Morphology of the Mara River: Assessment of the long term morphology and the effect on the Ecological Flow studies

Investigation after including long term morphological assessments in the Environmental Flow studies. It is the aim of this research to assess the influence of changes in flow regime on the long-term morphology for the Mara river basin, thereby concluding on the significance of these effects for the establishment of the EFAs. Both a long-term assessment and local EFA assessments are undertaken and the results are linked to each other. Analytical and numerical calculations for the long-term 1-dimensional sediment transport analysis were conducted. Four reaches along a portion of the main stream of the Mara River are distinguished in the analysis. First, a reference scenario was established based on current discharges and parameters measured during site visits. Scenarios for future discharge regimes, were derived from the EFR. Three discharge scenarios are considered; Q_EFA, floods build up by the full EFRs, Q_EFA,maintenance , build up by only taking into account the maintenance flows, and the 95 percentile of flow (Q_95), which is the legislative discharge set by the Kenyan government. The results of the analysis include changes in cross sectional averaged bed levels, velocity, and depth over the length of the river. Although the river shows signs of historical erosion, also seen from field observations, the current state of the river is expected to be stable, since the riverbed is controlled by bedrock. The result for the equilibrium sediment transport flux for the alluvial reach of the river was comparable to fluxes in other rivers in Kenya. A first indication for the corresponding characteristic time scale for adaptation of the bed level is 1000 years, which indicates the time it takes for half of the total perturbation of the riverbed to respond. These results support the assumption that the river is stable for the current situation. Comparing the calculated discharge scenarios, the Q_95 and Q_EFA,maintenance scenario show high response of the bed level. The QEF A, f loods shows the same pattern of steepening, but the bed level changes are much less. From this result it can be concluded that it is important to establish an EFR, that also considers the floods, recognising the importance of the morphological assessment of the EFA. This research has taken the first steps in the direction of assessment of river stability in perspective of EFA assessments under data limited circumstances. Unfortunately no further conclusions can be made upon the long-termassessment of the bed level response on a small time scale, therefore improvements of the numerical model are recommended to be made. It is recognised that 2-dimensional processes such as meandering and river widening are important to take into account as well. Moreover, for better understanding and possibly in the future quantification of the morphological processes, it is recommended to build up a data bases including standardized data collected fromfield surveys of the EFA ormonitoringmoments after flood events. This can greatly contribute to the understanding and verification of expected morphological processes in the river.Civil Engineering and GeosciencesWater Managemen