close to the soil surface, control by groundwaterdrainage systemsmay be required. The frequencyof measurementof depth to water table depends on the particular problem being studied. The frequency of readings may vary from - 11 - daily to quarterly; generally, however, readings shouldbe taken monthly. The objective of the measurementsis to establish a record of the water table fluctuationsover a period of time that will reflect all factors affectingthe water table. At least one full annual cycle of readings is needed before locating and designinga drainagesystem.

In water table surveys, interestprimarily focuses on long-termseasonal variations (short-term fluctuationsafter rain or irrigation may be studied separately,usually in connectionwith the functioningof a drainage system). The seasonal trends in water table levels are often closely related to the rainfall or irrigation regime in the area and may be identifiedadequately by observationsat a few selectedkey sites on key dates (selectionsmade on the basis of previous detailed studies). The prevailing seasonal water table regime can often also be deduced fairly accuratelyfrom the hydromorphiccharacteristics of the soil profile. The soil below the averagelow water table (the average summer/dry season water table level) typically has a uniform light gray to dark blue color associatedwith permanentnonaerated soil. The soil above the average high water table (the average rainy/winter season level) typically is uniformly brown to gray-brown. In between, a mottled gley zone may occur as a result of the intermittentoxidation and the reductionof iron and other elements. The upper and lower gley levels can usually be identifiedin a soil profile (after some initial guidance on the basis of availablewater table records), and these gley levels, recorded in soil surveys, provide valuable informationon the current water table regime in the soil. Hydromorphiccharacteristics in the soil change slowly, and it should be confirmedthat the observed gley features reflect the present groundwaterregime (fossil gley can occur in alluvial soils well below the present low water table levels, dating from a previous sedimentationand water regime period). Water table inforrmationcan be obtained from (a) records of observation wells, (b) piezometricreadings, and (c) water table data processing.

Water table levels (phreaticlevels) may be measured in bore holes reaching well into the groundwater. In a wide survey area, these observation wells may be placed in a grid or other regular pattern, althoughthe conceivedgroundwater flow pattern in the area should also be taken into account. Groundwaterflow will generally be down a slope, toward depressions and natural drains. A few judiciouslylocated observationwells may provide as representativea picture of the phreatic surface as that obtainedwith a dense grid system. Open water levels in wells and drains (natural or excavated) are also closely related to the phreatic surface. During periods of discharge, those levels will tend to be lower than the water tables in the adjoiningland. Only when there is static equilibriumseveral days after rain are these levels more or less the same. Well and canal levels may also be higher than the water table in the adjoiningland, e.g., when the well or canal walls have become sealed by sediments in the water.

Water table wells provide no informationon potentialdifferences within the groundwater body. Where such differencesare expectedto occur, piezometric studies should be carried out instead, as detailed in appendix 1. A piezometermeasures the hydrostaticpressure in a water body at the point where its filter is placed (see figure 1). Vertical water movementsbetween different soil layers can be assessed by installingbatteries of piezometers at different depths. The flow of water can be charted if the various hydrostaticpressures are known (see figure 2). These studies will also help to detect impedingand perched groundwaterconditions. Piezometers can also be used to study seepage flow from irrigation canals, reservoirs, and other water bodies.

The mere gathering of groundwaterdata is a needless expenseunless it is followed by plotting of the data in a form suitablefor study and interpretationof results. Interpretationbegins with the person gathering the data, who must remain alert to abrupt changes in conditionsand must - 12 - attempt to account for them. Water table data shouldbe analyzed to provide informationon the water table regime, such as the rates and directionsof groundwaterflow, groundwatersources and sinks, and regional and temporal trends. For this purpose the availabledata must be compiledin suitable graphs and maps as follows.

Hydrographdrawings may be made, showingthe elevationof the water table with respect to time for any single observationhole, well, or piezometer. Such a drawing clearly shows fluctuations in the water table as well as trends in water table movements. When analysis of the hydrographdoes not provide an explanationof certain problems, it may be helpful to superimposeadditional data onto the hydrograph, such as river stage, precipitation,periods of canal operation, and water deliveries. A useful tool in analyzinghydrograph data is to comparedepartures from normal weather data with hydrographfluctuations. The plot often explainsupward or downwardtrends in water levels.

All points at which groundwaterelevations were taken should be marked on a map of the area, and groundwatercontour maps rnay be prepared on the basis of reduced water table readings during key periods (e.g., the end of the dry and wet seasons). Water table maps show the direction of water movementby the shape and position of the contour lines, indicate the areas of recharge and discharge, and may provide some indicationof the relative hydraulic conductivityfrom the distance between the contour lines. The maps should also includeinformation on the constructionand depth of the well. This informationis useful in ensuringthat the water table maps show contours of hydraulicallyinterconnected groundwater bodies.

A contour map that shows the depth to water below the ground surface at any point can be prepared by overlayingthe water table contour map onto a topographicmap. Another method of preparing a depth to groundwatermap is to mark the measured depths to water from the ground surface on a base map at each measuriingpoint and to prepare a contour map from these values.

A depth to barrier map can be prepared in a manner similar to that used for the preparation of a depth to groundwatermap if sufficientinformation is availableon the location of the barrier. This type of map is useful in determiningdrain locations, estimatingquantity of groundwater movement, and providing other informationneeded for drainage calculations.

A water table profile can be made for a series of observationholes. The base profile is prepared by plotting the ground surface elevation;the location and depth of the observationholes; and any springs, canals, or ponds in the profile. A water table profile is more useful if it also contains informationon subsurfacematerial. The log data obtainedfrom installationof the observationholes can be plotted at each hole, and any other pertinent informationcan be plotted at its proper location. If soil texture data are available, it may be possibleto make tentative correlationsbetween observation holes. The elevationof the barrier in each hole should also be plotted on the profile, as this informationwill be helpful for locating drains and in calculatingother drainage requirements.

Readingsfrom several clusters of piezometerscan be plotted on a piezometricprofile drawn through the clusters. The elevation of the piezometricwater table for each piezometercan be plotted at the elevation of the bottom of that piezometer. Lines drawn through points of equal piezometric water table elevationshow lines of equipotential. Lines drawn from higher elevationsthrough lower elevationsand perpendicularto the equipotentiallines form a flow network and show the direction of movementof water and, possibly, the source of the water. This procedure is particularlyuseful in locating an artesian water source. - 13 - WaterBalance Studies. Generally, enough Flgure 1. Piezometersand Water Entry in Wells data and informationon groundwatershould be PEIZOMETER collectedto draw up a groundwaterbalance for the Tubeabout area whereby, ideally, a A 2 cm diam. distinctionis made between the situationwith and without the project. The \H\\ followingwater balance \ componentswould generally be involved. Watertable level The recharge by --- ersection rain water componentmay, powderedclay in some cases, be deduced from water table records (water table hydrographs). Waer Sophisticatedmethods of entry calculating recharge rates at tip - Coarsesand or from tensiometer readings is only finegravel generally confined to research projects. A _ _ reasonableestimate of orders of magnitude of deep percolation losses of infiltratedrainwater could also be obtained in some cases from soil moisture monitoringstudies or from soil moisture balance simulationstudies. Model studies could also sometimesbe used for this purpose.

The recharge by deep percolation losses componentgenerally varies with different soil, irrigation methods, and level of farm management. Orders of magnitudemay be estimated.from irrigation efficiency studies at the field level (monitoringof irrigation application,runoff losses, and soil moisture profiles).

To determine recharge by seepage from irrigation canals, empirical formulas or rules of thumb are generally not satisfactory,unless based on or supportedby recent measurementsunder comparableconditions. Generally,some differentiationamong the differenttypes of canals is also required (ined versus unlined canals, small versus large canals, recently constructedversus aged canals, different soil conditions).

The geohydrologicalsituation in the project area should be sufficientlyunderstood itoidentify the possible seepage pattern and seepage area, and the necessary studies should be underaken to arrive at quantitativeestimates of seepage influx rates.

Directions and rates of natural drainage to the natural sinks of the 'reg- u S lLs,-:S streams, valleys, and the sea) may generallybe derived from water table contuozmaps aBddwaer table profiles, together with informationon the geological/soilformation underlying the project area. Measurementof groundwaterdrainage discharge (baseflow)in the natural drains will also help to assess this component. - 14 - Other componentsmay Figure 2: Different Water Movementsas Indicatedby Piezometers includedirect evapotranspirationof 1 2 3 groundwater, groundwater _ l - pumping, or - i h - _ groundwater Layer I (root zone) drainage. Evapotranspiration _ becomes significant - _ - i when water tables are close to the soil D Layer 11 surface, and orders (poorlypermeable) of magnitude can _ - _ generally be establishedby calculationmethods. LayerIII Groundwater (highly permeable) pumping and artificial groundwater drainage should be Imperreablej base6e/ determinedon the basis of surveys (such as installed capacitiesand operation)and measurements. The contributionof each component and the balance between recharge and discharge componentsmay vary with time as well as with subarea.

Soil Drainability (HydraulicCGnductivity K-Values). Any drainage survey should involve an investigationof the top two meters of ithesoil profile with respect to the hydrologicalcharacteristics of the main soil layers or horizons. The best known method is determiningthe hydraulic conductivity in the field.

Hydraulic conductivityis one of the most importantsoil properties in relation to drainage because it largely determinesthe scope for drainage improvement,drainage measures to be used, and drainage costs. This importancejustifies the investmentof considerableeffort and expense and the use of the best methods to collect the necessary data. The drainabilityof the soil is also characterized by the hydraulic conductivityand the thicknessof the soil layers and by their position relative to the drain level. The hydraulic conductivityshould be determinedfor drainage investigationsby a suitable field method. Laboratory determinationsshould be completelyavoided because they do not reflect the field soil structure with its broadest mean, which includes water passagewayssuch as cracks, fissures, worm holes, and root channels. Moreover, the presence of such field water passageways may overshadowany expectedrelation, between the hydraulic conductivityand any soil physical properties. Therefore, estimatingand extrapolatinghydraulic conductivityvalues from other soil characteristicssuch as clay content or infiltrationrate should be totally avoided. The infiltrationrate is an irrigatabilityparameter that describesthe rate of water movementin the soil layers above the water table, and it has no direct impact on determiningthe drain spacing and depth. Also, many - 15 -

drainageresearch results have proved that the hydraulic conductivityvalues of heavy clay soils measured in the field may range from extremely low values to extremelyhigh values that exceed those of coarse sand.

It is unrealistic to expectthat any hydraulicconductivity survey will provide exact information about the actual hydraulic conductivityand its variation with soil depth, but the surveyor shouldseek to obtain a reasonableorder of magnitudethat can be substitutedin a drainage design formula. The Auger Hole Method introducedby Hooghoudt (1936) and described in appendix 1 is still considered the most common and reliable field procedure used in hydraulic conductivitysurveys. The observationpoints should be selected accordingto availablesoil maps and accordingto informationon the soil profile morphologyand its lateral variation pattern in relation to the physiography. The intensityof a hydraulic conductivitysurvey for the purpose of project design varies from one hole per 5 ha to one hole per 10 ha, dependingon the degree of homogeneityin the obtainablevalues. A higher intensityof one hole per 1 ha is used in establishingfield drainage testing or research fields. Maps should be prepared at a suitablescale (1:5,000 to 1:10,000 for final design work) showing the K-valuesdifferentiated, if possible, for K1above and K2 below the drain level and also showing the depths to the impermeablestratum. This process permits the identificationof areas that would be uneconomicalto drain.

All auger holes shouldbe drilled at least to the depths where the drains will be installed. It is advisableto extend one hole in ten to the relativelyimpervious layer (barrier), three holes in ten to a depth below the drain level, and six holes in ten to the depth of the drain level. The values of hydraulic conductivitycan be marked on the aerial photograph at points correspondingto the positions of the holes. The project area can be classifiedinto zones in which the hydraulic conductivityvalues are relativelyuniform (see figure 1-2 in appendix 1). The average value of hydraulic conductivity can be calculatedfrom this data. Additionaldata may be required if the hydraulic conductivityis highly variable. A farm on which the values of hydraulic conductivityare relativelyuniform may be given an average farm value by simple calculationof the arithmetic mean. It may sometimesbe necessaryto divide the farm into sections or portions, each having values that are relatively uniform but whose averages are different. Areas with an averagehydraulic conductivityof 0.10 m per day or less are marked "not suitable for subsurfacedrainage." The chosen value of 0.10 m per day is based on economicconsiderations that take into accountboth drain spacing and speed of reclamation. The hydraulic conductivitycan change through alternate drying and wetting of the soil or through leaching of excess salt. Some isolatedspots of very low hydraulic conductivitymay be encounteredon farms that are otherwise reasonablypermeable. These isolated areas are not included in the computationof the average. The drainage of such localizedimpermeable mounds is best achievedby draining the more permeable surrounding material into which these moundscan then dewater. If possible, installationof subsurfacedrains in such impermeablespots shouldbe avoided.

The followingsoils require special attention when assessingtheir hydraulic conductivityfor drainagepurposes:

* subaqueoussoils that may have very low permeabilityvalues in the unripened stage but that may easily develop very high permeabilityvalues in the ripened stage;

* substrata of river plains, which may have a high anisotropyfactor (K/K,, = 20 to 30);

* salty soils, which may loose much of their permeabilityupon leaching with fresh water; - 16 -

o heavy clay soils, which may have a stable crack pattern and high permeabilitybelow the wetting front;

* soils that derive a high permeabilityfrom old root channels (mangrove land, banana estates); and

* gypsiferous soil with piping problems.

Drainage Criteria/DrainageCoefficient of GroundwaterTable (q). Excess rainfall and/or irrigation water are the main sources rechargingthe groundwatertable balance, and consequently these sources determine the amount of excess water to be discharged. Normally, the free water body in the soil profile is the groundwater. The objectiveof drainage is to remove the excess water from the soil profile in a way that results in economicaland optimum control of the groundwatertable and soil moisture conditionswith respect to crop yields. A further objective in arid areas is to prevent the accumulationof salts in the root zone or to leach accumulatedsalts out of the salt profile. The objective of removing the excess water from the soil profile can be achieved only when the excess water occurs under a positive pressure as free water. When the water in the soil occurs under negative pressure (less than atmosphericpressure), it will not enter a drain and consequentlycannot be removed from the soil profile by drainage. The chosen combinationof the required drain discharge and the controlledgroundwater table represents the drainage criterion in the formulas used to describe groundwaterconditions. The required control of excess water is complicated,and any drainageproblem should be characterizedby its own special soil, agronomic, hydrological, and even economicconditions. Excess water control, therefore, requires a specialdrainage criterion based on the results of drainage investigations. Because it is almost impossibleto determine and take into account all these relevant conditions,however, identical and simplifieddrainage criteria based on empiricalknowledge are generally appliedon large projects. Empirical knowledgecan be gained only from years of experiencewith installed drainage systemsor from the results of field experiments.

Criteria for groundwatertable control and drain discharge can be culled from careful analysis of the agronomicand hydrologicalconditions in a project area. For steady state groundwater conditions,the drainage design formulas (which are discussed later in this paper) can be written in general form:

L2 = 8KD * hlq where KD stands for the soil mediumtcharacterized by hydraulic conductivity,thickness, and position relative to drain level of the various layers discerned, and hlq standsfor the chosen combinationof groundwaterlevel and drain discharge required to prevent the occurrence of excess water in the root zone. The term hlq is thus the drainage criterion for steady state groundwaterconditions. Under non-steady state groundwaterconditions, drainage criteria cannot be expressed in terms of a fixed water table elevation with a correspondingfixed drain discharge. Instead, the criteria are formulated in terms of a required rate at which the groundwatertable must be lowered. This can be seen in the modified Glover-Dummdrainage design equation(which is discussed laterin this paper):

L2 = X/2 . ./U . tlln (1.16 * h/h, ) where Kdl/ucharacterizes the soil medium, and the term tlln (1. 16 *hlh) standsfor the drainage criterion for non-steadystate groundwaterconditions. - 17 - Should steady state or non-steadystate solutionsbe applied in irrigated areas? As mentioned earlier, the appropriate choice of drainage criterion depends on hydrological, soil, climatic, and agronomicconditions. The complexityof the interrelationamong these conditionsmeans that a drainage criterion shouldbe regarded as no more than an attempt-though based on empirical knowledge and theoreticalreasoning-to express the aims of a future drainage system in a single value, such as hlq, which can be handled mathematically. Percolationlosses from irrigation are typically non-steadystate, but when an irrigationunit is irrigated over a full rotation period, the average percolationlosses become steady state. Moreover, the seepage losses from the canal system can also be considereda steady state. Accordingly, steady state solutionsare increasinglyapplied because they are easy to use and because they produce results that closely approximatethose obtained under actual field conditions. Whatever conditionsare chosen, a reasoned estimate of the drainage criteria should specify (1) a critical groundwatertable depth that should not, or not frequently, be surpassed; and (2) the quantity of excess water that should be discharged at a certain drainage rate or within a certain period.

The critical water table depth is often used as a criterion for determiningdrainage needs. Talsma (1963) defined it as the water table depth from which, at steady state, an evaporationflux of 1 mm/day- can be maintained. Peck (1978) stated that for dryland conditionsand a Mediterranean climate, a critical flux of 0.1 mm/day-' would be reasonable,compared with 1.0 mm/day-' for irrigated conditions. Moreover, Peck calculatedthe critical depth from the properties of ten soils and obtained a range from 0.9 to 6.6 m for irrigated land and from 1.6 to 3.1 m for dryland conditions. A shallow groundwatertable, in additionto numerousother complicatedfactors, may cause waterloggingand salinizationhazards within the root zone. Therefore, the choice of the "critical groundwatertable depth' mainly involvesusing empiricalknowledge; some critical water table depths under arid conditionshave been suggestedby Kessler (1970) and are shown in table 1. It is preferable not to use the conceptof critical water table depth in assessingthe performanceof a drainage system installed in a salt-affectedsoil. Instead, the change in soil salinity with depth provides a useful diagnosticparameter in assessingthe salinityhazards present.

Table 1. Critical Water Table Depths under Arid Conditions(cm)

Critical water table depths To prevent To minimizesalinization Soil texture waterlogging by capillarity Sandy 60 80 Silty loam 90 200 Clay loam 120 150 Heavy clay 150 120

Source: Kessler 1970. - 18 - The availabilityof computers makes it easy to calculate daily water table elevationsfrom long-termrecharge water records for specific soil and drainage system conditions,but these frequency distributionshave little meaning if they cannot be related to crop responses.

The types of crops to be grown and their drainagerequirements are important factors influencingthe prescribed drainage plianfor a given area. Drainage requirementsfor shallow-rooted crops are different from those for deep-rootedcrops. Some plants require well-drainedsoils; others are classifiedas "water-loving." For most crops, the groundwatertable limits the growth of plant roots and also the availabilityof nutrnlentsto plants. The response of specific crops to water table elevations,however, is not entirely known. The numerousfactors influencingcrop response render it difficult to determine the influenceof individualfactors. Many attemptshave been made to establish this relationship;the followingexamples are provided to aid in finding similar approaches.

Rather than establish frequency distribution,Sieben (1964) tried to arrive at a single value, termed the SEW3, value, through studies conductedon drained plots of young, light clay soil located in Zuiderzee polders in Holland. Tha SEW30value denotes the sum of all daily values of the amount (in centimeters)by which the water table exceedsthe level of 30 cm below the ground surface in the winter season. For example, if the water table is found at 25, 15, and 10 cm below the ground surface on three successive days, the SEW30value is equal to (30 - 25) + (30 - 15) + (30 - 10) = 40 cm. Sieben related the SEW30 values to crop yields and found no yield reductions for values of less than 200 cm, which were determinedin plots that can drain 7 mm per day at a water table depth of 30 cm below ground surface. The selectionof the base level of 30 cm did not appear to be significant; when levels of 10, 20, and 40 cm were used as standards, comparableresults were obtained.

Another attempt to relate waterloggingconditions to crop response was made in France by Poiree and Ollier (1962). Waterloggingin their studies refers to a water table considerablyin excess of 30 cm and 50 cm below the ground surface for grassland and arable land, respectively. Their results indicatedthat when waterloggingoccurred for three consecutivedays in March, yield reductions of 30, 10, and 5 percent resulted for potatoes, sugar beets, and winter cereals, respectively. When waterloggingoccurred for the same period in June, yield reductions for the same crops were 50, 10, and 20 percent, respectively.

Van de Goor (1972) reported field study results on the effect of different levels of groundwateron the yields of different crops grown on a clay soil (table 2). The obtainabledata show that the best production of grain, root, and tuber crops was experiencedwhen the groundwatertable was 150 cm from the ground surface. Another long-termexperiment carried out in a muck soil produced the results shown in table 3, which indicate that (1) the best yields of most crops were obtainedfrom the greatest depth of water table; (2) the averageyield of all crops grown with a water table height of 60 cm increasedby 35 percent comparedwith those obtainedwhen the water table height was 40 cm; and (3) the effect of good and deep drainage is partly due to the stimulationof nitrogen fixation and nitrificationprocesses, the increase of phosphate and potassium availability, and the presence of suitable soil temperatures.

The drainage criteria generally applied in the Netherlands,regardless of differencesin soil type, topography, or drain depth, are based on steady state flow conditions. The criteria state quantitiesof water to be evacuated in a given period of time at a certain fixed position of the water table (table 4). Table2. RelativeCrop Yields Obtained at GroundwaterLevels Ranging between 40 and 150cm belowSoil Surface

Grain, root, tuber crop yield (%) Actual Straw yield (%) Actual yield at yield at No. of Water table depth (cm) 100% Water table depth (cm) 100% (kg/ha) Crop years 40 60 90 120 150 (kg/ha) 40 60 90 120 150

Cereals 100 8,600 Wheat 6 58 77 89 95 100 4,600 59 75 84 92 93 100 5,150 Barley 5 58 80 89 95 100 4,100 57 76 84 100 5,850 Oats 3 49 74 85 99 100 5,000 60 82 89 98

Pulses 100 100 3,550 Peas 4 50 90 100 100 100 2,750 67 94 100 tO0 100 100 4,500 Beans 3 79 84 94 94 t00 3,100 86 95

Otlher 100 t00 5,100 Caraway 3 80 96 100 100 100 1,700 93 98 97 97 100 6,400 Rapeseed 2 79 95 98 98 100 2,500 70 84 92 Sugar beet 100 100 6,500 seed 1 75 82 96 96 100 4,250 78 94 95 n/a n/a n/a Sugar beet 2 71 84 97 97 100 40,500 n/a n/a n/a n/a n/a Potatoes 1 90 100 92 92 96 26,000 n/a n/a n/a n/a n/a = Not applicable. Source: van de Goor 1972. - 20 - Table 3. Relative Crop Yields Obtained at Groundwater Levels Ranging between 40 and 120 cm below Soil Surface Water table depth (cm) Crop No. of years 40 60 90 120 Potatoes 12 46 94 97 100 Corn 9 71 100 103 100 Peppermint 13 48 91 100 100 Onions 11 63 109 113 100 Sweetcorn 4 61 100 92 100 Carrots 4 59 93 96 100 Average 63 98 100 100

Source: van de Goor 1972.

In the United States, the required steady state drainage rates vary between 5 and 20 mm/day. Drainage systemsthere are designedon the basis of a falling water table. High water tables occur in the United States only after occasionalintensive rainfall, and the period between intensiveshowers is fairly long. Walker (1952) stated that when the water table rises in mineral soils up to some 15 cm below ground surface, it shoulddrop to 35 to 40 cm within 24 hours. Kidder and Lytle (1949) advised that when a water table after a shower has risen to the ground surface, it should drop from the surface to 30 cm within 24 hours and to 50 cm within 48 hours.

The determination of the drainage coefficient q is vital because it largely determines the capacity of the drainagesystem. Benefits increase with increasingvalues of q, but costs also increase. The choice of the drainage coefficientthus involves the optimizationof the expected

Table 4. Common Drainage Criteria in the Netherlands Depth of water table below ground Type of Drain discharge surface midway land use (mm/day) between drains (cm) Grass 7 30-40 Arable 7 40-50 Polder 10 30 Orchard 7 60-70 Vegetable 7 70

Source: van de Goor 1972. - 21 - benefits in relation to the costs. In land drainage, this optimizationgenerally comes down to choosing excess water (usuallyrainfall) over the land/crop that should be protected. Many countrieshave adopted standard drainage coefficientvalues to be used under different conditionsor have adopted standard methods for the determinationof q-values. These standardsmay be acceptablewhen they have been adequatelytested under the conditionsto which they are applied. Formulas and methods used to derive drainage coefficientsshould always be based on the hydrologicalprocess leading to the generationof discharge (usually rainfall-runoffprocesses). Generally, the following aspects requires clarification.

All relevant sources of excess water shouldbe analyzed(such as rainwater, floodwater, seepage water, snowmelt, and irrigationwaste), and the critical source should be defined and quantified. Regionaland seasonal variationsshould be investigated,as should differencesin water quality. When the source of excess water is stochastic,frequency analyses should be made. A differentiationmay be made between areas where the groundwateris recharged by rainfall and areas where the recharge by irrigation losses is most critical. In almost all temperate humid climates, subsurfacedrainage requirements shouldbe based on controlling water tables under rainfall recharge, whereas in almost all arid to semiarid climates, the water table must be controlled for rechargeby irrigation losses. In intermediatecases, a careful study must be made to identify which type of recharge is most critical. Design discharge may be based on experience, measurement,water balance studies, or groundwatermodel calculations. In countries with a long history of subsurface drainage, values to be used in design are usually well established. Water balance studies, discussed previously, can be used to establish groundwater surpluses over different time intervals. Considerableprogress has been made in modelingof groundwaterand water table regimes. These models can be used to establish required rates of groundwaterremoval.

Drainage coefficientsused in subsurfacedrainage design for water table control under rainfall recharge conditionsvary primarily within the narrow range of 5 to 10 mm/day. Values in this range have been found to represent a technical-economicoptimum for pipe drainage under these conditions. For mole drainage, higher values must be anticipated. For areas where subsurfacedrainage has been newly introduced, establishinga pilot scheme (describedlater in this paper) as soon as possible is advisable. The results of such a scheme, especiallyin combinationwith groundwatermodel studies and calculations,allow the verificationof adopted design criteria. Design dischargesfor main channelsshould be based on field drainage coefficients,although a differentfrequency of occurrence (safety factor) may be chosen. Other sources of excess water (such as seepage and sewerage disposal) must also be taken into account.

Drainage coefficientsfor use in subsurfacedrainage systems for water table control under recharge by irrigationlosses conditionsmay be determinedby groundwaterbalance calculations. Relevant guidelineshave been presented in FAO 1980. In addition, salt balance studies shouldbe made to determine leaching requirements. At a minimum, drainage coefficientsshould equal the leachingrequirements. The time period over which water and salt balances are to be maintained shouldbe analyzedcarefully. When annual balances are to be maintained,the course of the water table and soil salinity during the year should be investigated. It shouldbe demonstratedthat no adverse effects are expected.

Investigationsshould be made of present land use and expected future changes in land use in the area. The tolerance and sensitivitiesof the different types of land use to excess water should also be examined. - 22 - The design conditionsunder which the drainage system is designed to function should be spelled out clearly and explicitly. In particular, this requirement applies to:

* specificationof the critical rainfall event (designrainfall), indicating season of occurrence, frequency, and duration, all based on analysis of local conditions;

* probability of the joint occurrenceof excess water from two or more sources (e.g., the coincidenceof rainfall and irrigation spillage);

* deductionsmade for different types of storage (such as in the soil, on the field, in the drain system, in reservoirs), for evaporation/evapotranspiration,or for other items; and

* estimateddamage resulting from uncontrolledexcess water (preferablya quantitative assessmentbased on field observations,farmers' records, experiments; a qualitative assessmentmay also suffice in some cases).

Differentdrainage coefficientsmay be recommendedfor the differentparts of the drainage system. For example, a value may be chosenfor the design of the channel sections and another may be chosen for the structures. Allowancesmay also be made for, as an example, expected poor maintenance. In all cases, sound argamentsmust be advancedto support such special treatments.

For the steady or non-steadystate conditionsprevailing, a reasonableestimate of the drainage criteria should specify the critical groundwatertable depth that shouldnot, or not too frequently,be surpassed and also the quantityof excess water that shouldbe discharged at a certain discharge rate or within a certain period. Any method that can result in a reasoned drainage criterion has to (1) take into accountthe actual observed rainfall, both its depth and distribution, over a sufficientlylong observationperiod; and (2) treat the discharge as a non-steadystate process. The resulting method could then be:

1. Calculatewith a computerthe actual groundwaterstorage above the drain level from the original set of daily rainfadldata for a large number of years for several alternative drainage intensityvalues i(J). This storage value is equal to pL2/TKd (as described later in this paper). Any reduction in the amount of rainfall that is supposed to reach the groundwater table, due to surface runoff or storage in the soil moisture reservoir, should be introducedbefore the data are given to the computer.

2. Translatethe calculatedchanges in groundwaterstorage due to fluctuationsin groundwater heights above the drain level (groundwatertable hydrographs)by introducingrelevant values for the storage coefficient.

3. Characterizethe relation between groundwaterlevel and crop yields as Sieben (1964) has suggestedusing sum exceedencevalues above a chosen groundwaterdepth and for the critical drainageperiod. The relation might be characterizedby a permissiblefrequency of exceedencefor a certain groundwaterdepth, e.g., 1 time per year for a groundwater depth of 25 cm or 1 time per 5 years for groundwaterreaching the ground surface. This characterizationalso has to take crop type into account. The permissiblefrequency of 1 time per 5 years is the return period consideredwhen providing surface drains for rice production, a plant that can tolerate submergencefor up to 72 hours. Optimumrice - 23 - yields are obtained when the soil is alwayssaturated (groundwaterabove ground surface), assumingthat weed control is adequate.

4. For any selected drain depth and by using the computer-generatedgroundwater table hydrographs, find which drainage intensityvalue J will meet the conditionsof groundwaterdepth control (as discussed earlier). When necessary, the J-value can be expressedin the ratio qlh by substitutingthe relevant value of p so it can be introduced into the availablesteady state drainage formulas.

The criteria mentioned earlier are based on experience and consequentlycan be applied only to the conditionsfor which they were established. Comparison of criteria among countries is difficult due to differences in climate, soil, and croppingpatterns. Nevertheless, there appears to be a striking conformitybetween criteria applied in France and in the Netherlands. The falling water table criterion in the United States is also comparableto some extent to the Dutch steady state criteria. For arable land with drains at a depth of 100 cm, the Dutch standard discharge rate is 7 mm of water per day when the available water head is about 50 cm. If the water table rises to the ground surface, the water head becomes 100 cm and the discharge is about 14 mm per day. The average rate of discharge correspondingto a water table drop from the ground surface to 30 cm will be about 12 to 13 mm per day. The soil would need an effective porosity of 4 to 5 percent to release this amount of water, and this amount is common in Dutch clay soils.

Verificationof Drainage Parametersin Field Drainage Pilot Demonstration/DesignTesting Fields. Field drains are the only part of any drainage systemthat directly affects groundwatertable behavior, and consequentlythe drains affect crop yield. The design of an adequateand economic field drainage system should be emphasizedand is attainableonly when reliable drainage information is availableto rationalize the drainage design and to establish adequatesystems. In countries where few field drainage systemshave been installed, calculatedand estimated drainage intensitieshave to be verified and tested through field investigations. These investigationsshould be carried out on special experimental/testingsites carefully selectedto represent the prevailing conditionswithin the entire project area (as shown in appendix 3). Generally, any observationof the groundwatertable behavior in its relation to an installeddrainage system can be considereda drainage experiment. The obtainabledata can also be related to soil conditions,hydrological characteristics of the aquifer, land topography, climate, irrigation practices, and crop response. Drainage testing/researchfields (as described in appendix 3) must be suitable for both short- and long-term tests.

An important considerationregarding short-termtesting is that it is limitedvis-a-vis time, human resources, and funds and does not permit a high density of investigation. Within a short period, the testing scheme should provide reliable justified conclusionsrelated to practical experience in the area. The aim of short-term drainage experimentsis to:

* verify the initial assumptionsconcerning certain hydrologicalsoil characteristicsthat determine the groundwaterflow to the drains and water table, such as storage capacity, drainableporosity, and thickness of the phreatic aquifer; and

* test the groundwatertable behavior as inducedby the depths and spacingsof the field drains being tested.

Water table elevations and drainage discharge measurementsshould be carried out daily or at least twice weekly; in the latter case, measurementsmust be made daily during rainfall or irrigation. - 24 - In exceptionalcases-when the groundwaterreservoir reacts very rapidly-more than one observation a day may be needed during rainfall or irrigation. The frequencyof observationsactually carried out shouldbe governed by local conditions(hydrological, climatic, etc.) and the direct goals of the experiment. Generally, a short period of intensiveand completeobservations is preferable to a long period of infrequentand scattered observations. For the determinationsof soil-hydrological characteristicsand for the verificationof initial drainage concepts and related assumptions,the testing period may extend to only a few months in irrigated areas, whereas a full rainy season is often required in humid areas. Long-termifield drainage investigationsare an extended set of studies carried out on the same testing fields used for short-term studies. Long-terminvestigations can be operated for several years to:

* study the relationshipbetween crop yields and the groundwatertable regime;

* investigatethe effect of alternativefarm water managementpractices on the salt balance and the effect of the water balance in relation to crop response; and

* test the performanceof drainage materials and alternativeinstallation practices.

Discharge CapacityQ of Main Drainage Systems. Becauseof the relatively significant transformationof recharge occurring in groundwaterdrainage, field drainage discharges are generally much more attenuatedand moderated than those of main drainage systems. Moreover, design dischargesof main systemsmay generally be based on average high discharges rather than on peak discharges. Main system dischargesare formed by combiningdischarges from field systems, and therefore the design discharge of main systemsmust always be closely related to the adopted field drainage criteria, which specify, among other things, the rates of excess water removal. In rare cases, the design discharge may be straightforwardlyderived through statisticalanalysis of observed discharges. Usually, however, design dischargeshave to be calculatedusing one of several methods available. The most general method of calculatingthe design discharge Q of a drainage system is by using the formula:

Q = q4 where:

Q = design discharge in m3/s

qO = drainage coefficientin 1/s/ha or m3/s/km2

A = drainage basin in ha or km2

x = area reductioncoefficient (dimensionless)

The drainage coefficientis the design discharge per unit area for small basins. For large basins, the attenuationof the discharge hydrographdue to storage and resistance in the main system and due to the nonuniformityin rainfall coveragemust be taken into account, a process that generally results in lower design dischargesper unit area as the size of the basin increases. This effect is explainedby the area reduction factor, which is representedmathematically by an exponent of the area A (it always holds that x < 1.0). Because of the lack of analyticalmethods for deriving area reduction factors, these values must almost always be based on experience. Experience indicates that discharge - 25 - reductionsfor basins in temperate and humid climates are minimal; generally, no reductions shouldbe appliedfor basins up to 10,000 ha. For basins between 10,000 and 100,000 ha, reductions should generallybe not more than 10 percent. In the United States, an area reductionfactor of x = 0.8 has been satisfactorilyapplied over a range of conditions. Lower values on the order of x = 0.5 to 0.6 are used in arid climates. Some guidanceas to the x-values to be used may be derived from rainfall uniformity studies (these may be availablefor the region or for the climatic zone in which the project is located).

The above formula is most applicableto relativelyflat areas. For markedly sloping land, formulas of this type are not generally used. Rather, design discharges for such basins are determinedby methodsthat integrally capture the rainfall discharge process without explicitly recognizingsuch concepts as the drainage coefficientand the area reduction factor. Empirical methods can be used when they are supportedby substantialevidence of soundness. When such evidencedoes not exist, preference is given to methods that are based on a rational approach. In particular, the followingthree methods are recommended:

1. Unit hydrographmethod. A design storm is convertedinto discharge on the basin unit hydrographs, which are derived from observed hydrographsof the concernedor comparablebasins.

2. Soil Conservaion Service (USDA)method. The design rainfall is convertedinto runoff by means of the curve-numbermethod, while the calculatedrunoff is subsequentlyconverted into discharge by means of establishedhydrographic relationships(synthetic hydrograph or other).

3. Comprehensivemodels. These models realistically capture the entire hydrologicalprocess by which rainfall is convertedinto discharge.

Design dischargesgenerated by surface drainage are usually vital because magnitude,and therefore the cost of safe disposal, is often considerable;furthermore, considerabledamage often results when discharge capacitiesare inadequate. Surface drainage discharges, however, are also difficult to estimate. Many rational approaches, such as those cited above, have been developed based on careful analysis of the involved hydrologicalprocesses. None of these approaches, however, can be confidentlyaccepted as providingreliable results under all conditions. Therefore, measurementsprovide the best basis for estimatingdesign dischargesgenerated by surface drainage. Ideally, a series of about 20 years of measured peak dischargesshould be availablefrom which design discharges can be determined by statisticalanalysis. However, a short measurementprogram covering even just one good-sizeddischarge event is helpful, because thesemeasurementscan be used to calibratethe developeddesign discharge calculationmethods. Most of these methods rely on coefficientsand parameters that capture the responseof the basin to rainfall. For example, measurementcan be used to derive unit hydrographs, or parameters such as lag time as well as concentrationtime to peaks can be used to construct a synthetichydrograph for the basin. These measurementscan then be used to convert design storm runoff into discharge. Measurementsare also valuable in calibratingthe different methodsused to determine how much of the design rainfall will actually run off (expressedby runoff coefficients,graphs, and other parameters). All these checks on the reliabilityof the applied design discharge calculationmethods greatly improve the confidencewith which results of such calculationscan be accepted. Whereverpossible, therefore, such a measurementprogram should be conducted. Normally, the period between project identificationand appraisalwill suffice to completea short measurementprogram for this purpose. - 26 - The rationale of the design must alwaysbe clearly articulated. In particular, it must be clarifiedhow costs and benefits have been taken into account. All assumptionsmust be clearly described and defended (land use, critical period/season, choiceof frequency and duration of design rainfall, assumedstorage and other deductions,required evacuationtimes, assumedmaintenance and operation standards). The same applies to the relationshipbetween design discharge and freeboard. In some cases, it may be necessary to conduct a special study to analyzethe consequenceof damage causedby infrequent, uncontrolledrainfall events for the affectedgroup of farmers and to prepare a special relief program for these groups as part of the drainage project. A problem often encountered in a drainage project is the alteration of hydrologicalprocesses in the basin by the planned project measures, and such changes must be taken into accountwhen detrmining the design discharge. Predicted changesare most convincinglyestablished on the basis of experiences(preferably supported by measurements)from elsewhereunder comparableconditions. In some cases, it may also be acceptableto make predictionsby evaluatingthe effects of changing conditionson basin discharge based on well-establishedrainfall-discharge relationships.

Quite often, surface drainage projects include rice land. The hydrologicalprocesses and the drainagerequirements of rice land are generally differentfrom those of other dry food crops, and differentdrainage coefficientsshould be establishedin general. In the traditionalrice-growing areas in the monsoonclimates of SoutheastAsia, establisheddrainage coefficientsare still primarily based on traditionalpractices. Significantly,land use is often adaptedto the natural drainage conditions. As a result, rather low drainage coefficientscan be used. The introductionof improved rice varieties/croppingpractices, crop diversification,and intensificationof land use will require a thorough review of present drainagepractices in these countries. Generally, this review will result in higher drainage coefficients, though it may be difficult to provide adequateproof of cost- effectiveness. A regional consensuson drainagecoefficients for use in modern rice-growingprojects in monsoonalSoutheast Asia is being established,and project proposals should generally agree with present thinkingin the region. In this regard, exampleson the estimationof design discharge from peak surface runoff are provided in appendix4.

Financial/EconomicAspects

FinancialAspects. Three questionsmust be addressedwhen a drainage project is considered:

* whether there will be funds to completeit; * whether there will be funds to maintain it effectively;and * whether the financial and social benefits will make it worthwhile.

Having funds for implementationis not simply a matter of having cash in hand at the outset. A large drainageproject may be phased in over ten or more years. Financing failures preclude the planning of construction. Plans that are delayed when partially completedtend to languish and decay, and the disturbancein the schedule engenderswaste and disenchantment. In addition, there is little point in carrying the preliminarydesign beyond the stage where uncertainty in costing due to the design method is comparableto uncertaintydue to the vagaries of the tendering process. Costs for drainage are usually expressed,for convenience,on a per hectare basis and may vary widely dependingon such elementsas the scope of the project and the intensityof drainage. The costs of items involved may differ accordingly. Four examplesof cost estimates and actual costs for some drainageprojects implementedin several countries are given in appendix5. - 27 - Some specialistsfeel a project should aim to be self-liquidating,but few schemes, especially in poor countries, would be viable if this were regarded as a rigid rule. The benefits of drainage are both economicand social; some are direct, others are indirect; some are assessable,others are intangible. An authority may wish to take into accountthe demand created for services, materials, and equipmentthat stimulatesother sectors of the national economy or has a favorable effect on the country's balance of payments as a result of expandedfarm production. When drainage contributesto the settlementof land, it may be regarded as a social service that provides the unemployedwith a livelihood and restores the self-respectof the individual. If such factors are considered,farmers may be aided by tax relief on earnings and supplies or by any other suitablemeasure. A sound practice is for services related to drainage at the off-farm level to be charged to the farmer at a rate sufficientto provide revenue to cover the operatingcosts. This practice usually creates respect for the services supplied and promotes a sense of independencein the farmer. The capital cost of the scheme should be accepted by the state.

Economic Aspects.. The far-reachingnature of the effects of a drainage project renders full assessmentof the benefits virtually impossible,and the data availableare often insufficientto warrant a full economictreatment. A caveat is necessaryregarding the tendencyto justify drainage projects using bogus figures appliedto unrealistic criteria when there is confusedthinking over the aims of drainage. It is often unrealistic to apply the standard test of the annual benefit/cost ratio, and crude tests serve only as a means of comparisonbetween schemes. The costs used in benefit/cost studies are not necessarilythose used in estimatingconstruction costs. The costs of roads and other amenities necessary for the proper operationof the drainage system may be included, although they fall on some other body. Rates of interest on capital for constructionand rates for labor may be artificial as a result of governmentintervention and thus need to be adjustedto free market rates. The costs to the farmer of on-farm works and his or her labor costs shouldbe included, as well as his or her increasedexpenditure on seeds and fertilizers and the depreciationof farm machinery.

T-hebenefits of the project as a whole should be based on the farm prices for produce as applied to the estimatedproduction. The gross present value of production can be deductedfrom the gross future returns to provide the gross benefit. The increasedcosts of production, including maintenancecharges, can be deduc:edfrom the gross benefit to provide the net benefit. The tests that may be used are the ratios of gross farm benefit to investmentand of net farm benefit to investment. These are useful only for comparisonand are preferably applied to schemes with similar agricultural conditions.

The economicefficiency of a project is generally evaluatedby traditionaleconomic measures such as the rate of return (RR) and the net present value (NPV). A conventionalassessment should be made of the economicbenefits of the directly productive elementsof the project. The most common assessmentmethod is to measure the project's internal economicrate of return (ERR), which may be defined as the rate of discount at which the total present value of costs incurred during the life of the project is equal to the total present value of benefits accruingduring the life of the project. In an investmentproject, costs are typically bunched at the beginning of the project, whereas benefits begin to accrue only after a lapse of time. The applicationof a discount factor enables these costs and benefits to be comparedon the basis of their present value.

To calculatethe internal economicreturn, it is necessaryto construct a table showing the cost and benefit streams and the incrementalincome as they accrue each year during the life of the project. The cost streams used for economicanalysis should includethe capital costs of the project (including physical contingencies),as well as the incrementaloperating costs of farmers and of any project - 28 - authority. Maintenance costs are usually estimatedbased on a percentage of the capital costs of the works. If the life of a componentis less than the life of the project (whichmay be the case for pumping plants, for example),provision should be made for the cost of replacement. Residual values of project items at the end of the accountingperiod shouldbe taken into accountas negative costs when they are expected to be significant. The economicbenefits stream normallyincludes the value of the incrementaloutput of the project, valued accordingto projected economicprices and taking into account any changes in quality.

