Flood Control Problems* bv LUNA B. LEOPOLD & THOMAS MADDOCK JR.
General Considerations other. Under light population pressures HROUGHOUT the world, alluvial and low stages of development there is little soils are among the most fertile and conflict ; but as population pressures increase, T easiest cultivated. Alluvial valleys more and more of the flood plain is occupied, are routes for transportation either by water and greater and greater risks of loss from or by road and railroad. Rivers are sources flooding must be accepted. But sooner or of water, a necessity of life. But these river later a flood of unusual magnitude takes valleys and alluvial deposits, which have so place. This is thc time when those who have many desirablc characteristics and which gambled and won for many years must lose, have increased so greatly in population, are and the relationships between loss and gain periodically occupied by the river in perform- is balanced. ing its task of removing the excess of precipi- There comes a time when the losses, even tation from the land area and carrying away though they may be economically acceptable, the products of erosion. become unacceptable politically and socially. How a river behaves and how the river In such cases some Ievel of government flood plain appears depend on the relation- undertakes to provide a degree of flood pro- ships between water and sediment combined tection to reduce the risk of flood plain with the existing topography. Thus rivers occupancy. The greater the magnitude of and their alluvial deposits provide an endless the problem, the higher the governmental variety of forms which are shaped, to a large level which undertakes remcdial measures. extent, by the river flow during periods of In some areas only a national government rapid removal of debris and of excessive can be responsible. rainfall. The mechanics of river formation Basically, there are only a few ways in are such, however, that the highest discharges which flood protection may be accomplished. are not contained within a limited channel. The excess water may be confined between How much water a channel will carry depends levees or embankments or it may be stored in upon the frequency of occurrence of a flow. some place for release at non-damaging rates Low flows, which occur very frequently, are at a later time. Rut the most essential not important in channel formation. Neither factor which must be considered in flood are the infrequent discharges of very great control planning is that only very rarely is magnitude which, although powerful, do it physically possible to provide complete not occur often enough to shape the channel. protection against floods; and even more Channel characteristics, therefore, are depen- rarely is it economically feasible to provide dent on those discharges of moderate size complete flood protection even if it were which combine power with frequency of physically possible to do so. We know of no occurrence to modify the channel from. In case where complete flood protection has the highest discharges of a stream, water been provided against the floods which may rises above the confines of its banks and be expected from a river basin of appreciable flows over the flood plain. size. It must be considered, therefore, that The type of flood control programme floods are natural phenomena which are selected for a given area is, therefore, one characteristic of all rivers. They perform a which will conform to local conditions and vital function in the maintenance of river one which will yield the greatest benefits forms and out of bank flow may be expected with respect to its cost and at the same time with a reasonable degree of regularity. result in the least harm. The demands of the river and man’s re- For it must be recognized that there are quirements for land are in opposition to each always some undesirable features which
*With acknowledgement to the Chief Administrator, Kosi Project, Patna, Bihar.
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result from attempts to regulate rivers. and those characteristics associated with the Foremost among these is the fact that floods hydraulics of the stream having to do with have their benefits and elimination of floods width, depth, vrlocity and slope. eliminates benefits. One important benefit Thus we find there is general agreement on is the maintenance of fertility of the ovcr- some of the methods to be used to establish flowed arcas. Another is the increased crop certain criteria for flood control. Methods yield which may result from the natural are well standardized for determining the inundation of such areas. There are dis- size of a flood to be used for varions purposes advantages to methods of flood control. in project design. Procedures for routing Confining water between embankments leads flood flows arr generally uniform. But the to an increase in flood heights by reducing methods of analysing the flow of sediment- the amount of water held in temporary laden discharges in alluvial channels have storage in flood channels. The same process not been agreed upon. increascs flood problems downstream. Con- The applicability in a specific instance of finement of river flow may result in changes standardized procedures is limited by the in the relationships between width, depth, availability of basic data. Lack of basic velocity, slope and sediment load and may hydraulic data for ever plagues the planning result in modification of river sections, in engineer, because basic data increase in value the positions of bends and meandcrs, and with the length of record. Thus projects may cause or rrsult in scour or fill. Storage which are being plannccl today should have of flood watrrs requires land which may br records of precipitation, runoff, and sedi- valuable. l
