Flood risk assessment and mitigation along river Unyama:

Abstract

Unyama sub-catchment has an estimated drainage area of 1565 km 2 covering parts of Gulu and Amuru districts. The high stages of R. Unyama in Elegu flood plain are caused by heavy rains in Gulu and Amuru districts. This challenge has existed since 1960 but limited interventions have been done as businesses and other activities continue to be stifled. This research focused on flood risk assessment and mitigation along river Unyama in Elegu flood plain. Flood risk assessment was done through hydrologic and hydrodynamic modelling techniques. Since R. Unyama is not gauged, a rainfall-runoff model was developed using Arc-SWAT, Gumbel distribution method was used to analyze discharges for various return periods. Flood analysis was done using HEC-RAS software version 6.2, flood inundation maps corresponding to discharges of different return periods were developed and analyzed. A multi criteria approach was used to determine the most effective structural flood mitigation measure along river Unyama. Diversion and detention was found to be the most appropriate mitigation measure, Diversion channels were designed upstream of Elegu flood plain to divert the flood water before it reaches Elegu flood plain and store it in detention ponds such that it can be released at gradual and controlled intervals with an aim of attenuating the flood., the diversion and control structures were designed and simulated in HEC-RAS. Detention ponds were sized using hydrology studio, a flood routing-based software. The findings of the study were as follows; the discharges of 5, 7, 10, 25, 100 and 200-year return periods were 200m3/s, 233m3/s, 270m3/s, 360m3/s, 493m3/s, 558m3/s respectively. A 10-year return period discharge of 270 m 3 /s was considered as the design discharge of the diversion structure. Three diversion channels were designed and simulated, diversion channels one, two and three diverted, 29.3%, 33.7% and 15.6% of the total peak flow respectively from the river to detention ponds while 21.5% of the total peak discharge continued to Elegu flood plain where very minimal flooding occurred with maximum flood depth of 0.009m. Some of the recommendations included; the MWE should consider installing a river gauging station on River Unyama to ensure availability of accurate and reliable data, research should be done on willingness to pay for project implementation by the local people, the scouring effect of water on the gates and the channels should also be investigated.