LR706 East African Flood Model_ Fiddes - Transport for Development

THE TRRL EAST AFRICAN FLOOD MODEL
ABSTRACT
Four years ofdata from 13small representative rural catchmentsin
Kenya and Uganda were analysed to develop improved methods of
flood estimation forhighway bridges and culverts. Dueto the short
period of record and the very quick response time of the catchments,
Unit Hydrography techniques were found inappropriate. A technique
which made better use of limited data, therefore, had to be
developed. Rainfall and runoff were fitted to a simple three parameter
conceptual catchment model. The model was then used to
predict the 10 year flood using a 10 year design storm. A simple
technique is then developed for predicting the peak flow and base
time of design hydrography for ungauged catchments.
1. INTRODUCTION
A large proportion of the total cost of building a road in East Africa is for the construction of bridges and
culverts to cross streams from small catchmentsl. Wereas most of the larger rivers in East Africa have flow
measuring stations, very few smaller streams are so equipped. Design methods must therefore be based on
rainfall-runoff models.
Very few data are available for the development of suitable flood models. In 1966 the Kenya and Uganda
Governments and the UK Transport and Road Research bboratory co-operated in a project to instrument 12
representative catchrnents, results from which could be used to develop improved flood design methods. The
choice of location and instrumentation of these catchments is described elsewhere 2. This report describes the
programme of analysis of results and the design method that was developed. It was presented as a paper to a
Hood Hydrology Symposium in Nairobi, Kenya in October 1975 which was jointly sponsored by the Economic
Commission for Africa, the East African Community and the Transport and Road Research bboratory.
2. CATCHMENT MODELS
A number of possible rainfall-runoff models were considered,
2.1 The unit hydrography
A unit hydrography is a hydrography of unit volume resulting from a rainstorm of unit duration and
specified areal pattern. Hydrography for other storms of similar areal pattern can be constructed by superimposing
hydrography, suitably offset in time with ordinates proportional to the flow assumed to result from
the rainfall in each unit time period.
It will be seen that the unit hydrography simply distributes the runoff in time. The volume of runoff must
be estimated separately and this generally involves deriving a rainfall-runoff correlation.