WATER RESEARCH COMMISSION This Publication Emanates from a Research Project Entitled DESIGN STORMFLOW and PEAK DISCHARGE RATES F
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OBTAINABLE FROM WATER RESEARCH COMMISSION P.O. BOX 824, PRETORIA 0001 REPUBLIC OF SOUTH AFRICA ISBN 0 908356 78 1 This publication emanates from a research project entitled DESIGN STORMFLOW AND PEAK DISCHARGE RATES FOR SMALL CATCHMENTS IN SOUTHERN AFRICA which was conducted by the Department of Agricultural Engineering University of Natal Pietermaritzburg Head of Department Professor P Meiring Project Leader Professor R E Schulze This report has been reviewed by the Water Research Commission and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the Water Research Commission, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. Printed by Creda Press, Cape Town 1 P. 0 30X c'65 F-.'~G '• >'~:- 0001 PRETORIA I lei. (012) 841-3083 or 20A3 from Small Catchments - in Southern Africa, based on the SCS Technique By E J Schmidt R E Schulze Department of Agricultural Engineering University of Natal Pietermaritzburg 3201, South Africa AGRICULTURAL CATCHMENTS RESEARCH UNIT REPORT No. 24 Prepared for the Water Research Commission WRC Project No.155 TT 31/87 1987 PREFACE AND ACKNOWLEDGEMENTS For a number of years now there has been a need in southern Africa* for a versatile and objective, yet simple, conceptually based technique of estimating storm runoff volumes and peak discharge on small catchments, to be used for both design and natural storms. In 1979 the then Division of Agricultural Engineering requested the Department of Agricultural Engineering at the University of Natal in Pietermaritzburg to prepare a report, in the form of a user manual, on the United States Department of Agriculture's Soil Conservation Service (SCS) hydrograph generating technique, for use under southern African conditions. The report "Estimation of Volume and Rate of Runoff in Small Catchments in South Africa, Based on the SCS Technique" by Schulze and Arnold (1979) was subsequently published and since then has been used widely by State Departments, consulting engineers and other organisations responsible for the design of hydraulic structures. Since 1979 considerable further research has been conducted in the Department of Agricultural Engineering to improve the application of the SCS model. The research, funded by the Water Research Commission, has been directed primarily at the following four components of the model : a) synthetic rainfall distributions in southern Africa; b) adjustments to the runoff response coefficient (Curve Number) for antecedent moisture status; c) catchment response time; and the d) coefficient of initial abstraction. In addition to the research conducted locally, much research has been directed internationally in recent years towards improving estimates by the SCS model. Furthermore, the expansion of daily * In the context of this report southern Africa refers to South Africa, Lesotho, Swaziland and the national states. (i) rainfall data bases in southern Africa has also allowed for improved estimates of design daily rainfall totals for use with the model. It was decided therefore, that the 1979 SCS manual was to be rewritten, incorporating both relevant results from overseas and the research findings emanating from the Department of Agricultural Engineering at the University of Natal. The revised report is structured in a similar way to its predecessor, with the first section providing the background and theory to the SCS model, the second outlining procedures for the use of the model in Southern Africa in the form of a user manual and the third comprising maps, nomographs and tables for use with the manual. A separate volume containing appendices to be used in conjunction with the report has been published. In addition a simplified user manual (excluding the background and theory), more suitable for use in the field, has been published. The authors should like to acknowledge the assistance of Mr J van der Merwe of the South African Weather Bureau who supplied the autographic rainfall data used to regionalise the design rainfall distributions and Mr B du Plessis of the Direc- torate of Hydrology, Department of Water Affairs who supplied the extreme daily rainfall data used for mapping design daily rain- fall depths. Various organisations, including Steffen, Robertson and Kirsten (Civil) Inc, Rhodes University (Hydrological Research Unit), Witwatersrand University (Water Systems Research Pro- gramme), Department of Environment Affairs and the University of Zululand contributed to the project by supplying hydrological data in various forms. Many of these data, incorporated at great cost (in terms of time spent reformatting, checking, revision and processing) into the existing hydrological data files of the Department of Agricultural Engineering, were used in the verifi- cation of model components during the course of this project. The staff of the Computing Centre for Water Research (CCWR) at the University of Natal are thanked for their assistance and most of the data analysis and graphics were