PROBLEMS AND PROSPECTS OF HYDRAULIC MODELLING FOR ENVIRONMENTAL FLOWS ASSESSMENT STUDIES IN EAST AFRICA

Abstract

The objective of this paper is to document the problems and prospects of hydraulic
modelling for Environmental Flows Assessment (EFA) studies based on selected case
studies. Most of studies in East Africa use Holistic methodologies. An ideal data set for
defining river hydraulics for most of these methods would be six data points of stage
measurements over a good distribution of discharges, the stage of zero discharge and some
flood-related data. Besides, in East African region EFA studies suffer from data scarcity
(i.e., poorly gauged sites) and limited expertise and funding. The hydraulics studies
conducted by the authors entailed desktop research, limited fieldwork for data collection,
data analysis, and modelling. The hydraulic models (HEC-RAS and PHABSIM) used are
governed by Manning and/or Energy equation(s) to simulate hydraulics. The optimized
sensitive parameters include roughness number, expansion/contraction coefficients,
roughness modifier and Beta coefficient. Data collected at medium flow, bank full
discharge information at neighbouring flow gauging stations, information from previous
studies, field observations on flow regimes and professional experience validated the
performance of these models. The geometric characteristics for extended floodplains and/or
swamps were derived from a calibrated NASA Shuttle Radar Topographic Mission (SRTM)
Digital Elevation Models (DEM) of 30 arc-seconds resolution under HEC-GeoRAS GIS
extension Environment. The modelling results were considered satisfactory because the
relative errors for most of applications fall below 20%. The good performance achieved is
attributed to the instituted quality control measures right from suitable sites selection to
hydraulic modelling phases. Modelling results confidence rating of above 3 in a scale of 1
to 5 achieved depended upon the hydraulic complexity. Based on the satisfactory results in
the case studies, the authors would like to note that there are some prospects of carrying
out hydraulic analysis in the regions with inadequate data. However, professional input is
the key to successful modelling exercises. Therefore, follow research should use more data
to verify the approach adopted.