Effects of land use change on stream flow, channel erosion and river geomorphology: a case study of Motoine/Ngong river sub-catchment, Nairobi river basin, Kenya.

Abstract

Urbanization increases impermeable surface cover which causes streamflow variability, the most significant element in geomorphological processes of erosion and sediment transportation. Understanding the interaction among human activities, hydrologic and geomorphic processes, is fundamental in addressing environmental degradation. Motoine/Ngong river sub-catchment has suffered severe spatial land use change in the last three decades due to urban expansion. The study sought to assess effects of land use change on stream flow, channel erosion and channel geomorphology of Motoine/Ngong river sub-catchment. Specific objectives were to determine the land use changes within the sub-catchment from the years 1976 to 2012, to establish the relationship between land use change and stream flow, and assess erosion of gullies and river banks and the geomorphology of the river channel. It was hypothesized that there was no significant relationship between stream flow, gully and river bank erosion and changes in land use from 1976 to 2012 in the Motoine/Ngong River Sub-catchment. Primary and secondary sources of data were used in the study. Primary data on discharge was obtained from field measurements of cross-sectional area of the channel at the Motoine swamp outlet, Ngong Road Bridge, at the Nairobi dam inlet and at Kangundo Road Bridge. Stream velocity was measured using a current meter by wading. Geomorphology of the river and river bank erosion were obtained by observation and photography. Secondary sources provided information on stream discharge, land use and gully erosion. Analysis of land use change was done from satellite imageries and selfadministered structured questionnaires. Data was analyzed using percentages and averages and presented in tables and charts. The study revealed that there has been temporal and spatial change in land use in the sub-catchment. Clearance of forest and grassland vegetation increased bare surfaces resulting to gully erosion at the catchment. Built-up area and road pattern of land use increased from 22.78% in 1976 to 50.98% in 2012. This increased wet weather stream flow resulting to erosion of river banks to produce varied landforms such as river cliffs, river bank cavities, desiccation cracks and river bed outcrop rocks. Sediments produced from the processes of erosion were deposited in the river channel leading to the development of slip-off slopes and sand bars which resulted to the development of a braided channel pattern Correlation between built-up area and road and stream flow yielded a value of 0.8. The hypothesis that there is no significant relationship between stream flow and changes in land use between 1976 and 2012 was rejected at a confidence level of 95%, and the research hypothesis was therefore accepted. The study recommends that detention storage and infiltration of rainwater be undertaken to reduce overland and stream flow. There is need for gabions to be constructed to protect highly erodible sections of the river banks.