Flood control and assessment of Accra using the weap hydrological model

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January, 2018
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The increase in the frequency and intensity of flood in some parts of Greater Accra Region is becoming a matter of great concern to the entire nation due to its negative effects on the development of the region. These floods have over the years been attributed to numerous factors including; climate change, urbanization, poor drainage systems, sea level rise and over population. The study used Water Evaluation and Planning (WEAP) hydrological model to estimate the surface runoffs for the sub-basins and also to find the correlation between Standardized Precipitation Index (SPI) and the historical flood events in Greater Accra. The study also determined the surface runoffs generated within each sub-basin. The WEAP model was used to generate surface runoff from 1990 to 2040 taking 1990 as the base year. ArcGIS 10.3 was used to delineate eight sub-basins and thereafter deter mined the characteristics of the sub-basins. Rainfall normalization was performed using the Standardized Precipitation Index and the results were verified against historical flood events for the basin over the study period. The model output was compared to observed stream flow measurement. In general a good correlation of 0.81 and 0.71 were obtained for correlation coefficient and coefficient of determination respectively. The study re vealed that, a minimum rainfall of about 60 mm would cause flood across all the eight (8) sub-basins. Also, extreme rainfall with SPI of 1.5 or more would cause flood in Greater Accra. The month of June was found to produce the highest volumes of surface runoff while December generated the least volumes of surface runoff with average runoffs of 2.4 ii× 106 m3 and 1.0 × 106 m3 respectively. It was confirmed from the research that, higher volumes of rainfalls in the sub-basins generates higher surface runoffs and therefore there are higher tendencies for floods to occur when rainfalls are high.
A thesis submitted to the Department of Physics, Kwame Nkrumah University of Science and Technology, in partial fulfillment of the award of degree of Mphil( Geophysics)
Flood control, Weap hydrological model, Accra