Characterization of Surface Runoff in Port Harcourt City:Effect on Urban Flood Disaster Management

Abstract

Port Harcourt, a sprawling urban landscape in the humid tropics is prone to high rainfall runoff. The basic requirements in designing surface water projects for urban development, is the estimation of runoff resulting from precipitation. This study analyses the rainfall-runoff capacity of Port Harcourt and its implications to urban management. Mean monthly rainfall and temperature data for 40 years (1979-2018) for the study were sourced from Nigeria Meteorological Agency. The US SCS-CN and Khosla‘s models that incorporates rainfall and temperature data were utilised in the study and a correlation of the two outputs using Pearson‘s Product Moment Correlation Coefficient conducted. Results from the analysis showed that average minimum and maximum runoff depth for the entire area were 0.1 mm and <800 mm with runoff coefficient of 0.1. For the period under review, January and December had the least average monthly runoff depth for Port Harcourt at 14.2 mm and 15 mm, respectively; while July and September had the highest at 330 mm and 327 mm. Average annual estimated runoff depth for the area was minimum in 2011 at 127.8 mm and maximum in 2007 at 220 mm. There was a high positive correlation between the two methods used for this study up to 0.99 with standard deviations and coefficient of variations of 20.83 and 21.13; 11.98 % and 11.92 % for the SCSCN and Khosla‘s models, respectively. The estimated runoff depth for the area showed that average runoff values between 0–99 mm constituted about 33 % while values >99 mm constituted about 67 %. Land cover changes has been on the increase in Port Harcourt and will considerably impact periodic hydrologic variations. The implication of this study is that Port Harcourt will always be prone to intermittent flooding especially during intense storm if efforts are not made by policy makers to effect sustainable urban plan that mitigates flooding.