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 hydraulicmodelling for Environmental Flows Assessment (EFA) studies based on selected casestudies. Most of studies in East Africa use Holistic methodologies. An ideal data set fordefining river hydraulics for most of these methods would be six data points of stagemeasurements over a good distribution of discharges, the stage of zero discharge and someflood-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 studiesconducted by the authors entailed desktop research, limited fieldwork for data collection,data analysis, and modelling. The hydraulic models (HEC-RAS and PHABSIM) used aregoverned by Manning and/or Energy equation(s) to simulate hydraulics. The optimizedsensitive parameters include roughness number, expansion/contraction coefficients,roughness modifier and Beta coefficient. Data collected at medium flow, bank fulldischarge information at neighbouring flow gauging stations, information from previousstudies, field observations on flow regimes and professional experience validated theperformance of these models. The geometric characteristics for extended floodplains and/orswamps were derived from a calibrated NASA Shuttle Radar Topographic Mission (SRTM)Digital Elevation Models (DEM) of 30 arc-seconds resolution under HEC-GeoRAS GISextension Environment. The modelling results were considered satisfactory because therelative errors for most of applications fall below 20%. The good performance achieved isattributed to the instituted quality control measures right from suitable sites selection tohydraulic modelling phases. Modelling results confidence rating of above 3 in a scale of 1to 5 achieved depended upon the hydraulic complexity. Based on the satisfactory results inthe case studies, the authors would like to note that there are some prospects of carryingout hydraulic analysis in the regions with inadequate data. However, professional input isthe key to successful modelling exercises. Therefore, follow research should use more datato verify the approach adopted.