Morphometric features of a low-elevation urban catchment and the implications for flooding

Abstract

River basin morphometric is a geomorphological tool that applies to physical measurements about a river form, from which can be derived quantities that explain its characteristics, such as size, shape, stream network and discharge. This paper used basin morphometric to assess flood occurrence in Port Harcourt, Nigeria. Previous analysis of Port Harcourt region’s drainage as it pertains to flooding described the area as lacking significant displacement in relief; therefore, rivers were characterized by sluggish flow velocity, ponded in many parts and tidal in others. The consequence of absence of gradient as described by these authors is an area prone to widespread flooding; however, the basis of their observations was their reliance on topographic maps whose contour intervals were  15.24m in elevation. Extant topographic maps of the area were used in this study to render and extend contour lines to hitherto, flat surfaces at intervals of 3m. The result was a map with fairly crowded contour lines of the area described as Greater Port Harcourt city, which comprises the old city and all the on-going developments and proposed new city limits. The basin is a 3rd order basin; there were 6 first order streams, 3 second order streams and 1 third order stream. Flooding in the northern part of the  metropolis is caused by the ponding of depressions and human settlement
practices like roads and widespread concrete fences. In the south, very low elevation ranging between 0 – 12m and extensive tidal mudflats  transformed into human habitation predispose it to flooding. This study proposes that ponded areas be carefully drained and low lying areas  adequately planed and managed to check flooding in the city.

KEYWORDS: Basin Morphometric; Urban Flooding; Greater Port Harcourt; Ponded Depressions.