Abstract

The geomorphic response of a stream to urbanization is a common interruption to the channel geometry equilibrium with dire consequences of sedimentation of reservoirs, mass wasting processes and water quality deterioration. The channel morphology was investigated with respect to deforestation, an increase in built-up surfaces and bank instability within the Motoine/Ngong River sub-catchment of the Nairobi River Basin in Kenya. The study examined the relation between growth of built-up surfaces and channel morphology at four representative sampling points along the course of the River for the period 1976 and 2013 for which there was sufficient record. The findings indicate a steady spatial increase of the built-up surfaces by 50.9% during the period. The impervious surface reduced infiltration capacity, simultaneously increasing surface runoff and stream flow and seasonal flow variability. The increased discharge caused bank erosion in some places and sedimentation in the others, a sinuous channel morphology characterized by river cliffs, river bank cavities, collapsing overhanging banks, tension cracks, slip-off slopes, sand bars and a braided river channel. The changing storage capacity of the Nairobi dam is currently unknown due to lack of instrumentation and hydrological records.

Key words: Watershed/catchment, urbanization, built-up surfaces, erosion, sedimentation, channel morphology, Motoine/Ngong River.