A further economicindex often applied to projects is the NPV of benefits, derived by discountingthe net benefit stream by a factor equal to the opportunitycost of capital or to the investmentrate of interest. The analyses shouldbe strictly economicrather than financial-analysis shouldreflect the point of view of the economyas a whole rather than the standpointof the individual farmer or other entitiesparticipating in the project.

Because project estimatescan be subject to a considerabledegree of error, it is common practice to determinethe effect of changes in the levels of the most critical variableson the ERR. Such sensitivityanalysis should be based on meaningfulpercentage changes in the values of the individualvariables concerned. Althoughthe analysis may be directed toward revenue and cost streams, it is useful also to examinethe sensitivityof specificvariables (e.g., prices, yields, costs of major items, delays in receipts of benefits, carryover of stocks). The reasons for selectingboth the variables and the percentage of change appliedshould be noted.

Further details are beyond the scope of these guidelines; additionalinformation, however, is available in the literature (see, for example, Bergmannand Boussard 1976; Newman 1976; and Gittinger 1978).

Social/OrganizationalAspects

Technicalprojects often fail due to neglect of the human factor-failure to take into account such matters as the social organization, attitudes, and skills of project personnel and farmers, as well as prevailingproduction organizationsand farming systems. Even where, in general, drainage projects bring about less drastic change and interfere less with ongoing farming activitiesthan do, for example, irrigation projects, proper attention should be given to the human factor and its various ramificationsto ensure that the technicalworks are accepted and fully utilized by the intended beneficiaries.

Social Aspects. A large irrigation/drainagescheme imposesa major change on the communityand may initiate a sequence of changesresulting from affluenceand a new way of life. The full implicationscannot be predicted, but an attempt shouldbe made, with the present state of the society as the starting point. Factors to be consideredare the level of literacy, farming knowledge, and existing skills in farming, as well as attitudestoward change. Equally importantare the rural way of life and the social structure. The sort of mistakes that can and have been made through ignoranceof these elements can easily be visualized. A farmer who thinks that more water automaticallybrings better crops will break outlet structures to obtain it and will block field drains and drainage outlets. A farmer accustomedto having his or her own irrigation ditch will be unwilling to share the maintenanceof collector (trainswith others. When there is an intentionto introducenew settlers to farms in a project area, compatibilitymust be considered. Present farm practices must be known to provide the basis of a plan for development,and the nature of the farming populationmust be understoodif the rate of developmentis to match its capacityto adapt and learn. Social - 29 - developmentwill not crystallizeafter the inaugurationof a new drainagesystem. Farmers will probably expect a steadilyrising standardof living, and better education and farming expertisemay be anticipated. Channel systems are generally designedas permanent, and even with mediocre maintenance, their lifespan will see radical changes in farming methods and consumer demand. Prediction of these changes is virtually impossible. To compensate,it is desirableto build a water distributionremoval system that will allow flexibilityfor remodelingthe layout of farms as farming methods evolve.

Careful study should be made to determinehow the proposed drainage measures fit into the present farming systemsand how these measures will affect the present farming practices. The acceptabilityof the proposed measureswill also depend on how much they affect the farmers' incomes. If farmers are expectedto grow new crops, intensify their land use, or respond in another manner to achieve the projected returns from the drainage investments,the study should examine whether such expectationsare realistic, which potential constraintsmay be encountered,and how these constraintsmay be overcomeby extension services or by adaptationsin design or planning. The constructionor improvementof drainagesystems is likely to disturb ongoing rural life and farming activities. Serious problems may arise if, for example, ongoing constructioncuts irrigation supply and destroys crops, or, even worse, if it takes land out of production for some time. At the organizationallevel, the drainageworks may create disturbances,possibly due to changes in boundariesof drainageunits or to new technicalor administrativeduties.

Limitationsare imposed on irrigation/drainagedesign by present land tenure and current legislation. When the World Bank finances a project, the loan may be conditionalon the effectingof certain legislativechanges if existingpractice is prejudicialto the effectivenessof the project. In several countries, problems related to land tenure are attributableto land reform measures intendedto benefit peasant farmers. A fair but workable system of land tenure for an irrigation/drainagearea is difficult to find. Total state ownershipof land and control of irrigation and drainage systemsreduce the farmer to the status of hired laborer. Private ownership with full landholdingrights frustrates efficient irrigation/drainage,which is essentiallya social activity, unless excellent cooperationis achieved among all interestedparties. Regardlessof the legal status of land, the function of the planner is to observe, record, and recommend,and the function of the designer is to use this informationto overcome difficultiesand offer an effective system despite the imposed limitations.

Social benefits shouldbe presented in the context of the basic project objectivesas detailed in the feasibility study report. If the principal aim of a project is to develop a more equitable distributionof income in the target population,emphasis shouldbe placed on the extent to which it meets this goal. Similarly, if the objective is to generate employment,a thorough assessmentof the job-creating impact of the project should be made. Some of the following indicatorsmay be used in the assessmentof social benefits, and any adverse social effects shouldbe noted.

1. Income distribution. The extent to which, as a result of the project, the incomeof the poorest sector of the populationis improved, comparedwith that of other sectors, should be noted.

2. Employment. The extent to which the project reduces the underemploymentand unemploymentrates should be assessed. Generally,this can be quantifiedin terms of the number of man-yearsof work created by the project, with a distinctionmade between permanent employmentand employmentduring the constructionphase. When a - 30 - deliberate attempt has been made to substitutelabor for machinery, the cost (in financial terms) may be shown.

3. Access to land. If the project contains a land reform element, the distributionof land utilization rights, by type of tenure, before and after the project should be demonstrated.

4. Internal migration. In comntrieswith serious urbanizationproblems, it may be useful to attempt an evaluationof the probable impact of the project on rural-urban migration.

5. Nutritionand health. If the project site is in an area with recognized nutritionalor health problems, its expectedimpact on these problems shouldbe assessed. If a quantitative assessmentis possible, the; nutritionaleffects may be best described in relation to expected levels of daily protein and caloric intake comparedwith present levels. In evaluatingthe health impact of drainage projects, any potentiallyadverse effects should be noted.

Mellor (1969), in discussingfarmer innovationand change, stated: "A failure which drives a farmer under the margin of subsistenceor into debt in a systemof high interest rates may be disastrous and thus eliminatethe possibilityof playing probabilitieswhich would pay well over the long term." This does not preclude faLrmerinnovation and change; it simply slows the transition from low-input/low-yieldagriculture to a relativelyhigh-input/high-yield agriculture using improved farming techniques. Studieshave found that farmers are willing to change if they choose from among a limited number of alternativesand, as learningproceeds, if their subjectiveperception changes regarding the probability distributionover the various alternatives. This observation suggeststhat the governmentcan speed the diffusionof innovationby encouraging,for example, the availabilityof long-term credit at reasonable interest rates and of crop insurance programs to partially offset crop losses induced by lack of drainage. By doing so, governmentscan increase farmers' confidencein the stabilityof the planned production system and encouragethem to make the changes necessary to increase yields and profits. Nevertheless,it is importantfor the production system designer to include the considerationof an acceptablelevel of risk in the proposed implementationand managementstrategies. Consideringthe acceptablelevel of risk and governmentpolicy assurances, the systemdesigner should choose an appropriatetime frame for schedulingsystem implementation. Lengtheningthe implementationschedule delays the benefits and often will meet with objectionsfrom economists, on the grounds that it reduces investmentefficiency. However, lengtheningthe implementationschedule could make the differencebetween success and failure. For example, Nelson and Tileston (1977) found that farmers must want to make the changes necessary to improve their production. This requires that they understandthe new system, have confidencein it, and feel that they are an important part of it. Some past drainageprojects have fallen short of expectations due to poor coordinationof engineeringmeasures with other essentialinputs and services. Educating farmers and gaining their confidencetake time. This time can also be used for the gradual developmentof an effective system managementstrategy. Clearly, the design of drainage systems requires examinationof some "soft" social factors. Optimalproject size, the length of the implementationperiod and the implementationstrategy, and the formulationof socially acceptable uncertainty levels are related. Economicand engineeringanalyses can be used to help guide these tasks, but they alone are seldom adequate.

OrganizationalAspects. The organizationalor managerialaspect is an importantpart of feasibility studies. Many of these studies, especiallythose intendedfor submissionto the World Bank, include a section on the structure and competenceof the bodies responsiblefor organization and management. The engineer is responsiblefor being aware of managementproblems, even on - 31 - minor contracts at the field level, to avoid the difficultiesoutlined below, which can cause long and costly delays.

The constructionof a major project involves a number of contractsand subcontractsthat must be controlled and coordinated. A body without experiencein this type of organizationis liable to encounter serious difficulties. The constructionprogram involvescritical timing. One or two contractsrunning into trouble can dislocatethe entire project. The drainage authority in charge should avoid this problem by carefully selectingcontractors and by dealing with contractual difficultiesrapidly and effectivelywhen they arise.

Flexibility is needed in dealingwith unexpectedevents as they occur. It is also necessaryto take advantageof a contractor's particular expertise in methods of construction. It takes a reasonable and responsible contractorto cooperatewith the resident engineer, who acts on behalf of the employer, and to work within the framework of a specificationthat leaves a measure of freedom in the executionof the contract.

If the required standardsof workmanshipare above those previously achievedby the contractor, there are endless disputes and delays on site. Ultimately, completionis long delayed, low standardsof work are accepted, or work ceases. If the required rate of working is beyond the ability of the contractor, costs rise, timing is dislocated,and problems multiply.

The hazards of awarding contracts are one example of the problems involved in the control of a program of construction,and these hazards demonstratethe need for experiencedand decisive administration. Once the drainage system is in operation,the main function of the controlling authority is to maintain the drainage. Maintenanceof drainage systems is vital, but is generally more straightforwardthan that of irrigation. The responsibletechnical staff must be prepared in a timely and adequateway to operate and maintain the new systems. This process may require reorganization and training. The reorganizationmay affect several sectors, inside and outside the governmentand inside and outside the project area. To plan the adaptationof existinginstitutional arrangements, these arrangementsshould be "mapped"to take stock of the charters, roles, tasks, and competenceof the different agencies at various administrativelevels. The maintenanceof main drains is usually the responsibilityof the public sector, whereasthe maintenanceof field drains is the direct concern of farmers. However, drain collectors/tertiarydrains are often in no-man's-landas far as maintenance responsibilityis concerned, a situationthat inevitablyleads to a state of disrepair and unserviceability. In this regard, the followingparticipatory approach is proposed. Irrigation and drainage constitutean inseparabledimension of the project. The establishmentof irrigation associations/wateruser organizationsaround tertiary canals has to be encouragedby the centrally financedgovernment programs designed to carry out the on-farm developmentworks. The basic philosophyof such a proposal is that more active farmer participationin the irrigation-drainageactivities of the tertiary system, with some technicalguidance and financial incentivesfrom the public administration,is likely to yield lasting results, even though farmer participationmay slow the pace of physical progress. Promoting farmer participationis the most difficultpart of this approach. Where farmers are familiar with drainage, the task is somewhateasier because they are already convincedof the benefits that drainage can bring them, even if previous governmentactions may have accustomedthem to having the governmentprovide for them. A typical methodologyfor proceedingwith this work is to send a communityorganizer or institutionalofficer to the village to introduceto individuals,groups, and local leaders the subject of maintenanceand the conditionsof reusing drainagewater for irrigation. The officer, who should be familiar with the local situation,will assist in the establishmentof the - 32 - irrigation associationand also its legalization. Once all details are established,the officer continuesto guide and assist the group for some time.

Ernronmental Aspects

EnvironmentalImpact Assessment(ELA). The intendedeffects of drainage are well known, such as increased agriculturalproduction that results in the generationof income and food import substitution. In addition to the intended effects, however, other unintendedeffects that constitutea future problem may arise if these effects are not addressedproperly during the planning stage of the project cycle. When done properly, drainage planning leads to efficient drainage systemsthat will bring about only the positive intended effects. The objective of drainage planning is to compile informationfor the decisionmaker,wvho can use it in making decisions about project implementation, which may include:

1. choosingthe suitabledrainage method and implementingthe project as proposed. The decision on the timing of implementationhas to be consideredin the same fashion as postponementof it until actual drainageproblems have risen. The effect of such a postponementon the present values of costs and net benefits has to be weighed against the additionalbenefit of such postponementthat may result in a better design;

2. taking precautionsand selectingdesign measures to prevent or mitigate the unintended undesirable effects;

3. choosingan alternativedevelopment plan that achievesapproximately the same intended objectiveswith a less adverse effect;

4. abandoningthe project altogether.

Classesof Effects. In predicting environmentaleffects, attention shouldbe given to the following:

* direct and indirecteffects, orfirst order and higher order effects. There are chains of effects, such as draining the upstream part of a river catchment > change in river hydrology > change in the environmentof a downstreamfloodplain > change in economicpotentials (agriculture, fishery) and in wildlife habitat;

* secondavyeffects. The primary activity of a drainageproject may be extendedto include secondary activities, suchias industryfor processingagricultural products, which in turn will have certain environmentaleffects (e.g., polluted wastewater);and

* synergeticeffects. These effects include an increasedthreat to the survival of certain species of wildlife that are under pressure in several ways as a result of the same project, e.g., reduction in availablearea and degradationof biota through changes in river hydrologyand degradationof water quality.

Possible EnvironmentalImpact of Drainage Projects. Drainage projects may have a substantal effect on the environmentof the area where they are located, and a forecast should be made in the feasibility study report of the principal changes expectedto occur. The study should note possible adverse environmentaleffects, assess their seriousness, and weigh the extent to which other - 33 - benefits will compensatefor them. In particular, the effect of the drainage effluent on downstream water use and on downstreamecology shouldbe noted. The following list of possible changes is illustrativeonly and not intendedto be exhaustive.

I. Increased irrigation demand after the installationof the drainage system may constitutea serious problem if the irrigation water supply is limited.

2. Loss of wildlifehabitat should be evaluatedin relation to the human and wildlife environmentprojected upon completionof the project. Particular analysis should consider wetlandssuch as coastal lowlands,river floodplains,and marshes. Wetland value to society as well as to plant and animal habitats requires balancedanalysis and evaluation of alternativesto minimizeadverse impactson humansand wildlife, with plans for proper mitigationmeasures developedwhen needed.

3. The loss of traditional economicfunctions of the land, such as for fishing grounds or for grazing, should be considered.

4. If flood protectionof parts of river floodplainsis implemented,higher and increased flooding in the remainingunprotected areas may result.

5. Hydrologicalchanges in the more downstreamreaches of the river, most likely in the form of much sharper fluctuationsin both river dischargesand water levels, may result if a substantialportion of the upstream part of a river catchmentis drained. This change may, in turn, have secondary effects in the downstreamarea, such as:

- changingpattern of measuringerosion and sedimentation;

- changingnavigability of the river channel;

- fluctuatingavailability of irrigation water and dependabilityof supply;

* water regime change in downstreamfloodplains: more pronounced floods and droughts; possible degradationof ecosystems;

* risk of damagingfloods; and

* degradationof fishing grounds.

6. Many of the effects (usually adverse) on water quality are secondary, induced by the intensifiedagriculture made possible by the improveddrainage. Examplesof drainage- induced water pollution are:

* pollution by pesticide residues (will affect aquatic life and may be very detrimentalto fishing and fish farming);

* eutrophicationby fertilizer of manure surpluses;

* salinity; - 34 -

* pollutionby iron ocher;

* increasedturbidity;

* acidification;

* pollution by special substancesthat may be leached from soils of certain unusual composition. A famous case is the release of poisonous selenium (Se) upon drainage of an area in the San Joaquin Valley in California; and

* pollution by various detrimentalsubstances if wastewater of villages, towns, and industrialareas is evacuatedthrough the drainage system concerned.

In principle, there are potential positive environmentaleffects: for example, positive effects on human health through the eliminationof prolongedstanding water.

Methodologiesand Proceduresin ELA. As stated earlier, EIA is in essence an aid to planning and decisionmaking. In various countries, legislationon EIA has been establishedwith formal EIA procedures. The aim of these procedures is to give environmentalinterests full standingwithin the planning and decisionmakingprocess and to make a positive contributionto protect (and possiblyto improve)the environment. Legislationand procedures may vary somewhatby country. Usually the parties involved are:

* the proponent of the action (the drainageproject);

* the decisionmaker,usually a governmentalauthority;

* the reviewer: the person, agency, or board responsiblefor reviewing the EIA for relevance, completeness,objectivity, and compliancewith regulationsand guidelines; and

• the public, often in the form of special interest groups such as farmers, fishermen whose fishing grounds may beo3medegraded as a consequenceof the drainage project, and environmentalgroups.

Common EIA procedures include certain essentialelements. For example, alternativesto the proposed project, differentways to achieve the intendedobjectives, should be indicated. This process gives the decisionmakervarious options to considerwhen weighingintended benefits and estimating costs and environmentalimpacts.

The main steps in the EIA pirocessare:

1. Scoping. Scopingis the identificationof the project alternativesand environmental impacts to be considered. This step should involve the participationof all parties listed above. Ideally, scoping is a public process in which technical considerationsmay be contemplated,along with public hearings. Scopingwill result in specific guidelinesfor inclusionin the environmentalimpact study (EIS). (EIS refers to the document, whereas EIA refers to the entire process.) - 35 - 2. Drawing up the EIS. This study will containa descriptionof the proposed action (with alternatives),as well as relevant environmentalconsequences. Mitigatingmeasures for adverse effects should also be indicated. Generally,the responsibilityfor drawing up the EIS lies with the proponent. Guaranteesfor relevance and objectivityshould be obtained by a proper study reopening and review, both of which should be open to the public.

3. Submittingthe ElS for public review. The draft EIS is made public and is open to commentsby interestedparties.

4. Receivingadvice from the reviewingagency.

5. Accepting the EIS. The EIS is accepted (after possible adjustmentas advisedby the reviewer) by the responsible authority.

6. Choosingproject components. The decisionmakerchooses the project alternativeto be implementedand the measuresto be used to mitigateenvironmental impacts.

7. Implementingthe project.

8. Monitoring. During and after project implementation,the actual environmentalimpacts are monitored and comparedwith what was predicted in the EIS. This postauditis, among other things, a useful (and probably necessary)tool in improvingfuture predictions.

The ETAshould be a part of the earliest possiblestages of the decisionmakingprocess, when crucial decisions are still being deliberated. The process should certainly be carried out for policy decisions. For subsequentindividual projects, there may be EIAs of a smaller scope.

Presentationof EA Findings. For EIA to serve its purpose, the findings should be presented appropriately. The criterion is effectivenessof communicationwith the decisionmaker. This criterion is vital and not easy to fulfill, as extensiveexperience has shown. Informationshould be presented in an executive summary, with requirementsthat includethe following:

* avoid technicaljargon (the decisionmakeris usually not a specialist);and

* prepare a key issue matrix in which irrelevant environmentalimpact categories and unacceptableproject alternatives(project alternativesthat are certain to be rejected) are excluded. 2

Drainage System Design

Drainage for Control of GrounidwaterTable Levels:

Common Systems

Concept. As discussed here, drainagedesign refers to the determinationof the field drainage and the other effective drainage methods needed for the most common drainage practices, as well as to the geometry and dimensionsof the systems. Groundwaterdrainage can be either natural or artificial. Natural drainage takes place in soils with a deep hydrologicalprofile, i.e., with hydraulic conductivityincreasing with soil depth. In soils with a shallowhydrological profile, in contrast, an artificial field drainage system is needed to remove excess rainfall or irrigation water and to maintain the water table at the most appropriatelevel under the particular conditionsof soil, climate, crop growth, and use and passage of farr machinery. Artificial drainage can be either horizontal or vertical. Horizontalfield drains can be closed, like subsurfacedrains, or open surface ditches.

Field drains are the only type of drain that can control the groundwatertable and carry it to other types of drains, which are only water passageways,such as collectorsand secondary and main drains. Any design for field drainage, therefore, must provide for proper spacing between drains and for proper depth and direction of drains for adequatefunctioning. In the drainage design concept, the calculationof the proper spacing between field drains through drainage design formulas is important. As a rule, these equationsshow different combinationsof spacing and depth in such a way that a greater depth allows a wider spacing. In practice, however, drain depth cannot be chosen freely because it may be restrictedby factors such as water level in the main drains and the existence of unfavorablesoil layers, which can be impermeableor structurallyunstable. In soils with compacted, low permeable soil layers existingwithin the top soil profile, in combinationwith heavy rainfall or water application,the free water body that may accumulateon the soil surface is not the groundwater. It is a temporaryperched water table that can also exist at some shallow depth within the soil profile. Drainage formulasbased on the removalof excess water from the groundwatertable cannot be applied under these circumstances,and other practical solutionsbased on empiricalknowledge must be used.

It is not alwayspossible to make a functionaldistinction between the surface drains and the subsurfacedrains designed to control the groundwatertable. For example, an open ditch designed to control the groundwatertable may also receive water from surface runoff. Because most excess water is removedby surface drainage, an interrelationshipbetween surface and subsurfacedrainage may be expected. The results of studies on this subject carried out by Hoffman and Schwab (1964) are shown in figures 3 and 4. The fall of a water table midwaybetween the drains during the drainage process is greater when a surface drainage system is present. In the case studied and shown in figure 3, the spacing of the subsurfacedrains was 12 m. A differentspacing would give a different curve for the same conditions. Without surface drainage, the water table remains higher because excess rainfall water is stored in small depressionson the land surface. This water infiltrates into the soil, and consequentlythe water table does not drop as quickly as it would if this water source were absent. The rate of fall of the water table, starting from the soil surface as a function of tile spacing, is shown

- 36 - - 37 - in figure 4. For instance, if the drainage design requirement is defined as a desired Figure3. AverageWater Table Drawdown drawdown of the water table of 20 cm/day, after Irrigation the drain spacingrequired would be 10 m Soilsurface without surface drainage and 17 m with 30 ___"___v*__v -- ._ surface drainage. The conclusion drawn is S that with a surface drainage present, the * . 2 -- C ' no surfacedrainage permissibletile spacing could be increased by at least 50 percent. Such results can be obtainedonly by smoothingthe land surface 20 thoroughly. D . with surfacedrainage Up15 . Open ditches may be used to Eq 5 Tile 1940- Fescue provide subsurfacedrainage and are often 1962lCorn consideredfor use in flat fields where the 0 0.5 1.0 1.5 2.0 2.5 3.0 lack of grade depth of the outlet, soil Time(T) in days characteristics,or economicsdo not favor burieddrains. The ditchesmust be deep Source:Hoffman and Schwab 1964. enoughto provide for the escape of groundwaterfound in the permeable strata Figure 4. Computingrile SpacingsBased on or in water-bearingsediments. Spacing of Drain Oudlow the ditches varies with soil permeabilityand crop requirements. Because of their 105- required depth, the ditches usually have Equivalentrainfall for adequatecapacity to carryboth surfaceand * 3yearrecurrenced .terval 90 - TH = .9edet = 3.0 subsurfacewater. Advantagesin using open Tiledaepthe 4;° ditches include: 75 - =4 75- V * Open ditches usually have a smaller initial cost than buried .E 80 drains.

* Inspectionof open ditchesis l. * \ith w5 surtace easily accomplished. 32' rainage 30 2' * Open ditches can be used in soils where buried drains are not recommended. 15 no surface drainage * Open ditches may be used on a 0 flat gradient where the depth of 0 0.5 1.0 1.5 2.0 the outlet is not adequatetO Desired rate of water table permit gravity flow from drains drawdownin feet perday installedat the requireddepth Sowrco:Hoffman and Schwab 1964. and grade. - 38 - Disadvantagesin using open ditches are:

* Open ditches require considerableright-of-way, which reduces the area of land available for other purposes.

* Open ditches require moirefrequent and more costly maintenancethan buried drains.

* Open ditches limit the applicationof mechanizedfarming.

The question remains whether the total length of the surface field drains can be reduced by installing a good subsurfacedrainage system. Althoughthis is technicallyfeasible, the decision dependson economic considerations,taking into account the cost of land forming, the field drains, field laterals, and the subsurfacedrains and outlets, as well as the possible gain of arable land and the effect of various combinationsof drainage systemson crop yields.

Depth and Spacing of Field Drains

The placementdepth of field drains dependsmainly on the soil's hydrologicalcharacteristics. Generally, field drains shouldbe placed within the most permeable soil layer located above the relativelyimpermeable layer (barrier). For example, pipe drains laid at a depth of 1.8 m in a soil that is impermeableat 1.2 m from the ground surface-but with the trench permeable over its full depth-can be treated as a system in which the drains are laid at a depth of 1.2 m below the soil surface. Two opinionsexist regarding the suitabledepth of field drains to be laid in a soil of a homogeneousstructure: to install the field drains at deep levels or at shallowerlevels. The choice of deep drains is made mainly to prevent the upward movementof water to the soil surface, thus eliminatingany possible salt accumulationon the surface, particularlyunder arid conditions. This also permits wider spacingbetween drains. The U.S. Bureau of Reclamationpractice, for example, recommendsthe installationof field idrainsat depths between 2.1 and 3.0 m. Willardson and Donnan (1978) concludedthat the proper economicaldepth of placementof subsurfacedrains in arid regions has been accepted at 1.8 m or slightlydeeper. Maximumefficiency of both energy and material, as well as maximumwater table drawdown, can be obtainedby installingdrains at approximatelya 2.1 m depth. The excavationcost increases at deeper depthsto 3.0 m and tends to be balancedby a decrease in the cost of materials. Willardsonand Donnan's study also stated that lowering the drain 0.3 m, from 1.5 to 1.8 m, increases the spacing by about 126 percent, and lowering the drain to 2.4 m also increases the spacingby about 170 percent.

The opinion favoring the installationof drains at shallower levels is based on the premise that the water table does not control the level of salinity, but the direction of the flux does. It is possible to maintain downward flux even if the water table is very close to the surface. An upward flow can be a result of an external seepagethat may cause a serious salinizationhazard. Moreover, installing the drains at shallower depths will decreasethe irrigation requirements, a critical factor in irrigated agriculture. Doering et al. (1982) concludedfrom studiesin North Dakota that water table depths of approximately1.0 m provided maximumcrop yield with minimalsupplemental irrigation for several crops, and a drain depth of 1.5 m would be indicatedunder these circumstances. Oosterbaan(1982), in an evaluationof extensivefield data from Pakistan, found that the water table depth required to -prevent an adverse effect on the yields of sorghumand cotton was 60 cm. Such data provide some support for the suspicionthat the drain depth typically recommendedin Pakistan may be deeper than necessary. The equipmentneeded to install drains to a depth of 1.5 m by trenching is less expensive and easier than that needed for greater depths. Willardsonand Donnan (1978) indicatedthat with - 39 - each 0.3-m depth increment increasebeyond a depth of 1.5 m, the amount of excavationper unit length of drain increases by 20 percent. Moreover, with increasingdepth it is more difficult to hold the grade, to make lateral drain connections,and to provide effectivemaintenance and repair.

Over the past 50 years, numerousdrainage formulas have been developedthat can be used in calculatingthe proper spacing between field drains. These formulas, described in appendix2, can be classifiedinto two types based on water flow conditions. The first type applies to a steady state flow condition, i.e., continuousand steadily applied water or rainfall, discharged continuouslyand steadily by the drains, to maintain a state of equilibriumbetween supply and discharge. This situationrarely occurs in practice. The second type of formula applies to a non-steadystate conditionand is more commonlyencountered in practice, where every case has a non-steadystate condition, i.e., a rising and falling water table.

To use these formulas, two importantsets of data are required: the drainage intensity(which is known in quantitativeterms as the drainage design criteria) and the physical conditionsof the hydrologicalsoil profile. The drainage design criteria vary for steady state and non-steadystate formulas. For the steady state formula, the drain depth and spacing are dictated by the design discharge (q) and the maximumpermissible height of the groundwatermidway between the drains (h). Different combinationsof q and h can produce the same drain spacing. Thus, for a drain depth of 1 m, the combinationq = 7 mm per day and h = 0.5 m provides practically the same drain spacing or the same intensity of the drainage system as does the combinationq = 10 mm per day and h = 0.7 m. The physical conditionsof the hydrologicalsoil profile that must be known for steady state formulas are the hydraulic conductivityand the depth to an impermeablelayer. For the non-steady state formula, the drainage intensityis determinedby the required fall in the water table over a given number of days, starting from a given initial state (a fall from hk to k over a time t). The physical conditionsof the hydrologicalsoil profile that must be known for non-steadystate formulas are the hydraulicconductivity and also the drainable pore space.

The steady state formulas can be used for conditionsin which, as in Europe, rainfall is usually prolonged and of slight intensity. Experiencethen determineswhether the calculatedintensity of the drainage system is in fact sufficient. This decision can be inferred from the depth of the groundwaterduring the year and from the consequencesof this depth for crop production. In irrigated land or in areas of short and intense rainfall, however, there is a great difference in the intensity and duration of the supply and discharge of water. To calculate drain spacingsunder such conditions,it is desirable to use a drainage design formula based on non-steadystate or transientflow conditions. It is simpler, however, to calculatedrain spacingsby means of steady state formulas than by non-steady state formulas. Therefore, in many cases in which the non-steadystate conditions cannot be properly defined, or in which the hydrologicalconstants are only approximatelyknown, the use of steady state formulas is justified.

The importance of a good drainage formula for designingthe proper drainage spacing is often overestimated. The critical factor in the calculationof drain spacing is not the formula itself but the data substitutedin it: the drainage criteria and the physical conditionsof the hydrologicalsoil profile. Because the hydrologicalsoil data in particular may vary extensivelywithin a short distance in the field, their accuracy is often in doubt. It should thereforebe rememberedthat, in practice, the use of drainage formulas can never result in more than what may be termed a calculated estimate, i.e., a drain spacing that probably will be approximatelycorrect. The importanceof the formula, however, should also not be underestimated. The formula can be employednot only for making calculations, but also for providing a good idea about the various factors involved and their relative importance - 40 - under different conditions. Many different types of drainage design formulas as well as their nomographsare available; the two formulas shown in appendix2 were selected only as examples, with the objective of enabling designersand planners to ascertainthe relative importance of the various factors under steady and non-steadystate conditions.

Limitationsof Drainage Design Formulas

Adequate drainage parametersused in drainagedesign formulas, such as the hydraulic conductivityof the soil, rate of recharge to the water table, depth to an impermeablelayer, and spacing and diameter of field drains, allow predictionsof water table behavior. Drainage parameters that must be known to design a drainage system cannot be estimated accurately. In addition, there are limitationsto the measurementof the required soil characteristicsin the field, althoughthey may not be as severe as other measurementlimitations. Soils are often extremely variable, and reasonable values of hydraulic conductivityand (Irainableporosity are also relatively difficult to obtain. However, since drain spacing varies as the square root of hydraulic conductivityk/effective porosity p, as shown in appendix 2, the effect of even substantialuncertainty in the measurementsmay be greatly reduced in the calculateddrain spacing. The most reliable procedure to determine an adequate value for klp, as recommendedby van Schilfgaarde(1984), is to install a test section of drain and to determine the water table drawdownhi or the drain discharge q as a function of time. On semilogarithmicpaper, h or q shouldplot as a straight line against time, and the gradient of this line will be proportionalto ktp. This gradientleads to the parameter a = kdlpIl, referred to as the drainage intensityfactor by Dieleman and Trafford (1976); its inverse,j = l/a, was called the reservoir coefficientby Kraijenhoff(1958). Use of drawdown data for the determinationof k eliminatessome of the concerns with soil heterogeneity. It does not account for soil anisotropy, however, or for the effective anisotropy causedby layering or by the occurrence of lenses with different conductivity. Though anisotropytends to be the rule rather than the exception, and though its effect on drain performance is considerable,it is most often ignored. Boumans(1979) presented an example where drainage of an alluvial plain called for a drain spacing of 150 m when anisotropy was ignored and of 60 m when it was taken into account.

Drainage for Control of Groundwater Table Levels

Special Systems

Drainagefor ReclaimingSalt-Affected Soils. In the irrigated arid and semiarid regions, the rise in the groundwatertable as a result of poor drainage is often listed as one of the main reasons for the developmentof salt-affectedsoils in these areas. A commonclassification system for the diagnosisof salt-affectedsoils has been described by the U.S. SalinityLaboratory Staff, as shown in the followingchart (Richards 1954):

EC. ESP Soil Classification (mmho/cm) (%) s;4 s;15 Nonsaline, nonalkali s;4 -15 Nonsaline, alkali >4 < 15 Saline, nonalkali >4 < 15 Saline, alkali - 41 - The system is based on the EC. and the ESP values of the soil, and the adopted criteria are on an EC. value of 4 mmho/cm and an ESP value of 15. This system is general and can be used for phase-level differentiationof soil types in a systematicsoil mapping, as well as for single-valuemapping. It may or may not, however, be adopted at a project level where salinity criteria are normallygoverned by the prevailing climate-soilconditions, cropping patterns, and water managementpractices. EC. values higher than 4 mmho/cm can be used, and soil with an ESP value of 20 was found productive. Sodicity is not the only reason for alkalinity,and the exchangeabledivalent cation Magnesiummay have a dual role: it can behave once as an aggregatorsimilar to calcium and at another time as a dispersant similar to sodium.

Leaching of soluble salts accompaniedby a suitabledrainage system is used in reclaiming saline nonalkali soils. These soils might have an initial though temporarilyhigh permeabilitylevel resulting from false aggregation,and a careful leaching should always be consideredto prevent the developmentof any further alkalinityor the deteriorationof drainability. When reclaimingsaline alkali soils, the practice of leaching the excess salts is first done until the soil becomes nonsaline alkali and slightlypermeable. Then soil amendmentssuch as gypsumand lime can be added to amelioratethe soil structure. Many disused alkali soils are almost impermeableto begin with, and the added calcic forms, in addition to the alternateswelling and shrinkingprocesses, improve the soil structure and bring higher permeabilityvalues in the future.

The anticipatedchanges in drainage parametersused in drainagedesign formulasfor saline alkali soils do not favor the installationof a permanent system such as subsurfacedrainage during the land reclamationstage. The soil structure of this type of soil is significantwhen leaching. Zaslavsky (1979) stated that when an attempt is made to leach a leveled field of salt-affectedsoils by ponding, nearly all the flow takes place near the drains, and very little flow occurs near the centerlinebetween drains. Therefore, drainage planners should consider introducingopen surface drains to these soils with initial low values and then possiblychanging the drain locationsif required; for example, a surface drain can be refilled, and a substitutecan be excavatedat the initial midpointbetween drains. The spacing between these open field drains can be calculatedby traditional drainage design formulas after consideringthe intermittentleaching and its water requirements. Double the calculatedspacing can also be considered during this stage, as recommendedby Dieleman(1972). After this early stage of land development,when the hydraulic conductivityimproves to an economicand relativelystable level, subsurfacedrainage can replace the initiallyinstalled surface system accordingto the agriculturalconditions and requirementsdiscussed earlier.

Drainage of Swamp/MarineClay-Sediments

In many deltaic areas, in regions bordering coasts, and sometimesin river valleys, lowlands are continuouslyor periodicallyinundated by either fresh or marine waters. Such areas are usually reclaimedby constructingan embankmentand pumpingthe water out, and as a result a difference in hydraulichead developsbetween the embankedarea (commonlyknown as polder land) and the adjacent water areas. The resulting seepage inflow and any existingrainfall usually render the land susceptibleto waterlogging,and the installationof a specialartificial drainage system is required. The flat bottom of the embankedarea is commonlycovered with soft, pedogeneticallyundeveloped, subaqueoussediments mixed with a substantialamount of organic matter. At the moment of emergence, all the pores between the sediments are completelyfilled with water, and the sediments are consequentlyreduced. - 42 -

These sedimentshave to be changed into a normal soil passable to man and machineryon which crops can be grown and roads and buildings constructed. Initial soil formationor soil ripening is the term used for this initial change from mud to normal soil through decreasingthe moisture content, which initiateschemical and physical change. Initial soil formation is begun by draining a top surface crust about 2.5 cm thick using the biodrainagemethod, i.e., evapotranspirationcaused by the existingnatural vegetation or by reeds spread by aircraft. This process can take from one to two years; after that, the developedsurface fissures have to be extendedto cracks within the lower layers. Such crack formation requires the constructionof main and temporaryfield ditches. The dimensions of the field drainage system are related to the progress of crack formation. Because the crack formation is restructured at the beginning of the top layer, primary open field ditches are initially installed with a depth of roughly 30 cm, which can be extendedby an incrementof 30 cm every year to a maximumdepth of 120 cm from the soil surface. The permeabilityin this early stage of reclamationis very slow; therefore, a narrow spacing of 8 to 12 m between field ditches is usually applied. When the soil cracks reach a level of 60 to 70 cm below the land surface, a process that may take about four to five years after the installationof open ditches, the soil has suitable permeabilityvalues to allow the installationof subsurfacedrainage systems.

The depth of subsurfacedrains and their spacing cannot be determined with the method used for normal soils. As described earlier, the method for normal soils is based on the relation among the permeability,depth of laterals, drainage discharge, groundwatertable, and spacing between drains. The permeabilityvalues in ripening soils increasedramatically in relation to the progress of soil subsidencethat takes place over time. If the spacing between drains is calculatedaccording to a measured hydraulic conductivityvalue at the momentthe laterals are laid, the drain would be too narrow, and twice the spacing might be needed after a few years. Moreover, it is almost impossible to measure the hydraulic conductivityin such soils with wide cracks. The recommendedmethod for designing subsurfacedrainage for such soils is based on (a) studyingthe crack formation in soil profiles and predicting further crack ,developmentfrom the sequence of various layers, their clay contents, and the occurrence and rate of seepage; and (b) the results of field trials on the relation between drain spacingsand the groundwatertable and crop yield combinedwith the previous results. The drainabilityduring the first year after installingthe subsurfacedrains by this method is usually not optimal because the spacing could be too wide. Some of the open field ditches, therefore, may be left for some years to assist the performanceof subsurfacedrains until drainabilityimproves over time. Heavy-texturedsoils lose more water than do light-texturedsoils, and crack formation is more intensivein heavy-texturedsoils. Therefore, wider drain spacing of between 25 and 50 m is common in heavy-texturedsoils, whereas spacingsrange between 8 and 16 m in light-texturedsoils, with the recommendeddepth between 110 and 130 cm from the soil surface. The ripened structure of the top soil can be used as a trench backfill in clay soils, and then no drainage collar will be needed.

Drainage of Organic Soils

Peat is an accumulatedorganic matter formed in places where its accumulationexceeds its decay in a certain period. Organic soils are commonlyfound in low flat areas with poor natural drainage that are under a temperate and wet climate, such as Ireland, the U.K., Sweden, Germany, Poland, the northern isles of Japan, and the Netherlands. Peat soils can be found in subtropicaland tropical zones where water from surrDundingareas runs to them, such as in the San Joaquin Delta in California, in Florida, in the plain at Philippi in Greece, as well as in the polder land cut from the sea or lake bottoms in the Caribbean. - 43 - Peat has a spongy structure, a high pore volume, very high water retention, and a very low consolidationconstant. The drainageof peat soils causes dramatic changes in these characteristics. Examplesare (1) a shrinkage of the top layers via desiccation,which causes increasingcrack formationand permeabilityvalues; (2) oxidationof the organic matter; (3) compressionof the layers below the groundwatertable; and (4) an irreversibledesiccation of the organic matter leading to either a favorableblack dust with neutral or alkali conditionsor unfavorablehard and compact dry clods that are heavily humificated. The first three items together cause subsidence. Under a temperate climate, drainage of "living" peat gives a subsidenceof 25 to 30 percent of the original peat thickness. This means that the final height of the soils is highly dependenton the original thickness of the peat in areas under a temperate climate. Under subtropicalconditions, the layers below the water contributeslightly to subsidencebecause the peat there is more compact than that in the temperatezone, due to the temporarily low groundwatertable during the dry season. In the temperatezone, the water content per gram of peat varies between 8 and 16 grams, whereas it varies between 7 and 9 grams in areas under a subtropicalclimate. In the temperatezone, peat is a very loose sponge; in the subtropicalzone, it is more firm and compacted. The rate of natural oxidation is higher in the tropical zone than in the temperate zone, and the subtropicalpeat is usually eutrophic (rich in plant nutrients) and is more or less neutral (pH = 6.0 to 7.5). The peat of the termperate zone has a lower pH value, ranging between 3.5 and 4.5.

The prediction of the total amount of subsidenceand the differencesin subsidencemust be known before planning the drainage system and selectingthe pumping stations in the project area. The differencesin the peat thickness lead to differentland levels; therefore, it is important to map this thickness in advance. Predictions of the specificvolumes after subsidencein the area to be drained can be made through samplinganother drained area of a specific age that has the same kind of peat, land use, and drainage conditions. Since the prediction must include a certain degree of inaccuracy, the drainage plan has to be prepared to prevent the need for adjustmentsto compensatefor incorrect predictions.

The depth of subsurfacedrains is affectedby changes in peat soils, especially during the first years after installation,when the most subsidenceoccurs. It is recommended,therefore, that installationsbe postponedfor a few years. The peat soils disappear rapidly in subtropicalareas, and changing of the drain depth amountingto about 3 to 5 cm per year is a continuingprocess. The suitabilityof subsurfacedrains in these peat soils, therefore, becomes questionable, and the introductionof open ditches could be a preferable substitute. Field experienceshows that the groundwatertable can be maintainedin the grasslandof peat soils at an optimumdepth of 50 cm in subtropicalzones. Experience also shows that a spacing of 10 to 20 m between subsurfaceor field ditches is a common practice. Precisely the right distance can be found only through field trials.

Drainage of Acid-SulfateSoils

Drainage of continuouslyinundated coastal areas under water sometimesleads to unsatisfactoryresults. This outcome occurs when the depositionof sediments containingsubstantial amounts of organic matter takes place in water containinga certain quantity of sulphate ions that exist in the reduced form of ferrous sulphide. After a drainagesystem is installed, the ferrous sulphide can be oxidizedvery easily, through the penetrationof air, to ferric and aluminumsulphates. The latter salts have a weak base and a strong acid, so they react as an acid. In newly oxidized noncalcareous soils, the soil pH may drop to 2 or 3. Ferric sulphate looks more or less like cat droppings, and because of the yellowish spots in such acidic soils, they are called cat clay soils. Cat clay soils are extremely acidic, and they contain aluminumin amountstoxic for plant growth. - 44 -

Neutralizationof the acidic sulphate soils with lime is a common practice used to reclaim these soils. In most cases, the costs of liming are very high; it is wise to wait for some years before liming, since considerableamounts of the acidic matter can be leached during the first years. Smith (1970) reported that the lime requirement for a heavy cat clay soil in the Netherlands was 150 and 30 tons per ha when the determinationswere made in the first year and after several years, respectively. In making plans for reclaimingsuch soils, it is very importantto know whether cat clay soils will be formed after installingdrainage systems. The answer can easily be found after studyingthe sulphur and calcium carbonatecontents. If thlecalcium carbonatecontent is not higher than the S04 content, the soil may become acidic after drainage is introduced. A quick method to make such a prediction is described by van Beers (1962). When it is known that an area to be drained and reclaimedhas potential acidic sulphate soils, the first question that faces the planner is whether it might be better to cancel the plan. If the area has to be drained and reclaimed, it would be wise to drain it to a shallow depth so that potentially acidic soil layers remain as far as possible under unoxidizedconditions. This is practical, however, only when the contaminatedsoil layer is not close to the surface and when the water table can be kept fairly high and constant. In the humid climaticzone, it is advisableto use such a land as a grassland. In tropical and subtropicalregions, paddy would be the best crop to grow.

