THE IMPACT OF MEGA INFRASTRUCTURE ON THE HYDROLOGY OF RUIRU AND KAMITI RIVER CATCHMENTS

Ruiru, a town within the metropolitan area of Nairobi Kenya, has experienced rapid urbanization which has generated a lot of challenges and problems – economically, socially, and environmentally The Purpose of this research was to study the impact of mega infrastructure on the hydrology of Ruiru and Kamiti rivers. The catchment area was defined by River gauging station (RGS) 3BB12 on River Kamiti and delineated using the RGS as the catchment outlet and was based on SRTM void filled Digital Elevation Model (DEM). Data sets utilized in the study were landsat images, google earth images, river flow data obtained from from water resources authorities (WRA), river abstraction records, water production data and rainfall data. Various GIS methods employed to analyze the data included normalized difference built up index (NDBI), hydrological characterization using the threshold method, change detection, and GIS software (ILWIS and Grass GIS). The supervised classification built up index for the study area increased from 2.8% to 51.8% for the period under study. The mean flows over this period decreased from 2.2m3/s to 0.53m3/s. This indicated that there was a definite effect of change in land use to the river flow regime. However, to conclude that mega projects had a separate effect to ‘normal’ urban development would not have been correct. This is because the flow duration curves for the different periods (1958-1987, 1988-2000, 2005-2010, 2011-2016) showed a similar pattern with no abnormal changes seen in the previous 10-15 years. It is therefore recommended that developmental control be enforced. This will assist in mitigating against the negative effects of land use conversion. The present development control authorities need to be strengthened to enforce development control. The zoning guidelines and planning regulations form the basis for developmental control and should be enforced.