Geomorphic instability of small streams subjected to rapid urbanization in coastal Nigeria

摘要

The Ntamogba Stream experiences flood in the middle and lower reaches of the sub-basin each time it rains; it was discovered during the study of the area that, the stream has lost its flood carrying capacity which is brought about by the in-filling of the stream channel with refuse/garbage leading to streambed aggradations, erosion of stream banks and/or accelerated peak flow of storm water from the surrounding impervious basin. The inability of this stream to accommodate the flood water due to bed aggradations can best be described as geomorphic instability. Basin and channel characteristics including discharge were evaluated. We noticed that Reach 2 & 3 (Okija/Emekuku streets culvert) has an unusually large cross sectional area/wetted perimeter and discharge is quite high a reflection of large percentage of impervious drainage basin and beyond this point the cross sectional/Wetted perimeter is slightly reduced; this geomorphic imbalance results in large scale flood. The study also highlights that all natural flood receptacles have been destroyed by land speculators and because the catchment's characteristics determine sediment and nutrient inputs to the streams, it was observed that upland disturbance has affected the stream's geomorphic stability.