Drainage of Artesian Areas

The term artesian areas as used here applies to areas having drainageproblems caused by an upward seepage of water from a confined aquifer. In areas that are subject to artesian flow from permeable aquifers below the drain, relief-typedrains are usually installed to solve the problem. The proper depth and spacing for these drains are computedby using the formulas for relief drains; however, special considerationmust be given to selectingdrainage coefficientsthat are applicableto artesian conditions. In nonartesianareas, the drainagecoefficient is based on the expected infiltration from precipitationand/or from deep ;percolationfrom irrigation. In artesian areas, groundwater moves upward into the root zone, adding to the water that infiltrates from the surface, so the amount of groundwaterto be removed by drainage is greater. For this reason, drainage coefficients applicableto artesian areas are always greater than those applicableto nonartesianareas. This usually requires closer spacing of drains in artesian areas. The rate at which groundwaterunder artesian pressure moves upward into the root zone is a function of artesian pressure, the depth of the artesian aquifer below the drain, and the pernieabilityof the subsurfacesediments through which the artesian water must flow. Knowingthe values of these variablesfor a given situation, it is possible to computethe rate of artesian flow and to establish a drainagecoefficient applicable to the accretion caused by artesian conditions. This coefficient,when added to the drainage coefficientlocally establishedfor accretion from precipitationor irrigation, provides a drainagecoefficient applicable to artesian areas. Experience has indicatedthat this coefficientis in the range of one and one-half to two times the normal values used in nonartesianareas.

Recognizingthat there is not currently a precise method of computingthe value of accretions from artesian flow, the design of drainage systemsshould be conservative. Disposal drains, mains, and importantlateral drains should be designedwith some excess capacity. Lateral collector drains can be designed on a less conservativebasis as additional collector drains and installed to supplement the system if it is found to be deficient. This approach is often the most practical and least costly method in areas where groundwateryields from artesianflows are indeterminate. The important considerationis to provide for excess capacity in the disposal system so that additionalcollector drains can be added later. -45 - Several drainage design formulas have been produced under two different assumptions. The formulas of the first assumption,which take into accountthe spacing of drains given simultaneous upward seepage from the artesian aquifer as well as downwardseepage from rainfall or irrigation water, require a very close drain spacing. The formulas of the second assumption,which consider only the upward seepage from the artesian aquifer, provide a reasonablespacing within the range of practicality. A rather conservativeformula for designpurposes has been described, and its use has been illustrated by Luthin (1965) as follows:

H= oa + ln2 / (cosh4 :h/s + 1) ln cosh 2 nr/s-l cosh 2 z (r + 2h)s-1

where: H = water table height at the midpointbetween the drains

ja = hydraulic head in artesian aquifer

r = radius of drains

s = spacing of drains

h = distance from drain pipe line to artesian layer

x,y = horizontal and vertical coordinatesmeasured from the origin, which coincideswith the top center of a pipe drain

In = natural logarithm

The above formula has been derived for the following set of conditions. It is assumed that the area hydraulic conductivityis uniform and its soil is underlaidat a constantdepth by an artesian layer, that the water in the artesian layer is at a known pressure, that steady state conditionsexist, and that the soil extends an infinite distance above the drain lines. Becausethe last assumptionis contrary to fact and the water table heights predicted by the formula are slightlyhigher than the actual heights expected in the field, the formula is conservativefor design purposes.

Solutionsto the above equationhave been computedand are plotted in the graph shown in figure 5. They can be used as follows. Determine the depth to the artesian layer and the artesian pressure head in the field. The depth of the pipe drain line is frequentlygoverned by factors that are not under the control of the designer, e.g., the trenchingmachines available in the locality and the depth of the outlet. After decidingon the depth at which the drain is to be laid, the height of the water table above the drain line necessaryto give 3 ft (or whatever is deemednecessary) of unsaturatedsoil is calculated. The intersectionof the abscissa and ordinate on the proper curve for the artesian head will provide the required drain spacing.

This example is illustratedin figure 6. Supposethat 3 ft of unsaturatedsoil is required and that:

* the depth to the artesian layer below the drain line = 70 ft * the artesian head with reference to the drain line - 8 ft - 46 -

Figure 5. Determining Drain Spacing in Artesian Areas

1.0- __ __ _| + l

0.92

0 0.8

(U 0.7

0.8

r= 0.5

0.4

0.3

0.2V~~~~~~~~eth of reinlyrblw h ri ie(et

0.1

0 10 20 30 40 50 60 70 80 90 100 110 Deptihsof artesianlayer below the drain line (feet)

Note: Drain radius is 0.25 feet. All distances are with respect to the plane of the drain pipes. Source: Luthin 1965.

* the artesian head with reference to the soil surface = 1 ft * the depth of the drain line = 7ft

The water table must not be permitted to rise more than 7.0 - 3.0 = 4.0 ft above the drain line. From an examinationof the graphs, an 80-ft spacing of drain lines is correct.

If the artesian layer is a short distancebelow the soil surface, the problem can be solved by placing a horizontal drain through it. In some instances, drains as deep as 17 ft have been used successfullyto relieve the artesianpressure and to drain a large area. If the drain penetrates to the artesian layer, the graphs should not be used. Often a single drain will relieve the artesian pressure over an extensivearea. - 47 -

Pwnped-Well Drainage Figure 6. Determiningthe SubsurfaceDrain Spacing in Areas of Accordingto the Artesian Pressure USDA-SoilConserva- tion Service (1973), r pumped wells have . effectivelydrained land in some locations. 3' _ _ Thoughcostly and -v-- 7 _ restricted to favorable 4,4 geologicconditions, ° pumped wells are Drain ' . versatile and may have pipes an economicadvantage over other methods of lowering and maintain- ing a desirablewater 70 table level. Pumped- well drainage is based on the following principles:

1. Apumped | oOooab" -0o00 6 o0 0 0 c 00 well, like O oo0o CIO000 0 0 other forms of artificial Artesianaquifer drainage, Source: Luthin 1965. increasesthe flow energy gradient by creating a sink within a saturatedzone.

2. Energy that the well makes availableto the groundwaterflow system is derived from the motor, which lifts the water from the sink.

3. The increased gradient must extend to the crop root zone to a degree sufficientto control the water table within the desired area at the desired level.

4. The increasedenergy gradient may be in the form of drawdown, i.e., water table slope toward the well, or it may be in the form of a pressure gradient where the groundwater is confined. In either case, at a given point in the saturatedzone, the quantity (P2 ) is decreased in the expression:

Hydraulic gradient = (P1/W + ZI) - (P2/W + Z2)

L where:

PI and P2 = atmosphericpressures ZAand Z2 = elevation of point above a datum elevation - 48 -

W = specificweight of water L = length of path of flow between the two points thus increasingthe gradient toward the well.

alassesof Pwnped Wells. Pumped wells can be classifiedinto the following two groups:

1. Water table wells. Water table wells remove water directly from the free groundwater, creating a drawdown surface in the water table.

2. Confinedaquifer or artesian wells. These wells remove water from a fully saturated aquifer that is confinedby impermeableor slowlypermeable layers.

Theoriesof Flow into Pumped Wells. Flow into wells is a function of the drawdown and is usually expressedin the general form: Q = f (y, Y2,r 1, r2), whereYi and Y2 are the depths of water (or hydraulichead) at distances r, and r2 from the well, respectively.

The theories of flow into pumped wells can be described by the following equationbased on the approximationsof Dupuit: 2 y2 Y2Y Yi log I (r 2 /rl) where: Q = flow into well, with dimensionsL 3IT

K = hydraulic conductivity,LIT

Yl, Y2,r,, r2 as previously defined, in units of L (L = length and T = time)

This equation neglectsthe curvilinearflow due to the drawdownshape. The error is not large if r, and r2 are sufficientlylarge; the curvature is thus negligible. The equationmay be used to predict the drawdown curve and radius of effective influence. It is useful also for computingthe hydraulic conductivityfrom field pumping tests. Two or more observationwells are usually installed at different distances from the pumped well. The nearest observationwell should not be closer to the pumped well than 100 times the well radius.

Correspondingto the equation for unconfinedaquifers, the Dupuit equationfor confined aquifers becomes: Q= 2 Km - Y2 Y 1 logQamo (r 2 /rZ)

where: Q = flow into well, with dimensionsPIT

K = hydraulic conductivity,LIT - 49 -

m = thickness of aquifer, in units of L

Yi and Y2 = depth from bottom of aquifer to pressure surface, at distances r, and r2 from the well, respectively, all in units of L

Basisfor Design of Pumped-WellDrainage. The preceding equationsare for the equilibrium or steady state condition. Similar relationshave been derived for use in the non-steadystate, such as in situationswhere pumped wells continueto deplete stored water. Pumping from confined aquifers is usually steady throughoutthe season because the aquifers are deep and replenish slowly and uniformly. However, water table wells may be used for either short-timedrawdown or near-constant seasonal discharge. Their design, therefore, shouldbe based on two considerations:

1. Capacity shouldbe sufficientto lower the water table after irrigation, heavy precipitation, or other influentseepage, in a relativelyshort time to avoid crop damage.

2. Capacity shouldbe sufficientto remove at least the seasonal net replenishment,which is the ground water replenishmentless depletionsfrom causes other than the pumped well in question. Shorter pumping periods may be required for this analysis, such as periods of one, two, or three months.

Advantages of Pumped-WellDrainage. High initial and operating costs for a pumped well for land drainage may be offset by several of its advantagesover a shallow drain system, including:

1. The water table may be lowered to much greater depths.

2. Deep strata may be much more permeablethan those nearer the surface.

3. Productive land that would have been occupiedby open drains is saved.

4. Maintenancecosts are less than for open drains and may be less than for closed drains.

5. Pumped water may be a valuable supplementto the irrigation water supply.

Drainage to ControlSeepage of the GroundwaterTable from High Land to Low Land

InterceptorDrains. Interceptordrains may be planned as single random drains or as a series of parallel drains. They are used where soils and subsoils are relativelypermeable and where the gradient of the water table is relatively steep. Interceptordrains skim off or divert a portion of groundwaterflow, thereby loweringthe water table in the area below or downslopefrom the interceptor drain. The distance that the water table is lowered below the drain is directly proportional to the depth of the drain; therefore, it is desirableto make interceptordrains as deep as possible, consistentwith other factors. The upslope effect of interceptordrains varies with the hydraulic gradient, decreasingas the hydraulicgradient increases. This upslope effect of true interceptor drains is usually small and is often ignored.

An interceptordrain may be installed across the slope to intercept groundwatermoving downslopefrom the sources existing around the area, and it is usually placed where the surface of the barrier convergeswith the ground surface. This placementmeans the drain will be located at a break - 50 - in slope to control the water table on the lower slope. An interceptordrain installed on the barrier will effectivelyintercept most of the water moving downhill. The need for additionaldrains either upslope or downslopefrom the initial interceptor shouldbe determined. If the interceptor drain is to be located on a barrier, the discharge can be estimatedby using the followingbasic equation: q = KiA

where: q = discharge per unit length of drain (m3/s). This value of q is the total amount of water moving within the saturatedprofile above the barrier. For practical purposes, however, the drain can be expectedto intercept only that portion of the saturated profile above the water surface in the drain.

K = weighted average hydraulicconductivity of the saturated soil profile above the barrier (mis)

i= slope of the water table measured normal to the groundwatercontours (hydraulic gradient)

A = area through which flow occurs (m2), i.e., a vertical plane one unit of length wide from the water surface to the barrier. When the interceptordrain is going to be located above the barrier, the equationfor discharge from a unit length of drain becomes:

q=KiA Ad

where: y = height of maximumwater surface above the invert of the drain (m)

d = distance from drain invert to barrier (m)

The design discharge Q frorn an interceptor drain is equal to the discharge per unit length q times the length. Determinationof the required dimensionsof the interceptor drain for a given rate of flow Q, hydraulic gradient s, and channel roughnesscoefficient n can be made by solving the Manning equationto determinethe mean velocity v and by using this relation:

Q=Av

where: Q = rate of flow (l 3 /s)

A = cross-sectional area of the interceptor drain (n2)

Theoretically, a true interceptordrain lowers the water table downslopefrom the drain by an amount equal to the depth of the drain, and the distancedownslope to which it is effective in lowering the water table is infinite, provided there is no accretion to groundwaterin that distance. Under field - 51 - conditions,where there is infiltrationfrom precipitationor deep percolationfrom irrigation, there is always accretion to groundwater. The distancedownslope from the drain to which it is effective is governed by the amount of these accretions.

In the design of interceptor drains, it is usually necessaryto estimatethe downslopeeffect of interceptor drains to determine if one or several such drains are needed to lower the water table in the wet area. This problem is difficult, but can be approachedby use of an empirical equationor by progressiveconstruction. The equationis based on the assumptionsthat the drain intercepts all the flow upslope from it to the depth of the drain and that the distancedownslope to which it is effective is dependenton the depth of drainage required and accretion to groundwaterin the area below the drain. As shown in figure 7, this is the reach L from the drain to point m where the drain is no longer effective. For purposes of this discussion,a true interceptordrain is defined as one in which all groundwaterflow enters the drain from the upslope side. Based on informationcurrently available,true interceptionis thought to occur when the hydraulic gradientof the undisturbedwater table is in the range of 0.01 to 0.03 ft per foot or greater. Where the gradient is less than this, the interceptor drain functions more like a relief drain, and the spacingshould be computed usingthe ellipse equation:

Le = Ki (de- dw+ W2) q where:

L, = distance downslopefrom the drain to the point where the water table is at the desired depth after drainage (ft)

K = average hydraulic conductivityof the subsurfaceprofile to the depth of the drain (in.Ih) q = drainage coefficient(in./h)

i = hydraulic gradientof the water table before drainage, undisturbedstate (ft per foot)

de = effective depth of the drain (ft)

d, = desired minimumdepth to water table after drainage, based on agronomic recommendations(ft)

W, = distance from the ground surface to the water table at the drain (ft)

W, = distance from the ground surface to the water table, before drainage, at the distance (L) downslopefrom the drain (ft)

In the preceding equation,L. and W2 are interdependentvariables. In solvingthe equation, it is necessaryto estimate the value of W2and make a trial computation. If the actual value of W2at a distanceL. is appreciablydifferent, a second calculationmay be indicated. In those cases where the gradient i is uniform throughoutthe area, W2can be consideredas equal to W,. FIgure 7. Cross-Sectional Profile, Interception Drain, and Area Influenced

Interceptionditch

Ground surface

de

IF ~~~~~~~~~~~~~~~~~~m

-e L - 53 - Example. Refer to figure 7. Determinethe distance downslopefrom an interceptor drain that would be effectiveunder the followingconditions:

d = depth of drain = 8 ft

WI = 1.5 ft

K = 6 in./h (from auger hole tests)

i = 0.05 (ft per foot)

q = 0.004 in./h (locally establisheddrainage coefficient)

d, = 3 ft (from local agronomicexperience)

de = d-W 1 = 8- 1.5 = 6.5ft

Solution:

W 2 =WI=1.5ft

L. = Ki (de- d. + W2) q

L = (6) (0.05) (6.5 - 3 + 1.5) = 375 ft (0.004)

At a distanceof 375 ft downslopefrom the drain, the depth to the water table would be 3 ft. If two or more parallel interceptor drains are to be used, the spacing between the first and second drains would be 375 ft, as computedabove. The spacingbetween the second and third drain, between the third and fourth drain, etc., would have to be recomputedusing adjusted values for i and de, due to the change in the hydraulic gradientcaused by the first interceptordrain.

Multiple InterceptorDrains. Where it is necessary to install multipleinterceptor drains, and site conditionsare such that the above equationis not applicable,it may be feasible to install the systemprogressively and to avoid the uncertaintiesof estimatingspacing. This objectivemay be accomplishedby constructingthe first drain to protect the higher portion of the wet area and then delayingconstruction of the lower drains to allow time to evaluate the effect of the first one. Spacing of additionaldrains can be accuratelydetermined by exploring water table levels below the first drain to establish a spacing interval. As shown in figure 7, the second interceptor drain would be located a distanceL. below the first drain, where the desired drawdownis effected. In actual practice, this distance could be extended a short distanceto allow for some upslope drawdown. The upslope drawdownis a function of the depth of the drain and the hydraulicgradient, as previously mentioned. As a general rule, it can be consideredas equal to the reciprocal of the hydraulic gradient.

Drainage and Subirrigation

Concept. Subirrigationis drainage in reverse. With drainage, the excess water'flows through the soil toward and into the drains. Such a drainage system lowers the water table in such a way that - 54 - the developmentof the root system and the growth of crops are not limitedbecause of lack of air. With subirrigation,water is diverted into the drains and then infiltrates out into the soil. In this way, the water table is kept at a proper height to allow crops to derive their water needs from it. The drainage requirementscannot be neglected if the pipe system is used to feed water to the land. Subirrigationhas wider applicationin the humid areas of the world, in conjunctionwith existing drainage systems, than it does in arid areas. Salinizationof soils constitutesthe main danger where subirrigationis practiced in arid and semiarid areas. Salinizationof soils resulting from the upward movementof the water during the growing season can be entirely overcome under a humid climate by the downward movementof water during the noncroppedseason. The general requirementsfor subirrigationto be practical and successfulunder both arid and humid regions have been summarized by Criddle and Kalisvaart(1967) as follows:

1. The soil must be of a utLiformtexture, reasonablydeep, and highly permeable.

2. There must be a naturally high water table or a "tight" or restricting layer in the soil profile upon which a perched or temporary water table can be developedbeneath the normal root zone of the crops. The restricting layer may be clay, bedrock, or simply natural groundwater.

3. The area to be irrigated must be large so that lateral drainage and return flows will not be excessivein proportion to the water that must be delivered to the area. The land surface must be smoothand level or with only a gentle slope in one direction.

4. Adjacent fields must be in the same generalplane. To be practical and successful,the difference in elevationcf adjacent fields should not exceed about 10 cm (4 in.), and leveling may be required to achieve this limit.

5. The "floor" or restricting layer in the soil profile should be reasonably close to parallel with the ground surface. An effectivenatural or artificial drainage system may be necessary to allow for rapid lowering of the water table and for leaching salts. Checks in the drains may be necessaryfor proper control of the groundwater.

6. Both the soil and the water used for subirrigationmust be relatively free of salts, particularly if the lateral movementof the water is limitedand if excess water is not availablefor occasionalleaching purposes. The soil and the water must also be free of suspended silt or clay particles to minimizeclogging.

7. When the water table fluctuates regularlyin a soil and where the water contains appreciablequantities of certain salts, an imperviouslayer or lens of salt that may retard water movementtends to develop. This lens usually forms near the phreatophyticsurface; deep chiselingmay be necessary to shatter it.

8. Where annual precipitationis low, at least one annual surface irrigation may be necessary to leach out the salts. lJse of portable sprinklersmay be the most practical method of applying this water.

9. During the growing season, the water table must be controlledwithin limits, dependingon the crop growth cycle. Few crops will tolerate widely fluctuatingwater tables. Water - 55 - table elevations,therefore, should not be allowed to change materially, especially during the middle of the growing season.

10. Where independentgroundwater control on adjacentfarms is not possible, farmers must agree on the desirable position of the water table and then strive to operate on that schedule. Thus, subirrigationmay require special communitycooperation.

11. Special provisionsmay be necessaryto germinate a crop and to start seedlingson subirrigatedland. These objectivesmay require raising the water table to wet the surface by capillary action, temporary use of sprinklers, or localized surface irrigation. Regardless, the surface soil must be kept free of high salt concentrations.

12. The varying water needs and rooting habits of the differentcrops at various stages of developmentmust be known.

Subirrigationbecomes impossibleas (1) the permeabilityof the water-conductinglayer decreases, (2) the thicknessof the permeable layer decreases, and (3) upward seepage increases. Each of these three characteristicsrequires a closer spacingof the pipelines. Downwardseepage increases the water supply requirements. Seepage upward to the root zone in quantitiesequal to or surpassingthe evapotranspirationrequirements makes irrigation superfluousbut drainage necessary.

Controlof the Water Table in Pipe Lines or Field Ditches. The purpose of subirrigationis to maintain the water table in the pipe lines or feeder ditches at the desirableheight so that the water table under the cropped area is maintainedat the predeterminedelevation. The maximum quantityof water to be supplied to or dischargedfrom the field is determinedby the resultant of evapotranspiration,rainfall, seepage, and length and breadth (area) of the field. If the breadth of the field is fixed, the length and width (diameter)of the pipelines may be somewhatvaried to maintain the desired water table. Short and wide pipelinestend to approximatethe ideal, although a certain variation of the water table above the pipelinesmust be accepted. Here again, it is necessaryto accept a reasonablecompromise between the requirementsof the crops and the technicaland economicpossibilities. Especially importantis the admissiblelength and frequency of periods when the greatest deviationfrom the ideal water table will occur. The existingtopography of the area to be irrigated will also help influencethe choice. One must use standard sizes of tiles for economic reasons, and the choice of spacing between pipelinesor field ditches and their length is not alwaysleft to the designer. Instead the designer must take into considerationthe joint influenceof all factors on the groundwatertable and also on the economicresults of farming on the subirrigatedsoil.

A general plan must be consideredfor water supply and discharge for the irrigated area, of which the fields are a part. This plan must be a complete system for water supply and drainage. Figure 8 shows two possible systemsfor a flat area: for the first, the supply and discharge structures are completelyseparated; for the second, the same structures are partly combined. If areas of variable slope are to be subirrigated,then adjacentfields must be leveled so that a continuouswater table can exist from one field to the next with as little variation in depth as is practical. Slight and regular sloping of the land may be advantageous,because the natural slope can be used in designing the total supply and discharge systemof the area.

Design Criteria. As a basis for the design of a subirrigationsystem, it is usually necessaryto determine the substrata conditionsfrom test borings. These borings should be tied to a common datum, and the boring logs and samples shouldbe analyzedto obtain the following information:(1) - 56 -

Figure 8. Design for Subirrigationin HorizontalAreas

X ~A X Discharge - -- O

lines or

field

ditches

Supply _ _ road AL

lines _ __ _t

Discharge _ -_-_-

Flowdirection:

lo Supply - - - > Discharge \/ Weirs of different dimensions - 57 - existenceof any restrictinglayer and its topography,(2) contours of the natural water table, and (3) hydraulic conductivityof the various strata above the restrictinglayer. Land mapping and topography shouldbe obtained,not only for those areas proposed for irrigation, but also for adjacent areas that may be affected by the controlled water table. The locationof natural drainage ways or surface outlets must be shown accuratelyon maps.

Because satisfactorysubirrigation depends on the ability to control the position of the water table, there must be some provision for getting the water into the soil profile as needed. Experience has shown that under arid conditions,parallel feeder ditches or tile lines that run on the contour and are spaced sufficientlyclose to ensure proper control of the water table are often the most practical method. Water is run into these feeders under control and is allowed to seep out and feed the groundwaterbasin. Feeder ditches closed at one end are usually laid out nearly on contour with little or no appreciableslope. The quantityof water allowed into the ditch is held to what the soil will absorb under a given depth of water in the ditch. If either more or less water is desired to raise or lower the water table, the depth of the water in the ditches is varied accordingly. If this variation of water depth in the feeders does not provide the desired water table control, a change in feeder spacing may be necessary. The maximum spacingbetween feeder ditches or drains should be such that the depth from the land surface and the water table will not vary beyond defmed limits because of consumptiveuse or expected rainfall. This limit may be 6 in. or more for such crops as alfalfa or small grain, but it may be considerablyless for potatoes (Solanumtuberosum) or specializedcrops. The limit may also depend on temperaturesand consumptiveuse rates and on the amountsand frequency of summer precipitation.

If the lands were level and no precipitation,lateral flow, or deep seepage occurred, the amount of water necessaryto maintain a static water table would equal the consumptiverequirement of the crop. Such a condition,however, is not practicalbecause there must alwaysbe some lateral flow and drainage if the groundwaterquality is to remain good for crop production. Also, the designer has little or no control over the weather and must prepare for a certain amount of rainfall, except in a few extremely arid areas of the world.

After consideringall the factors influencingwater needs for the subirrigatedtract, there are other factors that must be considered in systemdesign. Since the slope of land may affect the water table position, an upslope feeder ditch or pipe will supply water to a greater portion of the areas between ditches. Slope tends to make proper control more difficultto maintain. Feeders may be required to carry considerablewater at certain times of the year so that the "sub" or water table may be raised rapidly. Thus, feeders shouldbe designedto convey several times the amount of water normally required to satisfy the maximumconsumptive use rate of the crop. In the arid areas of the United States, this ratio has sometimesbeen set as high as 20 times the consumptiveuse rate. Such capacity may not be needed in more humid areas. However, the size and the slope of the ditch should be such that it will convey the maximumflow of water needed. It is also desirablethat the depth of water in the feeder remain reasonablyconstant throughoutits length. Under these conditions,infiltration opportunity remains constant throughoutthe ditch length. This requirement, however, calls for nearly level feeders. The cross-sectionof the feeder ditch should be proportioned in such a way that adequate infiltrationcan occur, as discussedabove. The design of any open feeder cross-sectionmust consider the maximumstable slope that the soil permits and the farming requirements. In organic soils, ditches often have vertical sides and are quite deep. In mineral soils planted to pasture, the distributingditches are sometimesextremely wide and shallow. Ditches often have a tendencyto seal with use, and the banks may have to be reworked at intervals to permit adequate infiltration. In some instances, when a quick raising of the water table is desired, the feeder - 58 - ditches are permitted to overflow temporarilyonto the adjacent cropped area, if level. Because of the wide variabilityin soils and water supplies, field tests of existingditches in the area under considerationcan provide the best informationregarding the expectedrate of percolation to the adjacent land mass. From results of these tests and a considerationof other conditionsinvolved, a suitable cross-sectioncan be selected.

Drainage for Control of Surface Water

Drainage for Flood Protection

Concept. The threat of serious flood damage can psychologicallyimpede project development and must be taken into considerationduring the diagnosticstage of the planning process. Farmers threatened by floods may hesitate to invest in their land and to improve their farming practices without certain guaranteesfrom the government. Moreover, banks and financingagencies may feel obligatedto be cautious with their loans and thereforeunknowingly prevent low-incomefarmers from receiving assistancewith sufficientlyfavorable terms. The degree of safety is a policy decision involvingengineering, economic, and socioeconomicconsiderations. Generally, engineering considerationscan contributeto the establishmentof minimum safety criteria; economicand social considerations,however, should be the foundation and the justificationfor the implementationof safety standards, accordingto Kuiper (1965).

Flood Control. The flood control system of a project can be divided in two aspects: (1) control of water encroachingfrom outside the project boundaries (external water), and (2) control of water within the project (internal water).

Exernal Water. External water has three main sources: river and canal flooding, sea water intrusion from storm tides, and runofiffrom upland areas. Overbankriver and canal flooding occurs because river and canal channelshave insufficientcarrying capacity to handle the high flows. Levees increase the capacity of the river charmel,and the clearing of obstructionsbetween the levee and the river channel also increasesthe floodwater-carryingcapacity. Sea water intrusion into low-lying areas is common, and levees are only one rnethodthat can be used to control such an intrusion. Conservationfarming practices and correct watershedmanagement can reduce the amount of water moving across the land surface, thus reducingcrop destructionand erosion. Runoff from upland areas can best be handled by intercepitorchannels coupledwith small levees. The channels divert the floodwatertraveling overland into drains and streams, thereby preventingthe water from enteringthe project area.

Internal Water. Flood conditionswithin the project can be caused by heavy rainfalls, excessiveuse of irrigation water, or an inadequatedistribution system in the cultivatedarea. While project drainage systemsshould be designedto handle certain levels of rainfall, proper water managementpractices can help control flooding of low areas caused by excessive irrigation. The full benefit of internal drainage improvementcannot be realized until steps are taken to prevent flooding of the improved project area by external sources of water. Control of internal water has been covered in detail earlier in this paper. - 59 - Flood protectioncan take several forms, such as cutoff channels, conservationmeasures, and levee (dike) systems. In the long run, the most efficientmethod is to employ conservationmeasures throughoutthe watershed to decrease the peak runoff, thereby decreasingthe high water elevation in the lower reaches of the river. Potential floodwatersfrom the upper part of the river basin can also be diverted from the area by cutoff channels. A system of levees can be used to prevent the intrusion of water. Conservationmeasures can be appliedto improve the river channelcross-section and consequentlyits carrying capacity during flood periods. Of these three methods of flood protection, cutoff channels are generally costly and of relativelylittle use. Conservationmeasures, however, which can be very economical,are beginningto receive attention from fundingsources and are fundamentalto integrated regional development. Levee systems are widely used and are often quite economical,particularly as part of an overall land managementstrategy. These systemswill be covered in some detail below.

In levee design, one of the most difficult considerationsis determiningthe desired water level for the project site. For coastal regions, this is particularlydifficult because of the complexnature of waves and tidal movements. However, much work has been done in recent years to break down the total wave structure into componentsand to develop equationsthat adequatelydescribe each component. Buildinglevees (dikes) is the oldest and most frequentlyused measure against tidal intrusionsand floods. Levees, however, do have a limitedlifespan, and their failure often leads to disaster, as has been reported by the EconomicCommission for Asia and the Far East (ECAFE) (1972). An unwarranted feeling of safety is often induced by the presence of levees. Levees can never be built, however, to provide protection against the maximumpossible danger; the possibility always exists that a levee will fail. Careful planning, therefore, is required in determiningthe elevationof a new levee system. Pickles (1941) listed several of the effects of confining floodwaters between levees: (1) an increase in the rate at which a flood wave travels down the stream, (2) an increase in the water surface elevation of the river at flood, (3) an increase in the maximumidischarge at all points downstream, (4) an increase in velocity and scouring action through the leveed section, and (5) a decrease in the surface slope of the stream above the leveed portion. Once the decision to build a levee has been made, the first step is to analyze river discharge and water river records. For coastal regions, storm-tidewater elevationsare also analyzed. Next, the potential location is determined, and a complete soil survey of the levee site is made to ascertain the bearing capacityand permeabilityof the foundationsoil layers. The foundationcharacteristics will determineto a large extent the cross-sectionalshape of the levee. In that regard, Kuiper (1965) and Marsland (1966) have listed several levee componentsthat are in part determinedby the foundationmaterial:

1. A permeable foundationmaterial requires that seepage underneaththe levee be controlled. This requirementmay involve the installationof an impermeableblanket in front of the levee, normally about ten times the length of the head on the dike. A cutoff wall consistingof sheet piling or a clay trench dug down to an impermeablesubstratum may be necessary underneaththe levee. Drainage or relief wells near the toe of the levee may also be necessary. Levee failures resulting from underseepageoften occur unexpectedly because the bank erosionprocess takes place below the floodwater level and is therefore undetecteduntil the damage has been done. Accordingly, regular maintenanceof all flood preventionstructures must be included in project operation costs.

2. The strength of the soil foundationdetermines the berm base width. A deeper soft layer in the foundationrequires a wider berm. The critical slip circle increases in diameter with an increase in depth. Ditches near the levee increasethe effective height of the bank, thereby increasingthe likelihoodof slips. - 60 - 3. Care must be exercised when buildinglevees using materials significantlydifferent from those of the foundation. If the levee material is appreciablystiffer than the foundation material, i.e., if the foundationmaterial is more compressible,differential settlement could occur across the bank section, causing cracks to develop.

The side slope of the levee is in part determinedby the duration of exposureto high water levels. A levee along the coast is exposedto high water levels continuouslyfor short durations, whereas a river levee may be exposed to high water levels for several weeks during the year. A longer duration of exposuregenerally requires a flatter side slope and a more massive structure to compensatefor some weakening of the structure due to water absorption. The severity of wave attack and uprush will also play a part in the wet-slope determination. The slope of the levee, elevation of the top of the levee, and revetment materialused on the slope are interrelated. With the same slope, a revetment with a rugged surface would reduce the wave uprush and also allow a lowering of the top of the levee (Kuiper 1965). Care shciuldalso be taken that the dry slope of the levee is not too steep and is well protected. Several disastrous floods have been caused by steep dry levee slopes (ECAFE 1972). A grass cover can be used on the dry slopes if the levee is made of clay or has a clay blanket covering it.

Drainage for Control of Surface Water

On-Farm Water Management in Lowland Rice Fields

Rice Groth and Soil Water Conditions. Rice is grown in flooded basins in which the water level should be kept within certain limnits.During the growing season, the upper and lower levels should be kept in accordance with rice irrigation requirements. Rice reaction to soil water conditions has been described by many investigators,such as Adair et al. (1962) and Matsushima(1962). Rice apparentlyhas three critical periods wherein moisture stress reduces grain yield: (1) the seedling establishmentperiod, (2) the period of the tillering stage of growth, and (3) a period of 20 days before to about 5 days after heading. Stress may develop quite rapidly when submergedrice is drained because of the very shallow root system developedunder flooded culture. Thus, care is required to avoid sudden stress under these conditions.

Rice plants can tolerate excess water up to certain levels before production levels are affected. In this regard, Ghosh et al. (1975) found that a submergenceof 50 percent or more of the crop height was detrimentalto crop growth and yield at any stage from seedling establishmentto flowering. The significanceof the growth stage and submergenceduration on yield has been studied by ECAFE (1972) and Undan (1977). Their results showedthat the greatest flood damageoccurs at early reproductionstages around the time of panicle formation and that there is a significant increase in crop damage if submergencelasts for three days or longer. Therefore, rainfall records shouldbe analyzed for three-dayintervals during the critical period to determine the design flood level that will afford reasonableprotection for the rice plant against prolonged submersion. Van de Goor (1974) noted that the drainage of a rice production system shouldbe designed to discharge excess floodwatersoccurring during the crilicalperiod. The drainage system capacity should be such that after the third day followingthe onset of flood, water depth in the paddy will not exceed 20 cm. In addition, Fukuda and Tsutsui (1973) stated that the provisionof perfect drainage in paddy fields is usually not economicallyfeasible. Iherefore, it is the usual practice to allow for a certain submergencewhen designingdrainage systems. The allowablesubmergence depth and time differ - 61 - greatly with the crop growth stage; for example, if the greatest flood damage is most likely to occur at the panicle formation stage with submergencefor more than two days, crop growth will be seriously affected.

Drainage is practiced in rice fields to remove the water used in puddling, to prevent or cure diseasesor pests, to prevent lodging, to stop littering, to maintain certain water heights during the crop growth period, and to provide a suitable trafficabilitywhen completelydraining the field three to four weeks prior to harvesting. One mid-seasondrainage is practiced in Japan by draining the fields for three to seven days at the late tilleringstage. Water can be removed through surface drainage and percolation. The optimumrate of water percolation dependson the soil structure conditionaffected by puddling, the concentrationof toxic and salt substancesto be removed, the crop growth stage, and the farming practices, such as fertilizer applications. The optimumrates of percolationwere described by Fukuda and Tsutsui (1973) to be about 5 mm, 15 to 25 mm, and 8 mm per day in well- drained fields, swampy fields containingabundant toxic substances, and saline areas, respectively. Takahashi et al. (1978) reported that as a drainage criterion commonlyused for paddy fields in Japan, the groundwatertable shouldbe lowered from 30 to 40 cm from the soil surface in two to three days and from 40 to 50 cm within seven days after rain cessation. This recommendationmay be considereda safety measure to ensure the presence of a soil reservoir should more heavy rains occur.

Percolation may be horizontal or vertical. Horizontalpercolation (evee percolation)is usually predominantin terraced rice fields and is used to irrigate adjacent areas. The rate of this percolation may amount to ten times that of vertical percolation (due to greater hIL value), according to Fukuda and Tsutsui (1973). When fertilizer and water are in insufficientsupply, percolation is mostly consideredharmful. Moreover, Ponnamperuma(1965) found that percolation does not have such a great value in aerating the rice soil even with rates as high as 30 min per day; therefore, the benefit of a moderate amount of drainage in submerged rice soils shouldbe ascribed mainly to the leaching of toxins, including salts. Puddlinghas a great influenceon decreasingthe water losses through percolation, and van de Goor (1972) reported that the practice of puddling on a loamy soil decreasedthe percolation losses from 37.7 to 6.7 mm per day.

When rice is rotated among-other crops in an area that has a subsurfacedrainage system, the drain outlets are usually provided with a vertical stopper than can prevent or allow the outflow of drainage water when desired.

Controlof Surface Water on Paddy Fields. Flood damagein paddy fields results when rainfall brings more water than the allowablesubmergence depth. Design rainfall shouldbe determinednot only from a technicalviewpoint, but also for economicfeasibility. Usually, heavy rainfall that occurs at a frequencyof 1 per 10 to 1 per 30 years is taken into account. A five-year return period for internal drains and a return period of 25 to 30 years for main drains and rivers are commonlyused in developingcountries. Large-scale flood control projects, however, usually take into account a frequency of 1 per 50 to 1 per 100 years. Rainfall is taken into accountdaily, or sometimeshourly, where the area is subject to temporary submergencefor short periods, but continuousrainfall is considered prolongedflooding. When consideringdrainage schemes, there are two types of runoff coefficients:(1) the ratio of rainfall to correspondingrunoff, and (2) the ratio of maximum rainfall to maximumflood. The former is generally used for relativelylarge low-lying areas, whereas the latter is used to estimatepeak flood caused by rainfall concentratedin a short time period. -62 - In developingcountries, the farmermakes a temporaryopening through the earth bund and then closesthe openingafter the flood. In developedcountries, Wang and Hagan(1981) reported that to effectivelydrain excessstorm water from pa4dy fields without damaging bunds by overtopping, spillwaysshould be providedin eachbund. Spillwayscan be designedas broadcrestedweirs (figure 9), whichcan also be used to deliverirrigation water in cross-paddyflow. The dischargeequation for a rectangularbroadcrested weir is as follows:

q= CdLH 3 12 (1) where:

Cd 2 K-(h+H) 12 (2) as has beengiven by Doeringsfeldand Barker(1941);

h = spilway sill height from the paddy soil surface (m)

H = water head above the spillwaysill (m)

g = gravitationalconstant, 9.81 m/s;

L = spillway length perpendicularto flow direction (m)

K = constant,equalto 1.8

Doeringsfeldand Barkerfound that K = 2.0 for conditionsof free flow and K = 1.7 for conditions withstanding waves on the weir. For an operationas a spillwayin a paddybund, bothconditions will be encountered;therefore, an averagevalue of K = 1.8 is assumed.

Figure 9. Cross-Sectionof a RectangularBroadcrested Weir in a Paddy Bund

H D +

Source: Wangand Hagan1981. - 63 - Curtis (1941) found that a broadcrested ]Figure10. VariableCross-Section of BroaderestedWeir weir with a variable cross- section gave actual flow rate measurementsless than 2.0 percent different from those calculatedfrom 1.5' 9' \ is variable the preceding equations X40 (figure 10). This finding indicatesthat large Source: Curtis 1941. variability in the weir cross-sectionwill not significantlyaffect its discharge. Since these spilways will in most cases be constructedby a farmer, their insensitivityis very important.

The paddy bund spillwayconveys water in cross-paddyflow when all paddy fields do not have direct access to farm ditches. The discharge capacity is given by:

qw= DO* 10, 000 T where:

q = discharge required to supply soaking water to one hectare (m3 per hectare per day)

T = time allocated for applicationof q days

Do = land-soakingand standing-waterapplication (m)

Therefore, it follows that: SD, x 10,000 = 86,400 LFHm

and

4i = SDS8.64CHM (3) where:

S = rotational interval (days)

D, = maintenancewater requirements(meters per day)

L, = bund opening required for irrigation flow (m)

The maximumwater head can be given as:

H=D-Fd-h (4) - 64 - where:

D = height of the paddy bund (m)

Fd = freeboard allowance (im)

h = spillway sill height from the paddy soil surface (m)

By using the project design criteria, it is possible to calculate the paddy bund spillway length necessaryto deliver water to each hectare of paddy in the time allowed, as shown in the following calculationexample:

D. = 0.13 m, T = 1 day, D = 0.25 m, and h = 0.05 m

Let Fd = 0.08 m; H =D -Fd - h. Then H = 0.25 - 0.08 - 0.05 = 0.12.

From equation 2, Cd = 1.76. By solvingequation 3,

I, = 7 x 0.0065 = 0.072r m/ha/day 8.64 x 1.76 x (0. 12)3/2

The following assumptionsare usually used when designingfloodwater drainage:

* Water velocity in the paddy upstream of the spillwayis negligible.

* Water stored on the leaves of paddy vegetation and paddy bunds is negligible.

* Water at no time overtops the paddy bunds.

e The variable flow rate from other areas into the paddy of interest is equivalentto the variable flow rate at the paddy spillway.

* The water level downstreamof the spillwaydoes not prevent discharge and does not submerge the spillway.

* No irrigation water is provided during the rainfall period.

* Evapotranspirationand percolationrates are unchangedby the storm.

Paddy bund spillway design for floodwaterdischarge is a function of the rice plant growth stage at the time of flooding, which determinesthe allowabledepth and time period of crop submergence. The fact that the rice plant can be submergedfor a period of time without suffering significantdamage can be used to decrease the size of the discharge spillway. By allowing the rice plant to be submerged, storage is created in the paddy field. This storage can be divided into two parts: (1) dead storage below the spillway sill, and (2) temporary storage above the spillway sill. The average dead storage S over an area of paddy fields under rotational irrigation can be given as: - 65 -

A Sl =h- 2 Dt (5)

The temporarystorage (AS) can be written as:

AS2 = F - h (6) where: F = allowablesubmergence depth at the end of the design storm period (m). This depth must not be greater than the height of the paddy bund to avoid damageduring floodwater discharge

The paddy field of interest can receive water from other paddies, as shown in figure 11. The volume of water received from other paddies can be given as:

Vp = (R-ASl-AS 2 -TDT) Ap (7) where:

R = design storm rainfall, depth in m

V, = water from other paddies in e 3

T = storm duration in days

A = paddy area in e 2

The volume of water, in e 3, to be dischargedfrom the paddy of interest can be expressed as:

V = (R - AS1 - AS2 - 7DT)A + Vp (8)

Dividing V by T will give the averagevolume to be dischargedover the period of the storm, which must be less than or equal to the spillway discharge:

86,400 Q =V T

Substitutingequation 1 for q and solving for Ld,the bund opening required for the drainage flow gives:

Ld=V (9) 86,400 TCd h 3/2

It is expectedthat as one proceeds from the drainage ditch to the irrigation canal, the spillway required for flood discharge will be smaller and the opening needed for irrigation water delivery will be larger. Therefore, both sizes shouldbe calculatedand the larger used in the design layout, as shown in the following calculationexample: - 66 -

If R = 0.098 m per three-daystorm and a five-year return period for May, and A = 1.0 ha, A. = 5.0 ha, CD= 0.85, Ap = 0.5m2, D = 0.25 m, h = 0.05, D = 0.0065 m per day, S = 7 days, F = 0.2 m, and Fd = 0.0. Then:

From (5): AS1 = 0.05 - 7 (0.0065) = 0.027 m 2

From(6): AS2 = 0.20 - 0.05 = 0.150 m

From the previous example, the contributingpaddy can deliver 0.05 m of water in one day to the paddy of interest. Therefore, since 3 x 0.05 is greater than 0.098, it is reasonableto expect all excess waters to be, for the spillway between the two paddies:

From (7): V, = (0.098 - 0.027 - 0.150 - 3 x 0.0065) = 5000 = 275.5 m3 From (8): V = (0.098 - 0 027 - 0.150 - 3 x 0.0065) 10,000 + 4165 + 257.5 = 3437.5 m3 From (4): H = 0.25 - 0 - 0.050 = 0.20 m From (2): Cd= 2.06 From (9): Ld = 3437.5 = 0.072 m 86,400 x 3 x 2.06 x (0.20)3 2

The spillwayin the bund adjoiningthe drainage ditch should be at least 0.072 m in width to handle the design storm.

