Response of the Jakara stream channel to urbanisation

Abstract

This study assesses the morphological response of the Jakara stream channel to the effect of urbanization of its catchment. The proportion of the Jakara catchment under urbanization was determined using black and white air photographs taken in 1961and 1981, landsat   imagery of 1987, 1995 and 2006. These were used together with land use maps, road maps and layout plans and ground truthing and three sites were identified thus: An upper watershed dominated by urbanization – Urban; A middle section that is under going urbanization - Semi urban and - A lower catchment that is primarily rural –Rural. Detailed field survey was conducted in six reaches of the Jakara channel, two each in urban, semi urban and rural sites to measure morphological variable. Morphological variables of the Jakara channel reaches under different levels of urbanization were compared. Consistent and significant differences in the sites were demonstrated in bankfull width, depth, cross section and wetted perimeter. Jakara channel is demonstrated to be consistently larger in the reach under urban land use than those under non-urban land uses, with capacity ratio of 2.36, width ratio 1.94, depth ratio 2.25. Most studies in humid tropical areas of Nigeria and temperate areas elsewhere reported smaller urban river channels enlargement. Sinuosity was 68.58% less in urban than rural reach. Channel density increased by 28.6% due to storm sewers, culverts and other runoff removal. The results of the study has implications for urban channel management and suggest that different strategies may be required for  channel reaches based on the type of land use and that the selection of a mitigation strategy is dependent upon the extent to which the channel has been impacted by urban within the catchment.