170 LUNA & THOMAS - FLOOD CONTROL PROBLEMS ivailable from hill and mountain areas lcavc sediment load which are of greatest import- much to be desired, but this is the usual auce in determining channel characteristics situation. Too few people live in such ai ens (lo not even appear in a 0.6 depth sample. to permit the establishment of an adequate, It is suggested that sampling procedures hydrologic network. Constant pressure is used in the United States be adapted to meet required to expand the network to its prac- Indian icquircmcnts. Wc suggest, in parti- tical limits and methods must be deveIopec1 cuIar, the iise of equipnicnt equivalent to the to extrapolate existing records to areas of type of samplcm dcvclopccl by tlic United 110 data. Our discussions with the staff States Federal Agencies wh~chwill provide indicate that these problems are recognized samples of water ant1 scdimcnt moving nem- and efforts are being macle to solve them. the stream bed. For this purposc it is sug- The results of the cffort to expand the net- gested that an adequate number of samplers work in the upper Kosi river drainage has varying in size from depth-integrating hand been outstanding. snmplcrs for shallow streams ant1 recon- Stream flow records do not appear to bc riaisancc invcstigatioiis to the heaviest as adequate as rainfall records. Many deI’th-integrating samplers for us(’ on larger records are simply gage heights. These arc streams bc obtained from the United Statcs. of value but not as valuable as records of the It is not suggested that the large pointinteg- quantity of water flowing in the river from rating samplers hc secured nntil the scdiment day to day. There are essentially only three sampling programme is well developed. locations where discharges are measured in There is a series of publications, which North Bihar, and these are associated with sliould be madc available with the sediment collection of data for specific projects. The sampling equipment, reporting the results useful data collected at gaging stations is not of intei-agency studies in the sediment confined to records of discharge. Informa- field by the United States Government. tion on stream width, depth, velocity and These deal with an analysis of various types slope is of great value in many types of of samplers, the development of the recom- studies and these data should be sent to mended sampler, methods of securing samples central locations to supplement runoff data. in the field, methods of determining size of We suggest that consideration be givcn sediment particles, reports of field tests of to the centralization of complete records of sediment samplers and studies of volume- gaging station operations. The advantage wcight relationships of sediment. We sug- to be gained from such a procedure are: gest that the conclusions found in these (1) A celitral record body is built up which studies be given consideration in developing provides ready access to rccords of streams the Indian sediment sampling programme. having dissimilar characteristics. (2) Ready The presciit method of determination of access permits easy study of records for any sediment size appears to be satisfactory. purposc which may be desired. (3) Proce- We would recommend, however, that sedi- dures for the collectioii and analysis of data ment be ieported on n weight rather than on may be reviewed and the worth of the record volume basis. This suggestion is made evaluated. (4) Responsibility for uniformity bccnuse of the great vaiiations in sediment and adequacy of technical performance can weight which are observed under natural be placed in a single responsible officer. conditions p;irticularly in the finer sizes. The collection of sediment data is in its At rcasonably fr cquent intervals complete infancy in India as it is in other countiies. nicchanical analysis of tlie suspended sedi- Indian methods of data collection were ment should bc madc, tJting care to secure designed for the purpose or estimating rates analyws of sediment carried by n range of of sediment accumulation in reservoirs. discharges. Procedures are very satisfactory for this pur- It seems to LIS that while the Indian engi- pose, but if the uses of sediment data are to neer faces great problems, he also faces great be expanded, it will be desirable for a modi- oppoitunitics. Nowhere on earth is it pos- fication to be made in current procedures. sible to study the alluviation of streams As a case in point, the collection of sediment with such a wide range of discharges, slopes samples at 0.6 depth results in the ovcr-esti- and sizes of suspended load. Hence from mation of fine material and the under-cstima- the standpoint of developing the science of tion of the coarse material. Many sizes of river engineering there should be a well
171 JOURNAL OF SOIL & WATER CONSERVATION IN INDlA. VOL. 3, 1955
rounded and continuing investigational pro- the channel. The dependent factors are gramme which would cover the Himalayan then the channel width, depth, velocity and front. We are firmly convinced that the slope. Rugosity is a semi-independent fac- returns from such a study in the form of more tor, for it is governed both by the depth- precise planning, lower construction costs, particle size ratio and by sediment concen- freedom from operation problems, and lower tration. Under certain conditions the chan- maintenances costs will be many times the nel may inherit a slope and temporarily be expenditures necessary to finance such forced to accommodate to it, but in geologic studies. These studies are not research in time slope finally tends to be adjusted to a the abstract sense. They are highly prac- mutual interdependency with other depen- tical and have the definite objective of dent factors. aiding in the sound development of water Furthermore, the forces of erosion and resources. deposition tend to adjust and readjust in Such a programme would correlate field response to local variations, and these checks surveys to determine stream slopes, size of or balances tend to create and maintain a bed material, river cross-section ( which quasi-equilibrium which characterizes nearly should include rivers of such magnitude all channels. Though regime in natural that supersonic depth-finding equipment channels is seldom completely fulfilled, it is must be used in their measurement) with closely approached. Even when a natural hydraulic studies. The latter should deal channel is aggrading or degrading, braiding with stream width, depth, velocity and or meandering, the hydraulic factors still roughness, and with measurements of sus- tend to be nearly balanced, and divergence pended load and its size, together with the from regime is reflected in only small devia- flow of the streams, and the frequency of tions from values of hydraulic factors which various discharges. Enough work has al- characterize regime conditions. As a result ready been done in India to indicate that of these interactions, channels are similar such a programme would be effective. What and change in a similar manner if they have are now required are funds to defray costs, similar discharge and loads. A corollary to manpower to undertake the work and energy this is that one may judge the general to drive to a successful