Drainage for Control of Surface Water

Land Surface Drainage Figure L1. Cross-Paddy Drainage of Excess Rainwater

Concept. Surface drainage is defined as the removal of excesswater within Padd a given period of time from the land surface over or through the uppermosttopsoil layer to a proper, A open drainage system. The main concern in S draining flat areas is AS1 the timely removal of stagnant surface water often existing in depressions. When Source: Wang and Hagan 1981. draining sloping areas, the emphasis shouldbe on the removal of water without causing erosion. Surface drainage is needed to solve problems resulting from the combinationsof certain climatic, hydrologic, - 67 - topographic,soil, and land use conditions. Excess water caused by rainfall, surface runoff from upland areas, or overflow from rivers may result in ponding water on the land surface, especially when the topsoil has a lower infiltrationrate or lower vertical permeabilitythan that of the subsoil. Topography and land use may favor or hamper a timely discharge of water over the land. General relationshipsbetween rainfall and soil conditionshave a direct impact on surface runoff, and these concepts are generally considered:

1. Thresholdconcept: when the rate of rainfall does not exceed the soil infiltrationrate. In this case, the soil absorbs water until it is saturatedup to the soil surface, whereupon waterloggingoccurs.

2. Infiltrationconcept: when the rate of rainfall exceeds the infltration rate and stagnant water occurs before the soil is saturated.

Typical examplesillustrating the need for surface drainage have been reported in different regions, such as:

1. In the Ruzizi Valley, CentralAfrica, where a very heavy montmorilloniticsoil with a thicknessof several meters is overlying a highly permeable subsoil. The groundwater table remains at a great depth (10 to 20 m below land surface), althoughprolonged inundationsoccur during the wet season.

2. In the low-lying basin, clay soils in the delta of the MississippiRiver, UnitedStates, which is subjectedto a rainfall with high intensity(about 225 mm per day once every 10 years). The need for surface drainage is also evidenthere, and a subsurfacedrainage system is required for the control of the high groundwatertable.

3. Along the southeasterncoast of the UnitedStates, where very intensiverainfall (about 175 mm per day once every 10 years) causes a need for surface drainage on the permeable sandy soils. The average depth of the groundwatertable was observed at about 1 m below soil surface, and a storage capacitythat can be quickly filled was found to be about 75 mm. In this case, a subsurfacedrainage system alone is not sufficientto deal with such a large volume of water to be discharged.

4. In the midwesternUnited States, where large areas (under a rainfall intensityof about 100 mm per day once every 10 years) require both surface and subsurfacedrainage systems. The need for surface drainage is causedby the presence of a poorly pervious subsoil, which can be a result of frequent use of heavy farm machineryor a result of alternating freezing and thawing in winter.

5. Surface drainage is generally considereda prerequisite measure in many tropicaland subtropicalregions where rainfall with a high intensityis common, especially in heavy- textured, low permeable soils. When determiningthe extent of the need for surface drainage, both the general situationof the area and its surroundings, as well as the time of occurrence and duration of water ponding with regard to crops grown, must be taken into account. - 68 - Surface Drainage Practices (Systems)

Several terms related to surface drainage are used in these guidelinesto describe the same items indicatedby Philips (1963) and ASAE (1966). These are defined as follows:

* Crop rows: small channels between ridges on which crops are grown (more commonly called furrows);

* Row drains: small channelsperpendicular to crop rows at low places to collect water from the rows (sometimescalled interceptionor quarter drains);

* Field drains: shallow, graded, open channels with side slopes flat enough to allow crossing of farm machinery (sometimescalled collectiondrains). They collect water within the field from rows, row drains, or enclosed depressions. A distinctionhas to be made between single-fielddrains (or 'V" drains) and double-fielddrains (or 'W" drains). The latter consist of two field drains, parallel and close, with the spoil of the drains in between often used as a field road. Double-fielddrains are made when the spoil cannot be disposed of without blocking drainage into the ditch, mainly on flat land without irregularities; and

* Field laterals:principal outlet ditches for the adjacent fields. They receive water from the field drains and can be regarded as part of the main drainage system on the farm. They are often steep-sidedchannels that cannot be crossed with farm machinery without the existenceof a structure.

Various systemsof surface drainagehave been developedbased on topographic and soil conditions,crops to be grown, and preference of the design engineer and the farmer. Constructionof a surface drainagesystem on its own is usually not sufficientto guarantee proper drainage. As water remains pondingon the land, such as in pockets or depressions,the microtopographyof the land sometimeshas to be changed to enable a timely removal of excess surface water to the drains. Thus, reshaping the land surface is consideredin this chapter as a surface drainagepractice. Moreover, a distinctionis made between surface drainage systemsin flat and steep areas. A slope of about 2 percent is usually consideredthe breakpointbetween flat, sloping land and sloping areas. In flat areas, the lack of sufficientslope is a limitation in the designof the layout, while in steep areas, erosion hazards are the main cause oi limitations.

Land Fonning

Land forming is defined as the process of changing the topographyof a field to ensure the orderly movementof water over the land and to allow the introductionof mechanizedfarming. It is a relativelynew surface drainage practice, comparedwith its use for irrigation or erosion control purposes. The design of land forming shouldtake into accountthe time duration in which excess water has to be removed from the soil surface, the specific cultivationneeds of various crops and crop rotation, and the use of farm machinery. Furthermore, allowanceshould be made for soil structure, soil fertility, existingrequirements of irrigation, and erosion control. In additionto technicalfeasibility, the economicjustification of the design is an important consideration. In land forming, two phases are recognizedfor the improvementof surface drainage: land grading and land smoothing. - 69 - Land Grading. Land grading is a one-time operationcarried out by mechanicalmeans and involvingthe transport of earth accordingto specific cuts and fills based on a predeterminedgrade. The terms land grading, land shaping, and landforming are used synonymously,depending on local practice. Land shaping is a term often used in connectionwith erosion control: the layout of terraces, contour benches, etc. Land leveling is a term generallyused in the constructionof irrigation layouts. Land grading differs from land leveling for irrigation in that grade limitationsare less restrictive in land grading. Grades, especiallyrow grades, can be varied as much and as often as necessaryto provide drainage with the least amount of grading but shouldstay within soil erosion limits. The grade in the direction of drains must be continuous, with a minimumof 0.05 percent (preferably0.1 percent) and a maximumof 0.5 percent, dependingon erosion hazards. The grade of cross-slopes normally should not exceed 0.5 percent. A detailed topographicsurvey has to be carried out to locate the high and low areas with a sufficientdegree of accuracyto plan the required cuts and fills. Althoughvarious methods of land survey can be applied, the grid survey is generally the most suitable, especiallyon flat lands. The maximum allowabledepth of cuts depends upon the soil properties as determinedby soil surveys and also upon economicconsiderations.

Various methods of earth-movingcomputations for land forming have been described in the literature (for example, USDA-SCS1959), dealing only with design and computationalmethods for land leveling for irrigation, which requires a higher degree of accuracy in view of a uniform water distribution. The profile method is probably best suited for land grading to improve drainage. Land grading is carried out by heavy equipmentsuch as bulldozers and scrapers. A bulldozer can be efficientlyused for short distances up to about 100 m, and a scraper is usually the most suitable equipmentfor performing the work more accurately.

Land Smoothing. Land smoothingis the planing or smoothingof the land surface without changing the general topographyof the field. Minor differencesin elevation are eliminated,thus permittingeffective surface drainage. Land smoothingis also the finishing operationfor land grading and all other land-formingpractices to connectsmall surface irregularities. For proper land smoothing,the soil has to be dry and crumbly. The operation, directed by eye, can be performed best on an annual basis after completionof other tillage operationsfor seedbed preparation. Land smoothingis the cheapest and yet one of the most productivesurface drainage practices. The work can be done with a simple wooden drag behind a farm tractor or with more sophisticatedequipment, such as land levelers and land planes. With the latter equipment,depressions up to 15 cm deep can be filled if necessary. This practice, however, is not preferred because it impliesthe removal of large amountsof good topsoil from elsewhere. Motorgradersare not as well suited for land smoothing because they are too short to smoothmany irregularities.

Land SurfaceDrainage Systems in Flat Areas

Four different surface drainage systems commonlyapplied in flat areas are described in this section.

lhe Bedding System. In the bedding system, one of the oldest surface drainage practices in flat areas, the land is plowed into beds over a number of years and separatedby dead furrows running in the direction of the prevailing slope, as shown in figure 12. Apart from plowing (which is always carried out in a parallel direction), all farming operationscan be carried out in either direction across or parallel to the furrows. - 70 -

Figure 12. The Bedding System

-* Slope Fence Bed Dead furrow .- Turnstrip

Fielddrain

.//77777 7 . - Fieldlateral Bed lengthbetween 100 and 300 meters

Bed Deadfurrow

Width

Bed widthbetween 10 and 30 meters Bed heightbetween 15 and 40 centimeters Furrowwidth between 15 and 30 centimeters

Bedding has proved to be a practical solution when appliedon land with a slope of up to 1.4 percent. The bed width depends on land use, slope of the field and dead furrow, soil permeability, farming operations, and width of farm machinery. The following are reported recommended dimensions:

* 8 to 12 m for land with very slow internal drainage; * 15 to 17 m for land with slow internal drainage; * 20 to 30 m for land with fair internal drainage.

Bed length varies in practice from 10 to 300 m. Bed height-the distance between the bottom of the furrow and the top of the bed on pasture land-measures up to 40 cm, which is almosttwice the height usually applied on arable land. Field drains are shallow (average depth: 25 cm) and have almostflat side slopes (6:1 to 10:1) and a gradient of at least 0.1 percent. The bedding system is not a satisfactorysolution for surface drainage when crops are grown in rows parallel to the dead furrows. Crop ridges prevent overland flow to the furrows, and consequentlythe rows have to drain into the field drain or row drains have to be made and maintained. The bedding system is recommendedonly for pasture, hay, or any crop that allowsthe surface of the beds to be smoothed. At present, the parallel field drain system is preferred, even in extremely flat areas. Disadvantagesof - 71 - the beddingsystem are:

* The slopeof the furrowis not alwayssufficient, especially when made with only a plow.

* The topsoilis removedfrom the side of the beds to the middle,which may cause reductionin yieldsnear the furrows.

* The furrowsneed to be maintained.

* To a certainextent, the systemhampers mechanized farming. RandomDrainage System. From an economicpoint of view, fieldswith scattered depressions that are too deepor too large to be filledby land farmingpractices, but whichgenerally have good surfacedrainage, can oftenbe drainedwith random drains or ditches. Wherepossible, the drains connectone depressionto anotherin conveyingwater to a suitableoutlet. Drain depthdepends on the topographyof the area and on the dischargedesign and shouldbe at least 25 cm. Sideslopes shouldbe 8:1 or 10:1where they are to be crossed. If farmingoperations are carriedout parallelto the drains,a side slopeof 4:1 can be allowed. The spoilfrom the drainsshould be used to fill small depressionsnot connectedto the system. Effectivedrainage Is improvedif the fieldsare smoothed. ParallelField Drainage System. Thissystem is also calledthe land gradingsystem because surfacedrainage is improvedin the first instanceby landforming and not by drains,as in the random drainagesystem. Thismeasure by Itself,however, is not sufficientbecause the land surfaceis usually too uneven,and parallelfield drainshave to be constructedat convenientdistances. Drainspacing dependson the permeability,the soil, the cropsto be grown,the topography,and the gradientof the land after leveling. The gradientvaries in practicefrom 100to 200 m on relativelyflat landwith a break in the slopeon top of the ridge formedby grading,as shownin figure13. Althoughthe parallelfield drainagesystem is the mostexpensive of all systemswhen generallyapplied on land witha slopeof less than0.5 percent,It providesgood drainage to eachpart of the field. Otheradvantages are that fewer ditchesare involvedand that short and point rowsare eliminated,thus facilitatingmechanized farming operations. The methodcan be recommendedfor surfacedrainage of flat lands, whereveradaptable. Plowing Is carriedout parallelto the drains,and all other operationsare perpendicularto the drains. The rows leaddirectly into the drains,and they shouldhave a slopeof 0.1 to 0.2 percent. If the erodibiityconditions of the soil permit,the slopeof the rowsmay be as high as 0.5 percent. Field drainsusually have side slopesof 8:1 to 10:1and a grade varyingfrom 0.1 to 0.3 percent(never less than0.05 percent). A specialadaptation of the systemis land crowningon very flat landwhere earthis movedwith heavy, earth-moving equipment to makelow ridgesor crownswith field drainsabout 30 to 100m apart. In fact, the land is givenan artificialslope, which provides excellent drainage but involvesconsiderable earth movingand the maintenanceof manydrains. ParallelOpen-Ditch System. This systemcan be introducedto soils witha moderate permeabilitybut in need of surfacedrainage. A systemsimilar to the parallelfield drainagesystem may also be appliedto controlthe groundwaterlevel (subsurface drainage). To this end, the field drainsare replacedby openditches at least 60 to 100cm deep and are givensteep side slopesof 1:1 or 1-1/2:1,depending on soil conditions.Maximum spacing varies in practicebetween 60 and 200 m. Becausethese ditches can no longerbe crossedwith farm machinery, all farmingoperations are carriedout parallelto the ditches. Dischargeof excesssurface water fromthe rows is madepossible - 72 -

Figure 13. Parallel Field Drainage System Row directions

Fence

4 t 9 Turnstrip

Fielddrain

7'Z// /77/7/ Fieidlateral

Upto400m -Upto -

200 m

Original - Landsurface

Note: Field drains are up to 200 rn on land sloping in one direction. Field drains are up to 400 m on land slopingin both directions after grading. Rows are constructedon graded and smoothed land perpendicularto the drains. by row drains. This method is applicableon peat and muck soils in need of surface drainage and, in the meantime, requiring groundwatertable control. For mineral soils, it is more convenientto apply the parallel field drainage system for surface drainage and the pipe drainage system for subsurface drainage.

Land Surface Drainage Systems in Sloping Areas

Surface drainage methods in slopingareas are closely related to problems of erosion control. Suitable conditionsshould be effectedto regulate any occurring overland flow, which has to be interceptedbefore it becomes dangerousas an erosive force. A descriptionof three different surface drainage systemsfollows.

Cross-SlopeDrainage System. The cross-slopedrainage system is the most widely used - 73 - practice of surface drainage on lands with a slope of 2.0 to 4.0 percent, too steep for the application of flat land practices because of erosion hazards. This system is appliedwhere overall slopes are rather regular and long, but where many shallowdepressions occur. Field drains are constructed parallel to the contourson a uniform grade, varying between 0.1 and 1 percent, accordingto the topography. Drains are also known as cross-slopeditches, and, when serving erosion control purposes, they are called channel-typeterraces or Nichols terraces. Spoil from drains is used to fill depressionsor is spread out on the downslopeside, though in a layer that is never higher than about 7 cm above the natural ground level. This requirementimplies that the major part of the drain (80 to 100percent) is below the original land surface. Smoothingthe land between the drains is essentialto the good operation of the system, especiallybecause all farming operationsshould be parallel to the drains. The distancebetween the field drains applied in practice dependson the slope, rainfall intensity, soil erodibiity, and crops. The distancevaries between 30 m (4 percent land slope) and 45 m (2 percent land slope). The maximumlength of a field drain slopingto only one side is 350 to 450 m. Depth of the drains is at least 15 to 25 cm, and the top width can be from 5.0 to 7.5 rn. The cross-sectionalarea can vary from 0.4 to 7.0 in2.

DiversionDrainage System. This system is a measure to protect flat areas from waterlogging caused by upland runoff. The system is implementedby constructingindividually designed channels or standard diversion drains near the toe of the hilly area across the slope. The interceptedflow is thereupon conductedto an adequateoutlet. The term diversion drain or terrace is used for a drain that is to collect surface runoff, whereas the term interceptiondrain is used for a drain that is to collect subsurfacewater before it seeps to the surface of the field. When diversion drains become deep enough, they can also collect subsurfacerunoff.

Terracesfor Soil and Water Conservaton. Water erosion is caused by rainfall on land with too steep a slope or by an accumulationof too much water from a too lengthy overland flow distance. This erosion can be prevented by the constructionof terraces in a system that closely resembles the cross-slopedrainage system and that is often used as such. Terraces are categorizedinto two types, accordingto their construction:

1. Bench or step-type terrace. In this type of terrace, original relativelysteep slopes are convertedto several vertical steps separatingthe horizontalplots of land (an exampleis the sawahs in Asia).

2. Broad-base terrace (also ridge-typeor mangum terrace). This type of terrace is used on slopes of up to 10 percent. It has a broad-surfacechannel and differs from the channel- type terrace used in the cross-slopedrainage system in that the spoil from the drain is used to build a relatively high ridge on the downslopeside. Only 50 percent of the cross- sectional area of the drain is below the original land surface. Usually soil material is hauled from the downslopeland to enlarge the ridge.

Terraces can be constructedwithout any grade or with a decreasinggrade in the upstream direction for the purpose of creating an approximateconstant water depth in the channel to promote water conservation. Terraces with reasonably constant water velocity, created by an applicationof an increasinggrade in the upstream direction, are sometimesused in erosion control practices to prevent silting of the channel. Grades appliedusually vary between 0.1 and 0.6 percent. The side slopes of the channels vary accordingto the slope of land, i.e., 10:1 on a 2 percent slope and up to 4:1 on a 10 percent slope. The channeldepth depends on the length of the terrace (which is usually between 340 and 450 m) and the slope of land, and it varies between 20 and 25 cm. In designingterraces, the -74 - distancebetween channelshas to be deiterminedfirst based on local practice. Then the location of terraces is determined, keeping in mind that:

* The area of the uppermostterrace should not exceed 1 to 1.5 ha. * Natural impedimentsand sharp curves shouldbe avoided. * Channels shouldbe located just above a sudden break in the slope.

The terrace length is mainly governed by the possibilityof finding a good and nonerosive location for water disposal. Once the area is known and a rate of runoff established,the total discharge can be determined. The dimensionsof the drain can then be calculatedwith the Manning formula by using a roughness coefficientof 0.025. Velocityof the water at the outlet should not exceed a critical value-usually 0.6 m/s (0.45 m/s for sandy soils and 0.3 m/s for peat soils). A freeboard of 10 cm shouldbe allowed, and the cross-sectionalarea should not be smaller than 0.55 m2 (triangular)or 0.9 e 2 (trapezoidal),depending on constructionand maintenanceconditions. For the same reason, the cross-sectionalarea shouldbe kept the same over the entire length of the terraces. Constructioncan be carried out with a motorgraderor with scrapers if the soil has to be moved over longer distancesto fill depressions. Regular maintenanceof field drains is important to keep them in proper working condition.

Mole Drainage as an Intermediatie System

Mole drainage is the constructionof an undergroundchannel without digging a trench and withoutusing tubes. It is used mainly in soils with a dense, impervious,fine-textured subsoilin undulatingareas. The problem is not the control of a groundwatertable (which may be very deep), but the removal of excess water from the field surface or from the topsoil. Mole drainage, therefore, can be considered an intermediatesystem between surface drainage and subsurfacedrainage. The water reaches the mole channelmainly through the fissures and cracks formed during installationof the channels. The picture of oufflowsfrom mole drainage systemsdiffers considerablyfrom that of subsurfacedrainage systems controllingthe groundwatertable. As a rule, the outflows from mole drainage systems show a quick response to rainfall or irrigation water and high-peak flows. As the water applicationceases, the outflows die away over a short period. The time lag between water applicationand drain outflow is a few hours at most. Many researchers have reported on the positive results that can be obtainedafter applying adequatemole drainagesystems. For example, Feichtinger (1965) indicated that fissuring of the soil brought about by mole drainage will gradually create a better soil structure, possiblythe main benefitof moling the field. Cieslinskiand Wanke (1978) stated that mole drainageleads to favorablechanges in the physicohydrologicaland biological properties of heavy soils.

Mole drainage systems cannot be applied routinely. The following guidelinesprovide basic principles, but their applicationwill vary with the physical and human resources encountered.

Types of Mole Plows and Power Requirements

The four most common types of mole plows are:

1. trailed mole plows; 2. semi-tractor-mountedmole plows; 3. fully tractor-mountedmole plows; and 4. attachmentsand modified plows for special purposes: -75 - * one-disc coulter attachment(to reduce draft); * one hollow blade (for use in peat); and * one subsurfaceexcavator (for use in peat).

Hudson et al. (1962) stated that the determinantsof the size of the tractor required to carry out mole drainage work vary. The major factors influencingdraft are the heaviness and moistnessof the clay, size of plug, depth of pulling, and the settingand maintenanceof the plow. Incorrect adjustmentand poor maintenanceof the plow will increase draft appreciably. Measures to improve performanceinvolve hard facing the whole point of the torpedo and keeping the blade perfectly straight and its front edge well sharpened. Contractorsreport that a wheel tractor is not likely to be as satisfactoryas a crawler. By doing the work when conditionsare favorable, however, the individualfarmer will be able to carry out satisfactorymole drainage work even with a medium- powered wheel tractor, provided that the correct hitch height is used on the tractor's drawbar.

Soils Suited to Mole Drainage and Their Characteristics

For mole drains to persist and function properly, they shouldbe drawn in a layer of the subsoilthat is moderatelyhigh in clay. The determinationof the amount of clay present in a subsoil is not in itself a satisfactoryguide. Fine-texturedsoils vary considerablyin their structural stability indices, and one soil higher in clay than another might in fact be less suited to mole drainage. Sand, gravel, or ironstone patches may render an otherwise suitable mole drainage soil an impracticable proposition.

Preparationand Layoutfor Mole Drainage

The clay in which moles are drawn shouldbe sufficientlyplastic to take the shape of the plug without cracking or scaling the walls of the channel and to allow for the use of minimumpower in drawing the plow. The time at which the clay is moist enoughto be plastic will depend on climatic and soil conditions.

Irregularities in the ground surface, either natural or caused by tillage operations, may constitute a serious cause of breakdown in moles, especiallyon gently sloping ground. On steeper slopes, irregularitiesare less important. One shouldthink about eliminatingsurface irregularities when the ground is being cultivated. Few of the usual cultivatingimplements will leave the surface free from humps and hollows. One of the final operations before sowing should be smoothingor leveling with any effectivelytowed leveler.

Ironstone may occur as solid sheets several meters in diameter and several centimetersin thickness, as soil lumps, or as a thick, rubbled layer where seepage water concentrates. When they exist, these ironstone patches are an important considerationin the selectionof outlets to mole drains. The breakdown of mole drains at or near their outlets means the destructionof the entire system above the breakdown,and where there is a possibilityof such an occurrence, the system should be as restricted as possible. When contemplatingmole drainage, the potential existenceof ironstones, stones, gravel, sand, and pipes should be checkedusing a probe to determine the presence, depth, and approximatedensity of such obstacles.

Direction and Gradientof Mole Drains and Speed to Pull Moles

Hudson et al. advisedthat the smaller the natural slope of the ground the more mole drains - 76 - should approach running parallel to the directionof the slope, and the steeper the slope, the more the direction of the drains should approach a right angle to the slope. Crossing steep slopes is desirable, not only to ensure maximumefficiency of drainage, but also to avoid grades that may cause scolaror erosion of the moles.

Nicholson (1937) suggesteda fall of 1 in 200 or more. Blackaby(1936) stated that estimates of the minimumfall required for the satisfactoryfunctioning of mole drains vary considerablyand have been placed as low as 1 in 500 or 600 and that it would be useless to attempt mole drainage where the fall is extremely small unless the surface is entirely free of irregularities. An important principle to follow is that the fall frorn the highest point in a mole drain to its outlet should either be uniform or increase as the outlet is approached. Nicholson (1934, 1937) explainedthat because of the angle of cracks caused by passage of the blade and torpedo, moles should be drawn uphill so that the fissures can facilitate entry of drainage water into the channel.

Blackabyalso stated that it is an axiom of good mole drainage that the work should be carried out at a slow and steady speed. The faster the cartridge of the mole plow travels through the clay, the more likely it is to tear the walls of the drain. In addition, the results of long-term studies conductedby Nicholson(1934) indicate that no significantdifference exists between drains pulled at just under one mile per hour and those pulled at three miles per hour.

Depth of Mole Drains

The depth at which mole drains are pulled shouldbe governed by the soil profile, as mentionedearlier. The best depth of a mole drain in soil will combinea reasonably long life with a considerabledegree of efficiency. This objectiveleads to the suggestionthat deeper mole drains remain in a better state of preservationas long as they are not so deep as to be below a good permeable clay. Sometimes,however, increaseddepth might be expectedto slow the penetration of water into the drain because the epoxyingof cracks in the clay is likely to decrease with depth. In practice, several considerationsmay influencethe depth at which drains will be pulled. Where the farmer has his or her own tractor and plow and is able to do the work at moderate expense, he or she may prefer to make drains shallow, fully aware that he or she will have to renew the system more often. In many cases, the power of the tractor will limit the depth of draining and also the size of plug used. If the work is being done by a contractorat somewhatgreater cost, it is preferable to put the drains deeper to get a longer life, although it might be necessary to sacrifice some efficiency.

The experimentalwork at Massey College (New Zealand), described by Hudson et al. (1962), indicatesthat moles pulled at various depths of 16, 19, and 22 in. do give different rates of flow. In the early part of the drainage season and right throughthese seasons when drain flows are not great, the deeper drains give greater total outflows, although not alwaysgreater peak flows. As the soil becomes thoroughly wetted, the shallowerdrains give greater total flows and more intensiveflushes.

Spoor (1975) pointed out that the determinationof critical depth is an importantfactor in mole drainage. His work shows that when a relatively deep and narrow line is pulled forward, the soil moves out of the way, taking the least line of resistance. At shallow depths, the least resistance is offered by a shear failure in a forward and upward direction. Thus, an implementworking relatively shallow will produce soil shatter and loosening to its whole working depths. If the same implementis made to work relatively deep under exactly the same soil conditions,however, the line of least resistancefor the soil is forward and sideways. This result impliescompaction and plastic failure of the soil. A channel is left behind, whose shape is controlledby the shape of the implementdrawn in - 77 - the soil. The transitionpoint between the two different modes of soil failure has been called the critical depth. The importanceof critical depth is that the mole drainageis possible only at depths greater than critical. Bailey and Trafford (1978) reported that for conditionsin England and Wales, this critical depth is about 300 to 400 mm. Thus, all mole plows are working below the critical depth, and soil cracking is largely confined to the topsoil.

Size of Plug

The size of plug, torpedo, or cartridge is referred to, rather than the diameter of the drain because mole drains are always smaller in diameter immediatelyafter pulling than the diameter of the plug used. Plugs may increasein size under the influenceof hot and dry seasons but, unless they scour, will ultimately decrease. The most popular size of plug among farmers is 2-1/2 in. in diameter, although 3-inch plugs are also commonlyused. Contractorsoften use 4-inch plugs, especiallyfor major mole drains. Increasingthe size of a plug intensifiesdraft appreciably, and the power of the tractor availableto farmers often determinesboth the size of the plug and the depth of pulling. Hudson et al. (1962) favored using a 3-inch plug and renewing it when it is worn to a diameter of 2-1/2 in. They also suggestedthat the plug size shouldbe progressivelyreduced at depths less than 18 in., specifyingthat the plug shouldnot exceed 1-1/2 in. in diameter if moles are pulled as shallow as 12 in. A large plug used at shallowdepths will cause excessive shattering, leadingto breakdownof moles.

Spacingsbetween Mole Drains

The closer the drains, the more effective and certain drainage will be because failure of some of the moles is not likely to have such serious consequences. Hudson et al. (1962) mentionedthat distances between mole drains range from 5 to 30 ft in England, and the practice in New Zealand is to place moles at intervals of about 6 to 9 ft. In some systems, it is desirableto bring the lower ends of the minor mole drains close to one another at 2 ft and 6 in., where they connectwith major mole drains for conveniencein concentratingthe discharge from major drains at suitablepoints. In such systems, the general distanceover the major portion of the area may still be 6 to 9 ft. On land with well-definedtillage marks on the surface, moles are sometimesdrawn parallel to the marks.

Length of Mole Drains

Opinionsvary as to the maximumlength of mole drains. In reviewing the results from a large series of demonstrations,Blackaby (1936) stated that it is possible to make them too long, particularly where the diameter is less than 3 in. Davies (1931, 1932) consideredthat if the fall in the direction in which the moles will run is good, they may be up to 600 ft in length; if the field is nearly flat, however, they should not exceed 100 yd before discharging into the main. Hudson et al. (1962) recommendedthe followinglengths of mole drains on the assumptionthat 2 1/2- or 3-inch plugs are used and that the surface is reasonablysmooth:

GradientLength of Mole Drain

From "flat' to 1:100 From 130 to 200 ft From 1:100 to 1:60 From 200 to 330 ft From 1:60 to 1:40 Up to 2,000 ft

Because of the danger of erosion in cases where the drains are steeper than 1:40, lengths shouldbe - 78 - greatly reduced to lower both the volume and velocity.

Methods of ConnectingMinors to Ma,forMole Drains

Intersections. Various methods are in use that aim at reducing the number of drain outlets by concentratinga number of minor drains into a major drain:

* intersectionsof minor and major mole drains at the same level but at different angles;

* pulling the major drain first, followedby minor drains at varying distances above it; and

* pulling the minor drains first, followedby the major drain at varying distances below them.

Previous experienceshows that systems of mole drainagethat entail running minor drains into major drains-placed on the same level as or below the minor drains-necessitate either the clearing of a clay obstructionor the provisionof a channelthrough the obstructionthat will enable water to flow freely from the minor drain to the major drain. Possibleexceptions to the above are 30 to 400 junctions and cuttingof the top of the major drain with the minor drain.

Systems of Mole Drainage. Three common mole drainage systems in use, which have been reported by Hudson et al. (1962), are summarizedas follows:

• single outlet and one outlet for each pair of moles to open ditches or gully sides, as shown in figure 14. The system is most suitable where there is a slight convex slope to the ditch or gully side; and

* McLeod system: the system has been described by Hamblyn and Galpin (1937) and is shown in figures 15 and 16.

The adaptabilityof the McLeod Systemto a large variety of topographicconditions is greater than with most other systems. The features of the system are:

* By collectinga number of minor drains to a major drain, and by bringing several major drains to one suitableposition for an outlet, multiplicityof outlets is avoided and maintenanceis thereby reduced.

* Major drains are connecteddirectly with pipes, and thus the outlets can be protected and rendered secure.

* Very few pipes are ordinarily required.

* Minor drains are connectedto major drains using an unobstructedjunction that can easily be made-when there is sufficientwater rnwning-by clearing the clay blockage, as shown in figure 17.

* If moles are drawn when there is no excess water in the soil, all the junctions of a series of minor drains to a major drain are completeand merely plugged, except the one marked A6 farthest from the outlet. The hole at this junction is cleared carefully, and a bucket of - 79 -

water is poured into it to help clear any pieces of clay. All holes are filled in when it is certain that the work is satisfactory.

Two-way pulling is possible except with the major drains, which are always pulled away from the outlet.

Method of SpearingIntersections Figure 14. BringingTwo Moles to One Outlet in an Open Ditch or of Minor and Major on a Gully Side Drains. The procedure recommendedis to first pull major drains to N1 300 1st PULL favorableoutlets. 7 4 2nd PULL Often a 3-inch plug is 7'__ used at depths up to 30 > in. The major drains are placed at distances ranging from about 66 to 200 ft apart and are run in the direction of greater fall. The minor 400 1stPULL drains, 6 to 9 ft apart, 2nd PULL are then pulledmore or / less at right angles to _0 the majors. Because of the power required to I pull the deeper drains, the system is used more _ often by drainage contractors (who usually have powerful 1st PULL tractors) than by farmers. 0 60ri 2ndPULL

Methodsof Joining 3rd PULL Moles to Other Systems 4th PULL Apart from the method of joining major moles to pipes at Source: Hudson et al. 1962. outlets as described under the McLeod System, it is sometimesnecessary to connect individualminor drains from a group of minor drains to pipe drains. This may occur where the fall is small and where no suitable outlets to the moles can be provided. In such cases, pipes to which the moles are led can be given fall across a flat area by varying the depth of the pipe trench. Under any conditionwhere mole drainage is done or is likely to be done in conjunctionwith pipe drainage, the depth of the pipes should be such that their tops will be at least 2 in. below the bottoms of the moles to obviate the risk of breaking pipes when mole - 80 -

Figure 15. Details of Pulling Minors in the McLeod System, Where Two or More Majors Run RoughlyParallel to One Another and Fairly Close Together

Openingout graduallyto 6 to 10 feet

m5 / 2C/l/3//m7 / < Directionofpull 'm4l / 7(6( / ofsurface rn1 m

0 2 4 6 8 10

Scaleof feet

550TO750 2' 6' to 3' 0"

Source: Hudson et al. 1962. drains are pulled across them. Provision shouldbe made for water to pass from the moles to the pipes through highly pervious materiial,which will prevent free movementof soil into the pipes.

When joining moles to establishedpipe systems and in the absence of gravel or other pervious material over the pipes, an easy, effective, and not unduly expensive alternative is to use individual gravel junctionsfor the moles. This process entails locating the lines of pipes, marking their positions, drawing the moles over them, and digging holes with a narrow spade or boring them at the junctions of moles and pipes. Boring may be done with a diameter hand- or power-operated5-inch posthole borer. A well-gradedaggregate with a 3/4-inch maximumstone is then placed in the hole until it reaches to about 1-1/2 in. below the mole, which can be seen on the side of the bored hole. A piece of broken field drain pipe is then placed at the end of the mole to keep stones from entering - 81 - it, and additionalaggregate is added until its height is about 1 Figure 16. Drainage Plan of a Portion of a 32-Acre Field in. above the top of the mole. The hole is then filled with soil and is lightly tamped. When a continuouslayer of gravel has been placed over pipes at the time of installation so that the layer is within mole drain depth from the surface, drawing the moles across the lines of pipes and through the backfihled gravel is an easy method of providingwater from for the movement moles to theof pipes.

Where the moles have been pulled before the pipe trenches are opened, the interval between moling and pipe instalation shouldbe Drawing by B. McLeod. minimized. 'te cutting of the Source: Hudson et al. 1962. trenches will cause the moles to be sealed where they enter the trench. The ends should be cleared and, if the pipe is not perforated,holes shouldbe formed in the pipes below the moles. A 'box" is then formed by placing a neatly cut turf or edge on each side of each mole and hole in the pipe. Each turf shouldhave the grass side inward and must be fmrmly wedgedbetween the walls of the trench with its top edge about 2 in. above the top of the mole. The procedure of forming a gravel connectionfrom this stage onward is the same as described previously. Where the moles are pulled soon after the pipe system is installed, they should be pulled through the pervious materialplaced over the pipe. In some countries, a long-usedpractice persists of placing the soil directly over the pipe with the expectationthat when the mole is pulled through this layer its permeabilitywill provide for sufficientlyrapid movementof water to the pipe joints. The probability of the soil's collapsing,leading to blockageof the mole, is ignored. Covering the pipes with gravel is preferred, and the moles can be pulled at any time after the trenches are backfilled.

Hudson et al. (1962) described a techniquefor minimizingthe number of junctions of moles to pipes based on the McLeod System. Instead of joining each mole to a pipeline, moles may be brought to it in groups by using major moles. The joining of the minor moles to the major moles may be done by any of the methods described earlier. The principle of keeping a continuousor increasingfall in the drains should be maintained. Figure 18 illustrates an alternativearrangement of minor and major moles.

Life of Mole Drains

Nicholson(1934) pointed out that a wide range of opinions exists on the life of mole drains, which seems to vary from 2 to 50 years. He added that these limits are obviously extreme: at the lower limit, the soil was admittedlyunsuitable, and at the higher limit, the author was an enthusiast. - 82 -

Figure 17. How Clay Blockinga' Minor Is Cleared

Bladeslits -

. \ I, X ~~~~B . \ Minor f :

Source:~~~~Huso eta.92

Source:~~~1Huso al 162 \t

T'he majority of opinions give mole drains a life of 10 to 15 years, and most of the work has been done at depths of 20 to 24 in. The loDwerlimit may be reduced to a matter of a few days if particularly heavy rains follow immediatelyafter pulling in wet ground. Moreover, Hudson et al. (1962) stated that the frequent passageof a heavily loaded vehicle over a mole-drainedarea or the occasionalmovement of such a vehicle when the ground is softened by rain may cause blockageof moles. If such vehiclesmust be run over moled land in routine movement, a regular track shouldbe used and a pipe drain installed on the upslope side of the track to collect the water from the moles.

Main Drains and Their Stiructuires

GeneralInformation

Main drainage systemsare co:mposedof different elements. Most main systems are earth- lined open canal systems. Structuresare mostly crossing structureswith roads, railways, irrigation, and other infrastructuralsystems and outlet structures. Typical crossing structures are bridges and culverts, while outlet structures are usually of the sluice type. In sloping land, the main drainage system may also includedrop structures or other means of energy dissipation. Further, main systems may includepumping stations for drainage of low-lyingareas or for final disposal. Disposal alternativeshave been discussed earl'ier in this section as part of drainage main systems. - 83 -

Open Main Drains

For hyd- raulic as well as Figure 18. Methods of Minimizing the Number of Junctionsof Moles administrative/mana to Pips (Iles) gement reasons, the design of main drains should aim at a structured, Minor hierarchical canal mole order. The canals of different orders (third, second, and primary) should be Majormole planned to drain o basins of the same f T r I I I I I I I order. In most a) Moles at rightangles (approx.) to tiles landscapes, this will naturally lead to well-spacedand ordered sys-tems. Other factors, Majormole however, also need Minormoles to be taken into account (such as present and planned physical infrastruc- ture, costs,and Pipes operation). / i the area served by a main b) Molesparallel to tiles drainage system should preferably constitute an independenthydro- a. Should be used when the fall of pipes is greater than that in the minor logical unit. If the moles. system only serves b. Shouldbe used when the fall of pipes is slightlyless than or about the part of such a unit, same as the fall at right angles to them (approximately). remedial or compen- Source: Hudson et al. 1962. satory arrangements are normally re- quired to safeguard the drainage of the part outside. When a system concernedonly with the upper part of a natural basin is passing through the lower part to the outlet, provisionshave to be made to ensure that the drainage of the lower area is not impeded.

Main drain alignmentsare often dictated by the existinginfrastructure (such as roads, railways, existingbridges, and culverts) and/or by administrativedivisions (such as property or district boundaries). There may be a preference to plan the new drains along existingroads because - 84 - this facilitates inspectionand maintenanceand also avoids unnecessaryfurther divisionof the land. In contrast, new drains may be sited away from the roads for reasons such as safety and to avoid abuse. The latter considerationsalso apply with respect to the alignmentof main drains relative to settlements. Alignmentof main drains along property and administrativeboundaries divides the land loss evenly between the two parties anademphasizes the communalcharacter of these drains.

Wherever feasible, existingdrains shouldbe used (excavatedor natural). Natural drains as a rule follow the predominantgradient of the land, often through the depressions, and as such can normallybe integratedrather well into the main drainageplan. The addition of irrigation surface waste (or in some cases subsurfacedrainage flow) often changes a normally dry natural stream to one with a continuousflow, at least for the irrigation season. This result correspondsto a change from an ephemeral stream to an intermittentor perennial stream. The continuouswetting of the natural channelbanks may result in an unstable conditionwhen a floodflowoccurs. The stability of the natural channel used as an open drain should be checkedby a tractive force analysis based on particle size analysesor plasticity indices of soil textures. Stabilityshould be determinedfor five-year frequency floodflowplus irrigationwaste flow. The tractive forces used to check stability, in addition to being affectedby wetted banks, are also adjustedfor the type of sedimenttransported by the channel. If instabilityis indicated, control structures will be required. Someminor canalization and/or realignmentcan always be done to improve the fit of these natural drains into the desired layout. In general, the scope for incorporatingthe natural drainage system into the new plan will increase toward the outlet. The location of the main outlet is also a determinantin the planning of the main drainagesystem. The location determinesthe overall orientation of the system, particularly of the primary canals. Generally, the shortest route to the outlet point shouldbe sought.

Flow velocities should be designedto cause neither serious scouringnor serious siltation. For the low-order canals, it is generally sufficientto show that the flow velocity under design conditions is within the generally acceptedrange of safe limits (these limits for different conditionscan be found in most handbookson hydraulicengineering). For large canals and structures, a more thorough analysis is required. For canals, tractive forces should be analyzed for different flow regimes: at design discharge, but also at anticipatedextremely high and low discharge. The hazards of scouring near structures must be assessedby analyzingflow velocities under different flow regimes. On the basis of these analyses, it may be decidedthat special protectionmeasures are required. When vegetativeprotection measures are proposed, the suitabilityof these measures must generally be proved by presenting evidenceof eitier successfulapplication under similar conditionsin the region or successfultrials on the project during the project preparation stage. Althoughin most projects the silt load of the drainage water is rather small, it can create a considerablemaintenance burden; therefore, serious siltation should generally not be allowed to occur. Critical velocities may be based on handbookrecommendations and/or local experience. Specially designedstructures to collect sedimentmay be required under certain conditions. Where these structures constitutea major investmentand are vital, prior model studiesshould be undertakento establish design features before embarkingon a large constructionprogram.

Cut sectionsshould be based on hydraulic efficiency,side slope stability, and cost- effectivenessconsiderations. Handbookstandards may be relied upon partially, but local experiences (especiallyon side slope stability)should also be considered. Constructionand maintenanceissues should also be taken into account explicitlyin the canal design. The design should be compatible with the availableconstruction and maintenanceequipment or the equipmentthat will be acquired. Curves shouldbe provided where the canal changes directionto prevent excessivebank erosion. The radius of curvature dependsupon the velocityof flow and the stability of the side slopes. If gradual - 85 - curves will not eliminate erosion in the channel, it may occasionallybe necessary to decrease the velocity by increasingthe width or side slopes or to provide bank protection. The junction of one channel with another should also be such that serious bank erosion, scour holes, or sedimentationwill not occur. Where the general directionof the main drain is perpendicularto that of the lateral drain, the lateral drain may be curved near the junction. For small channels with low velocities, the angle at which the two channelsjoin is less importantthan for larger ditches with higher velocities. The bottom of the main and the lateral drains shouldjoin at the same elevation. If the lateral drain is shallowerthan the main drain, the overfall at the junction may be eliminatedby increasingthe grade of the lateral drain in the reach near the junction over the entire reach.

Pumped Outlets

Pumping is necessarywhenever the outlet water level (river, sea) is higher than the desired water level in the area drained. Where the outlet water level is extremelyvariable, the extent to which dischargeby gravity is possible shouldbe investigated. It is often advantageousto combinea pumping station with a gravity outlet and to use the pump only when necessary. The size of the area served by one pumping stationmay vary widely from a few to several thousandhectares. The small pumps are often installedby private farmers if the water level maintainedin the main area does not suit their particular requirements. Small stations may also be installedat the outlet side of the collector of a compositepipe drainage system, especiallyif the pipe outlet is very deep (which is common with drainage for salinitycontrol in arid regions)and the soil is of low stability. The pumping stations at each collector outlet are an attractivealternative to deep, open main drains that would be very difficult, if not impossible,to maintain.

If different water levels have to be maintainedin differentparts of an area, the area may be divided into subunits. Regardingthe design of pumping units, an optimum solution shouldbe worked out between two extremes:

- allow the water of the entire area to flow by gravity to the lowest part and lift it with one large pumping station, which requires numerousand deeper drains; and

* divert the high water directly to the outlet point, where it is evacuatedby either gravity or pumping. Low parts of the region are each provided with small pumping stationsthat may lift the water to the 'high level" or directly to the river or sea, which requires more pumps of lower capacity and fewer and shallower open drains.

