Environmental flow assessment using HEC-EFM and GIS: A case study of Kibos river

Abstract

Historically, water has been managed from a supply perspective with an emphasis on short-term economic growth from the use of the water. In this respect many municipalities, water service boards and other local authorities strive to supply water in abundance to their community. This has led to unprecedented environmental degradation. This has been witnessed in the over-consumption of Upper Athi River and Upper Tana River for supply to the City of Nairobi. There is a danger of similar situation recurring on the Kibos River for abstraction to the City of Kisumu. The water-resources planners such as water supply, hydropower, and irrigation engineers, need to give due emphasis to understanding of the need for environmental flows required to maintain the health of the ecosystem of these rivers. Most of the ~ethods developed so far are project specific or basin specific and cannot be readily applied in Kenya as hydrological and physical characteristics of the rivers/basins for which the methods are developed, are different from that of Kenyan rivers/basins. There is therefore a need to select standard methods and software/s which can be used at national level irrespective of the type and scale of project under consideration. his research has used HEC- Ecological Functioning Model (EFM) an open source software in water resources planning (in the Kenyan context) through modeling of Kisumu Water Supply and Sanitation Long Term Action Plan, using Kibos River as source of water. The three environmental indicators employed for the research are fish (Labeo, Clarias and Barbus), micro-invertebrates in general and Nyamasaria swamp as wetland. The methods applied for assessment of risk level are modified method derived from Davies and Humphries (1996) for Risk Levels Assessment based on Modified Key Ecological Variables and the method developed by Tannent in 1976 for identified critical minimum flows required for Fish, Wildlife and Recreation in streams. . The research has revealed that there will be environmental change on Kibos River due to the proposed intake/diversion weir on Kibos River. It is expected that there will be significant migration of fishes from the affected reach of the river to the reach upstream of the diversion weir and to the river reach downstream of Awach and Kibos confluence. This can only happen if the run-of-river. scheme treatment plant is operated at 48,000 m3/day throughout the year. July 2013 Page xii of xii If the city is supplied at 36,000 m3/day as run-of-river scheme and if necessary mitigation measures are taken the water supply project can be compatibly integrated in the ecosystem.