undertaken on the CCWR (ii) computer. Special thanks go to Mrs S Neuwirth who wrote a large number of computer programs and was involved in all phases of the project from data capture and processing, model verification and design analysis through to the final computer graphics. Thanks are also expressed to Mr S J Dunsmore and Mr J P Weddepohl who assisted in the project whilst completing their masters degrees. The assistance of Mr D R Weston in data preparation and model testing is also acknowledged. The staff of the Department of Agricultural Engineering, especially Messrs G R Angus, M C Dent, S D Lynch, and H D Tarboton are thanked for their contributions towards the project especially with regards to the establishment of the 712 climatic data sets. The department's secretaries, Miss J M Whyte and Mrs K M Temple are thanked for the typing of the document. The research reported in this document was undertaken in terms of the following research project which was funded by the Water Research Commission (WRC) : Design stormflow and peak discharge rates for small catchments in southern Africa. The Steering Committee responsible for the project comprises the following people : Mr D W H Cousens Jrfater Research Commission (Chairman) Mr P W Weideman Water Research Commission (Secretary) Dr P J T Roberts Water Research Commission Dr D A Hughes Rhodes University Mr B J Middleton Steffen, Robertson and Kirsten (Civil) Inc. Mr C T Crosby Department of Agriculture and Water Supply Mr J H Varkevisser National Transport Commission Mr Z Kovacs Department of Water Affairs Prof P Meiring University of Natal Mr P W L Lyne University of Natal Please note that throughout this report the decimal point rather than the comma has been used, in order for nomenclature to be compatible with that from computer printouts, which form an integral part of this document. (iii) EXECUTIVE SUMMARY Background There is a need in southern Africa for hydrological information which will assist engineers/hydrologists responsible for the planning of hydraulic structures, in making economic and safe design decisions. Estimates of design flood characteristics (e.g. peak discharge, runoff volume and hydrograph shape) from small catchments (<8 km ) in southern Africa are generally derived by the use of mathematical runoff models, since only limited gauged hydrological data are available. A major problem associated in the use of such flood estimation models is the scarcity of adequate input information to the models and the lack of familiarity with the various techniques. In 1979 a report, "Estimation of Volume and Rate of Runoff from Small Catchments in South Africa, Based on the SCS-Technique" was published by Schulze and Arnold of the Department of Agricultural Engineering of the University of Natal, Pietermaritzburg, to address the above needs. The report presented the theory of the SCS model, and the procedures to applying this design flood estimation model, developed originally in the USA, to southern African conditions. Since 1979 the report has been used widely by State Departments, consulting engineers and other organisa- tions responsible for the design of hydraulic structures. In the 1979-1984 Water Research Commission (WRC) funded project "Hydro- logical Investigation of Small Rural Catchments in Natal with Particular Reference to Flood Events" considerable effort was expended into researching/modifying individual components of the SCS runoff equations. In June 1984 the present contract was entered into between the WRC and the University of Natal with two major objectives : a) A complete update and revision of the SCS manual for the estimation of design floods in southern Africa was to be undertaken, following further testing using expanded data (iv) sets from diverse climates, land uses and soils and incorpor- ating/ integrating research findings from previous projects. The revised manual was to be presented at two levels : a simple user manual for field use and a comprehensive manual, including theoretical and research aspects for use by the professional designer. b) Research on the effects of the joint association of rainfall and catchment moisture status on runoff generation was to be conducted with an aim to providing realistic estimates of design runoff for different regions of southern Africa. General methods currently used in southern Africa (and in most parts of the world) for estimating design runoff, apply design rainfall of a given recurrence interval to a catch- ment, assuming "average" antecedent moisture conditions for the catchment, to estimate design runoff for the same recur- rence interval. Such an approach does not account for the regional variation in what may be termed "average" conditions in an area of vast heterogeneity with respect to soil mois- ture conditions and hydrological response. Furthermore, such an approach does not account for the likelihood that rainfall of a given return period does not necessarily generate runoff of equivalent return period, there being a possibility of a large flood being produced by a relatively small rainfall event's falling on a wet catchment.