The selection of the most suitable type of pump is based on efficiency characteristicsfor the required combinationof discharge and operatinghead. Such pump characteristicsfor any pump are availablefrom manufacturers. For example, some common informationrequirements are:

* easy accessibility,such as for fuel supply (applies especiallyto combustionengines and maintenance);

* type of fuel used;

* provision of a no-return valve at the outlet side (usually in the pressure pipe) to prevent backflow of water;

* availabilityof pumpingstation foundationdesign and constructiondrawings; -86 - * trash rack at the inflow side; trash and debris removed regularly; site availablefor temporary depositing;and

* a structured maintenanceschedule and person(s) or institutionresponsible.

The power and drive of commonpumped outlets can be classifiedas follows.

Windmills. Windmills are mostly used for drainageof comparativelysmall areas. For a given project, the site should be evaluated for sufficientwind with sufficientreliability during critical periods. Windmillsare usually provided with an automaticdevice to switch on and off as dictated by the intake water level and can thus operate for prolongedperiods without any supervision.

Electric Engines. Advantagesof electric engines are (a) simplelogistics (no transport of fuel) once power is supplied to the site, and (b) no need for continuousattendance because of the possibilityof an automaticon-and-off switch.

Internal CombustionEngines (Usualy Diesel). These engines are used where electricity is not easily available. They have the potential to automaticallyswitch off.

ChannelStructures

According to the U.S. Bureau of Reclamation(1984), open drain structures (figure 19) consist of the following.

Inlets. Inlets for surface water shouldbe made of corrugatedmetal pipe with a design coefficientof roughnessno greater than 0.021. The pipe can be galvanized, asphalt-dipped,or asbestos-bonded,depending on the corrosivity of the soil. The corrosivity can best be determined by experiencein the area with highway culverts or existingdrainage structures or by similar means. Velocity in the pipe shouldnot exceed 10 ft per second, and the minimumpipe slope should be 0.01. The outlet end of the pipe shouldextend 12 in. beyond the edge of the normal water surface in the drain so that water from the pipe will not drain onto and erode the bank of the drain. This end of the pipe should also be at least 18 in. above the normal water surface elevationin the drain. Multiple pipes may be used if required. Headwallsmay not be necessary, althoughriprap is an inexpensive safety feature. Earth backfill shouldbe compactedaround the pipe for its full length and for 1 ft above the pipe. One collar is required for each pipe.

Drops and Chutes. Conventionalchute structures may be used where appropriate. Drop structures should be used as follows: Differential drop in water surface (ft) Structure O to 2.0 No structure but some riprap 2.0 to 5.0 Rock cascadedrop with sheet piling 5.0 and over Baffled apron or rectangular-inclined(RI) drops

Crossings. Crossing structures can be of either metal or concrete pipe, depending on the importanceof the crossing, which is measured by the capital loss that would result from its failure. In chemicallyactive soils and in waters that would be corrosiveto the pipe, the pipe should be protected with an asphalt or similar coating. Crossing structures for major highways, railroads, and Figure 19.Typical Drain and Collecting Ditch Sections

e- (i Ditch e iDitch Blend excavated material smoothly into Blend excavated material smoothly into terrainor spreadon roadwayas directed 1 1\2:1 A terrainor spread on roadwayas directed 1 ~~~~~~~~~~~~~A 1<

Existingground surface Existingground surface

TRAPEZOIDAL V-TYPE TYPICALSURFACE DRAIN AND COLLECTINGDFTCH SECTIONS

Spoilbank as required 6' oras directed 5' Min.from toe 4l, 40'Max 12' Min 5 Min fom too R.O.W.Line 4, -1 1 1\2: 1 Windr.. ~~~~~~~~OperatingRoad 112. R.O.W.Line

-,k -_sl------'A ' I 00 csi~ ~~~~11 2~~~~ & 1 or flate /Eit ng groundsurface c4 _1 Moveback about 6 to form 7 bermfor stabilfty,if crirected

OPERATINGROAD-ONE SIDE ONLY

6' or as directed OperatingRoad OperatingRoad 6 or as directed R.O.W.Line R.O.W.Line .L > | 12'Min. Over40' t |12'Mn 5!WiOM n.rom too ^5' Min. from to1 I M M,nr o

L 4 r S=0.04,< e_9t OtDrain ) ;S0.04- ___J i2 ; - _~~~~~~~~~~~~~~~~-t------

¢9 | \ Spoil bankas required c4 Spoilbank as required 11/2: 1 or flafter Existingground surface

Source:USDI Bureauof Reclamation1984. OPERATING ROAD-BOTHSIDES - 88 - canals should be designed for flows from a 25-year storm; for less major crossings, flows from a 10- year storm can be used, and flows from a 5-year storm can be used for roads within a field or for farm ditches. Circular pipe culverts can be placed with a maximumof 50 percent of their diameter below gradeline; however, 25 percent or a 1-footmaximum is the preferred limit. Pipe-arch corrugatedmetal culverts, if justified, can be placed with about 20 percent of the "rise" value below gradeline. The pipe should extend well beyond the toe of the fill, and collars shouldbe placed on the pipe as required. Maximumvelocity ifora full pipe shouldbe about 5 ft per second. A siphon-type structure should not be used for drainage crossingsbecause of the variation in flow. During low flows, any transported sedimentwill be deposited in the siphon, and without scheduled maintenance, the crossing will become plugged.

Outlet and WaterLevels. Smedemaand Rycroft (1983) provided the following review of outlet and water levels. They indicatedthat the outlet point of a drainage system is normally a convenientlylow point on a river, lake, sea, or any other componentof the hydrologicalsystem that may be suitableto act as a recipient of drainage water, e.g., a nearby (marshy) depression. An outlet into a river is often located sufficientlyfar downstreamto create a good gradient in the main drain system and to allow gravity discharge, Prevailingwind directions, foundationconditions, liability to scouring siltation, accessibility,and proximityto powerlines should also be considered. In humid climates, the natural surface drainage system is usually well developed, and finding an outlet does not generally pose a problem. In arid or semiarid areas where there is much less runoff and fewer and less-well-developednatural drains, outfall drains of irrigation/drainageprojects may have to be extendedfar outside the area to end up at a suitable outlet point. In some cases, the discharge may be led into an evaporationpond, or a vertical outlet may even be used, i.e., discharge into the underlying substrata. For example, the latter solution is suitablewhere an enclosed area is underlaid by a well-drainedaquifer. The level difference between the regional drainage base (= critical outer water level at the outlet point) and the field drainagebase (= water level in the field drains) constitutesthe total availablehead for drainageflow out of the area (P,-P3 in figure 20). The field drainagebase should be low enoughto allow efficient collectionof the excess water in the field; dependingon the field drainage systern used, these base levels vary from about 30 to 40 cm (shallow drainage)to about 120 to 200 cm (groundwaterdrainage) below the soil surface. Starting from this level (P1 in figure 20), the inner water level at the outlet (P2 in figure 20) is determinedby the hydraulic gradeline adopted in the main canal system, where H = h, + h2 ...... = total headloss in the primary, secondary, etc., canals (includingthe structures).

When P2 > P3, a gravity outlet may be used, but pumping is usually required in situations where P2 < P3. Often the outer water level will vary in time. This is the case, for example, with outlets into the sea or into the lower reaches of rivers subjectedto tidal influences. River levels also depend on discharge that will often vary seasonally. In all these cases, the frequency and duration of high outer levels shouldbe studied, specificallyfor those periods during which the drainage discharge from the area occurs. These periods usually coincide with high outer water levels. Some gravity drainage will continue even when P2 < P3, so long as P, > P3. The outflow is likely to be less than design outflow, but a submergedoutlet is quite acceptablefor short durations. On the basis of these investigations,it may then be decidedwhether a gravity or pump outlet is most feasible. Various combinationsshould also be investigated,as well as the potential scope for temporarily storing water in the area during short periods when gravity discharge is not possible.

Various outlet structures, suchIas those shown in figures 21-23, may be used. Where the outer level is always well below the ilmer level, an open, free-fall structure is suitable. Various types of gated structures are used to stop the intrusion of water resulting from high outer levels. The flap - 89 -

Flgure 20. ImportantWater Levels in DrainageSystemi

xP1 o_T _ t uP1 P field drainage base

_ _ _ _ _ P '2 inner water level at the outlet P3 outer water level V~~~~ Tertiarydrain o ai)

Primary drain Outlet

Source: Smedemaand Rycroft 1983. gate is popular because it allows water to flow out of the area when the outer water level is below the inner level but closes automaticallywhen the outer level rises above the inner level. Flap gates may be placed in a small outlet sluice or may be incorporatedinto a culvert outlet passing through a dike (e.g., they could be fitted in the culvert as shown in figure 23). In the latter situation, the gate is usually installed at the outer end of the culvert. The gate may hinge freely with a vertical rest position, or it may rest slightly slanted outward. The slanted design ensures the best closure (and as such is often preferred where salt intrusion is liable to present problems), but it requires more pressure from the inside to open the gate. Automaticgates are suitable for small tidal outlets. Where the outlet is on a river with a mainly seasonalfluctuating water level, a manuallyoperated gate is preferable because the operation of the gate is not much of a burden and because the infrequent operationof the gate makes automaticoperation risky. Automaticgates may still be the best solution, however, for remote outlet points. Large outlets may comprise single- or multiple-barrelreinforced concretebox culverts, or they may take the form of a sluice. With tidal outlets, these structures may function automaticallyby providingthe culverts with flap gates and the sluice with self-movingdoors. Very large outlets may have specificoperating rules or otherwise warrant nonautomaticoperation. The self-primingoutlet siphonis a less commontype of outlet structure. Here, the excess water is siphonedthrough a pipe over the dike. Outlet siphons may be consideredin tidal outlet situationsas an alternativeto the more conventionaloutlet culvert. The advantageto an outlet siphon is that it is not necessaryto make an opening in the dike. Poor foundation conditionsfor the culvert may also make the siphon an attractive solution. Pump outlets may vary from small sump installationswith a capacity of 20 to 50 I/sec7lto large stations occupiedby several pump units, each with a capacity of some 10 to 20 mn/sec71 . Often the pump outlet and gravity outlet are combinedin one outlet - 90 - structure, with the pumps mainly comiinginto operationduring prolongedperiods of high outer water levels.

Drainage EffluentAlternatives Figure 21. Small Outlet As a rule, drainage effluent is dischargedinto the Sluice with Vertical Doors nearest natural water course (river) or directly into the sea, whichever solution is cheapest and most convenient. The need to find alternative solutionsusually arlisesin areas with irrigated Outer side agriculturewhere the effluent is saline to some degree. On the one hand, if irrigation water is in short supply and the drainage effluentis of acceptablequality for irrigation (possiblyafter mixingwith irrigationwater), the effluentshould not be led directly to the sea, even if the area is near the coast, but should be routed back into the irrigation network either in the same area or (possiblyvia a river) to areas located further / -.-- - downstream. The mixing ratios need to be carefully assessed l .- \ and continuouslymonitored and shouldbe related to quantity and quality of both irrigationwater and drainage effluent. On the other hand, if the effluentis not suitablefor irrigation, it should first be investigatedwhether diischargingthe effluent into the river system or a freshwater lake would be acceptable. Inner side Factors to be consideredin this respect are: Source: Smedemaand Rycroft 1983. * What is the effect on the qualityof the river/lake Ry_roft_1983_ water in the short, mediumn,and long term?

* What is the river/lake water used for (e.g., irrigation, drinking water), and to what degree will the potential use be affected?

* What will be the cumulativeeffect of additionalsimilar drainage schemes? (For instance, it is possible that the freshwatersystem can accept the effluent of one drainage scheme but not that of, for example, iFouror five that have been planned for the future.)

Alternative solutionsfor effluent disposalthat deserve to be explored are:

* a special outfall drain to the sea or to salt marshes;

* a wastewaterdisposal pond, effectiveonly in areas where the pond can be bottomed in permeable sands and gravel with an adequatenatural outlet. Inlet structures to bring surface wastewater from the fields to the pond have to be constructedalong with settling basins or silt traps ahead of the inlet structure;

* an evaporationbasin (e.g,, to be built in a nearby desert depression). The required area shouldbe properly assessed on the basis of such factors as quantity of effluent and evaporationrate; and

* a temporary basin (seasonalholding base) from which the effluent can be released into the river system at times and quantitiesat which it will do no unacceptableharm, such as at high river discharges (or) when there is no withdrawalof irrigation water from the river. -91 -

Figure 22. Flat Gate

r _ Counterweight

I__ /// / /z /

Frontelevation section

Source: Smedemaand Rycroft 1983.

Drainage Water and Safe Recycling

ReturnFlow. Reuse of drainage water is becominga prerequisitefor expandingavailable water resources in many tropical and arid regions and thus an element of current irrigation/drainage planning and design. In this regard, Houk (1951) mentionedthat measurementsin the Sacramento Valley in California showed that the annual return from irrigated lands, expressed as a percentage of annual gross diversions, could be estimatedat 42.5 percent and that return flows during the irrigation season could be estimated at 75 percent of the annual return for all crops, includingrice. Houk's principle conclusionsinclude:

1. On large, well-establishedirrigation projects that are amply supplied with water, annual return flows may vary from about one-third to two-thirdsof annual diversions.

2. Annual return flows on well-establishedprojects may vary from less than 1 to more than 3 acre-feet per acre of irrigated land, dependingon soil conditions,diversions, conveyance, and irrigation efficiencies.

3. On lands exclusivelyproducing rice crops, nearly all excess waters return to streams during the irrigation season.

4. Return flows from irrigation projects constitutea valuable source of water supply for additional arable lands that otherwise could not be utilized in crop production.

Return flow from drainage systemspermits the use of the availablewater supply several times over as it moves progressivelydownstream through a project. The number of times drainage water can be diverted for reuse is limitedonly by its chemicalquality and the geography and topography of the area. When return flows cannot be utilized by the project where they originate, they should be - 92 -

Figure 23. Flat-Gated Culvert Outlet througl a Dike

Maindrain

Sea

Source: Smedemaand Rycroft 1983. diverted to irrigate lands further downstream. In many projects, a gradual increase in the use of return flows has facilitated the developmentof new areas that otherwise could not have been supplied with water. Investigationsof return flow shouldbe carried out continuouslyby measuring the flow during the irrigation season or the rainy season when return waters are reaching the main drains at relativelyhigh rates. The monthlydistribution of annual return flow in the main drains is then presented in diagram form for reuse planning.

Drainage Recovery. The actual value of the water recovered seldom, if ever, justifies costly drainage improvementsrequiring long and intricateditch systems. However, when such improvementsare necessary to reclaim waterloggedareas and prevent further damage to project lands or to save water when it becomes an expensivecommodity, the water recovered constitutesa net gain in supplies availablefor use in other areas. In central New Mexico, for example, the drainage returns from about 59,000 acres of irrigated land varied from 0.9 to 4.8 acre-feet per acre (8.7 to 46 percent of gross diversions). This water became availablefor irrigation expansionprograms. -93 - The importationof excess irrigation water from main drainage canals is known as recycling of drainage water. Drainage water recycling shouldbegin as small-scaleand inexpensiveprojects that collect runoff from drains. Limited drainage recovery by pumping can be carried out by farmers individually. Later on, as demands for water increase, larger and more expensiveprojects can be built. In this case, the drainagesystem has to be designed in a special way to allow water recirculation. All main drains have to be provided with adequateautomatic locks or sluice gates. Other structures may be required at the end of collector drains to prevent the loss of water when it is not required. In this case, drainage canals should be designed accordingto peak dischargesand a suitable water level estimatedto allow for further storage space. Collectionconduits, tunnels, and other engineeringworks must at times be provided so that the irrigation-drainagenetwork can be treated as a reservoir. Drainage recovery shouldhe secured by installingmany small pumping plants at carefully selected locations. Moreover, the influenceof tidal movements,if existing, should always be considered when preparing a design that includes recycling of drainage water.

Reuse of Drainage Water and QualityImprovement. Water resource developmentplans should address the possibilityof mixing controlledquantities of drainage water, if brackish, with fresh water. The actual suitabilityof brackish water for leaching salt-affectedsoils or irrigating normal soils depends on the specific conditionsof use. Highly saline water used for leaching saline soils is allowed if the salt content is lower than that of the soil. Regarding the use of brackish water for irrigation purposes, the chemicalconstituents of the water, type of crop, stage of plant growth, soil characteristics,on-farm water management,and climatic conditionshave to be taken into account. Paddy fields require a ponded water condition, and this provision minimizesthe osmoticand matrix potentials and allows for relatively safe use of brackish water. FAO (1975) reported that paddy rice can tolerate a water salinity level of up to 1,400 ppm without any detrimentaleffect. Rice plant susceptibilityto salinity stress varies accordingto growth stage. Kaddah et al. (1973) found that after the seeding stage, rice tolerance to salt increases gradually throughoutthe vegetativegrowth stage.

Available, updated values of crop tolerance to salinity and toxicity are commonlybased on experience in areas other than a given specific project area; therefore, caution and a critical attitude should be taken when applying these values to local conditions. In this regard, Ayers and Westcot (1976) presented guideline values for evaluatingthe suitabilityof waters for irrigation (appendix6). These guidelinesconsider the effect of excessive sodium, total salt load of the water, a modified SAR concept, specific ion toxicity, and soil permeability.

On-farm water managementpractices developedfor irrigation with nonsaline water were examinedfor their applicabilityto irrigation with brackish water by Shainbergand Shalhevet(1984). The practices examinedwere the interval and quality of irrigation scheduling, includingleaching requirements, irrigation method, and mixing of different-qualitywater resources. The following conclusionsmay be drawn from the availableinformation and experimentalresults:

1. The bulk of the evidence shows no interactive effect on yield resulting from irrigation interval and irrigation water salinity. Decreasingthe irrigation interval resulted in the same relative increasein yield for waters of various salinity levels. This finding is contrary to the commonbelief that the effect of salinity can be moderated by increasing irrigation frequency.

2. The problem of leaching for salinity control under practical field conditionsis complicated by the variability of soil characteristicsand water distributionand by the inevitableloss of water below the root zone, except under high-frequencyor deficit irrigation. In many - 94 - cases, leaching may be detrimentalrather than helpful for annual crops. Research findings on the leaching requirementfor specific crops, obtained under steady state and/or high-frequencyirrigation, may not be directly applicableto current irrigation practices.

3. Four options exist for using various sources of water that differ in salt content: (1) using good-qualitywater for sensitive crops and poor-qualitywater for tolerant crops; (2) blending the sources to an acceptablewater quality; (3) applying good and poor water alternately on the same field; and (4) applying good water for part of the season and poor water for the rest of the growing season. Little experimentalevidence is availableto assist in making an educatedchoice among the options.

Disposal of Drainage Water. Ochs (1987) stated that the disposal of drainage water can entail high cost and serious concern that has to be consideredin drainage planning and design. An excellent on-farm and project drainage systemhas no value unless an adequateoutlet exists for disposal of the drainage water. Drainage water collectedby field drains or removed by the pumping of wells can, dependingon its quality, be a total or partial substitutesource of irrigation water or can be disposed of in ways that do not reduce the quallityof surface water and groundwaters. In many cases, however, the disposalof drainage water in a way that does not harm streams and groundwater is not practical. There may be several reasons for this: the cost is prohibitive;the drainage water is neither collectednor pumped but moves by undergroundflow to streams and the groundwaterbasin; or the quality of the drainage water, though somewhatdiminished, is such that it still has value for irrigation or other purposes. Ochs also mentionedthat where the flow of agriculturaldrainage water to bodies of surface water and groundwatercannot be eliminated, quality degradationcan be reduced by minimizingthe amount of salt leaving the root zone. That amount can be reduced by decreasing water applicationor by removing accumulatedsalts in the smallest volume of water compatiblewith the leaching requirement. Simultaneously,reducing total project evapotranspirationand the amount of water applied decreases the amount oiFsalt that must be removed from the drainage water. This objective can be achievedin a variety of ways, such as using closed water conveyancesystems, eliminatingnonbeneficial vegetation, and growing crops with lower evapotranspirationrequirements resulting from the season or length of their growing period. Removalof excess dissolved salts consistent with the leaching requirement maximizesthe salt concentrationof the soil solution and drainage waters and helps produce harmless, slightlysoluble salts (ime and gypsum) in the soil.

One of the main problems in surface drainage is the safe conveyanceof discharge from the shallow field drains to the main drainage system. Runoff from a row, row drain, or planed field should not be allowed to flow directly into a field lateral; this would cause erosion of the side slope. Runoff, therefore, shouldbe collectedin a field drain with flat side slopes and conveyedfrom there to the main drainage system. The difference in bed levels, as well as in water levels, between field drains and field laterals will cause scouring of the bed of field drains near outlets. This can be avoided by installingdrop structures made at the site from wood or prefabricatedsmall-drop spillways or pipe-drop inlets. Sometimesit is advantageousto collect the runoff from several field drains into a somewhatdeeper type of field drain or lateral drain (preferablya grassed waterway) and to convey the water from there into the main ditch through one single-dropstructure. Water conveyancecan become a major problem in sloping areas where the field drains are usually constructedparallel to the contours and the field laterals run down slope. This slope is nearly always too steep to construct laterals without drop structures in view of severe scouring. Structurescan often be avoided, however, when grassed waterwaysare used and are characterizedby moderate to steep grades and a permanentvegetative cover on the flat side slopes (4:1 and flatter) and bottom. Permanent and dense sod-forminggrasses of locally availablevarieties are widely used. The velocities allowed in the - 95 - design reported by Raadsma (1970) are shown in table 5. The cross-sectionscan be calculatedwith the Manningformula, using a roughnesscoefficient of at least 0.035 to ensure that the allowable velocity will not be exceeded. Sufficientfreeboard should be given to avoid overtappingof the banks. A minimumbottom width of 1 m can be applied to allow the passageof farm machinery.

The vegetation in a grassed waterway can be preventedby constructinga concrete or asphalt- lined furrow at the bottom of the waterway or by installinga subsurfacedrain where the problem mainly occurs. If the slope becomes too steep for the use of grassed waterways, lined channels with drop structures (chutes)will be required. The choice of a method depends on local conditionsand on economicconsiderations. Difficultiesmay arise with the water disposal from larger depressions, where the cut would be too excessiveif an open ditch were applied (the random ditch system). In this case, a surface inlet to a subsurfacedrain, at the lowest spot of the depressions, could be made. In many cases, however, the capacity of the drain will be insufficient, and a special tube outlet will have to be constructedto convey the water to another depressionor a main ditch. Under certain conditions,a vertical drain or an invertedwell may be feasible where the true groundwaterlevel is located at great depth and where a sufficientlypervious subsoil is present. Otherwise,the water has to be collected in a sump and pumped out.

Table 5. Allowable VelocitiesUsed in Designing a Drain Cross-Section Velocityvalues (cm/s) Soil structure Stable soils Easily eroded soils Slope 0-5% 5-10% 0-5% 5-10% Dense, sod-forminggrass 5.08-6.01 4.57-5.08 3.81-4.57 3.05-3.81 Other types of vegetation 2.29 1.52

Source:Raadsma 1970. 3

Project Preparaition,Installation, and Maintenance

Final Project Preparation and Specifications

The final project preparation phase of a drainageproject involvestwo main functions: (1) the technicalwork, and (2) the administrativeand organizationalarrangements. At the beginning of this phase, decisionmakersneed to determine and concludehow the project will be installed. Commonly, the installationor constructionof drainageprojects is carried out by (a) qualified construction contractors, who work under a contract awarded after a formal tenderingprocedure; or (b) a "force account" arrangementunder the direct responsibilityof the implementingagency. Under the latter procedure, there is normally tenderingfor the purchase of some equipment and materials. An early decision on the manner in which a project will be installed is importantbecause the differencesin the two installationapproaches affect the content and expressionincluded in the job specifications.

The scope of a drainagepro.ject may vary widely. It may deal only with drainage, such as field drains and main drains, or it may be includedas a principal componentof a comprehensive scheme such as land consolidation,land reclamation,soil improvement,or soil rehabilitation. Accordingly,the scope and makeup of the final project preparationphase for a drainage project are not rigidly fixed. The following,however, is a generalizedpicture of the final project preparation phase. This phase precedes the project constructionor project execution, and the parties usually involved are:

- the implementingagency, normally a governmentalauthority bearing the overall responsibilityfor the project;

- the consultant/engineer,who assists the implementingagency and prepares the final designs and tender documents. Usually, this party also should be engaged to supervise the ensuingconstruction; and

* the financingagency, which most often is an international,multilateral, bilateral, or national bank or institutionthat provides funds accordingto their specificity.

TechnicalWork

The principal componentof the technicalwork is the final design analysis and the preparation of the technical specificity, which includes the final design documents. The final design activity normally is preceded by field surveys and the gatheringof data, which include:

* the drainage parameters, as discussed earlier, which are required for final calculationof field drain spacings and idepths;

* soil borings along the projected routes and alignmentsof open drains and collector lines, informationthat serves design assessments;and

- 96 - - 97 - ground surveys, typically profiles and cross-sectionsof projected open drains, information that serves the design analysis and calculationsof earth-movingvolume. All elevations should be referencedto a commondatum or benchmark, e.g., meters above (or below) mean sea level (MSL).

The technicaldocuments includethe engineeringdrawings, constructionand materials specifications,contract bid list, engineer's cost estimate, and contract performancetime estimate. Except for the engineer's cost estimate, these documents are incorporatedwith the contract provisions and tender documentsif contractbids are invited, and the constructionis carried out using constructioncontracts. When the work will be done by force account, all documents are still prepared, with the exceptionof the contract bid list and contract performancetime estimate. These items are replaced with (1) organized work quantitiescomputations (earthwork and materials), listed and summarizedin convenientpartials and segmentsof the various project features to be installed; and (2) a listing of the probable pieces and types of equipmentand supporting items needed for carrying out the work.

EngineeringDrawings. Engineeringdrawings for drainageprojects usually consist of three types of presentations:maps, plotted profiles, and cross-sectionsand drawings of structure details.

Maps present the spatial or horizontal associationof various project area features and may picture such informationas ground elevations(contours), hydraulic conductivity,depth to sand, depth to barrier, or other informationsuch as land partitions and farm ownerships, existing infrastructure, and projected project layout. A rational numbering system and distinguishablesymbols should be utilized on maps to identify drains, roads, structures, or other area features. Map scales shouldbe selected to serve the clarity of the detail presented. Broad project-widemaps that present project layout might normally be on a scale of 1:25,000, whereas interpretationalmaps (such as contour maps) may need to be on a scale of 1:5,000 to 1:10,000. On more rare occasions, a larger scale on the order of 1:2,500 may be useful to display importantinformation such as a critical horizontal control and location of some project elements.

Plotted profiles (sometimescalled longitudinalsections) are used to display (1) site features such as the bed of existing channels, ground lines, and location of structures; and (2) design dimensionssuch as gradelinesand slopes, hydraulic gradients (water surface profiles), drain junctions, and location of new structures. Cross-sectionsof drains depict dimensionsand shapes. For open drains, they also often picture the magnitudeand span of the excavations. Typical sections shouldbe used to present repetitive elements.

Structure details are drawings used to present the design shape and dimensionsof structures and serve to indicatehow the structures will be built. Scales for structuraldetails are usually on the order of about 1:25 to 1:50. Standardizedor typical drawings are useful where several similar structures, such as manholes, are to be installed. A table of controllingdimensions should accompany a standard drawing that is intendedto depict several similar structures.

Specifications. The specificationsare written stipulationsthat augmentthe drawings and make provisionsfor requirementsthat are not suited to graphicalpresentations. Specificationsconsist of constructionspecifications and constructionmaterials specifications.

Construction specificationsshould be written to address each major and distinct item of work, e.g., excavationof open drains, installationof closed field drains, installationof pipeline drain - 98 - collectors, or concrete construction. Constructionspecifications should be titled and numbered. They must be clearly written, specific to the job at hand, and unambiguous. Constructionspecifications can be written in one of two modes. In the first mode, they express the quality and results that will be realized by the work. The constructionprocedures and equipmentused for obtainingthose results are not stipulatedexcept where strictly necessaryto clarify or amplifytheir meaning. Normally this mode of writing is preferred when constructionis tendered to competentcontractors, resulting in a formal constructioncontract. With the second mode of writing, constructionspecifications express the general results to be obtainedbul they accentuateor emphasizethe constructionprocedures and/or equipmentto be used for accomplishingthe quality and results. Usually this mode is preferred when constructionwill be carried out by force account.

Construction materials specificationsstipulate the performancerequirements of construction materials (e.g., cement, concrete aggregates,steel reinforcement,drain pipes, filter materials). These specificationsare written to control the quality, shipment, on-job storage, and handling of materials. Wherever possible, it is desirable and convenientto reference standard material and materialtesting specifications,such as those of the American Society for Testing and Materials (ASTM), DIN (Germany), KOMO (Netherlands),and British Standards. Constructionmaterials specificationscan be written as a clause within the constructionspecifications or as a separatelynumbered series of specificationsthat are referenced, as appropriate, within the constructionspecifications. All manufacturedmaterials delivered should be accompaniedby a manufacturer's or supplier's written certificationthat the materials conform to the specifications.

ContractBid List. The contract bid list, sometimescalled the bid schedule, consists of the items of work upon which the pricing and cost of the job are established. The items of work are thus called bid items. The bid items should be broken down and separatedsufficiently to (a) provide a distinguishingtype or kind of work that represents a distinct work effort or procedure, (b) constitutea measurable or quantifiableitem or uniitof work, (c) provide control of the progress and payment of the work, and (d) create a systematicmeans for the inspectionand final acceptanceof the work. For each item, the bid list must include:

* a bid item number;

* a descriptivetitle (e.g., excavation,main drains);

* the estimated quantityof the item;

* unit of measurement(e.g., cubic meters, linear meters, kilograms each, or lump sum);

* a blank for the bidder to enter his or her unit price;

* a blank for the bid item cost (an extension of the quantityand unit price); and

* a blank for the bid summary cost (the sum of the cost figures for all items).

In addition to these items, the identificationnumber of the specificationsthat govern or control that particular item of work should be provided for each bid item.

Engineer's Cost Estimate. The engineer's cost estimate is developedat the end of the final design activity and is the final and most refined cost estimate. The engineer's cost estimate shouldbe - 99 - presented in the same format as the contract bid list. It is used as the primary documentfor evaluatingthe prices of the bidders' response.

ContractPerformance lime Estimate. The contractperformance time estimate is developed near the end of the final design activity. The estimate is a technicalexercise and not an administrativedetermination. It is an estimate of the tota. calendar days reasonablyrequired to complete the constructioncontract. Normally, the estimate is based upon daily production rates of the pieces of equipmentthat would create a reasonableconstruction activity for the size of the project at hand. Daily production rates must be tempered consideringthe local on-site conditions. The contract performancetime estimate includes considerationof (a) tasks and equipmentoperations that can be carried out concurrently, (b) nonworkdays (weekendsand holidays), (c) anticipatedweather shutdown days, (d) abatementof the interferenceto ongoingcrop production, (e) expectedmaterials delivery schedules, and (f) mobilizationtime allowances.

Special TechnicalInputs for Force Account Work. Drainage projects that will be installedby force account proceduresrequire some added technical input that is normallynot necessaryfor formally contractedinstallations. The major additionaltechnical input is advice and recommendations concerningthe equipmentand ancillary features for doing the work by force account. At this point, a decision shouldbe made on whether subsurfaceclosed drains will be installedby hand or machine methods. The following equipmentshould be consideredwhen determiningthe equipmentneeds for installationsby force account:

* trenchingmachines (for field drains);

3 excavatorsor trenchingmachines (for collector drains);

* laser control instruments;

* draglines/excavators(for open drains);

• bulldozers (for clearing, backfill, spoil bank spreading);

* motorgraders (for smoothingspoil and berm areas, blading access roads);

* cranes (for handling concretepipe, prefabricatedunits);

* tractors with front-end loaders (for handling filter gravel and concrete aggregates);

* forklifts (for loading/unloadingpalletized materials);

* dump trucks (for gravel/aggregatetransport);

* trucks or tractors with trailers (for transport of pipe);

* low-bedtruck (for transportingheavy equipment);

* dewateringequipment for critical trench excavationsand wet foundations aetting pumps, piston pumps, well-pointsand risers, manifoldpipes, couplingsand fittings, filter cloth, bailers and sump pumps for localizedwater removal); -100-

* pipe manufacturingmachine for concrete pipe (where concrete pipe manufacturingand marketingdo not exist locally);

* concrete mixers;

* fuel tank truck/on-sitefuel storage;

* mobile workshop, with hand tools;

* welding equipment;

* portable electric generators;

* portable air compressor;

* topographicsurvey equipment;and

* vehicles for transport of personnel.

Administrativeand OrganizationalArrangements

The implementingagency normally is responsiblefor most of the administrativeand organizationalarrangements. The financingagency may, from time to time, provide valuable help and advice. It is essential, however, that the implementingagency be prepared and qualified to administerand managethe project. 'The principal administrativeactivities can be grouped into four main categories:

1. satisfying local legal requirementsand issues;

2. contractingfor the necessaryprofessional services;

3. programmingand schedulingthe construction;and

4. administeringthe constructioncontracts.

SatisfyingLocal Legal Requirements. Legal aspects to be arranged include the acquisitionor expropriationof lands for the permanent drainage works. Normally, land ownership titles in perpetuity are taken for all works that utilize or occupy the land surfaces, works that include such features as open channels, pump stations, access roads, and maintenancetravel ways. Works such as subsurfaceclosed drains, however, which do not occupy or utilize the land surface, might be installed with land rights only in the form of land easementsor similar land entitlements. It is importantthat the implementingagency and the financinginstitution agree on the legal arrangement concerningland rights for the project.

Further local arrangementscenter around such items as (a) permits for ingress/egressto perform surveys and on-site investigations,and (b) compensationto farmers for crop damages due to crop destructionor a missed crop season because of constructionactivities. Permits are temporaryor short-term agreementsof consent and cover both formal and informal actions. With respect to crop damages, the first emphasisof the project administrationshould be constructionphasing and - 101 - schedulingto mitigate crop losses. In areas of double cropping, however, particularlyin monocultures,it is often impracticalto try to avoid all crop losses. When managingconstruction phasing to the point that contractors' delays and downtimecreate excessive constructioncosts, it may then be better to accept and plan for some crop damages. The equitabledetermination of crop damages is an important legal concern.

ContractingProfessional Services. Frequently, the implementingagency is not able or suitablystaffed to carry out the technicalprofessional services needed for the final project preparation of a drainage project. In such cases, a qualified consultant/engineermust be engaged. The implementingagency, usually with the advice and agreementof the financinginstitution, must be prepared to contract professionalservices by taking the followingbasic steps:

* develop and clearly write out the specificity(terms of reference)for the services;

* broadly solicit interest;

* accordingto an establishedbody of criteria, screen and appraise the respondingfirms and select a short list (usuallythree to six firms);

* formally invite a technicalproposal from each of the firms on the short list;

* appraise the technical proposals received and rank them accordingto professionalmerit in light of the services solicited; and

* carry out contract negotiations,beginning with the number-oneranked firm, and award a contract when successfulnegotiations are reached.

After the professionalservices are contracted, the contract administrationtasks of the implementing agency normally are not greatly demanding. Subsequentadministrative tasks involve such items as making background informationand reports available,assisting on any additional survey tasks, facilitatingthe logistics of the consultants' work in accordancewith the provisionsof the contract, and processing progress payments to the consultingfirm.

Programmingand SchedulingConstruction. The makeup of the program and schedule of constructionwill differ, dependingon whether the constructionis performed using formal construction contractsor force accountprocedures. In either case, however, the programmingof constructionis an administrativeactivity that can have a significanteffect on the success and cost-effectivenessof the project installation.

For construction by contracts, a decision is often necessaryon whether the entire project should be executedusing one prime contract or whether several contractswould be advantageous. Each approach may have distinct merit. On some projects, tentativedecisions on this matter may be based upon informationfrom earlier feasibilitystudies and preliminarydesigns. In such cases, the work approach and progressive steps of the final design activitymight be fitted or phased to agree with the separate contracts anticipated. In other cases, a sound decision concerningone or several contracts may not reachable until significantfinal design work is completed. Programming and schedulingthe constructionprovide for efficiencyand should considerthe following: - 102 - 1. The status of land rights acquisition. When some of the land rights are unresolved, it may be suitableto begin installationsof some separate project componentswhere land rights are resolved, but this should be done only if the future resolution of the lingering land rights is fully ensured.

2. The readiness of the final design. Normally, at least the design work for the first year of constructionshould be completebefore constructionbegins.

3. Establishmentof governmentalfacilitiesor assistance. The implementingagency should fully establishbeforehand such steps or procedures as custom clearances, work permits, and similar concerns.

4. The availabilityoffunds. In some instances there may be a progressive disbursementof constructionfunds based solely upon financingconcerns or funds availability. In such cases, the disbursementschedule may significantlyaffect the constructionprogramming, and all make-readyactivities should be phased to agree with the establisheddisbursement schedule.

5. The availabilityof competentprime contractors. A good prime contractor can relieve the implementingagency of rnany administrativetasks that are intrinsic to a series of separate contracts. The prime contractor arranges and overseesall of his or her subcontracts.

6. The advisabilityof constructionusing several constructioncontracts. Normally, there shouldbe sound reasons for installingdrainage projects using several contracts. These reasons may includesuch factors as the unlikelihoodof responses from competentprime contractors, the physical need for some project componentsto be installedsignificantly earlier than others, and the opportunityto better control potential crop damages caused by constructioninterference. When separate contractsare used, they are normally distinguishedby the type of works, e.g., main open drains, pump stations, collector lines, field drains, and major structures such as sluices and bridges.

Construction by force account procedures impliesthat the drainage project installationwill be done by several partials of work. For a force account approach, the programmingand scheduling of constructionmust necessarilyconsider and managethe following:

1. the sequencesand componentsof the installation,e.g., open drains, collector lines, field drains, structures, and pumps;

2. a procurementschedule ILor the constructionequipment, spare parts, fuel, and supplies (see earlier in this chapter);

3. a procurementschedule iforconstruction materials, e.g., drain tubing and filter fabrics;

4. a training program for equipmentoperators and support personnel;

5. a scheme, using either in-houseprovisions or service contracts, for the maintenanceof constructionequipment; - 103 - 6. a scheme for constructionlayout surveys and constructioninspection, carried out by either qualified in-housepersonnel or a professionalservices contractor; and

7. a system of documentationand records management,including financial accounts, procurementrecords, personnel and training records, and technical records (e.g., engineeringnotes, constructioninspections, materials certifications,and as-built drawings).

Administeringthe ConstructionContracts. Contractadministration is a vital administrative and organizationalactivity. Good contract administrationcan do much to provide for efficient construction;conversely, poor contract administrationcan create many constructiondifficulties. Good contract administrationconsiders the primary concernsof a three-memberteam: the engineer (inspections,technical conformance,and acceptance);the contractor (productivity,construction efficiency, and cost control); and the implementingagency. The implementingagency, as a contractingorganization, must realize that the contract belongsto the engineer and contractor and will be controlledby the contractingofficer. Contract administrationinvolves many tasks and considerations,commonly including:

1. Assemblingthe tender documents. This step includes the technicaldrawings and specifications,the contract provisions,the bid document, and instructionsto bidders (e.g., bid bonds, customs restraints, and labor rules, if any).

2. Issuing invitationsfor bids. This includes published announcementsas required, but may also includedirect invitationsfor bids to a known and acceptable list of bidders.

3. Evaluatingbids. Bids are evaluated, and an abstract of bids is normallyprepared. The evaluation includes the comparisonof the bids as well as comparisonwith the engineer's final cost estimate (see earlier in this chapter). If a bid bond stipulationwas includedin the tender documents, then the acceptabilityof the bid bonds is evaluated. When a bid bond, which is a surety for the bid offer, is required, the amount of the bid bond normally is 10 to 20 percent of the bid amount. Bidders should be disqualified,without consideringtheir competitiveness,if (a) the bid was received after the established deadline, and (b) the bid bond, if required, or other required representationsand certificationsare not completed and included. Normally, bids are not disqualifiedfor simple arithmetic errors on the bid list. The correction is made, but no unit price can be changed. The bid evaluationwill establish the apparent winningbid as the lowest summary bid of all qualifiedbidders.

4. Inspectingthe contractor'splant and equipment. After bid opening and before contract awarding, it is often, though not always, necessaryto inspect the plant and equipmentof the apparent low bidder, who will become the contractor if all further requirementsare satisfied. The contractingofficer (or his or her contract specialist)and the project engineer normallymake this inspection, which is intendedto affirm the suitabilityof the contractor's equipmentresources for constructionof the project.

5. Awarding the constructioncontract. Before the contract award, the chosen contractor presentshis or her performanceand paymentbond. The performance and paymentbond normally is a 100/50bond, which means the surety for the contract performanceis up to 100 percent of the contract amount, and direct paymentof any outstandingcontractor's - 104-

obligations(unpaid supplies,materials, etc.) is ensuredup to 50 percent of the amount of the contract. As needed, 1heimplementing agency and the financinginstitution usually will have an understandingconcerning all bondingrequirements. With the performance and payment bond approvedand all other matters in order, the contract is signed and thus awarded. Letters of credit and other financial issues are arranged.

6. Sending the notice to proceed. Usually a preconstructionconference is held between the contractor and the implementingagency. Pertinentissues such as custom clearances, constructioncampsites, mobilization, work days, constructionsafety, and an array of mutual concernsmay be discussedat this meeting. After this conference,the contracting officer normally will deliver or send to the contractor a written notice to proceed. This notice specifies a date to proceed, which becomes the official starting date of any specified contract performance(see earlier in this chapter). The notice to proceed may also documentmatters of agreementfrom the earlier preconstructionconference, such as the delivery/mobilizationperiod.

7. Arrangingconstruction layout surveys. Contractadministration includes arranging all constructionlayout surveys that are stipulatedby the contract to be performed by the implementingagency. Often, the contract may provide for all broad layout controlsto be performed by the implementingagency and all detail staking (e.g., cuts and fills) to be furnishedand performed by the constructioncontractor. Also, the implementingagency often performs the surveys for final acceptanceand quantitymeasurements or surveys for contract payment.

8. Supervisingand inspectingconstruction. Contract administrationincludes arranging and furnishingconstruction supervision and inspection. Normally these tasks require a competentproject engineerand one or more experiencedconstruction inspectors working under the supervisionof the engineer. The number of inspectorswill depend on the size and complexityof the project. Constructionsupervision by the engineer entails constructionoverview, technical interpretationsof the contract to the contractor, technical acceptanceor rejection of the works (includingconstruction materials), liaisonduties with the contractor for the contractingofficer, and general supervisionof the inspections. Constructioninspection includes detail examinationsof the work and materials, on-site samplingand/or testing, and field measurementsfor conformancewith the technical requirements. Constructioninspection should be treated as both continuousand intermittent. Work such as clearing, open channelexcavations, and similar activitiesmay be inspectedintermittently. Work such as subsurfacedrain connections,laying and bedding of concrete pipe, constructionof intricate concreteforms, placement of intricate concrete steel reinforcement,all concretingoperations, and similar activitiesshould be continuouslyinspected.

9. Modifyingthe contract. Good engineeringinvestigations, design, and specificationswill minimizethe need for contract changes. Even so, a large, complexcontract is usually completed with some changeto some part of the works. When a contract change is needed, even a so-called field change, such a change should be documentedby a signed contract modification. Modificationsare made to add or delete elements of work, change lines or limits of prescribed features, expand or alter contract conditions,and facilitate similar needs. Contractmodifications should be concise statementsthat identify (a) the item of work being changed and the purpose of the change; (b) the contract authority or It

- 105 -

clause under which the change is being made; (c) the magnitudeof the change, such as quantitiesadded or deleted; (d) the amount of the negotiated cost change; and (e) a line for the signaturesof the contractingofficer and the contractor. Attachmentsto a modificationmay include such documentsas the engineer's technicalfact-finding brief (technicalnecessity for the change) and the contractor's cost developmentfor the change. The cost of the contract change must be equitable, i.e., fair to all parties. Contract changes should not be handled as oral instructionsfollowed as red-lined changes on the as-built drawings. Good contract administrationdemands that each contract changebe handled in a timely way with a written contract modification,a contract addendumthat documentsa new or added contract agreement and does much to obviate contract disputes and misunderstandings.

