Integrated Flood Hazard Mapping : A Case Study of the Upper Athi River Basin
Abstract:
Flooding is one of the most catastrophic natural disasters globally, significantly impacting communities, infra structure, and economies. Upper Athi River basin, is one of the river basins in Kenya that is under increasing risk of f looding due climate change, rapid urbanization, and unsustainable land use practices. The objective of this study was to assess and map the flood hazard in the upper Athi River Basin. The study used HEC- HMS to simulate the hydrological processes of the basin and to generate the flood hydrographs that were then routed in the river channel using HEC-RAS model. Flood hydrographs with return periods of 25, 50 and 100 years were modelled (using flow routing methods) through the river channel. HEC-HMS and HEC-RAS models were calibrated and validated using observed discharge and water surface profile data, respectively and before they were used for simulation. Model performance, statistically assessed using Nash-Sutcliffe Efficiency, was found to be very good. Flood hazard maps developed from the study showed that a 25 year flood would cause an inundation of 17 meters from the river bank which increased by 29% and 106% for 50 and 100 year floods respectively. The water level (depth) in the main channel increased by 50% and water velocity tripled for a 100-year flood compared to 25-year flood. The results show increasing potential risks of flooding posed to agriculture, infrastructure and people living in the mapped f lood zones. The findings highlight the need for flood hazard maps to be incorporated into land-use planning, emer gency response strategies, and policy frameworks to enhance flood risk management. To mitigate future flood risks, the study recommends establishment of early warning systems and implementing sustainable land management practices, such as afforestation and pervious pavements.
