FLOOD VULNERABILITY ASSESSMENT OF WASTE TREATMENT SITES: A CASE STUDY OF THE ASAFO SEWERAGE TREATMENT PLANT IN KUMASI, GHANA

Abstract

The study assessed the flood vulnerability of the Asafo Sewerage treatment site within the Kumasi Metropolitan Assembly in Ghana using the HEC-RAS model for 1- and 100-year floods. Three distinct scenarios (S1, S2 and S3) relating to the point where the Gee River joined the Subin River and another that combined S1 with a built levee (S1+Levee), were tested to ascertain the most effective way of preventing floodwaters from entering the treatment site. Adopting S2 would increase the extent of inundation from the natural regime by 12.3 19.4% for 1-100-year floods in the site. The result depicts a direct relationship between flood inundation areas and designed streamflow for the different return periods within the treatment site, with the natural regime already experiencing inundation at least once a year. The observed flood inundation could be due to the low ground elevations and the natural depression created by the diversions of the Gee and Subin Rivers from their natural state within the treatment site. The results revealed a significant decrease in the 1-year flood extent under S3 (71.58%) and S1+Levee (94.94%), despite a slight rise under S1 (3.24%). The extent of inundations in the 100-year flood was estimated to drop by 39.0%, 40.4%, and 97.0% under S1, S3 and S1+Levee, respectively. The treatment site recorded low flood depths (0.0-0.6 m) and velocities (0.0-1.0 m/s), whilst high flood depths (> 1.0 m) and velocities (> 4.0 m/s) were within the main Subin River channel. It is worth noting that not all the inundated areas within the treatment site are susceptible to high-risk flood levels, with most of the designated high and very high flood-risk zones located within the main Subin and Gee Rivers channels. Based on the study, S1+Levee is considered the best option to adopt for reducing the amount of flooding within the treatment site, followed by S3 and S1 in that order.