Critical Hydrological Dynamics and Flooding in U-Shaped Valleys Within Urban Areas: A Case Study of Some Stream Channel Systems in Yaounde and Its Environs, Southern Cameroon Plateau

Abstract

Roger Bissaya, Richard Tanwi Ghogomu, Bernard Njom and Mero Yannah

This paper examines the hydrological dynamics and flooding of U-shaped valleys in urban areas, with projection to some stream channel systems in Yaounde and its environs. So, it relies on analysis of unmodified (or natural) stream channel systems, assuming that the quantity of water of an unmodified stream channel system is directly related to the size it usually carries and that it tends to bring back during high discharges or floods. The drainage basins are usually elongated, with the time of concentration depending on the trunk stream length, e.g., 9.35 hours for the Mefou watershed and 5.21 hours for the Mfoundi sub-watershed. The streams continuously adjust the shape of their channels; for instance, in the Mefou River, the floodplain straight channel segments do occur with sinuous line, thus the sediments are deposited to form bars. Such path and deposition of sediments result in meandering. During extremely high discharges, the water flows in a braided pattern around islands and bars. Although generally characterized by dense, dendritic drainage patterns, the stream channel systems sometimes exhibit a remarkable centripetal drainage pattern associated with flood basins. Since the river discharge usually exceeds the capacity of the channels at U-shaped valley junctions, such as the low-lying areas at Nkolbisson, Midtown, Nguem, Etoa, and Mehandan, floodings are recurrent.