10. Paying the contractor. Normally, constructioncontracts are paid in periodic progress payments, usually monthly,based upon estimatesof the work done during the period. Progress pay estimates are made using work items (see earlier in this chapter). When materials are delivered and accepted at the job site, that material cost usually is included in the next pay estimate without waiting for installationof all the material. Payment retention is made on each progress payment in accordancewith the paymentretention provisions, if any, of the contract. Progress payments shouldbe managedexpeditiously by the contractingofficer. It must be realized that the contractorhas significantoutlays of cost for equipmentoperations, labor, and materials. The contractorshould not be expectedto wait unduly for contractpayments where the delays stem from poor contract administrationand willful neglect of the contractingofficer. Final payment (includingany retentions)is made at the end of the contract and after final inspectionand acceptance.

11. Documentingthe contract. Good contract administrationentails good documentationand records management. Documentationis the responsibilityof the contracting officer and includes the documents of both the contractingofficer and the engineer. Good practice requires retaining and preserving these records. The followingitems commonlyare filed in the contract final documentation:

* Originalcontract documents;

* records of conferencesbetween the implementingagency and the contractor;

* contract modifications;

* progress and final paymentrecords;

* final inspectionand acceptancecertification;

* survey records and engineeringnotes;

* constructiondiary (kept by the engineer/inspectors);

* inspector's daily or weekly reports;

* material certifications; - 106- * test records; and

* as-built drawings.

Installation

Installationof Subsurfaceaosed Drains

Subsurfaceclosed drain installationcan be done either by hand or by machines. Hand trenching is practiced where trenching machineryis not availableor where machine operation is impractical due to the small size or location of the proposed project. Drain tubes can be either tiles or corrugated (plastic), and trenching machinescan be either classic or modern.

Installationby Hand. The trench is usually staked at intervals of 10 to 20 m. Hubs are offset a short distancefrom the centerlineof the trench and driven to about the ground level. A guard stake is driven near the hub and marked with the station and the depth of cut from the top of the hub to the trench bottom.

Batter boards are used to assist the excavator in staying on grade. Batter boards are constructedby driving one stake at the hub and another on the opposite side of the proposed trench from the hub. Select a convenientdepth (1.5 or 1.8 m), and clamp or nail a horizontal crossbar on the two stakes at an elevation such that the top of the crossbar is the distance selected above the trench bottom. If 1.8 m is selected and the cut on the hub is 1.2 m, the top of the crossbar shouldbe placed 0.6 m above the top of the hub.

Crossbars are set in the same manner at each hub. A chord line or wire stretched over the top of the bars shows the slope of the proposed trench bottom. The chord line should be straight if it does not include a grade break. The trencher can then measure from the chalk line to the bottom of the excavationto see if the trench is on grade. If three batter boards are set, the grade may be checked by sighting over the crossbars. When a grade break occurs, a crossbar must be set at the station where the break is to be made and additionaltargets set beyond this point to obtain the correct slope of the line of sight.

The following tools are needed for constructingshallow field drains by hand:

* drain spade with a long narrow curved blade, used to dig a narrow ditch 10 to 15 cm wide;

* drain scoop, used to smooth the trench bottom; and

* tile hook, used to place the tiles in the trench.

It is not necessary for the laborers to stand in the trench when using these tools; by this means, a narrow trench can be excavated. The excavationshould start from the open outlet ditch when working in a wet soil so that the excess water can be dischargedimmediately.

Trenchingfor drains begins at the outlet and proceeds upslope. Two or three spadingsmay be necessary, the number depending upon the depth of the trench. A trench width of 30 cm is - 107 - sufficientfor drains with a diameter of 10, 12.5, and 15 cm. Careful excavationof the top spading will prevent alignment difficultieslater as the line and width of the trench are establishedby the first spading. An accurate line should be stretchedbetween the stakes to ensure good alignment. The trench excavator faces the outlet and casts the first spadingwell back from the edge of the trench to provide space for the other spadings in such a way that the soil will not roll back into the trench. The excavator may cast the excavationon either or both sides of the trench.

The last spading should be to a depth about 5 cm above the finish grade of the trench. The last few centimetersof the soil should then be removed from the trench by a drain cleaner or round- pointed shovel. The bottom of the trench should be shaped so that about one-fourthof the circumferenceof the drain will be in contact with the soil. Care must be taken to smooththe trench bottom to the exact grade. If a section of the trench is accidentallyexcavated below grade, the section should be backfilled, tamped, and reshaped to grade. If unstable soil is encountered,the bottom of the trench shouldbe firmed with stable soil, sand, straw, sod, gravel, or other material. Sometimesit is necessary to cradle the tile by the use of boards, using the rail and cleat method of forming the cradle.

The tile or concrete drains may be placed by the use of a hook or by hand. If the drains are slightly warped, they should be turned so that they fit tight at the top. Care should be taken to ensure that the drains are placed in good alignmentand that the gap recommendedin the plan is maintained. Kickingthe drain into place may result in too small a gap. Placing the drain with the hook will usually ensure more uniform gapping. If a curve in the line is too sharp to lay the drain without causing a wide gap, the end of the drain pipe may be chipped so that it will have the intendedgap. A monkey wrench or cutters may be used for this purpose. Junctions of lines are usually formed by manufacturedfittings, and cutting and concretingthe junction is usually not necessary. "Y" fittings are ordinarily recommended. Research has shownthat a junction at any reasonableangle will not materially retard flow.

As soon as the drains are placed and inspected, they shouldbe secured by putting friable soil around them and then blinding them with topsoil to a depth of about 25 cm to preserve the alignment. Any filter material should be placed before the blindingis done, provided filter material is to be used. On steep grades or where the topsoil contains very fine sand, use heavier soil from the sides of the trench in blinding. Envelopes and surroundscan be placed on or around the drain.

Backfillingthe trench shouldbe done at the end of each day's work to eliminatethe possibilityof damage from surface water if heavy rainfall shouldoccur. The end of the drains should always be blocked at the end of each day's work to prevent the entrance of silt and debris. The backfilling of a hand installationis frequentlydone by mechanicalmeans.

Installation by Machines. Staking drains is begun by placingthe beginning or the 0 + 00 station near the outlet end of the line. This location may be at the end of the outlet pipe. ]Hubs should be offset the proper amount to allow for passage of the machine and to the correct side of the centerlineof the planned trench. They should set about every 30 m or less and at all changes in grade or direction of the line. A minimum of three stakes is required to sight a grade. With a short tangent or change in grade, it may be necessaryto set extra hubs on the extendedtangent or extended grade to provide a minimum of three sighting stakes. It is good practice to set hubs at control locations, such as maximumcuts, undergroundconduits, and centers of ditches. Hubs are set on long curves under normal conditions,but the machine operator will usually take care of the short curves by sighting across to the next tangent. - 108 - The hubs shouldbe driven on the required offset from the trench with sufficientaccuracy so that the elevation control targets will line up when they are set. The targets generally used are a metal rod to which is attached an adjustable crossbar painted bright red. The machine operator usually sets the targets, presses the rod into the ground at the stake, and then adjusts the crossbar to a fixed distance above the grade of the trench establishedby the hub.

The elevation of the hubs shouldbe taken to the nearest 0.3 cm and the cut marked on each witness stake. The cut at a point is the differencebetween the elevationof the hub and the planned grade elevationof the drain at that point. Cuts are usually marked in meters and tenths of a meter, but some operators prefer to have the -ut given in meters and centimeters. Cut sheets for the machine operator should be prepared in duplicate and should show stationing,hub elevation, grade elevation, and the difference, which is the cut that is marked on the stake. The sheet should show the grade and size of drain. Specialnotes may be put on the sheet to describe any unusual conditionsuch as soils, depths of cuts, undergroundconduits, and any special protection required for the drain.

Within certain constraints, subsurfaceclosed drains can be installedby machinesthat create open trenches during installationor by machinesthat plow the drain into the ground. The two installationmethods are thus known as;the trench or trenchless methods. The trenchless method is an adaptation of mole drains. A tine (bullet-shapedpoint) is fixed to a vertical shank, all in the configurationof an integrally mounted plow on a tract-type tractor. The trenchless method gained some early prominencewith the advent of perforated corrugatedplastic drain tubing, which could be fed easily into the tine of the plow through a round hollow shaft formed in the vertical shank. The trenchless method of installationhas practical limitationswith respect to types of drain conduit materials, drain sizes, and depths of placement. Due to the tremendouspower requirements, trenchless drain placementsdeeper than about 1 m are rarely practical. The trenchless method has serious shortcomingsfor drains whose purpose is salinitymanagement, where much deeper placement depths are often advisable. Because of the far more numerousadaptations of the trench method, only the machines for that method are herein discussedin detail. There are three basic types of trenching machines: (1) the bucket wheel type, 1(2)the bucket ladder type (often called a chain trencher), and (3) the backhoe excavator. The most common types are the bucket wheel and the bucket ladder. The backhoe is usually a special-purposemachine for conditionsnot suited to the other two types of trenching machines.

The bucket wheel type of trencher has a large wheel mounted on a frame at the rear of the machine. The wheel can be moved up and down by power to keep the trench on grade. Attachedto the wheel are excavatingbuckets. Just behind the buckets are a cutting shoe and a shield to keep the loose earth from falling back into the trench. The cutting shoe shapes the bottom of the trench for the drain. The shield is long enoughto allow the drain pipe or tubing to be placed in a clean trench within the shield. The excavatingbuckets carry the excavatedmaterial upward and deposit it on a conveyor, which deposits it on the ground at one side of the trench. Different sizes of bucket wheel trenchers are availablefor various depths and widths of the required excavation. They may be mounted on wheels or on semi- or full-crawlerframes. Bucketsmay be changed to fit the type of soil in which the excavationis to be made. Some machinesare equipped with a tile chute that carries the tile sections down into the trench shield, where the tile layer standing in the shield checks the laying of the tile. Some machinesare equippedwith various types of cutters to cut the topsoil into the trench after the drain has been laid to blind the drain. Odters are equippedwith conveyorsthat catch the excavatedmaterial and carry it baek of the trench shield to completethe backfilling. Many prefer this automaticbackfilling because the installationis completedin one operation. The bucket wheel - 109 - machine is of a somewhatrigid constructionand cannot follow a very sharp curve; however, the grade on moderate curves is maintainedaccurately if rocks are not encountered.

The bucket ladder type of trenchingmachine has a ladder-typeboom around which the excavatingbuckets move on an endless chain. The excavatedmaterial is placed on a conveyor belt that deposits it beside the trench. Trench shields are used behind the buckets to keep the spoil and crumbs in the trench so that the material can be removed by the buckets. The depth of the trench is maintainedby the raising and lowering of the ladder. This machine is a fast excavator and can follow a rather sharp curve, but it is difficultto keep on grade on curves and the grade requires constant checking. Various widths of trench can be cut with this type machine because the buckets can be changed as desired. This machine is capable of excavatingdeeper trenches than the bucket wheel type of machine generallyused in farm drainage.

The backhoe machine is used where drains are to be laid in trenches deeper or wider than can be excavated economicallyby other trenchers. The trench-hoebucket is in the form of an inverted dipper that is drawn toward the operator like a hoe. Usually the smaller-sizemachines are used for drain excavation.

Installing a drain by a classic excavatorinvolves the followingoperations: trench excavation, tube or tile laying excavation, installationfilter material, and backfillingof the trench. When using modem drainage machines, the first three operations are performed simultaneouslyas the machine moves forward. The backfillingof the trench is sometimesdone by a separatemachine of either an auger or a bulldozer type. Other drainagemachines are provided with a backfilling implementfixed on the front of the machine, and the backfillingoperation is performed as the machine moves backward to begin excavatinga new trench. Some general informationon these machinesfollows:

* The commontrench width is 20 to 25 cm, and wider trenches are possible.

* The commonmaximum trench depth is 2.0 m; however, machinesfor trench depths to 2.8 m are available. Much more rarely available, and usually only by customized procurements, are large special machinesfor trench depths to approximately3.6 m.

* Common engine horsepower is between 100 and 150 hp.

* Commonmaximum working speed is between 800 and 1,200 m per hour, dependingon details such as soil working conditionsand excavatingdepth.

* Commonmaximum network capacity is between 100 and 400 m per hour, and in many developingcountries, a capacity of 100 m per hour may be more realistic.

* Tube laying can be either for clay tiles or plastic pipes. Clay tiles are placed by hand on a chute mountedbetween the check plates, and the tiles move automaticallyinto the right position on the bottom of the trench. Plastic pipes are fed through a conducting-pipe mountedjust over the digging chain.

* Filter materials "in bulk" (such as gravel) are often spread out by hand. Rolls of filter mats either can be mounted on the machine so that the mats are applied to the pipe drain without labor requirements,or they can be prewrapped around pipe drains. - 110-

Modem trenchingmachines are equipped with laser control, which has significantly contributedto efficiencyand accuracy in the installationof subsurfacedrains. Laser control techniquesgreatly alter the traditionalgrade control surveys and staking, as describedpreviously in this chapter. Most trenchingmachines equipped with modem laser control are chain trenchers (bucket ladder type), since this type of machine is prevalent and usually equipped with modem hydraulic controls. However, older bucket wheel machinescan be retrofitted with laser equipment, particularlyfor a semiautomaticmode of operation. Laser control equipmentfor trenching machines commonlyconsists of the following features:

1. Laser transmitter. The transmitter is a precision instrument,battery-powered and usually mounted on a portable tripod. The instrumentemits a conformedlaser beam, and the emitter rotates in a horizontal plane, usually at a rate of 300 revolutions per minute. The instrumentis equippedwith a self-levelingfeature. Angles from the level horizontal plane, plus or minus, can be dialed into the instrumentfor the x or y axis or both. The axis positions can be selected to any desired field orientation. When in operation, the result is a plane of laser light tilted from the horizontal in a preset x and y grade.

2. Laser receiver. The receiver usually features an interceptionwindow on a vertical mast that is affixed to the excavatingstructure of the trenching machine. The vertical intercept of the receiving window normallyis about 12 cm, with a maximumsignal sensor at the middle (cross-hair)position. In the automaticmode, at the startup of each drain line, the height of the mast (receiver)is placed in a fixed and locked position for the proper depth. The receiver is electronicallylinked to a transducer (control box), which in turn operates the hydraulic control valves of the trencher as the laser receiver "homes"on the laser beam. Thus, the digging depth of the machine automaticallyis constantlyand accurately at a predeterminedand fixed distancebelow the laser beam plane. The semiautomatic mode is similar. The differenceis that as the receiver "homes" on the laser signal, the control box displays a steady "on-grade"indicator light, a blinking "above grade" indicator light, and a blinking "below grade" indicatorlight. The trenching machine operator manually operates the hydraulic controls of the machine to maintain the "on- grade" response of the indicator lights, and thus the digging depth is kept on the true and proper grade. Usually, laser control equipmentincludes another piece of receiving equipment, which is a sirnilar receiver windowthat is affixed to a portable staff or rod. This receiver is used in much the same fashion as a survey rod. It facilitates such tasks as (1) backsightingon benchmarksto establish the instrumentheight of the transmitter, (2) checking the setup and accuracy, and (3) establishingor confirmingthe start-up depth at the beginning of each drain line.

Various brands of laser control equipmentmay differ slightlyfrom the functions and configurationsdescribed here; however, the basic performancecapabilities of the equipmentbrands are essentiallythe same. The use of laser control equipmentis particularly adaptedto geometric drain patterns such as grid layouts. The x and y grade settings corresponddirectly to the desired collector line grade and the grade of the lateral lines. The settings also are easily computedfor other geometric patterns, such as the herringbone. Usually, one setup of the laser transmitter will accommodateseveral lateral lines.

Inspection of Drainage-RelatedActivities. Inspection of the drain installationshould be carried on periodicallythroughout the constructionstage to ensure conformancewith plans and specifications. The followingitems should be checked: - ill -

* quality of tile, tubing, pipe, and other materials;

* alignment, depth, and grade of drain;

* trench width at top of drain;

* joint spacingof tile;

* connections;

* bedding;

* filter or envelope materials and installation;

* blindingmethods;

* backfillingmethods;

* outlets; and

* auxiliary structures.

Reasonableallowance should be made for errors resulting in small variations from the planned grade for slight unevennessin the diameter of individualdrains; however, a reverse grade should never be permitted in drain lines.

Inspectionof materials and handling include drainagepipes, filters, and envelopes as well as structures.

T-hesepipes should be inspected:

* clay pipe, with or without collar;

* concrete pipe, with or without collar, plain or reinforced, ordinary or sulphate-resistant cement;

* corrugated PVC or PE pipe for laterals, end pipes;

* PE or PVC pipe for collectors; and

* couplers and end caps.

Specificationscovering the physical requirementsand testing methods have been developedby many organizations (e.g., ASTM). Inspectionshould be carried out in areas related to the delivery of pipes, including:

* manufacturer's certificate;

* insurance for damage during transport (both overseas and overland); - 112 - * damagedpipe;

* protection of plastic pipes against direct sunlight;

* blind pipe requirements(under watercoursesand risers for horizontal dewatering), outlet sections;

* delivery scheduleversus storage requirements; and

* manufacturingat project site (e.g., concrete pipes).

Inspection should be carried out in these areas related to filters and envelopes:

* selectionof filter material (synthetic,mineral, organic);

* survey for suitable quarry for gravel, transportationdistance, need for and location of screen plant; and

* screening and grading of gravel accordingto design specifications.

Inspection of these structures should be made:

* manholes, prefab or made on site;

* drain bridges;

* junctions; and

* outlet structures.

Problems in Drain Installation. Most problems in drain installationare rather minor where good soil conditionsexist and where groundwateris not present. A good planning and stakingjob can eliminatemany problems. Good1planning will allow the recognitionof many of the difficulties that may be encounteredduring construction,and the contractorcan be prepared to meet them. Installationshould be done when the water table is at its lowest level.

Wheneverthe plans indicatethat there is a considerableamount of fine sand, especiallywet sand, the installationshould be delayed until the water table is at its lowest elevation. Dry sand presents a problem, but fluid sand that runs into the trench and covers the drain before filters can be installed presents the most difficult situation. In many drain lines, sand pockets may be found that make constructiondifficult, but if the problem is limited, it can be solvedby using some special procedures.

It may be possible to plan the drain through the sand layer at a shallowerdepth to lessen the problem of installation. Caving of the trench sidewalls is always a problem. In some cases, the drain shield may be made longer to protect a greater length of the trench. The drain should be laid as soon as possible and before the shoe has passed. The protection of the shield may provide enoughtime to wrap the joint, provided the trencher advancesslowly. The drain shouldbe blinded as soon as possible, and the trench should be backfilled. - 113 - When the sands are saturated, the machine should be kept moving. Stoppingthe machine in saturated sand will permit the sand and water to build up over the drain in the shoe and make it impossibleto keep sand out of a drain already installed. If the trencher is stopped, the shoe will settle in the wet sands and cause a low spot in the grade. A poor foundationresults when the trencher is started again. Blindingthe drain should be done with great care to prevent the pipes from being knocked out of alignment and grade. If the trench walls cave in such a way that the caving material falls vertically on top of the drain, very little damagemay be done, but if they settle vertically or slip down, the drain will be pushed out of line. The damage should be repaired immediately. The conveyor shouldbe set to throw the excavationas far from the trench as possible to relieve the weight on the trench side walls and to prevent the wet sands from runningback to the cutting wheel.

If lateral drains are to be installed in this portion of the main drain, sufficienttime should elapse after installationof the collectorsto permit the groundwaterto drain out. It is importantthat the drain laying and backfillingbe done quickly followingexcavation of the trench. It is good practice to have no more than 12 ft of trench open at a time because of the possibilityof the banks' sliding in and causingthe trench bottomto be forced up. If the job is interrupted, the work should be completedas far as the trench is opened.

If it becomes apparent that the foundationcannot be stabilizedby placing good mineral soil or gravel in the bottom of the trench, then ladders may be installed using the board and cleat method to install a mechanicalsupport for the drain. In lieu of this procedure, a continuousplastic, bituminized fiber or metal pipe may be installed. The continuousconduit will usually solve the grade stability problem, but the problem of laying it to grade may be difficult. Crushed rock has been used in many installationsto stabilizethe grade. After crushedrock has been placed in the bottom of the trench, the grade must then be reestablished.

Installationof Open Ditches

Constructionplans for individualfarm ditches usually include drainage plan maps, specifications,inspection procedures, and disposalof excavatedmaterial. The plans should be discussed with the landowner or his or her representativeto make sure the landownerfully understandsthe proposed work. Cost estimatesshould be provided on request. Maintenancealso should be discussed and methods of maintenanceand locationof a travel way agreed upon. The maintenanceplan for group jobs should show an annual maintenanceschedule and should specify the practices to follow. It should cover details for carrying on maintenanceoperations and for periodic inspection. The method for paymentof maintenancecosts must be agreed upon and includedin the group agreement when a group job is involved.

Drainage Plan Maps. Drainage constructionplans for large jobs should include a map of the proposed improvement. The completedmap should show the following:location of proposed ditches, bridges, culverts, farm boundaries and names of owners where necessary, and watershedboundaries and areas; existingland use and irrigation facilities; nearby towns, roads, railroads, townshipand section lines; and location of maintenancecenters, roads, drop structures, and other features affecting the design, construction,and maintenanceof the planned improvements. On many jobs it is convenientto show the detailed plans on standard plan profile sheets. Where this is done, the scale for the plan should be the same as the horizontalscale used in plotting profiles. In such cases, a general location map shouldbe includedto show the generallayout of the system and to index plan - 114 - profile sheets. This map should show the entire watershedarea within which the drainage problem area is located.

Plans for all ditches of the drainage system down to farm laterals should include profiles. The profiles should be plotted on standard-size,transparent profile or plan profile paper, and the completedprofiles should show:

* normal ground line and elevationof isolatedlow points in the field into which the ditch will drain;

* existingditch bottom;

* hydraulic gradeline;

* proposed ditch bottom;

* existing and proposed culverts, flumes, and other structures, and proper identificationof each structure. Also note if an existing structure is to be removed;

* points of entry of significantditches;

* elevation of high water for design storm at outlet;

* width of ditch right-of-wayto be cleared (notationby reaches);

* datum used and descriptionof importantbenchmarks;

- logs of soil borings; and

* elevationsof water table and dates of reading if encounteredin the soil borings.

The number of cross-sectionsrequired depends on the variations in cross-sectionsof existing ditches and on the uniformityof topography along the proposed ditch location. The manner of payment also governs this matter.

Cross-sectionsof proposed ditches are superimposedon original cross-sections,and the amount of excavationis computed. At least one typical ditch cross-sectionshould be shown on constructionplans. Where the land surface is reasonablyuniform, the depth for new ditches may be obtained from the profile and the excavationcomputed from geometric tables. Typical cross-sections are plotted directly on the profile sheet when excavation quantitiesare computedfrom tables or by computerfrom field notes. For others, cross-sectionsshould be plotted on standard-sizetransparent cross-sectionpaper. Ownershipboundaries should also be included.

Sufficientsoil profile informationshould be obtainedthrough soil borings to locate any unstable soil conditionsthat may exist along the planned ditch route. It may be possible to reroute the ditch to bypass the unstable area.

The calculationsmade for the design of all ditches of the drainage system shouldbe recorded on a standard ditch design sheet and made a part of the plans. - 115 -

Usually the structures for small- to moderate-sizedrainage projects will be of standardsize and design. For these structures, a copy of the detailed design should be obtainedand includedin the plans. If a structure is to be designedby another agency, it may be so noted on the profile or plan map, and the required elevation of the invert, or flow line, and the minimumcapacity required should be specified. If a structure is not standardand plans are prepared by another agency, a set of plans and specificationsshould be included. Plans for structuressuch as bridges, culverts, chutes, flumes, floodgates, drop structures, watergates, levees, dikes, and pumpingplants that are a required part of the drainage project should be included. For bridges and culverts to be used for public roads, size and the invert and road elevations are all that need to be shown.

Specifications. Written specificationsare prepared for each item in a job proposal, and standard specificationsare desirable for the more common types of work. For contract work, specificationsbecome a part of the contract along with the plans. On force account work, specificationsshould be used by the person in charge of the job to ensure that constructionis in accordancewith the plan and with required standards. Specificationsneed to be detailed enough so that there can be no reasonablemisunderstanding of the type of job desired. Nonessentialdetails should be omitted. Each work unit engaged in open ditch drainage work should have available standard specificationsfor the followingitems of construction:

1. Specificationsfor and required amount:

- draglines;

- hydraulicexcavators (profile buckets);

- bulldozers;

- tipper trucks;

• road grader (for finishing inspectionpath and maintainingaccess roads);

e low-loadersand prime movers;

* water bowsers;

* fuel tankers;

* mobile workshopand weldingequipment;

* concrete miners (for structures);

* transport for personnel;

* topographicinstruments; and

* small tools. -116- 2. Other importantissues include:

* general and special conditionsof contracts;

* erection at site and training of operators;

* guarantees;

* timely delivery of spare parts;

* inspectionof equipmentat factory; and

* acceptancecertificate.

Inspection. Standard inspectionsare usually made for clearing, grubbing, channel excavation, spoil bank spreading, pipes for culverts, installationof culverts, concrete, steel reinforcement, and seeding ditchbanks. Moreover, inspectioncovers quantitysurveys (verificationof invoices) as well as levels, graders, and profiles.

Installationof Drainage Works in TidalLands

Agriculturallands in coastal areas, located along rivers, estuaries, bays, and the open sea, are subjectedin varying degrees to overflowand restricted drainage causedby tidal water. Protection from overflow usually is obtained by the enclosure of such areas with dikes. Drainage may be obtainedby establishinga system of internal drains, with water discharged over the dikes by pumps, gravity flow through gates structures, or a combinationof pumps and gated structures.

Forebay ChannelSection. The forebay channelsection must have adequate capacity to deliver the design drainage discharge to the forebay and tide gate, and it must also have capacity to pass the peak discharge of the tide gate. The peak discharge may be several times the drainage inflow.

Discharge Bay ChannelSection. The discharge section of the outlet bay should be designed with ample capacity to discharge the peak flow to deep open water without causing backwateragainst the tide gate. Usually low foreshorebanks have adequate overflowarea to accommodatesuch discharge; however, this should be checked.

Tide Gates, Pipes, and Outlet Structures. As a rule, tide gate structures must be installed in wet, swampy areas and tidal flats where constructionis difficult at best. The structure is usually placed at a minus elevation so that the gate functionsas a submergedorifice, at all times. Structural sites and approach channels are usually excavatedby dragline, which also serves as a crane for hoisting gates, pipes, and other structural members into place. During construction,the site is isolated from surface waters by encirclingthe site with spoil banks that act as temporary dikes, and it may be necessaryto dewater the excavationby pumping. Under very difficult conditions,it may be necessary to shore the excavationwith sheet piling and encircle it with a battery of dewateringwells connectedto a manifoldand pump.

An outlet structure is usually necessaryto protect the gate and connectingpipe from scour and floating debris. Treated timber structures, when used, shouldprove to be a very practical and economicinstallation. This type of structure can be prefabricatedin whole or in part, depending - 117 - upon the size, and set in place by dragline for attachmentto preset piles that have been driven or jetted into the foundation. Concreteor masonry cast in place is not usually satisfactoryfor this kind of site unless the excavationcan be kept free from salt water long enoughfor the cement to set.

Corrugated metal pipes are commonlyused in tide gate structures and must be asbestos- bonded to resist salt water corrosion. The pipes should be 10 gauge or thicker to ensure long life under these severe saline conditions. Experiencehas shown that thin-gaugenoncoated pipes have a very short life in brackish water. The pipe conduit and gate, unless very small, shouldbe installed as separateunits. Usually a short or stub section of conduit is fabricated to the gate and joined to the pipe after both are placed. Camber shouldbe provided in the excavatedtrench bed to allow for consolidationand unequal loading by the superimposeddike. Conduit and gate must be set true to line and grade if the structure is to operate as designed. Prefabricatedcast iron flap gates, usually called tide gates when used for tidal installations,have bronze bushings and are coated with a water- resistant rust-preventivecompound at the factory and have a long life, even under brackish water. Flap gates are also availablewith stainless steel hardware as a further precaution against salt water corrosion.

Where tide gates must have a large capacityto dischargethe drainage flow during the low tide period, it is usually desirable to install a battery of small gates rather than one or two large gates. As a general rule, tide gates are placed so that the top of the gate is at about elevation -30 cm, to provide for submergenceat all times and to provide for full utilizationof the availablestorage reservoir. These conditionsdictate that the pipe and gate be placed entirely below zero elevation, which in turn imposes difficult constructionand maintenanceconditions. From this, it is obviousthat small- diameter gates and pipe can be installedmuch more easily than larger-diameterstructures. For example, five 64-cm gates in a battery will have about the same discharge as one 120-cmgate and will reduce the depth of trench excavationby 2 ft. This 2-feet reductionin trench depth may materially simplifyconstruction and save more than the additionalcost of multiplegates.

SupplementalPump Installations. Pump installationsfor pumping drainage water in connectionwith tidal drainage are usually low head, high-capacityinstallations. They are used in situationswhere the tidal range seldom exceeds2.5 m, except during periods of severe storms. The average pumping lift is usually small, in the range of 0 to 4 ft, and pumps selected for this condition should have a high efficiency at low head operation. Propeller and mixed-flowpumps are best suited for this situation. As previouslymentioned, pumps used in tidal drainage situationsare usually operated only during the gate-closedperiod, to reduce the storage requirement. They can also be operated during the gate-openperiod; however, this is seldom done because the only benefit is to reduce the load on the tide gate, permittinguse of a smaller gate. Generallythe cost of pumping during this gate-openperiod, for the life of the project, is far in excess of the cost of a larger gate.

Maintenance

It is vital that the drains have a good outflowinto the open watercourse. This objective requires maintaininga sufficientlylow water level in open outlet ditches and ensuring the functioning of tube drains. The functioningof open drains can be verified by visual observationswithout measurementsduring the wet periods, after consideringdrain dischargesand field conditions. The functioningcan also be verified by calculatingthe loss of the water flow path (correspondingflow resistance)by using piezometers installedto measure the water pressure inside the tube drain. - 118 -

Tube Drains

Tube drains can be clogged by soil particles (siltingup), chemicaldeposits (iron), or plant roots.

Caoggingby Soil Particles. Many field and laboratory investigationssuch as those carried out by Bishay and Dierickx (1975) have shown that soil particles may move into the drain from above and below the drain level. Silting up is likely to occur in soils of low structure stability. The rate at which silting up takes place is extrernelyvariable. Drains that have been installed for 30 years under dry conditionsin heavy-texturedsoils have shown a layer of deposits of only a few millimeters. Drains that have been laid in sandy soils, however, may be more than half filled with sand after a period ranging from a few days to a few months. The largest amount of soil particles invade the tube drain shortly after installation. Then the soil structure around the tube drain stabilizesduring the succeedingdry period so that the rate of silting up is significantlydecreased during the followingwet period.

Prevention of silting up can be accomplishedas follows:

1. Tube drains can be protected by filter materials either by covering the top and bottomof the drain by voluminousmaterials such as gravel or by wrappingthe pipes completelyby using mat filter materials such as coconut fiber.

2. Cleaningof drain tubes can be done by flushingthe drain with water by using special flushing machines. Water is pumped into the drain through a reinforced hose provided by a special nozzle and both are moved through the entire drain line.

3. The natural flow of water in the drain can be regarded as a third measure if a minimum water flow velocity required to move soil particles of about 0.35 m/s becomes available in the drain line. Several recommendationsregarding the minimumslope of the drain required to provide a sufficientflow velocity inside the drain are available. The validity of such recommendationsshould be accepted with some caution, because since the rate of flow in a drain starts at zero and increases in the flow direction, the required flow velocity occurs only near the outlet. So self-cleaningmay take place over a short length of the drain close to the outlet, whereasthe soil depositswill stay further upstream. In flat areas where slopes are less than 0.1 percent, the minimumrequired slope cannot be obtainedwithout considerabledifficulties, and self-cleaningdoes not take place to a considerableextent.

Cloggingby Iron. Clogginglby iron cannot be preventedby filter materials because the Fe+ dissolvedin the groundwaterenters the drain and then can be oxidizedto an insolubleFe'+ + form that precipitates inside the drain.

Maintenancemeasures needed are as follows:

1. Iron deposits can be removed by flushingusing low pressure machines.

2. Good results can be obtainedby pushing a dry plastic hose provided with a head through the drain during periods when the drain is flowing. The hose loosensthe iron deposits, which can then be removed by the drainagewater. - 119 -

3. Iron can also be removed by dissolvingit using sulphur dioxide (SO) dissolvedin water.

4. Iron deposits can be avoided by preventingair from enteringthe drain by keeping the drain outlet under the water level of the open ditch so that no oxidationcan take place.

5. Iron depositscan also be avoided by forcing the iron to precipitatebefore it enters the drain by using lime (CaCO3).

Cloggingby Plant Roots. The roots of certain species of trees and shrubs can enter through the points and perforations of tube drains and may cause a completeblockage of the drain within a short time. Windbreaksgrown along orchards (frequentlyconsisting of populousspecies) may also cause considerableroot growth inside the drains. Observationsdone on orchards showed that fruit trees are not likely to cause root growth into drains if the drains are placed at a depth of one meter or greater from the ground surface.

A good remedy has proved to be the use of nonperforatedplastic pipes in places where the drain lines cross a border, trees, or shrubs, such as those around sports grounds.

Maintenanceof Open Field Ditches and Drains

Disposal of excess surface and subsurfacewater, the control of groundwaterlevels, or a combinationof these situationsrequires proper drainage. Sedimentbuildup, damage caused by tillage equipmentpasses, and vegetativegrowth in open ditches and drains can reduce their design capacities. Timely maintenanceis needed for drain systemsto function over their 10- to 50-year design life. Studies of drainage ditches show that without maintenancethe useful life of most ditches is seven to ten years.

Key maintenancetips in this case includethe following:

1. Timely removal of sedimentsand annual chapping or mowing are usually required. Farm tractors with mountedblades or scrapers are ideal for cleanouts.

2. In croplands, a vegetativefilter strip is desirableon each side of the drum to remove sedimentfrom surface inflow. Tillage operations should be performed in a manner that maintainsthe filter strip.

3. Undesirablevegetative growth inside the drain can be controlledby mowing. Since ditch banks and berms are a favoritehabitat for many wildlife species, the timing of maintenancemowing is very important. Mowing can be delayed until after the principal rearing season has passed. Dependingon the vegetation and climatic conditions, undesirablevegetation can be controlledby mowingabout every two to five years. In warm climates mowing is required at least once a year, and in tropical areas it has to be done at 6-month intervals. A good practice is to mow one side one year and the other side on alternate years.

4. Controllinggrowth of vegetation by grazing is also practiced, but it requires very careful management. Too often livestock damagethe desired vegetation and ditchbanksby trampling. - 120 - 5. All tillage equipmentshould be lifted when crossing the waterways.

6: Biologicalmaintenance is successfullypracticed by growing certain selected fish. Appendix1

Identificationof Soil Drainability

Auger Hole Method for Determining Hydraulic Conductivity in Situ

The auger hole method, shown in figure 1-1, provides a compositevalue of hydraulic conductivityfor the soil layers between the water table and a level at few centimetersbelow the bottom of the hole.

The relation between the hydraulicconductivity K in meters per day and the rate of water rise Y/t in centimetersper second can be expressed as: AY At where the C-value can be obtainedfrom the graphs described by van Beers (1963). The K-value can also be obtainedfrom the followingtwo formulas describedby van Beers:

K = 4000P AY when S > 1/2 H (H + 20r) (2 - BY At H and

K= 3600,2 _ _Y whenS = 0 (H + lOr) (2 -f At H where:

H = depth of the hole below groundwatertable in centimeters

Y. = distancebetween the groundwatertable and the elevation of the water surface in the hole after removing the water at the time of the first reading in centimeters

Y,, = the same as Y0 but at the end of the measurement(five readings are usually taken) in centimeters

AY = the rise of water level in the hole in centimeters = Y. - Y. during the time of measurementt in seconds, and the measurementshould be completedbefore AY- 1/4Yo

Y = distance between the groundwaterlevel and the average level of the water in the hole during the time of measurement = Yo- 1/2 AYin centimeters

- 121 - - 122 -

r = radiusof the hole in centimeters

D = depthof the impermeablelayer belowthe holebottom in centimeters

Interpretation of ObtainableHydraulic Figure1-1. DeterminingHydraulic Conductivity by the Auger ConductivityValues Hole Method For drainage design,it is necessaryto Measuringtape knowthe hydraulic with float conductivityfor a Standard greaterpart of the Groundsurface profile. It is particularly W importantto determine whetherthere is any relativelyimpermeable Groundwaterlevel layer that shows a- _____ markeddecrease of -i hydraulicconductivity | withdepth. Theterms H permeableand impermeableare Y relative. The Yo impermeablelayer may be takenas that havinga 7 hydraulicconductivity of one fifth, one tenth,or AY lessthan that of the overlayinglayers. The impermeablelayer does ______not necessarilyhave to 2i be of a more fine- texturedsubsoil. It may D be a productof other soil formationfactors, such as a compression R v ipel layer by the weightof the -l glaciersduring the ice age, a stratificationof the alluvialsediments thathave been depositedfrom the streamsin the irrigatedareas, or the formationof water passageways,such as crackswithin the soil profile. The influenceof the impermeablelayer depends on its depthbelow the field drainsand also on the spacing. Therewill be waterloggingabove the impermeablelayer if the rainfallrate, or the rate at whichwater is addedto the soil, exceedsthe permeabilityof this layer. The flow patternof the watermoving toward the drainswill be altered drasticallyby the impermeablelayer if H < 1/4L the drain spacing. The drainswill have to be placedclose together to achievethe effectthey would have in a deeppermeable soil. Hooghoudt - 123 - (1936), however, shows that if H 2 1/4 L the drain spacing, the flow system can be treated as if such a layer were entirely absent. Thus, it follows that a knowledgeof the precise location of the impermeablelayer becomes less importantas the spacing decreases, that is, as the average hydraulic conductivityof the soil decreases. It then becomes sufficientto predict its stratificationfrom what is known of the soil stratification.

When the depth of the impermeablelayer has been determined and the average value of hydraulic conductivityis calculated,the drain spacing can be calculated(as described in chapter 2). The design engineer determinesthe feasibilityof installinga drainage system with appropriate spacingsfor different sections of the farm. A compromisespacing is frequentlynecessary, and some over- and/or underdrainageis then inevitable. Table 1-1 shows average values of hydraulic conductivityfor each of the example farms shown in figure 1-2, the holes for which the hydraulic conductivitywas too low for inclusionin the calculations,the depth of the impermeablelayer below the laterals, and the drain spacing calculatedby the HooghoudtFormula. The depth of the laterals below the ground surface is assumedto be 1.8 m for all farms. The q-value is 0.005 m per day when the static water table midway between drains is 0.45 m below the ground surface. The depth of the relatively impermeablelayer was determinedand found to be 4.8 m below the soil surface or 3.0 m below the lateral. The following points about each farm can be made after studyingtable 1-1 in conjunctionwith figure 1-2. Farm 788 has been omitted from table 1-1 because subsurfacedrainage is not recommendedbecause the average hydraulic conductivitywas so low. Figure 1-2. Six Example Farms

PortionA PortionB

20 19 ~~ ~ ~~~~~~~~~2822 21 16 15 81 7

(tIRY) (DRY) (DRY) | (DRY) 2 1 113) A (N.M.) (008) (1.2) (04)

I* \ (DRY) 27 23 17 14 9 6 1 } 22 21 ~~~~~~181 (0.b) \ 9\(2.0) * / * * * * \ 22 21 * IDRY) ; P1C}12 3 (1.2) (0.7) (DRY) (DRY) (0.7) (0.5) (0.8)

(N.Ui3 ) I (2.9)* F 9 (1.3) (1.4) (0.9) (1.3) (NM ) (02) (1.1) 116 1 (0.2) 71 1\\ I* / * / 5 I 25 19 12 11 4 3 e, (DFY)I 14 (0.7) * \ 0

Partmos1794 & 486 1.{ (DR) (03) I (0.8) (1.9) (0.1) p.5) (1.4) (DFY) Portions179 &486 0 '; OY *;0 18ha) 110RY) (0-9) / < ~~~Farm1864 Portion1 00 (18 ha)

PortionA PortionB

6 7 ~~~ ~~~~1213 19 24 25 21 9 * 0 0 0 5'S 0 0 7 * -I (0.9) (0.6) (17 (0.8) (0.5) (03) (0.1) 7.2) (0.6 (J4) (1.5) (DRY) (o.14) O} * * (1.7) / 15~~~~~~~~~~~~~~~~~~~~~~1 2 58 11 14 12 23 26 30 27 16 2 I'\@) N( /;L (°) /oH 1 322l0.3) (1.1) (0.1)A (04) (0.) (0.3 0.7) (3.0) (1.5 (1.5) .)(.2) (0.60.8) (0.6

3 s>010 15NM) (5 (03)(0 24 18 17 ( 101 6 ((1) (06 03 03i (.)0(0.7) ~ )(1.4)(08) 0.)(1.0) (2.0) (0.15)/ N.) P (V0 25 23 12

I (1~~~ ~ ~~~~~~(0.8) ~~.8) (0.1) Farm1295 Farm1182 Portions821 &1032 Portions856 & 1102 (32 ha) (36 ha)

Note: Positions of auger holes are marked. The figures in brackets are hydraulic conductivity values, and the other figures are hole numbers. Dry: hole was dry at 2.00 m depth; N.M.: not measured because there was insufficientwater at 2.00 m; and L.V.: very low hydraulic conductivityvalues. Figure 1-2 (continued)

/ 1?) (003)0-1) 21 PortionB 28 29 30~~~~~ 24204(02

(03) '17 0 , 19 (0.2) (.2J (0.4)() 24 (0.3) (8) (0.9 (N.M.) |* '.(0.1)1 is 13 25 0 21 ~~~~~~~29 28 (0.1) 16 0 (0.2) 2) 0 ~ ~~~~0 112.30(0.4) '(0.4) --- (.6 12 0 12 15 ; (05) I (0.26 1!8 19( 20 206 14 13 (0.4 (N.M.) (L.V.)

A (LV) (~~0.4 * (0.34 2 22 27 Portin /PouP2)p)p)f4(L)tEX~~~~~~(.M. i,l)(LV

0 0 04 ( 2 (0.1)(0.02)2(03) (0.5)120 (03) 2 (.3 1 \ 2 2 8 2 2 22rm27 C0.A) 16 15 .1 * (N.M.) (L.V)

(N.M) (L.V) (L.V) (L.V) ( ha)

P 978 \ / 8 7 Farm ~ ~ 0.2 (0.02) .0 .8 Porthon612\ 41~ e * ** (27 ha) \, 0.02) (0.05) (DRY

Farm788 Portion92 (29 ha) - 126 -

Table 1-1. Average HydraulicConductivity, Depths of the hnpenneable Layers below the Subsurface Drains, and Drain Spacingsfor ExampleFarms

Farm Portion Section Aiverage Holes not included Depth of the Calculated number number hydraulic in calculating the average inpermeable drain spacing conductivity K-value layer below drain (m) (in/day) level (m)

219 261 A 0.10 26 1.20 19 B 0.36 Nil 1.20 25 C 0.93 Nil 1.20 60

978 612 0.30 2,5,9, and 16 0.60 28 1,182 856 & 56 1,102 0.71 22 and 28 1.20

1,295 821 0.42 13 1.80 44 1 1,038 0.96 21 2.40 76

1,864 179 A - AU - 486 B 1.26 10, 14, dl24 3.00 93 166 0.96 3, 7, 10, 14, 3.00 81 ______17, and21

Farm 219: The spacing recommendedfor section A of this farm is very close. Wide spacing can be used for the remainder.