conclusion. character of discharge and load by inspection of the channel characteristics. Principles and Observations River pattern is defined as the plan view of Relative to River Morphology the channel as seen from the air. There are The authors have been personally con- three general types of patterns. The first is cerned with research in the mechanics of meandering, which is characterized by a rivers, the inter-relation of discharge, sedi- relatively small value of width-depth ratio, ment load, and the shapes and patterns of as compared with other patterns of rivers river channels. Though our experience in having equal bankful discharge. The second India has been insufficient to analyse the is the braided pattern distinguished by many operation of these factors in the rivers of anastomozing channels separated by bars or India, we believe that an outline of some islands. This pattern is typified by rela- observations made on American rivers might tively large total width of water surface provide some analogies applicable to Indian relative to mean depth as compared to other conditions. patterns at equal bankful discharge. Chan- A river builds its flood plain and carves its nels which are not divided by islands and do channel in response to the discharge and not meander may be called the “normal” sediment load provided by the drainage pat tern. basin. The amount and distribution of Comparing rivers of equal bankful dis- water and the quantity and size character- charge, braided patterns only occur when the istics of the sediment debris are functions of channel gradient exceeds a given value, and the precipitation, geology, topography and meandering patterns when the slope is less physiography of the drainage basin. These than that same value. “ Normal” pat- qualities of the watershed determine the dis- terns, however, can occur in either range of charge, sediment concentration and size of channel gradient. the sediment particles. These can be con- Our own observations on natural rivers sidered independent factors with respect to and experiments in the laboratory have led
172 LUNA & THOhIAS - FLOOD CONTROL PROBLEhIS us to coiiclude that tlie primary cause of braided channel generally is determined by river braiding is selective deposition. When tlie load and the flow pattern rather than the load introduced into the reach is rela- vice versa. tively small in quantity, bed particles tend Meandering 017 the other hand represents to move in a band over the centre of the a condition in which there appears to be little channel, the width of the band increasing segregation of particle size in the process of with increasing quantity of sediment intro- deposition, and there is less tendency for duced. When the load includes particles niateriai to move in the centre of the channel. slightly too coarse to be kept in motion, these Central bars do not form. Deposition is come to rest near the middle of the channel primarily on tlie point bars on the convex and in doing so trap or hide some of the bank of a channel in a bend. finer particles. Meanders tend to move downstream and By this process a central bar is gradually the whole river wanders from one part of the built up, which on the average is somewhat flood plain to another. Because deposition coarser than the mean of the introduced load. is nearly equal to erosion, general aggrada- Despite the decrease of depth immediately tion occurs more slowly than in braided over the central bar, velocity actually re- channels and the movements of the channel mains the same or increases somewhat at are, therefore, slower. In this sense the that place as a result of decreasing roughness meandering pattern is more stable in that it over the bar, and the particles continue to can more easily be restricted within a given move over the bar instead of in the deeper meander belt. portions of tlie channel which flank the Meanders are formed by processes which central bar. The building of the bar is also occur in all channels, for the wave length of accompanied by increasing water surface meanders is directly proportional to river slope. As the bar builds closer to the water width, and this proportionality also holds surface, the velocity then decreases, bed sedi- true for pools and alternate riffles which ment over the bar ceases, at which time the occur in all natural channels. band of moving sediment is diverted into It is believed that meanders become deve- one of the deeper segments flanking the bar. loped only under certain combinations of At such a time essentially two channels width and velocity-depth ratios. If one exist separated by a bar which becomes an considers the change in velocity, depth, and island when the water surface is lowered. width downstream in most rivers at bankful These central bars force the flanking stage, it will be noted that width increases as channel to erode the channel boundaries the square root of discharge as found by with the result that discrete channels sepa- Lacey, depth increases as the 0-4 power of rated by islands are formed. The central discharge and velocity increases as about the bars grow by addition at their downstream 0.1 power of discharge. The slope, however, ends and are often cut across by new chan- tends to decrease as about the 0.4 power of nels as the process of channel shifting con- discharge causing in general decrease in tinues. The shifts often make separate size of debris downstream. Thus decreasing channels rejoin, so the net result is a limited slope and decreasing size of material down- number of channels, continually dividing and stream with a concomitant slight increase in rejoining. velocity demonstrate that in general the That the slope of such a braided stream load and discharge tend to govern the width, should be steeper than that for a single depth, velocity and slope rather than the channel carrying the same total water and velocity governing the load. The decrease the same size of particles follows from the of particle size downstream in a large river is general consideration that channel slope the result of sorting of coarse particles from tends to vary inversely as a power function fine as well as breakage and abrasion. The of discharge. depth increases downstream faster than Thus a channel becomes braided because velocity and this is one factor which tends to of progressive deposition of the coarse por- make a meandering pattern in downstream tion of the debris load, and the slope of the recahes of a river system.
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