River Flood Modelling with Mike 11: Case of Nzoia River (Budalangi) in Kenya

Abstract

This paper presents a one-dimensional unsteady flow hydraulic model used for the simulation of flow in rivers: the MIKE 11 model from the Danish Hydraulic Institute (DHI). The approach used for this model leads to unsteady flow simulations along river channel reach. The study case applied to the model is the lower Nzoia River, about 25 km in length. The study’s focus was the development of a MIKE 11 river model based on surveyed river cross-section data. The flooding problem in the lower Nzoia (Budalangi floodplains) in Kenya has been perennial each time causing a reversal of gains on economic and social development. The main objective of this study is to implement a one dimensional hydrodynamic model for the lower part of Nzoia river using the MIKE 11 modelling software. This was done with an aim of investigating the nature of the 2008 Budalangi floods. Two scenarios were considered, namely; reference case (intact dyke) and the breached dyke case. Longitudinal river stretch profiles were produced for the two cases and it was conclusive that the 2008 flooding was mainly necessitated by dyke breach. Further, the optimal computational spatial step (?x) and time step (?t) for the model were found to be 500m and 1 minute respectively. For this condition no instability of the model simulation was observed.

Key words: Nzoia River, MIKE 11 hydrodynamic model, Budalangi, floodplains, river hydraulics, unsteady flow