Farm 978: There is quite high variation among the values of hydraulic conductivity. Four low values have been discarded from the calculationof the average. If possible, the drains should not be laid in the area representedby the three holes 5, 9, and 16.

Farm 1,182: The values of hydraulic conductivityare uniform, and the average is above that generally found in the area. There is one extremelylow value and several other values that are somewhatlow. The low values are confined to an area in which there appears to be a general lowering of hydraulic conductivity.

Farm 1,295: Values of hydraulic conductivityvary, but the farm is obviously divisibleinto two sections that are reasonably uniform.

Farm 1,864: Portion 166 has reasonablyuniform values. Some holes are dry. Section B of portions 179 and 486 has quite high and reasonablyuniform hydraulic conductivityvalues. Again, there are some dry holes. Appendix 2

Calculating Drain Spacing by Drainage Design Formulas

Steady State Conditions

Hooghoudt (1936) gives the following formula for computing the spacing between horizontal field drains under a steady rainfall rate and in soils with an impermeable layer at an intermediate depth, as shown in figure 2-1.

2 2 L = 8K2 dh 4Kh (Hooghoudt's Formula) q q

Figure 2-1. Description of Hooghoudt's Formula

q

bhK * S s~~~~~~~~~~~~q

L s C. .,

D

7L p-

- 127 - - 128 - where:

L = drain spacing in meters

K2 = hydraulic conductivityof the layer below drains in meters per day

K1 = hydraulic conductivityof the layer above drains in meters per day

h = height of the water table above the drain level midway between drains in meters

d = thicknessof the equivalentlayer-a value depending on the drain spacingL, the pipe drain radius r and the depth D of the impermeablelayer below the bottomof the drain in meters. The d-values can be directly obtained from special tables prepared by Hooghoudt, or they can be calculated, as described later in this report. With open field ditches, the d-values can also be obtained when consideringthat the wetted perimeter U of the drain U = irr

Discussionof Hooghoudt'sFormula

The formula is based on the presence of two layers of differentpermeability values (K2 and KJ), and the drains occupy the interfhcebetween these layers. In this case, the first part of the formula relates to the flow below the drains, and the second part relates to the flow above the drains. However, K2 is equal to K1 in a homogeneoussoil.

The drainage coefficientq is here expressedin m/day, which is the same as m3 per m2 area drained. For example, when qu = 0.005 m/day (0.005 nr3/m, the drain spacing is 40 m, and the drain length is 100 m; the discharge per area drained will be 0.005 x 40 x 100 = 20 mr/day or 20,000 1 per 86,400 seconds = 0.23 l/s, and the discharge per hectare will be 0.23 x 10,0001(40x 100) = 0.58 I/s/ha.

The value of the availablehydraulic head h can be calculatedfrom the minimumpermissible depth of the groundwater(a in figure 2-1) and the depth of drains b. The latter is a questionof economicsand depends on such elements as the position of suitable soil layers, the available level of outlets, and the salinityof the groundwater.

If the groundwaterflow occurs mainly below the drains (i.e., a high KRDvalue), a variation in q, K2, or h will result in a variation in the drain spacingL that is proportionalto the square root of these three values. For example, a variation of + 100 percent in q, K2, or h will result in a difference of about 40 percent in the drain spacing (V'2 = 1 . 41 ) ; a variation of +50 percent will produce a difference of about 25 percent, and finally a variation of -50 percent will give a difference of 30 percent (V/-5 = 0 .70) . It can also be seen from the formula that the drainage coefficientq has relativelythe same effect on the calculationof the drain spacing as does the hydraulic conductivity K. However, the q- and the K-valuesdiffer extensivelyin their importance as a source of possible errors in the calculationof the drain spacing required. The q-values, relating to the rainfall discharge or the leachingperiod to prevent salinizationin irrigation projects, do not usually show much variation, and normally an approximatecalculation of the q-value does not differ very much from a - 129 - carefullycomputed value. The possible variations in the K factor, however, are manifold, and the estimate of this factor may be extremely inaccurate. In addition, another considerablevariation may result when determiningthe depth of impermeablelayer D. The q-values can be approximately calculatedand discussed, but the hydrologicalsoil factors K and D cannot be calculated,but have to be determinedin the field after the various hydrologicalsoil units have been outlined. The above- mentioneddifference in the influenceon the calculationsof q on the one hand and AD on the other hand might explainthe fact that the importanceof the q-value is often overemphasizedand that of the KD values often underrated, especiallyfor the layers below the drains.

The main feature of Hooghoudt's analysis is that the groundwaterflow can be Figure 2-2. GeneralPrinciple of Hooghoudt's schematizedas a horizontal flow up to a Formula distance of 0.7D from the drains (making the width of this zone = L - 1.4D) and a radial flow from a distance of 0.7D up tou the drain. Hooghoudtthen specifiesthat the h - - - sum of the horizontal resistanceRhand the fT radial resistanceP., should be equal to the D I horizontalresistance in an equivalentlayer D -- having a thicknessd as calculatedover the entire length L, as shown in figure 2-2. I Consequently,the formula L2 = 8Kd-hlq, .7D+ 0.7DI which was primarily intendedfor an almost L - -L…----… entirely horizontal flow, can be appliedto cases in which the radial resistancecannot be neglected. When a simplifiedformula is used for the radial resistance, this d-value I can be easily calculatedas follows:Rb ------(D,L - 114D)+ R, (d,L). The groundwater d flow to the drain can be expressed in the d following general form: h = qL x R, in \\X\\\\\\\\ which h is the availablehydraulic head, qL is the rate of groundwaterflow from two * … sides into a unit length of drain, and R is the total resistance, which is equal to the horizontal resistanceRh plus the radial resistanceR,. The radial resistanceR, can be calculatedby a long formula given by Hooghoudt in which, however, the first term l/irln(0.7D/r) predominatesto such an extent that the remainder of the formula may be omitted without impairing the required accuracy. The horizontalresistance Rhcan be calculatedaccording to Hooghoudtto a distance of 0.7D from the drains. Hence, this equationbecomes (L - 1.4D)V8DLand d = Ll8(P + 1,).

CalculationExample on the Use of Hooghoudt'sFormiula

Hooghoudt's formula is very simple, but calculationswith this formula entail a laborious process of trial and error. This fact can be illustrated by the followingexample:

Problem: A soil with an impermeablelayer located 3 m below the drain level (D = 3 m), K2 = 1 m per day, K, = 0.5 m per day, q = 0.005 mper day, h = 0.6 m, r = 0.10 m. Determine the proper drain spacing. - 130 - Solution:

1st Trial: AssumeL = 40 m. From Hooghoudtd-values tables: for 9D = 3 m, L = 40 m, and r = 0.10 m, the d-value = 2.16 m.

Hence, L2= 8 x 1.0 x 2.16 x 0.6 + 4 x 0.5 x 0.36 0.005 0.005

1600 = 2074 + 144 = 2218orL > 40m

2nd Trial: AssumeL = 45 m; the d-value = 2.23 m.

2025 = 8 x l.Qx2.23 x0.6 + 144 = 2285orL > 45m 0.005

3rd Trial: AssumeL = 50 m; the d-value = 2.29 m.

2500 = 8 x 10 x 2.29 x 0.6 + 144 = 2343 or L < 50 m 0.005

4th Trial: AssumeL = 48 m; the d-value = 2.26 m.

2304 = 8 x 1.0 x 2.26 x 0.6 + 144 = 2314 0.005

The obtainable value of the fourth trial is close enoughto the 48 m assumed. Hence, the theoretically required spacing is about 48 m. To simplifythe calculations,different monographsare available.

Non-Steady State Conditions

During the irrigation of a field or the leachingof saline soils, there is considerablepercolation of water in a short period. As a result, there is a rapid rise in the groundwater, which gradually falls again over a period of days. In this case, the issues are not a given intensityof rainfall and a given water table (combinationof q and h), but the fall in the water table required over a given number of days starting from a given initial level (a fall from k0 to h) over a period of time t, as shown in figure 2-3.

The method of calculatingthe drain spacing under a non-steadycondition that is adopted here is the one presented by Dumm (1960): PL 2 /'n 2 KD=Jandh./h.= 6.2 (1-0.813)-et;= 1.16et/-7 where: - 131 -

Flgure 2-3. Falling Water Table or Transient Flow

Groundsurface

|~~~~~ . .

' 'IDo D

I I I I aX//X,Sm,, mperffiet,belaryerMDDt

L drain spacing in meters

Kt hydraulic conductivity of the soil in meters per day t = time in days p = drainage porosity (other terms used are specific yield and effective porosity). The magnitude of this factor depends on the characteristics of the distance between a given soil layer and the groundwater level, and it is in the nature of an average value expressed in ratios per volume in a customary manner (dimensionless) ho and h, = the water table heights at midpoint between drains at the beginning and the end of a drainage period in meters d = average depth of flow in meters j the reservoir coefficient in days e = the base of the natural logarithm = 2.72 -132- Discussionof Non-Steady State Fornulas

The formulas employedfor a non-steadystate conditioninclude the drainage parameter drainableporosity p that has to be measured. If no measurementsof this parameter are available, then they can be estimated in most cases accordingto van Beers (1969) by employingthe formula p = /K in whichp is expressed in ratios per volume and K is the hydraulic conductivityin cm per day. As an example, if K = 100 cm per day, then p = v=lW 10 percent, and it has to be written in the formula in the customrarymanner as p = 0.10.

A further difference in the non-steadystate formulas is that they do not include the drainage coefficientq employedin steady state formulas. If a given amount of water has to be drained off, say, 20 mm in 10 days, this figure iisconverted into an equivalentfall required at the midpointheight between the drains. Also, the KD value shouldbe known when using non-steady state formulas. The average depth of flow D (the averagethickness of the soil transmittingthe water to the drains) is not constant for the transient case of a falling water table. It varies with the slope and position of the water table. This relation can be best approximatedby D = Do + (ho + h)/4, where D. = the distance betweendrain level and the impermeablelayer. A further approximationhas been prepared by the U.S. Bureau of Reclamationto calculatethe average thickness; D = Do + (hJ/2). By that means, the problem of includingthe unknown h, is avoided. The differencebetween (ho + h0)/4 and (ho/2)increases with decreasingthe h,/h/ ratios. Therefore, the approximationprepared by the Bureau of Reclamationshould be appliedonly if the ratio h/lhois sufficientlyhigh. However, a possible error in the average hydraulichead will have a negligible influenceon D when Do is relativelylarge when comparedwith h,,. Moreover, the value of D. may be replaced by a Hooghoudt's d-value to includethe radial flow nearby the drains.

The relation between hlho and KDtlpL2 as based on the above equationis presented in figure 24. This figure may be used as long as ho is relativelysmall when comparedwith Do, i.e., as long as the drains are high enough above the impermeablelayer. When the drains are placed on the 2 impermeablelayer or when they are very close to it, however, the relation between k/h, and Kh0t/pL can be obtained from the following equationgiven by Dumm (1954) as L = 9khtl2p(holh,-1), presented in figure 2-5.

CalculationExample on the Depth and Spacing betweenDrains for a Peak Irrigation Season

Problem: Assume the followingconditions.

AgroclimatologicalConditions

1. The evapotranspiration(ET) during the peak irrigation season is 225 mm per month or 7.5 mm per day.

2. The rainfall is negligible. Figure 2-4. Relation between ht/h. and KD,/pL2 When Drains Are Placed in Aquifer above Impermeable Layer

h t ho 1.0-_

0.9 0.8 0.7

0.6 0.5 0.4

0.3

0.2 0.1

0 0.001 0.01 0.1 1.0 KDt pL2

Source: Kessler 1970. Figure 2-5. Relation between 1u/h. and Kht/pL 2 When Drains Are Placed in Aquifer on hnpermeable Layer

h t

ho 1.0

0.9 +- 0.8

0.7- -

0.6 - -

0.5--

0.4 --

0.3 - - 411 0.2--

0.1

0.001 0.01 0.1 1.0 10 Khot pL2

Source: Kessler 1970. - 135 - WaterBalance Conditions

3. No foreign water (artesian, seepage, surface runoff) is flowing into the project area.

4. There is no natural drainage.

Soil Conditions

5. The soil has a silty clay texture.

6. The soil moisture contents, on a volume basis, at the field capacity (Mf0) and wilting point (M.,p)are 36 and 16 percent, respectively.

7. The saturation capacity on a volume basis (M.) is 65 percent.

8. The drainable pore space (p) of the zone of fluctuatingwater table is 0.1 (10 percent).

9. The hydraulic conductivity(K) of the upper 4 m of the soil profile is 1 m per day.

10. An impervioussoil layer is found at 4 m below ground surface.

11. The leaching efficiency (fi is estimated at 0.6.

Salinity Conditions

12. The maximumpermissible salt concentrationof the soil saturationextract (C.) = 4 mmhos per cm.

Water Table Conditions

13. The maximumpermissible water table height is 100 cm below ground surface.

Irrigation Conditions

14. Irrigation water is applied by the check flooding (basin)system.

15. There are no surface water losses.

16. The field irrigation efficiency (E,) = 0.7 (70 percent).

17. The salt concentrationof the irrigation water is (C.) = 0.9 numhosper cm, and the water is free from any toxic ion.

18. The depth and frequency of irrigation are governed by the soil moisture characteristics and the consumptiveuse requirementsonly.

19. The permissibledepletion of the availablesoil moisture storage is 50 percent. - 136 -

Required-A designof horizonitalfield drainagesystem by pipesof a wettedperimeter () = 0.4 m. Solution Fromcondition 6, the amountof availablewater in the soil = 36 - 16 = 20 percent,i.e., 200 mmof waterin a root zone of 1 m thickness. Fromcondition 19, the amountof waterto be replacedat each irrigation= 0.5 x 200 = 100mm.

Fromcondition 18, irrigationis practicedevery 100/7.5= 13 days. Fromconditions 1 and 16, the grossamount to be appliedto the field = 100/0.7= 143mm.

Fromcondition 15, the waterlosses by deeppercolation = 143- 100 = 43 mm per 13 days or about 100mm per month. Fromconditions 6, 7, 11, 12, and 17, the leachingrequirements are:

LR= X ET = 54 mm per month

f(| M.x c)- Tlheirrigation water losses through deep percolation resulting from an applicationof an efficiencyof 75 percentare considerablyhigher than the leachingrequirements. Since deep percolation losses are usuallyassumed to be uniformlydistributed under basinirrigation systems, it is not recommendedthat theybe addedto the leachingrequirements. The designof the drainagesystem in this casewill be basedon the percolationlosses only. Fromcondition 8, the deeppercolation losses of about45 mm (43 mm after beingrounded off) will causethe groundwatertable to rise by 45/0.1 = 450 mm = 45 cm = 0.45 m duringthat irrigation interval.

Whenthe maximumpermissible water table of 100cm belowground surface is attainedat the end of the last irrigationin the peak season,it followsthat the watertable just beforeirrigation has to be at least 100 + 45 = 145cm deep. The drainswill have to be placedat a minimumdepth of about 1.5 m and at a practicaldepth of 1.6to 1.8 m. Computationsof the requireddrain spacings for three differentdrain depthscan be summarizedas shownin table2-1.

Calculating Drain Spacing by Steady State and Non-Steady State Formulas

A comparisonbetween the drain spacingscalculated by a non-steadystate and a steadystate formulacan best be illustratedin the followingexample. Thedrainage conditions used in the - 137 - Table 2-1. Computationof Drain Spaungs for Different Drain Depths

Drain depth (m below surface) ho (m) h (mi) /ho KDtIpL2 L (m) (1) (2) (3) (4) (5) (6)

1.50 0.50 0.05 0.10 0.250 32 1.65 0.65 0.20 0.31 0.135 45 1.80 0.80 0.35 0.44 0.100 55

Column 2: ho = drain depth minus maximumpermissible water table 9 .50 - 1.00 = 0.50 m, etc. Column 3: h, = hominus recharge from irrigation = 0.50 - 0.45 = 0.05 m, etc. Column 4: k/ho = 0.05 = 0.1, etc. 0.50 Column 5: When hlho is known, then KDtIpL2 can be obtained from figure 2-4. Column 6: For a drain depth of 1.5 m, the KDtIpL2 value is 0.25 and when applyingthe equationL 2 = KDt/0.25pwhere K = 1 m per day, t = 13 days, andp = 0.1 (conditions8 and 9), then: L = 520D since D = d + (ho+ h.)4 = d + 0.14, L2 = 520 (d + 0.14) = 520d + 73. The d-value can be obtained from figure 2-5 and the spacingL can be obtained by the trial and error method.

example shown in appendix 3 will be used in calculatingthe drain spacing by the steady state formula:

L2 = 8Kdh q

When K = 1 m per day, the drain depth is 1.5 m, h = h0 + ht = 0.28 m, q = 45 mm per 13 2 days = 0.0035 m per day, and V2= 640d.

By the trial and error method using a d-value (obtainedfrom the d-values figures), the required drain spacing is 35 m, and when adoptingdrain depths of 1.65 m and 1.80 m, the resulting drain spacings are 42 m and 50 m, respectively. The obtainabledrain spacingscalculated by a non-steadystate formula and those obtainedby a steady state formula are shown in table 2-2. The difference between the obtainable results is within a 10 percent range, which is slight under field conditions.

When the calculationsare carried out with a steady state formula, a satisfactoryorder of magnitudeof the required drain spacing can usually be obtained.

The calculationsof the required drain spacingswere based on the water losses during the peak irrigation season. Two questionsthen arise: (1) whether the selectionof this season as a basis for the -138-

Table 2-2. Drain SpacingsObtained by Steady and Non-SteadyState Formulas

Drain spacings (m) Drain depth below Using a non-steady Using a steady state ground surface (m) state formula formula

1.50 32 35 1.65 45 42 1.80 55 50

Source: Kessler 1970.

calculationsis sufficientlyaccurate, and (2) whether it is necessary to evacuate all water losses resulting from one irrigation applicationduring the period between this applicationand the next one (13 days in the last example employed). The answers depend on many factors related to soil characteristics,agroclimate, cropping pattern, and irrigationpractices. The deeper the drains are installed, the greater the possibilitiesof storing water temporarilywithin the zone above the drains. When using this zone as a buffer, there might be no need to discharge all the losses during the interval between two successive irrigations. Instead the water table could be allowed to rise gradually in the course of the irrigation season in such a way that it will reach its maximumpermitted height just at the end of the season or at the end of the peak period. During the slack or the nonirrigation period, the water table will fall to a certain low level. The criterion in this case is that the annual discharge is at least equal to the annual recharge. If it is less, then the water table will tend to rise in the course of a few years until an equilibriumis reached at unsatisfactorywater table heights to crops. Thus, the designer has to develop a drainage system that will lead to a dynamic equilibriumat specific maximum water table heights and under specificconditions related to crops, soils, irrigation, etc. The term dynamic equilibrium,as introducedby the U.S. Bureau of Reclamation,refers to a state of equal annual discharge and recharge, while during the peak irrigation season, the recharge will exceed the discharge and thus cause a rising water table. In this regard, Dumm and Winger (1963) studied the fluctuationof the water table for several crops in a five-yearrotation. The drains were placed at a depth of 2.4 m and at a spacingof 480 m. The soil was a sandy loam with a K- value equal to 11.4 m per day and a p-value equal to 0.23. The maximumallowable water height

Table 2-3. Water Table Buildup during Irrigationof SaMower

Irrigation no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Days between irrigations - 14 12 10 10 10 10 10 10 10 10 10 10 10 h' (cm) 31 42 53 64 74 83 92 100 107 114 120 125 129 133 a. Deep percolation losses of each irrigation = 35 mm. Source: Dumm and Winger 1963. - 139 -

was 1.2 m belowthe groundsurface. Theyfound that during the peakseason of onlyone safflower crop, the maximumallowable water table height was slightly exceeded. During the alfalfacrop growthperiod, the water tablereached a levelclose to the limit. Thus, underthe specificconditions of crops,soils, and climateprevailing in the area, a spacingof 480 m at a depthof 2.4 m seemed correct. A calculationof the drain spacingbased on the peakirrigation of safflower,however, gives only428 m whenK- andp-values are as above,h, = 1.20m, the deeppercolation losses = 35 mm per irrigation,and h, = 1.05. Thiscalculation is basedon the dischargeof all losseswithin the irrigationinterval, and their resultsindicate that such considerationwas not needed. Theirresults indicatethat the watertable rises throughoutthe irrigationseason and thus that onlypart of the losses are removedbetween two irrigations,as shownin table 2-3. Appendix3

EstablishingPilot Demonstration(Testing/Research) DrainageFields and VerifyingDrainage Parameters

EstablishingPilot Demonstration (Testing/Research)Fields

Layoutof DrainageTesting/Research lields

The problemsto be investigatedin drainagetesting fields should be carefullydefined before establishingthe fieldsand their networkof observations.Results obtained from thesefields must be applicableto a muchwider area, and thereforesite selectionmust be representativebased on an availablesoil surveyand land classificationmaps. The sizeof the field dependslargely on the nature of the problemand its relatedconditions. Frequently, an area of 5 to 20 ha is consideredsatisfactory. Generally,however, the total acreageshould not exceed50 ha to avoidthe effectof soil heterogeneity and to providean acceptableorganization for the observations. Regardingthe selectionof drain spacingsand depthto be tested, Dielemanand Trafford (1976)recommended that for relativelyclose spacings,the test spacingsshould include some that are at least 100percent wider and 100percent smaller than the theoreticallycalculated spacing. If, for example,the calculatedor otherwiseestimated spacing amounts to 40 m, spacingsof both20 m and 80 m shouldbe included. Normally,three test spacingsare sufficient.Wider calculated spacings to be testedfor short-termdesign information should be limitedto 75 m becausea widerspacing, such as 100 m, makesthe experimentalplots too wideto be irrigatedat the sametime and will not result in a uniformwater table rise. Onceadequate information and understandingof the hydraulicsof the area havebeen developed on plotsof 75 m, extrapolationto actuallyneeded wider spacings is possible,and widerplots will then serveto verifyconclusions and to supportlonger studies in later stages. The water flowto the drain linesshould be as uniformas possibleover the lengthof the drain. Therefore,water applicationshould be uniform,and this goal canbe reachedby designing plotsthat are longin relationto their width. A length:widthratio of 4:1 is considereda minimum, and it is preferable,if possible,for the ratio to be between5: 1 and 10:1. Regardingthe drain depth to be tested, normallyone test depthis sufficient.The selecteddepth is governedby the characteristicsof the hydrologicalsoil profile. Where,however, available machines and soil conditionspermit greaterdepths, two test depthscan be considered. To reducehydrologic interference between adjacent plots as much as possible,the following measuresare suggested: * introducea bufferplot betweentwo test units(figure 3-1), the widthof whichis at least equalto the larger drainspacing of two adjacentunits;

* keep differencesin drain depthbetween adjacent units, if any, to less than 40 cm; * makedifferences in drain spacingsbetween adjacent units correspond to the smalleststep. For example,if test spacingsare 20, 40, and 80 m, then unitsof 20-mand 40-mspacings or of 40-mand 80-mspacings should be adjacent,rather than thoseof 20 m and 80 m.

- 140- - 141 - Often, it is difficult to find or to mobilize a machine to lay pipe drains in the drainagetesting field, especiallyduring the identificationstage. In this case, open field drains or hand trenching can be used instead. Regarding the drain depth to be tested, normally one test depth is sufficient.

The drain lengths are usually not less than 150 m, and the upper limit set for flat areas is between 250 and 300 m. Figure 3-1 (case a) shows that all drains flow individuallyinto the collector ditch and the outflow is measured individually. Another alternativeis combiningthe inner drains into a one-end pipe and measuring the outflow from the one-end pipe (figure 3-1, case b). The drainage unit normally comprisesfive drains. Drains 2, 3, and 4 of unit A; 7, 8, and 9 of unit B; 12, 13, and 14 of unit C; and 17, 18, and 19 of unit D may be expectedto yield reliable observations.

Network of ObservationPoints

Basic observationsin any drainage experimentalfield are those of drain discharge and water table elevations. Natural drainage or seepage supply, rainfall, and any volume of water added to the soil must be known to identify the water balance. Other measurementssuch as soil moisture retention, soil salinity, crop yields, soil structure, and groundwaterquality can also be examined.

GroundwaterTable Levels

The recommendedcommon technique is to install piezometersto the required depth to be investigated,up to 2 to 3 m from the ground surface. Three lines of piezometersof small diameter (about 20 mm) can be installed at 1/4, 1/2, and 3/4 the drain length, and a minimum requirement would be two lines at 1/3 and 2/3 the drain length. Each line consistsof piezometers installed above the drain center, just outside the trench backfill (40 cm from the drain center), at 1/4 and 1/2 the drain spacing, and at the upper and lower ends of the plot to observe border effects. The land level and the piezometertop level should be recorded in relation to an absolute datum. The height of the piezometertop above land level should be recorded, and all piezometersshould be lined up to the same absolute level to reduce conversionerrors. Water table elevationsare often measured by a normal spring steel ruler fixed to an electric device or a mechanicalsounder. The functioningof all piezometers should be tested and confirmedbefore measuring.

DrainflowMeasurement

The simplestway of measuring drain dischargesis by means of a bucket of known volume and a stopwatch. Discharge recorders attached to drain outlets and weirs are also used. The drain discharge is measured in drains 2, 3, and 4 of unit A; in drains 7, 8, and 9 of unit B, etc. (figure 3-1, case a), or in the end drains (figure 3-1, case b). The discharge capacity of the collector drain should be large enoughto keep its water level below the field drain pipes during periods of high discharge.

Frequencyof Observations

The frequency of observationsis governedby local conditions(hydrological, climatic, etc.) and the goal of the experiment. Generally, it is better to have a short period of intense and complete observationsthan a long period of infrequentand scatteredobservations. No strict rules can be provided, therefore, but the number of observationsmust be sufficientfor the processing of steady and non-steadystate conditions. Sometimeswater table elevationsand discharge can be measured once a day or at least twice weekly. When the groundwaterreservoir reacts very rapidly, however, more than one observationper day may be needed during rainfall or irrigation. Dieleman and Figure 3-1. Setup of a Drainage Testing Field

Bufferplots Bufferplots

1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 A B 1A~(LL End drain

Collector Collector

drn AL|k0 tAL AL| drain

11 12 13 14 15 16 17 18 19 20 11 12 13 14 15 16 17 18 19 20 Casea Caseb

Source: Dieleman and Trafford 1976. - 143 - Trafford (1976) have suggestedthe followingfrequencies: (a)for water table heights: one measurementbefore water application,one measurementat the end of water application,three times per day during periods of high water table elevationsfollowing recharge, and twice daily during the remainingdays; and (b)for dischargemeasurements: three times per day during periods of high discharge following recharge, twice daily during the remainingdays between two irrigations or in rainy climates, and once daily in periods of low and more or less constant flows.

TestingProcedures

Under irrigation conditions,water should be applieduniformly to as large a part of the test area as possible. The volume of water to be applied shouldbe large enough to cause the water table to rise to the ground surface at any midpointbetween the drains. After turning off the water supply, the water levels and drain outflows are measured during one recession period, i.e., the period in which the water table at the midpointbetween drains falls uninterruptedlyfrom a point near the ground surface close to the drain depth (usuallyone to two weeks). These data should be processed immediately.

Under nonirrigationconditions, the water table rise is governed by rainfall, and the chance of having a quick rise to the ground surface is small. In this case, it is important to obtain a wide range of water table elevationsand their correspondingoutflow rates from a series of rainfall events in one season. On-the-spotdaily measurementsof rainfall are strongly recommended.

Length of ObservationPeriods

Observationsduring two drainage events in both the rainy and the irrigation seasons are usually adequateto evaluate drain performance. A good drainage event may result from rainfall or irrigation applicationsand should be characterizedby high water table elevationsand high drainage rates at least equal to the levels and rates used in the design. The drainage event should be spread over a drainage or irrigation season. One event is likely to last for one week after heavy rainfall and two weeks after an irrigation application. The stability of the drainage system can be evaluated after comparingthe results obtained from the different events. Records shouldbe studied after each event, and a completeevaluation must be made at the end of the drainage season.

Processing and Analysis of Water Table Elevations and Drain Discharge Data

The principle of evaluatingfield data on water table elevationsand drain discharge is based on the applicationof the formulas describing the flow of groundwaterto drains. Calculationexamples on data processing and analysis are mentionedin appendix2. Instead of introducingdata on soil permeabilityand drainage criterion to calculate the required drain spacing, the actual drain spacing can be introducedwith the objectiveof discoveringthe actual soil hydrologicalcharacteristics from the measured discharge and water table elevations. In drain spacing calculations,it is usually justifiable to simplifythe actual non-steadystate conditionsto steady state conditions. In evaluating field observations, however, the actual conditions,which are usually non-steadystate, have to be taken into account, and thus the analysis shouldbe based on formulas describing the non-steadystate flow to the drains. Both procedures are described and analyzedbelow. - 144-

Steady State Conditions

Measureddischarges and correspondinghydraulic heads observed during periods of approximatelysteady state water table and drain discharge conditionsare plotted in figure 3-2, with the discharge q in mm or m per day on the vertical axis and the hydraulic head h in mm or m per day on the horizontalaxis. The processingof these data under steady state flow in homogeneousand isotropic soils is based on equation 1 below.

q- 8Kdh + 4Kd 2 (1) L2 L

F'igure3-2. q--hLines

q

h L~~~2)

h

Source:_Kessler _1970.

The explanatorynotes on this equation were given earlier. If the drains are placed on an impervious layer, then D. and also d-values are reduced to zero, and equation 1 becomes: - 145 -

q 44Kh2 (2) which then refers to a flow above the drains only. If, however, D. and d-values are large compared with the h-value, the second term on the right-handside of equation 1 will be negligibly small; then: q MM8Kdb (3) L2

Thus, equation3 expresses a flow below the level of the drains. Equation 1 may be written as:

q = Ah + Bh2 (4)

or

h = A + Bh (5) h where:

A- 8 Kd L 2 andE- 4

Equation 4 shows that the q-h relation will approach a straight line when the value of BhEis small, compared with the value of Ah, as shown in figure 3-2, curve 1. This straight line indicates a larger transmissivityof the layers below drain level than that of the layer above drain level. When the flow above the drain level is not negligiblysmall, the q-h line will be curved and its shape will depend on the relative contributionmade by each of the two parts of the right-hand term in equation 1. The stronger the curvature, as shown in figure 3-2, curve 2, the larger the contributionmade by the layer above the drains. The interpretationof curved q-h lines is often difficult; therefore, the q-h values may be plotted against the h-values, as shown in figure 3-3. This relationshipcan now be presented by a straight line forming an angle with the horizontal axis.

By transposingequation 1, we obtain:

tan a = 4K (6) L2

If the value of 4K is relatively small, the qlh line will be horizontal.

If the soil profile consistsof two layers of distinct hydraulicconductivity values, equation 1 is still applicable. If the boundary of the two layers is at the level of the drains, it is then written as: where K, and K2 are the hydraulic conductivitiesof the soil above and below the drain level, respectively. If the boundary of the two layers is located below the drain level, equation 1 is not - 146-

8EKdh 4Kh 2 L2 1) (7 )

applicableand the processing of data shouldbe based on different expressions, such as those suggestedby van Beers (1969).

Figure 3-3. qlh - h Relation

q/h days- 1 x10-3

6.0-_

q =-8Kd + 4Kh h L 2 L 2

4.0 - - Tan °f = -2

_q = Kd 2.0 h L 2

0 0.2 0.4 0.6 0.8 1.0 h(m)

Source: Kessler 1970.

InterpretrationExamples

Example 1. (Source:Dieleman and l'rafford 1976.) Assume that a drainagetesting/research field is provided with subsurfacepipe drains with a radius r = 0.05 m that are placed at 2.0 m depth at a spacing of 100 m. The discharge rates and water table depths have been measured frequentlyduring periods of litfle change in water table positions, and the observationshave been plotted in figure 3-4 (A and B). The correspondingvalues derived are given in table 3-1.

Figures 3-2 and 3-3 show a plot of q versus h and of qlh versus h. The q-h relation is a slightly curved line which indicates that the greater part of the excess water will flow to the drains through the soil layers below drain level. The qlh-h relation is a straight line whose tangent is read as tan oa= 0.4 x 1I3. This is the value of B in equation4, and by applying equation 4 or equation6 with L = 100 m, the hydraulic conductivitycan be found as: - 147 -

L2 tan a = 1002 x 0.4 x 103 = 1 mperday. 4 4

The value of A = 8Kd/L2 can be read from the intersectionpoint on the vertical axis as 1.6 x lof3. With L = 100 m and K = 1 m per day, it follows that d = 2 m.

With known values of the drain spacing L Figure 3-4. Discharge versus Time (A) and Hydraulic Head 100 m, drain radius r = versus Time (B) 0.05 m, and the equivalent q thickness d = 2 m, the (r/day) actual thickness of the imperviouslayer D. can be found by using the nomographshown in figure 0 A 3-5. Working backward on the nomograph,and using as wetted perimeter U = irr = 3.14 x 0.05 = 0.16 m, the problem of finding the DJu 0 value can be solved on the Days left-handvertical axis, 0 2 4 6 8 which is about six times the D. value on the right-hand h vertical axis. This result appears to be true for D. = 2.3 m. In practice, the value of the wetted B perimeter is often taken as larger than what would follow from the drain radius. Particularly when highly pervious envelope Days material is used, the wetted 0 2 4 6 8 perimeter is calculated as the sum of the trench bottom width and twice the pipe diameter. In this example, if the trench width is 0.25 m, the wetted perimeter amountsto 0.45 m and the D. value is then found at about 2.2 m below the drains.

The relation between the discharge rate and the water table fluctuation is not always clear. For example, when the trench backfill is highly permeable, an interflow may cause high discharge rates. Water flows horizontallyto the trench backfill through the pervious topsoil, which overlays a less permeable subsoil. There it seeps downwarduntil it reaches the drain tubes. This continuesuntil the permeabilityof the backfill decreases in the course of time. In cracked heavy clay soils, the drain tubes often start to discharge long before the soil is saturated. Cracks may run full, and the piezometers may indicate a sharply rising water table that cannot be understoodfrom the volume of rain. The reason is that though cracks are easily filled with water, it takes time for water to infiltrate in the soil body between cracks. This type of infiltrationtakes place during rainfall and continues - 148 - Table 3-1. Discharge Rates and CorresponldingHydraulic Heads (Based on Figure 3-4) q h qlh (m/day x 10V) (m) (days-' x 103) 4.23 1.8 2.35 3.60 1.6 2.25 3.00 1.4 2.14 2.52 1.2 2.10 2.00 1.0 2.00 1.53 0.8 1.91 1.10 0.6 1.83 0.70 0.4 1.75 0.33 0.2 1.65 Source: Dielemanand Trafford 1976. after the cessation of rain. After that, the water table drops rapidly due to the continuingwetting of the soil. Water tables may also react:sharply when entrappedair is involved. In these cases, observationsshould be examined carefullybefore they are processedusing the equations,which may fail.

Non-SteadyState Conditions

Under irrigation, water table movementsmay become fast enoughto make the applicationof non-steadystate formulas preferable. They have the added advantagethat calculationscan be based on either water table heights or drain outflow rates. Therefore, if data on one parameter appear unreliable, data on the other remain availablefor processing. However, it is strongly recommended that both parameters be processed, if possible.

Irrigation recharges the groundwaterreservoir, and the water table rises as a result. When irrigation is completed,the recharge stops and the water table begins to fall. The relationbetween hydraulic head and discharge during a rising water table is complicatedand differs from that during a dropping water table, as shown in figure 3-6.

At a certain time tAafter cessationof the recharge, this relation will become approximately constant (tail recession), as shown in figure 3-7; the following expressionsbecome applicableto subsequentstages of the falling water table:

at =2.3 log ho (8) ht where:

t = lengt]hof observationperiod, during which the water table drops from positiionh. to k (days);

h. and ki = available hydraulic head at beginning (t = 0) and end (t = t) of any - 149 -

Figure3.5. Nomographfor the Cikcuaton of Hooghoudt'sd-Valu Accordingto d- Do

SDo In Do 1 iL u

DO,d Touse: 1.0 12 1.4 1.6 1.8 1.SeeCt appropriate values for Do / / 1 l / / / / 2.0 D. /u andDo. u 2.1 100 -2 2 D,d 2. Connct selectedDo lu on the -2I / -2* bft-handscale with Do on the so 80 2.3 rfght-handcale. 2A 60 7/ 25 50 2.5 3.Findi pointP wherea linedrawn / 2.6 fromDo lu to Do,dinterects L 40

/2.8 30 / 4. Readvalue of P onthe nght-hand 1/)3,/ 3.0 DO,d-scaleas HooghoudVad-value. 3.2 20 3A

3.6 15 3.8Exnpe

/ 4.0 f Do u .15 andD o 10 m,then 10 withL -40 m,read d 3.7.

1 T L 40 Do-d

I 5 3.6 N ~~38 dm3.7

11022030 5080t501

2 b-

1.5 ~~~~~~~~~30Note' if Do-c2, use ERNST or calcuiat d withthe aboveformula. ffDo ',1/2L,

1 ___40860 0020 useDoau1/2L 10 20 30 50 80150 L Source.Kessler 1970. - 150-

a = r2 KdL +pL2

the drainage intensityfactor (days -1);

p = drainable pore space, also termed effective porosity or specificyield, in the zone of fluctuatingwater tables. It representsthe volume of water that is drained from (or taken up by) a unit volume of soil when the water table drops (or rises) over unit distance. Some values of p (on a volume basis) can be given for orientation as 1 to 3 percent for heavy clays, 6 to 9 percent for medium-texturedsoils, and 10 to 15 percent for sandy soils. d, K, and L = as described previously.

Similarly: at = 2.3 log qJ/q, (9) where:

q. and q, = drainage rates at the beginning (t = 0) and at the end (t = t) of the selectedobservation period.

There are two additionalusefal equationsthat follow from equations8 and 9 by expressingq in h:

q= 2aPh (10) 7';

q=:.2nKdh (11) L2

Note:

1. The point of time in a water table recession below which these equations are applicable, i.e., where the relationbetween discharge rate and head is constant, is given as tA = 0.4 = 2 days. For example, if a = 0.2 days -1, which is common in irrigated areas, then the usable period of observationsbegins at tA = 0 . 4 0.2 = 2 days after completionof irrigation or after cessationof rain. Since the value of a is one of the test objectivesthat is unknown at the beginningof the test, intensive measurementsshould start immediatelyafter the end of water application.

2. The equationsfor non-steadystate flow require a constant thicknessof the aquifer through which water flows toward the drains. This requirementimplies that an imperviouslayer, if any, should be at a considerabledepth below the drain or, more accurately,that the transmissivity(product of hydraulic conductivityand thickness)of the part of the aquifer below the drains should greatly exceedthat of the aquifer above the drains. - 151 - Example of Procedure and Analysis under Non-SteadyState Conditions. The following procedureand analysishave been demonstratedby Dielemanand Trafford(1976).

A drainagetest/research field has been drainedby pipesat a spacingof 30 m. The pipeshave a radiusr = 0.05 m and havebeen placed at a depthof 1.8 m, as shownin figure3-8. The soil investigationsshow a thicklayer with a plasticconsistency whose upper boundary is at a depth of 4.8 m belowground surface. Fromhydraulic conductivity measurements and additionalobservations on the seasonalfluctuations of the watertable, it is concludedthat the transmissivityof this layer is very smallcompared with that of the overlyingsoil, and the layer,therefore, may be consideredan imperviousfloor. The volumeof water appliedin an irrigationis Flgure 3-6: Shape of Water Table during Recharge (1) and 140mm, of which40 mm during Most of Recession(2) percolatesbelow the root zone, accordingto design Groundsurface assumptions.It is assumed TN\ //VI"X\ '1 I the entire40 mm recharges l the phreaticaquifer on that 1 sameday. Duringthe day of rechargeand the 2 followingdays, the water table depth and the dischargerate are measured severaltimes a day. Amongother things, the experimentwill serveto Source: Dielemanand Trafford1976. identifythe drainage intensityfactor a and to collectbasic informationon such individualdrainage parameters as hydraulic conductivityK, transmissivityAD, and effectiveporosity p.

Calculationof the Drainage Intensiy Factor

To arrangethe field observationsand calculatethe drainageintensity factors, the following stepshave to be taken: 1. convertthe observeddischarge rates into millimetersor metersper day and draw these valuesversus time; drawvisually a 'best fit' linethrough the points,as shownin figure 3-9; 2. convertthe observedwater table height into hydraulichead values (mm or m), plot these valuesversus time, and draw visuallya best fit linethrough the points,as shownin figure 3-9.

3. fromfigure 3-9, readthe correspondingvalues at the end of the days and composetable 3-2. - 152 -

Figure 3-7: Water Table Rise and Recession (Equations 5 and 6 and Applicable to the Recession Stage on Right t)

Watertable height

(in)~~ ~ ~ ~ ~ ~ ~~~~~~~~~Dy

Periodof A recharger I mcharge~I

,' '_ ,;'i IDD DD |Figure 3-8. Drainage Conditions of Worked E xample on Non-Steady State Flowl

G3rund wxac .xxx 009~, 7M,~ ~ ' Cx xxx )00( K

1.8 m t + ~~~~~~~~~~~~~~Water table s~~~~~~~~s h zt posdchbeor .;------_------V --- ~---

Do=*3.0Om . L=*30 m - 153 - 4. plot qt and/or h, values from this table versus time on semilogpaper, and obtain the lines of figure 3-10. Note that accordingto equations8 and 9, which apply to tail recession, these lines shouldbe straight and parallel to one another.

5. Calculatethe drainage intensityfactors. A practical calculationmethod is to use equations 8 and 9, which may also be written as:

2 .3 ((log ho - log h.) t (12) anda 2.3(log q0 - log qt) (13) t

In both cases, the result is a = 2.3 tan a (where tan a is obtained from figure 3-10).

Observe that ht, qt and h., q. are points on the straight part of the lines. They can be selected freely, after taking into accountthat h.,q0 presents an earlier date than ht, q,. To obtain tan a, it is practical to select one full logarithmiccycle on the r or the q axis, i.e., from 700 to 70 Oog 700 - log 70 = log 700/70 = log 10 = 1).

The position and direction of the straight parts of the two lines shown in figure 3-10 are clear from the points obtainedbetween the third and eighth day, and the value of tAcan be easily calculated from the curve as: tan a = 1 t 2 - tl.

(from figure 3-10)

1 =- .105 9.5

Table 3-2. Recharge r, Discharge q, and Corresponding Hydraulic Heads h (Based on Figure 3-9) t 1 2 3 4 5 6 7 8 days r 40 ------mm qt 14.4 5.9 4.4 3.4 2.6 2.0 1.6 1.2 mm/day h, 495 430 340 265 205 160 125 100 mm

Source: Dieleman and Trafford 1976. - 154 -

Figure 3-9. Water Table Position and Discharge Rates Observed and Converted into HydraulicHeads (mm) and Discharge Rates (mm/day)

Hydraulic Discharge head rate (mm) (mm/day) 800- 16

_S X~'

A~~~l

eoo -7 1 12-7

0 2 4 6 8 10 Days

Source: Dieleman and Traffordt1976.

