Flood mitigation modelling in the River Nzoia basin through storage reservoirs

Abstract

Nzoia River in Western Kenya is prone to frequent floods particularly in the flood plains of Budalangi. These floods result in displacement of people and destruction of property. The implementation of flood control works through the rehabilitation of dykes and river training has remained a challenge because these dykes are constantly breached due to inadequate capacity of the river channel to contain high flows. To mitigate this problem three dams have been proposed: Anyiko/Rambula (42A), Rongai (34B) and Kipkaren (35). The aim of this study was to evaluate the effectiveness of the proposed reservoirs as a flood mitigation measure in the Nzoia River basin using MIKE 11 model. MIKE 11 is a software package for simulation of one dimensional fully dynamic wave flow in rivers. It has the ability to simulate different flood mitigation scenarios before and after dam construction. The overall methodology involved rainfall-runoff modelling for each of the reservoir catchments using NAM module in MIKE 11, and MIKE 11 HD model for river flow simulation. Good results, with a coefficient of determination above 0.85, were obtained for both calibration and verification. The calibrated model was evaluated on its ability to predict extreme events, and the performance was satisfactory based on graphical probability plots. In order to quantify the flood magnitude for each return period for the scenarios with and without reservoirs, flood frequency analysis was undertaken. In the scenario investigation, the reservoirs were analyzed individually and the flood effect based on the dam implemented was evaluated at Rwambwa (1EF01), a downstream river gauging station. With the implementation of dam 42A, 34B and 35 the simulated peak flows were 320m3/s, 491m3/s and 601m3/s respectively. Based on flood thresholds of 298m3/s and 568m3/s for overtopping the river banks and the dykes respectively, it was evident that even after flood regulation, dam 35 was least effective in controlling the floods. The flood peak from dam 42A will overtop the river bank but is not sufficient to overtop the dykes. From the flood frequency analysis it was observed that implementation of the individual dams reduced the incidence of flooding significantly. The return period of the dyke crest level flood improved from 1.7 years for no dam to 13 years when either dam 35 or 34B was implemented, but increased to 31 years when dam 42A was implemented. Among the proposed dams, dam 42A was found to be more effective in flood mitigation. It is recommended that it is constructed with gates to control the outflow from the reservoir.