Accordingly, a = 2.3 x 0. 105 = 0.242 days. Figure 3- 10 indicatesthat the lines become straight at the time

tA = 0a4=- 0.4 =1.7 days after the recharge. The tAvalue in figure 3-9 refers to a point in time after the cessation of recharge when the line will be straight from t = 2.7 or t = 3 days after the start of irrigation (allowingone day for irrigation). It often happens, however, that the observationsappear somewhat scattered in the lower region of the lines where dischargerates are low and move slowly. The inaccuracy of the observationsmay then have a considerableimpact. The uncertainty about the beginning and end of the straight part causes the need for frequent and accurate observationsduring the period between about the second and the sixth day after water application. - 155 -

Figure 3-10. Plots of Dischargeand Head versusTime (Table3-2)

Hydraulic Discharge head rate (mm) (mm/day)

1000io ' 10

500- - 5 400- _ . ;* _ - 4 300 Discharge 3

2n _ _~~~~~~~~~~~~~~~~ 50 1 30 --- -oa - -. - 40 30 Hed0.3 20

10- j I I I i -0.1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Timein days

Calculationof hydraulicConductivity and Transmissivity The hydraulicconductivity K canbe calculatedby plottingqt versush, valuesfrom Table3-2 and Figure3-11.

As mentionedearlier, the qlh relationshipyields a straightline whenmost of the waterflows to the drainsthrough the soil belowthe drain. The variationin the watertable position then has only a minoreffect on the actualthickness of the phreaticaquifer D, and the non-steadyequations are applicable.By applyingequation 9, which,after transposing, reads:

Kd_, L2 h 2si the hydraulicconductivity and the transmissivityvalues can be calculated.For example,with L 2 = 900, the Kd value = 1.8 m2 per day. The hydraulicconductivity value K can be obtainedafter findingHooghoudt's d-value from the d-valuesgraph. For u = 0.3 m, L =30 m, and D. = 3 m, the - 156 -

Figure 3-11. Discharge Rate versus Hydraulic Head

t=1 whenq= 14.2 (Data are from the calculation example) 10- A

8

8- _

t=2 0 E 6 E

-. - .4 3 044

5

2 7

t =8

O-/ I . ~ ~ ~l ~ ~ ~ ~~~~I 0 100 200 300 400 500 Hydraulic head (mm)

d-value = 1.97 m, and consequentlyK = 0.9 m per day. The transmissivityKd = 0.9 x 3 = 2.7 m2 per day.

Calculationof the Effective Porosity

The effective porosityp may be calculatedfrom the expression:

72 KCd pL 2

If a, L, and Kd values are known, or by substitutingfrom equation 8 the values for a and Kd found before and L = 30 m in the first expression,then p = 0.08. The same value can be obtained from the second expressionby substituting:

q =°. 0127 h - 157 - and a = 0.242.

The effective porosity may also be found from the volume W of water released by the soil when the water table drops from position h. to position h, during a known interval within the tail recession period accordingto the expressionW = 0.7p (ho - h), where W is calculatedfrom measured discharged rates.

The effectiveporosity can also be calculatedfrom the water table rise as induced by recharge. Table 3-2, for example, shows a rise of 495 mm as a result of a recharge of 40 mm and p = 40/495 = 0.08. This method of calculationcan be used under certain conditions:(1) the soil shouldbe at field capacity prior to the recharge, (2) the occurrenceof entrappedair that may cause the water table to react differentlyfrom one part of the plot to another should be avoided, (3) the recharge rate should be slow, and (4) there should be no water pondingon the surface. Therefore, the calculation should probably be limited to humid areas with steady rains in periods of low evaporationand to irrigated areas where water is appliedby sprinkler.

Conclusion

Field drainage systemscan be rationalizedonly when based on a thorough understandingof the project's hydraulic characteristics. In the predesign stage of drainagesystems, the number of certain survey data collected from piezometers, auger holes, and soil profiles shouldbe kept low for practical reasons, and the sample sizes shouldbe relativelylimited when comparedagainst the project size. The use of these data in design, therefore, is still always questionable. When obtainedfrom test/research fields, however, these data are derived from samples the size of the test plot; therefore, they provide signific.nt support in interpretingthe survey results and constitutea strong foundation for the selection of designparameters such as aquifer transmissivity,the position of impervious barriers, and effective porosity. Moreover, the test/research fields will show whether one of the experimentaldrainage intensitiessuch as drain spacing and depth satisfies the agronomiccriteria and how this informationcontributes to the design of adequatedrainage systems. Test fields show which combinationof drain spacing and depths is right or wrong, and they demonstratethe reasons for these results. By understandingthe hydrologicalcharacteristics of the project area, the design being tested can be adjusted to satisfy the other parts of the project area that might have slightlydifferent conditions. Differences in soil hydrologicalcharacteristics, cropping patterns, and irrigation methods lead to differences in the recharge to the groundwaterreservoir. These recharge differencesare possible to predict, as is the behavior of the water table in other seasons when the crop water requirementsand rainfall pattern differ.

As an illustrationderived from the previouslyshown calculationexample, the water table recedes from h. = 43 cm on the day following irrigationto h, = 16 cm on the sixth day, i.e., a drop of 27 cm in four days. Supposethat agronomicrequirements were found to allow such a fall to take place in six days. What would this mean in terms of drainage intensityor drain spacing? To answer this question, these values of h. and h1 can be substitutedin equation 8, obtaining:

at = 2.3 log 43/16

With t = 6 days, we find the drainageintensity factor a = 0.17, which is considerablybelow the previouslyfound value of 0.242. - 158 - When we introducea K-value of 0.9 per day and an a-value of 0.08, which are not influenced by the relaxed time requirement, in the expression:

a72 Kd pL 2

we obtain L2 = 710d. By applying successiveapproximations with D. = 3 m and u = 0.45 m, we find L is about 40 m. By means of the tested spacing of 30 m and the correctingtesting procedure, we can concludethat a spacing of 40 m would be an adequatelyeconomic spacing. Appendix 4

Estimating Design Discharge from Peak Surface Runoff

Example 1: Discharge Rates in Paddy Areas

For the design of surface drainage systems,the expectedpeak surface runoff has to be determined. Different empirical methodshave been developedto computerunoff from watershedsin sloping areas, and any suitable one of them can be used. These methods may assist in estimating peak flows for small agriculturalareas on flatter land, but any method has to be adaptedto local conditionsof rainfall, topography, and land use. The following examples,used in designing surface drainage in Japan, have been given by Fukuda and Tsutsui (1973) to estimate the runoff in low-lying paddy field areas. Design rainfall is determinednot only from a technicalviewpoint but also for economicfeasibility. Usually, heavy rainfall occurring at a frequency of 1 per 10 years to 1 per 30 years is taken into account. Large-scaleflood control projects usually take into account a frequency of 1 per 50 years to 1 per 100 years. Rainfall is taken into account daily or sometimeshourly where the area is subjectedto temporary submergencefor short periods, but continuousrainfall is considered prolonged flooding. In consideringdrainage schemes, there are two types of runoff coefficients:the ratio of rainfall to correspondingrunoff and the ratio of maximum rainfall to maximumflood.

The former type of coefficientis generallyused for relatively large low-lyingareas. Table 4- 1 shows the runoff coefficientsconsidered for rivers in Japan (table 4-2 shows the runoff coefficient of paddy fields). The latter type of runoff coefficientis used to estimate a peak flow Q caused by rainfall concentratedin a short period of time accordingto the following equation:

Q = 10 FR/360T where:

Q = the peak flow in m3/s

A = the catchmentarea in ha

F = the runoff coefficient

R. = the maximumrainfall during n hours

T = the length of drainagetime (t) in hours

In estimatingthe runoff in a low-lyingpaddy field, Tables 4-3 and 4-4 are used. Table 4-5 shows an example of a calculationusing these tables, assumingthat the rainfall during five days amountedto 300 mm.

- 159- -160 -

Table 4-1. Runoff Coefficientfor Rivers In Japan Topography of rivers Runoff coefficient(%) Steep mountains 75 - 90 Forest 50 - 75 Flat upland 45 - 60 Paddy field under irrigation 70 - 80 Small rivers in plain 45 - 70 Large rivers in plain 50 - 75

Source: Fukuda and Tsutsui 1973.

Table 4-2. Runoff Coefficient of Paddy Flelds Area of paddy field n t F About 50 ha of terraced land 4 4 0.4 - 0.7 Smaller than 100 ha 24 24 0.5 - 0.8 Smaller than 500 ha 24 24 0.4 - 0.7 Smaller than 1,000 ha 24 28 0.6 - 0.8

Source: Fukuda and Tsutsui 1973.

Table 4-3. Total Rainfall and Runoff Coefficients

Total rainfall (mm) 0-10 10-30 30-50 50-100 100-200 200-300 300

Runoff coefficient(%) 0 10 30 50 80 90 95

Source: Fukuda and Tsutsui 1973. - 161 -

Table 4-4. Single Rainfall and Runoff Pattern

Amount of single rainfall 1st day 2nd day 3rd day 4th day Total

Under 30 mm 100 - - - 100

30 - 50 mm 70 30 - - 100

50 - 100mm 60 30 10 - 100

100 mm and over 50 30 15 5 100

Source: Fukuda and Tsutsui 1973.

When time and funds are available,experimental fields shouldprovide the answers to the questionsconcerning the amountof excess surface water to be expected. In the meantime, the maximumlength of rows and plane fields can be establishedas related to the slope of the land from the point of view of erosion hazards. On such fields, the amount of runoff and peak flows are measured for different slopes and row lengths during different times of the year. Also, additional informationon climatologicaldata, soil moisture changes, and the influenceof different land surface practices (land forming) have to be collected. Most difficult to assess is the crop response to drainage and the damage caused by flooding expressedin changes in yield. This assessmenthas to provide the tools for the economicjustification of the designed drainage system.

Discharge rates can also be estimated in paddy areas by summing dischargesfrom paddies along the ditch or by using the 'rational formula": Q = 2.778 CU where:

Q = discharge capacity (m3/s)

C = the runoff coefficient,defined as the ratio of the peak runoff rate to the rainfall intensity (dimensionless). It dependson the infiltrationrate, surface cover, channel and surface storage, and intensityof rainfall (Schwab et al. 1966)

I = mean rainfall intensity(cm/h). The rainfall must be for a duration equal to the time of concentration;thus, I = RCT' = time of concentrationin hours (time necessary for runoff from the most discharge area portion to reach the discharge point)

A = the drainage area (km)

The above procedure is described in detail by Sokolovet al. (1976) and by Gray (1973), who reported on some of the shortcomingsthat were observed in paddy rice areas. For rice irrigation, the most importantdisadvantage of the procedure is that no allowancehas been made for water retardation due to storage. Paddy rice areas are characterizedby flat topography covered with many small reservoirs, and consequentlystorage can prevent a significantportion of storm rainfall from Table 4-5. Daily Runoff Caused by Continuous Rainfall

(1) (2) (3) (4)* (6r*

Day Rainfall Accumalated Rumnff Runoff caused by single (mm) rinfll (mu) cocffici t (daily) rainfall (mm) Daily runoff (mm)

Ist 2nd 3rd 4th Sth 6th

lt 40 40 30 12.0 8A 3.6 - - -

2nd IS0 190 80 120.0 - 60.0 36.0 18.0 6.0 -

3rd 1S 20S 90 13.5 - - 13.5 - - -

4th 70 275 90 63.0 - = - 37.8 18.9 6.3

5th 25 300 95 23.8 - - - - 23.8 -

Total 300 232.3 8.4 63.6 49.5 55.8 48.7 6.3 (4)* Obtainedfrom table 4-3. (5)** (2) x (4). (6)***= (5) x the rail pattn shown in table 4-4. Source: Fukuda and Tsutsui 1973. -163- reachingthe drainageditch. If great care is not exercisedin the use of the rationalmethod, the drainagesystem could be greatlyoverdesigned and can becomemore expensivethan necessary.

Example 2: Discharge Rates in Humid Flat Lands

Stephensand Mills (1965)described a formuladeveloped for use in the humidflat landsof the UnitedStates, as follows:

Q= CA 6 where:

Q = dischargecapacity (m 31s)

C = coefficientrelated to the drainagearea and magnitudeof storms = 0.21 + 0.0744 R

R. = 24-hour rainfall excess (cm). For storm periods longer than a day, the total rainfallexcess is dividedby the lengthof the stormperiod in days. This averageR. is then usedto calculateC

M = drainagearea in km2 Appendix5

Cost Estimatesand Actual Costs for Some DrainageProjects

Drainage costs, and accordingly the costs of items involved, vary widely, depending on the scope of the project and the intensity of drainage. Four examples of cost estimates and actual costs, expressed on a hectare basis, are given in this appendix.

Example 1: Lateral Pipe Costs

Smedema (in a 1987 unpublished memorandum on lateral pipe costs) analyzed the costs of a plastic corrugated pipe drain, including the costs of materials such as drain pipe plus envelope and the costs of installing these materials. Other miscellaneous costs (outlet/crossing/connection structure, engineering and supervision, overheaid, profit, taxes, etc.), which may comprise 20 to 30 percent of the total costs, were not included. Furthermore, a distinction was made in this study between drains that were shallow (1.0 m) and narrow-spaced (10 to 20 m; type W") and those that were deep (about 2.0 m) and wide-spaced (50 to 150 mr;type 'B"). The conclusions are as follows.

Pipe

Both the PE (polyethylene) resin and the PVC (polyvinyl chloride) resin are commonly used as raw materials. The world market price of PE or PVC resin has been almost stable during the last 10 years, presently ranging between IUS$700to $1,000 per ton. The costs of the most commonly used size of corrugated plastic drain pipe in different countries are shown in table 5-1.

Table 5-1. Ex-Factory Costs of Corrugated Plastic Drain Pipe in Different Countries Delivery cost (US$) per meter for pipe with given diameter Country Year 60 mm 100 mm The Netherlands 1987 0.50 1.40 Spain 1984 0.40 Egypt 1984 0.35 Iraq 1982 2.00 Pakistan 1981 2.30 India 1987 2.10 Peru 1982 1.80 USA/Canada 1985-87 0.90 Source: L. Smedema 1987 (unpublished memorandum on lateral pipe costs).

- 164- - 165 - Table 5-2: Cost Indications for Envelopes around Pipe Drains, 1985-1986

Type of envelope Qualityof Priceof raw materil Coa of envelopematerial needed for a 4 tpe requiredmaterial (USS) A drain pipe (US$)

Raw material Wrappedand/or installed

Cocos 1000-1500lg/m 0.3-0.5/kg 0.1.0.3 0.5-0.7 Syntheticfiber 300-900g/m1 1.0-2.0/kg 0.1-0.6 0.8-1.2 Gravel 0.02-0.4 ri/m 10-20/rn 0.2-0.8 0.6-1.0

Note: Cocos and syntheticfiber costs are the market prices paid by the wrapping factory. Gravel costs are the market prices of graded material, includingtransport to the project site. Source: L. Smedema 1987 (unpublishedmemorandum on lateral pipe costs).

The costs in North America and Western Europe are often lower than in the developing countries, where drain pipe manufacturingand installationare at an early stage. Costs may fall in the developingcountries as experience is gained and as the market expands.

Envelope

No dominantstandard envelopematerial has yet emerged. The study consideredonly the three types widely appliedin Europe and North America: cocos fiber, syntheticfiber, and gravel. Cocos fiber and gravel are widely used in Europe for types A and B drains, respectively,while syntheticfibers are still under development. Some cost indicationsare shown in table 5-2.

Installation

Smedemastated that the costs of installationby machine, which includes the cost of backfill, are mainly governed by the depth of installation. Deeper installationreduces the working speed and/or requires more powerful, costly machines. The trench width is also a cost-determiningfactor, although it is largely coupled with the installationdepth, as shown in table 5-3.

Table 5-3. Cost Indications for Lateral Pipe Drain Installed by Machine in Trenches of Different Depths (1985-1986)

Indicators Installationdepth (m)

1.0 1.5 2.0 2.5 3.0 3.3

Trenchwidth (cm) 20 20-30 30-35 30-35 35-40 35-40 Type of trencher (KW) 75 100-150 100-150 200-225 200-225 250 Working peed(mfhr) 400 250 150 150 100 100 Relativelaying codt includingbackfill (USS) 100 200-300 400-S00 800 1200 2000

Source: L. Smedema 1987 (unpublishedmemorandum on lateral pipe costs). - 166- The installationcosts in Western Europe and North Americahave tended to decrease over the last five years. The installationcosts for Type A drains under well-managedoperation amountedto US$0.30 to $0.40 per meter in the Netherlandsand US$0.70 to $0.80 per meter in Canada in 1986 and 1987. Costs may easily increase by 100 to 200 percent under poor organization and logistics, with harder or sticky soil, or with the existenceof many surface obstructions. Recently, trenchless installationhas become a feasible alternativemethod of installationwhen performed to a depth of about 1.5 to 1.8 m, whereby the costs are somewhatlower than those of conventionaltrench installationmethod.

Total Costs of a Lateral Pipe Drain

The total costs of a lateral pipe drain in the Netherlandsbetween 1980 and 1985 were as follows:

Installation depth = 1.0 to 1.3 m Drain spacing = 10 to 30 m Installationmethod = Machine (trencher) Drain pipe type = Corrugated plastic Drain diameter IC = 54 mm Envelope material = Cocos fiber, prewrapped Costs in guilders per m Pipe = 2.20 Envelope = 0.50 Installation = 0.75 Total = 3.45

The drainage costs in the Netherlandsare close to the lowest possible economiccosts. Costs in the United States are naturally higher than those in the Netherlandsdue to the use of gravel filter instead of cocos fiber, the use of a larger-sizepipe of 100 mm instead of 60 mm, and the excavation of wider trenches in the United States than those in the Netherlands. In the United States, the breakdownin pipe, envelope, and installationcosts are on the order of 1/3:1/3:1/3. Installation costs in the Netherlandsalso constitute about 1/3 of the total costs when cocos fiber envelope is used. The other 2/3 goes toward the materials, of which 1/3 and 2/3 go for the envelopeand the pipe, respectively. These breakdownratios change drastically when a syntheticfiber is used (50 percent goes to the envelope, and the rest is equally divided between the pipe and the installationcosts). Total drainage costs in the developingcountries are usually much higher than those in Western Europe. Manual installationin some developingcountrles is more expensivethan machine installation in Western Europe. For example, the contractorrate was US$1.1 per meter length of Type A drain installed in India in 1986 and 1987, while the machine installationin the Netherlands amountedto about US$0.35 per meter during the same period.

Drainage costs are expectedto be steady for some years to come. The situationdiffers in developingcountries, which only recently began pipe drainageprojects. The current high costs there may be expected to fall as the drainage business/industrygains experience and organizes and as the drainage market expands and steadies. Some technologicalimprovements, such as low envelope costs, could also contributeto this future trend. - 167 - Example 2: Project in Pakistan

The exampleproject in Pakistanis the EastKhairpur Tile DrainageProject (EKTDP). This WorldBank-financed project covers a totalarea of about 18,000ha in SindProvince. The main constraintsfor agriculturethere werewaterlogging and salinity. The projectobjectives were to: * installa pipe drainagesystem; * improveirrigation;

* improveroad networks;

* providecredit facilities for the agriculturalpopulation; and * improvethe agriculturalextension services.

Onlythe drainagecomponent will be consideredhere. The generalfeature of the drainage componentis that field pipe drainsflow out intopipe collectors,from whichthe water is evacuated throughsmall electric pumping stations into shallow open disposal drains. The followingdata representthe plansand cost estimatesas of June 1981. In a later stage,the planswere modified, especiallyregarding the specificationand Installationof the collectors. The area coveredby drainageIn this projectamounted to 14,000ha, and the general specificationsof the drainageItems and the quantitiesused are outlinedbelow. FieldDrains (aterals) - corrugatedPVC pipes, diameter100 and 91 mm (outsideand inside,respectively), manufacturedelsewhere In Pakistan;

* spacing:range, 50 to 175 m; average,115 m (totaling1,415 km for the projectarea, correspondingwith an averageof 105m/ha);

* depth:average 1.8 m;

* averagelength of singlelines: 665 m; * all field drainpipes surrounded by gradedgravel, amount0.1 m3/m;and * installationby trenchexcavating drainage machine; * graveltaken from a nearbyquarry (about 5 km outsideproject boundary), processed in a speciallyerected screening plant, and transportedto siteby trucks (to roadside)and gravel hoppers(feeding the drainagemachine). Transport distance for gravel:maximum 30 kIn; for 80 percentof projectarea within20 km. - 168 - Pipe Collectors

* concretepipes, manufacturedlocally (within the project area);

* diameter range: 9" to 18" (230 to 460 mm);

* depth: maximum 3 m (at outlet);

* total length for project: 1517km, correspondingwith 14.6 m/ha and

* installationaided by a hydlraulicexcavator after previous dewatering (sloughingsoil conditions). Dewateringthrough a horizontalPVC pipe below the future trench bottom (total depth about 4 m) installed by a special trencher and connectedwith a pump. Vertical well pointingalso used in places.

Disposal System

* pipe collectorsdischarge into open drains through small pumping stations, each serving one or two (opposite)collector lines;

• open drains have comparativelyshallow excavation: average 15 m3/m; and

• total length of open drains: 44 kIm,corresponding with 3.3 m/ha of project.

These costs were calculatedin June 1981 when project executionbegan. The project was completedin 1986. The local currency is the rupee (Rp), and the exchangerate in June 1981 was 1 Rp to US$0.091 (US$1to Rp 11). The costs in this example are given in Rp and US$.

Laterals (FieldDrains)

Cost per meter of field drain:

Pipe material (ex factory) 24.40 2.25 Transport of pipes to site 0.50 0.04 Gravel filter and transport to site 21.00 1.95 Installation of pipe and gravel 2.50 0.95 Land clearing and trench backflilling 1.60 0.15 Repair of crossingswith roads, canals, etc. 2.75 0.25 Transport of equipment 1.65 0.15

Total cost per m of field drain 54.40 5.74 Total cost of field drains per hectare 5,712.00 602.70 - 169 -

Pipe Collectors

(a) Costs of dewateringper m of collector length:

Pipes with nylon envelopes 38.00 3.45 Transport of pipes to sites 0.50 0.04 Installation(special trencher) 10.50 1.00 Pumping cost 94.30 8.60 Land clearing and leveling 2.10 0.20

Subtotalper m 145.40 13.30

(b) Costs of collectorsper m

Material (concretepipes) 63.45 5.75 Transport 17.30 1.60 Excavation, installation,backfilling 78.30 7.05 Land clearing and leveling 50 0.50

Subtotalper m 164.05 14.90

Grand total per m of collector 309.45 28.20

Grand total per hectare of project area 4.517.97 411.72

Connectionsbetween laterals and collectors(manholes)

Cost per unit 1,800.00 163.50 For project (1718 units) 3,092,400.00 280,893.00 Per hectare of project area 220.00 20.00

Pwnping stations with swnps (excludingpumps)

Cost per unit 160,000.00 14,545.00 For project (41 units) 6,560,000.00 596,345.00 Per hectare of project area 470.00 43.00 - 170 -

Open disposaldrains

Costsper m of drain: Excavation 132.00 12.00 Structures 220.00 20 Totalper m of drain 352.00 32.00 Cost per hectareof projectarea 1,161.60 105.60

Total costs of drainage (per hectare of project area)

Field drainsOaterals) 5,775.00 525.00 Collectorsincluding dewatering 4,518.00 412.00 Connections(manholes) 220.00 20.00 Pumpingstations with sumps (excluding pumps) 470.00 43.00 Opendisposal drains 1,155.00 105.00 Totalper hectare 12,138.00 1,105.00

The costsgiven thus far representthe estimateddirect costs. Thesecosts wereincreased by extra and Indirectcosts as follows: Contingencies 10% Contractor'soverhead 10% Contractor'sprofit and risk 10% Foreigntechnicians 2% Supervisionand accounting 5% Crop compensationfor farmers 7% All theseextras total 44 percent. The implementationof the projectinvolved the purchaseof a substantialamount of drain installationequipment from abroad. 'rhe costshad been estimatedat an earlierstage and are givenin table5-4. The estimationwas madein July 1978(differing from the total estimategiven above) when the exchangerate wasRp 1.00 = US$0.10. The costsinclude customs duties and are dividedinto localand foreigncurrency, both expressed in Rp.

Example 3: Project in Peru ThisWorld Bank-financed project covers about 3,300 ha locatedin a few noncontiguous subareas,all situatedin river valleysalong the Pacificcoast south of Lima. The main constraintfor agriculturewas soil salinity. A generalfeature of the drainagecomponent of the projectis the installationof fieldpipe drains,mostly having a directoutflow into an opencollector drain (about80 - 171 - Table 54: Detailed Cost Estimates for Drain Installation Equipment ('000 Rp)

Ittm No. Local Foreip Total US$ ('000)

Screeningplant 2 717 2,350 3,067 307 Concretemixer 3 200 - 200 20 Electric submersiblepumps 8 115 122 237 24 Dewateringpump 24 783 2,417 3,200 320 Mobile dieselpump 6 177 205 382 38 Self-primingdiesel pump 3 91 103 194 19 Generatorset 4 149 401 550 55 Vertical well-pointing et 1 715 836 1,551 155 Horizontaldewatering machine 1 506 1,664 2,170 217 Drainagetrenchingnmachine 2 735 2,410 3,145 315 Laser control asuembly 1 1Oo 176 276 28 Wheeltractor 1S 1,067 4,074 5,141 514 Crawler tractor 70-80 BHP 3 230 8S5 1,115 112 Track type front loader 2 301 1,685 1,986 149 Rubber-tiredfront loader 4 632 2,471 3,103 310 Ripper track, 8 tons 5 1,572 1,685 3,257 326 Wheeledhydraulic excavators 2 302 977 1,279 128 Tacked hydraulicexcavators 3 457 1,481 1,938 194 Trailer, 5 tons 9 498 436 934 93 Dumper 4 1,042 937 1,979 198 Water tank trailer 2 n 3 149 - 149 15 Fuel tank trailer 2 235 225 460 46 Low loader 1 194 178 372 37 Gravel suply trailer 5 6 554 1.160 lIS Subtotal 11,573 26,272 37,845 3,738 Spare parts (20%) 2,315 5,154 7,469 747 rection, training I S00 650 65 Total 14,038 31,926 45,964 4,550 Note: July 1978 price level. percent of the area). In about 20 percentof the area,pipe collectors flow out into open disposal drains. The open disposal system dischargesinto the sea, either by gravity or through pumping.

The general specificationsof the drainage items and their quantitiesare as follows:

Pipe field drains

* corrugatedPVC pipes, diameter 100 and 91 mm (outside and inside, respectivelly);

* spacing: range,40 to 150 m; average,80 m;

* total length:442 kim,or 134 m/ha;

* depth: average,1.80 m;

* length of drainlines: average,280 m; - 172 -

* all field drains surroundedby gravel, quantity 0.1 mI/m;3 and

* installationby trenching drainage machine.

Pipe collectors

* concrete pipes;

* diameter range 8" to 12" (200 to 300 mm);

* depth: maximum2.50 m; and

* total length in project: 23.5 mn.

Open drains

* excavation:average 15 m3/m; and

* total length: 100 km.

The actual costs were estimated in November 1982 in the local currency (the sol). The exchangerate was 100 soles to US$ 0.113 (US$1 to 880 soles). The costs shown here are generally both in soles and the US$ equivalent.

M0US$

Pipe field drains

Pipes 1,593.00 1.81 Gravel, includingtransport to site 333.00 0.38 Installation 1,777.00 2.02 Land clearing and trench backfilling 90.00 0.10 Repair of crossingswith roads, canals, etc. 210.00 0.24 Transport of equipment 175. 0.20

Total per m 4,178.00 4.75

Per hectare 559,852.00 636.50

Pipe collectors(concrete)

Cost per m (all inside diameter) Diameter 8" (200 mm) 20,000.00 22.70 Diameter 10" (250 mm) 23,647.00 26.90 Diameter 12" (300 mm) 29,270.00 33.25 Average 24,305.66 27.61 Total project (23.5 km) 620,400,000.00 750,000.00 - 173 -

Connecnons(total project)

Lateral - collector (manholes)400 pieces (each $186) 74,400.00 Lateral outlets: 1,170 pieces (each $40) 46,800.00 Collector outlets: 350 pieces (each $105) 36.750.00

Total 157,950.00

Open drains, includingstructures

Costs per m 24.00 Total project (100 km) 2,400,000.00

Total costs

(a) Total project Field drains (laterals) 3,000,000.00 Pipe collectors 705,000.00 Connectingstructures 158,000.00 Open drains 2,400,000.00 Total 6,263,000.00

(b) Total drainage system per hectare 1,900.00

Example 4: Project in Egypt

Several World Bank-financedpipe drainage projects have been implementedin the Nile Delta, as well as in the Nile Valley. The combinedproject area coven severalmillion feddans (1 feddan = 0.4 ha). Execution of Project No. 1 beganin 1971; projectnos. 4 and5 are under construction. The general features of these projects are that field pipedrains flow out into pipe collectors, which in turn discharge into open disposal drains. The waterfrom the disposaldrains is generallyevacuated through large pumping stations.

The general specificationsof the drainageitems and their quantitiesactually under constructionare as follows:

Field Drains

* corrugatedPVC pipes, diameter80 and 72 mm (outsideand inside, respectively);

* depth: about 1.25 m;

* length of drain lines: about 200 m; -174-

* cover material: sometimesgravel; sometimesnone; quantitiesof gravel unknown; estimated on the order of 0.025 to 0.05 m3/m;

* installationby trench-excavatingdrainage machine; and

* spacingsrange between 30 and 60 m; average about 40 m; thus about 20 m/ha.

Pipe collectors

* concrete pipes;

* diameter range 6" to 16" (150 to 400 mm);

* length of collector lines: up to 2 km; and

* total length: around 30 m/ha.

Connectionsbetween Field Drains and Collectors

* majority: direct (blind) connections;and

* about every fourth connection: manhole(thus, one manholeabout every 160 m of collector or about 0.2 manholesper ha).

The costs of drainage are based on the fact that drainage works are executedby contractors. In 1986-87, contractor's prices were in the range of LE 200 to 250 per feddan. The works to be executed includepipe field drains, pipe collectors, and structures (connections,manholes, and outlets into open drains, includingsupply of materials, installation,backfilling, necessary repairs, etc.). The exchangerate in March 1988 was LE 1.0 to US$0.46. Thus, accordingto the official fixed rate, the costs per ha were in the range of US$1,818to US$2,273. ADDendiX6: Crop Tolerance Tables 0% 10% 25% 50% Maxi- Crop ______r__um EC0 ECW LR' EC. EC, IT EC. WEC, ECdd

Field crops Barley* (Horuesn vulgare) 8.0 5.3 10% 10 6.7 12% 13 8.7 15% 18 12 21% 56 Cotton 7.7 5.1 10% 9.6 6.4 12% 13 8.3 15% 17 12 21% 54 (Gossypiumhirstwum) Sugarbeet 7.0 4.7 10% 8.7 5.8 12% 11 7.5 16% 15 10 21% 48 (Bla vulgaris) Whear& 6.0 4.0 10% 7.4 4.9 12% 9.5 6.4 16% 13 8.7 22% 40 (macwn aesdvwn) Safflower 5.3 3.5 12% 6.2 4.1 14% 7.6 5.0 17% 9.9 6.6 23% 29 (Cauihns finctorius) Soybean 5.0 3.3 17% 5.5 3.7 18% 6.2 4.2 21% 7.5 5.0 25% 20

Sorghun 4.0 2.7 7% 5.1 3.4 9% 7.2 4.8 13% 11 7.2 20% 36 (Sorghm bicolor) Gronmdnut 3.2 2.1 16% 3.5 2.4 18% 4.1 2.7 21% 4.9 3.3 25% 13 (Aradis hypogaea) Rice (paddy) 3.0 2.0 9% 3.8 2.6 11% 5.1 3.4 15% 7.2 4.8 21% 23 (Ozyzasatwva) Sesbania 2.3 1.5 6% 3.7 2.5 8% 5.9 3.9 12% 9.4 6.3 19% 33 (Sesbaniamacrociarpa) Corn (rain) 1.7 1.1 6% 2.5 1.7 8% 3.8 2.5 13% 5.9 3.9 20% 20 (Zeamays) Flax 1.7 1.1 6% 2.5 1.7 8% 3.8 2.5 13% 5.9 3.9 20% 20 (Linwnusitatissimum) Broadbean 1.6 1.1 4% 2.6 1.8 7% 4.2 2.0 12% 5.8 4.5 19% 24 (Viciafaba) Cowpea 1.3 0.9 5% 2.0 1.3 8% 3.1 2.1 12% 4.9 3.2 19% 17 (Vignasinensis) Beans (field) 1.0 0.7 5% 1.5 1.0 8% 2.3 1.5 12% 3.6 2.4 19% 13 (Phaseolusvulgaris) Crop tolerance tables (continued) 0% 10% 25% 50% Crop EC.- ECWb LR0 EC. ECX LR EC. EC, LR EC. EC; LR ECdxd

BVegabe crops 4.0 2.7 9% 5.1 3.4 11% 6.8 4.5 15% 9.6 6.4 21% 30 (Beta vulgaris) Broooli 2.8 1.9 7% 3.9 2.6 10% 5.5 3.7 14% 8.2 5.5 20% 27 (Brassicaitalica) Tomato 2.5 1.7 7% 3.5 2.3 9% 5.0 3.4 13% 7.6 5.0 20% 25 (Lycopersiconescuktmwn) Ccumbeer 2.5 1.7 8% 3.3 2.2 11% 4.4 2.9 15% 6.3 4.2 21% 20 (Cknais sativus) (>- alnupe 2.2 ;1.5 5% 3.6 2 3.8 12% 9.A 6.1 19% 32 (Ciwus melo) Sroinch 2.0 1.3 4% 3.3 2.2 7% 5.3 3.5 12% 8.6 5.7 19% 30 (4hma okraca) CNb,. 1.8 1.2 5% 2.8 1.9 8% 4.4 2.9 12% 7.0 4.6 19% 24 (Braca oeracea capatata) Pobtto 1.7 1.1 6% 2.5 1.7 8% 3.8 2.5 13% 5.9 3.9 20% 20 (Soklnwn tuberoiwn) Sweet corn 1.7 1.1 6% 2.5 1.7 8% 3.8 2.5 13% 5.9 3.9 20% 20 (Zea mays) Sweet potato 1.5 1.0 5% 2.4 1.6 8% 3.8 2.5 12% 6.0 4.0 19% 21 (Ipomea batatas) Pevver 1.5 1.0 6% 2.2 1.5 9% 3.3 2.2 13% 5.1 3.4 20% 17 (Casiaum f rutescens) Lettuce 1.3 0.9 5% 2.1 1.4 8% 3.2 2.1 12% 5.2 3.4 19% 18 (Lactucasativa) Radish 1.2 0.8 4% 2.0 1.3 7% 3.1 2.1 12% 5.0 3.4 19% 18 (Raphanussativas) Onion 1.2 0.8 5% 1.8 1.2 8% 2.8 1.8 12% 4.3 2.9 19% 15 (Allium cepa) Carrot 1.0 0.7 4% 1.7 1.1 7% 2.8 1.9 12% 4.6 3.1 19% 16 (Daucuscarota) Beans 1.0 0.7 6% 1.5 1.0 8% 2.3 1.5 12% 3.6 2.4 19% 12.5 (Phaseolusvulgaris) Crop tolerance tables (continued) 0% 10% 25% 50% EC. EC LRS EC. EC, LR EC. EC. LR EC. EC; LR ECdxd

Drati ms 4.0 2.7 4% 6.8 4.5 7% 10.9 7.3 11% 17.9 12 19% 64 (Phoenix dacjylifera) Fii (Ficuscarica) 2.7 1.8 6% 3.8 2.6 9% 5.5 3.7 13% 8.4 5.6 20% 28 olve (Oleaeuropaea) Pomeranate (Punciagranatum) Gravefruit 1.8 1.2 8% 2.4 1.6 10% 3.4 2.2 14% 4.9 3.3 21% 16 (CiOg7paradis7. rangeo 1.7 1.1 7% 2.3 1.6 10% 3.3 2.2 14% 4.8 3.2 20% 16 (Citruissinensis) Ixtnon 1.7 1.1 7 % 2.3 1.6 10% 3.3 2.2 14% 4.8 3.2 20% 16 (Citruslimonea) Apple"Pyrus malus) 1.7 1.0 6% 2.3 1.6 10% 3.3 2.2 14% 4.8 3.2 20% 16 Pear (Pyrus communis) Walnut 1.7 1.1 7% 2.3 1.6 10% 3.3 2.2 14% 4.8 3.2 20% 16 (Juglansregia) Peach 1.7 1.1 9% 2.2 1.4 11% 2.9 1.9 15% 4.1 2.7 21% 13 (Prunuspersica) Agncot 1.6 1.1 9% 2.0 1.3 11% 2.6 1.8 15% 3.7 2.5 20% 12 (Pyrs armeniaca) Grape 1.5 1.0 4% 2.5 1.7 7% 4.1 2.7 11% 6.7 4.5 19% 24 (VWtsspp.) Almond 1.5 1.0 7% 2.0 1.4 10% 2.8 1.9 13% 4.1 2.7 20% 14 (Prunusamygdalus) Plum 1.5 1.0 7% 2.1 1.4 10% 2.9 1.9 14% 4.3 2.8 20% 14 (Prunusdomestica) Blackberry 1.5 1.0 8% 2.0 1.3 11% 2.6 1.8 15% 3.8 2.5 21% 12 (Rubusspp.) Boysenberry 1.5 1.0 8% 2.0 1.3 11% 2.6 1.8 15% 3.8 2.5 21% 12 (Rubusspp.) Avocado 1.3 0.9 7% 1.8 1.2 10% 2.5 1.7 15% 3.7 2.4 20% 12 (Perseaamericana) Raspbnery 1.0 0.7 6% 1.4 1.0 9% 2.1 1.4 13% 3.2 2.1 19% 11 (Rubus i1aeus)

Strawberrv 1.0 0.7 8% 1.3 0.9 lO0o 1.8 1.2 15% 2.5 1.7 21 % 8 CroD tolerance tables (continued) 0% 10% 25% 50% Crop ______m ax. EC. ECWB LR0 EC. ECR LR EC. EC, LR EC. EC, LR ECud Forage crop T Wheatgrs 7.5 5.0 8% 9.9 6.6 10% 13.3 9.0 14% 19.4 13 21% 63 (Agropyronelongatun) Wheat gras (fairway) 7.5 5.0 11% 9.0 6.0 14% 11 7.4 17% 15 9.8 22% 44 (Agropyronelongatmn) Bermudagrass 6.9 4.6 10% 7.4 5.7 13% 10.8 7.2 16% 14.7 9.8 22% 45 (Cynodondactylon) =Brley(hayr 6.0 4.0 10% 7.4 4.9 11% 9.5 6.3 16% 13.0 8.7 22% 40 (Hordeumvulgare) Perennial rye grss 5.6 3.7 10% 6.9 4.6 12% 8.9 5.9 16% 12.2 8.1 21% 38 (Loliwnperenne) Trefoil, birdsfooe (narrow leaf 5.0 3.3 11% 6.0 4.0 13% 7.5 5.0 17% 10 6.7 22% 30 aLcorniculaus tenuifolims]) Hardinggrans 4.6 3.1 9% 5.9 3.9 11% 7.9 5.3 15% 11.1 7.4 21% 36 (Phalaki5tuberosa) Tai fescue 3.9 2.6 6% 5.8 3.9 8% 8.6 5.7 12% 13.3 8.9 19% 46 (Festulaetior) Crestedwheatgrass 3.5 2.3 4% 6.0 4.0 7% 9.8 6.5 11% 16 11 19% 57 (Agropyrondesenorsun) Vetch 3.0 2.0 8% 3.9 2.6 11% 5.3 3.5 15% 7.6 5.0 21% 24 (Vicia sativa) Sudangrss 2.8 1.9 4% 5.1 3.4 7% 8.6 5.7 11% 14.4 9.6 18% 52 (Sorghumsudanense) Wildrye, beardless 2.7 1.8 5% 4.4 2.9 7% 6.9 4.6 12% 11.0 7.4 19% 39 (Elms triticoides) Trefoil, big 2.3 1.5 10% 2.8 1.9 13% 3.6 2.4 16% 4.9 3.3 22% 15 (Lotusuliginosis) Alfalfa 2.0 1.3 4% 3.4 2.2 7% 5.4 3.6 12% 8.8 5.9 19% 31 (Medicagosativa) Lovegrms' 2.0 1.3 5% 3.2 2.1 8% 5.0 3.3 12% 8.0 5.3 19% 28 (Eragrostisspp.) CrOD tolerance tables (continued) 0% 10% 25% 50% Crop ______M ax. EC: EC,, LLR EC. EC. EC L EC. EC, LR ECad

Forage crops Corn (forage) 1.8 1.2 4% 3.2 2.1 7% 5.2 3.5 11% 8.6 5.7 18% 31 (Zeamays) Clover, berseem 1.5 1.0 3% 3.2 2.2 6% 5.9 3.9 10% 10.3 6.8 18% 38 (hlfoliwn alandhium) Orchard grass 1.5 1.0 3% 3.1 2.1 6% 5.5 3.7 11% 9.6 6.4 18% 35 (Datylis glomta) Meadowfoxtail 1.5 1.0 4% 2.5 1.7 7% 4.1 2.7 11% 6.7 4.5 19% 24 (Alopecwwspratensws) Clover, alske, ladino, 1.5 1.0 5.5% 2.3 1.6 8% 3.6 2.4 12% 5.7 3.8 19% 20 red. strawbeffy, (liWoUwn spp.)

Noe: Based on data reported by Maas and Hoffman(m press), Berstein (1964), and Universityof California Committeeof Consultan (1974). Figures indicate expectedyield reducticn for the particular crop due tiothe soil or irrigation water salinity shown. a. EC indicateselectrical conductivity of the saturationextract of the soil (USSL), reportedin miflimhosper centimeterat 25°C. Valuesreported are from Maas and Hoffman (in press) and Bemstein(1964). b. EC,, indicateselectrical conductivity of the irrigation water, in miflimhosper centimeterat 25C. This assumes a 15-20% leachingfraction and an average soil water a1inimtyoqua to about three times that of the irrigation water applied(EC,, = 3EC,,) or about twice that of the soil saturationextract (EC, = 2ECj). From the above, EC = l.5EC,. c. LR mdicates leachingrequirement or the minmuwmleaching fraction that can be relied u to control salts within the tolerance of the particular crop grownand consideringthe quality of water used. LR is determinedfrom the equation lR = ECMEC,,. d. MaximumECdW is the maximumsalinity of the percolating water draining from the root zone that can result due to removalof water by the particular crop to meet its water requiremnt for growth (if all the root zone soil water were at this maximumECd, yield reductionwould beo100% since the crop would be unable to extract water from mhevery salty soil water). This is the value used as ECdwin the LR calculation(LR = EC*/ECp). For the given cron and quality of water indicated aJpplicationof irrigation water to exactly meet the evapotranspirationdemand of the crop plus the LR to control sat should result in maximum efficiencyof water use. At this efficiency,percolating water draning from the root zone would be minimal rCprng quantitybut at a maximumregarding salinity and should approachthe maximumECdW as shown on these crop tolerance tables. EC,, = e. Barley wheat, sgar beets, and severalother crop are less tolerant of salts during germinationand early seedling growth. For germiation of beets, soil saminityin the seed area should not exceed TC. = 3 mmhos/cm; for barley and wheat, EC. should not exc 4 or 5 mhoslcm. f. Tolerace data may not apply to semnidwarfvarieties of wheat. These are often more tolerant. Bermudagrass varieties. Suwanee and Coastal are about 20% more tolerant; commonand Greenfieldare about 20% less tolerant. h. roaleabidsfottrefoil t ~~~i~~~i~~sfoot aprstoappears be~ less~ tolerant ~ than~ narrow~ lafleaf. i. Averageof Boer, 'Wilman,Sand, and WeepingLovegrass. Lehmanappears about 50% more tolerant. Sow"r: Ayers 1976. - 180 -

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