Assessment of Stormwater Drainage System for Small Urban Watershed: Case of Shambu Town, Oromia Region, Ethiopia

Stormwater runoff in a small urban watershed is one of the major challenging issues facing many countries where financial capacity is very weak to provide sufficient drainage systems. This study aims to assess the stormwater drainage system in the small urban watershed as a case study of Shambu town, Oromia Region, Ethiopia. The main objective of this study was to assess whether this excess stormwater runoff is due to hydrological change or insufficient hydraulic design of the existing drainage system in the town. The existing drainage system in this small urban watershed was analyzed by statistical analysis and the main purpose of this was to know whether the designed drainage systems were sufficient hydraulically. The existing drainage system in this small urban watershed was analyzed by statistical analysis and the main purpose of this was to know whether the designed drainage systems were sufficient hydraulically. The existing design document was taken in this analysis and field analysis also made to check whether the drainage was constructed according to the specification. The area also analyzed hydrologically by delineating the watershed using ArcSWAT and the peak discharge was computed the outlet of the watershed using Rational method. Some data such runoff coefficient and manning’s values were also generated from the watershed based on the reality on the ground. The two results from hydrological and hydraulic models were compared and that of peak discharge obtained from rational method became greater than of the Peak discharge obtained from existing drainage measurement. Therefore, in this study it was concluded that since the watershed is steep in slope, it is contributing additional runoff to this small urban watershed, additional new 50% of the existing drainage system and rehabilitation works should be implemented to modify the draining capacity of the existing scheme. Additionally, managing and improper construction alignment problem in the existing system were identified, so to control this challenge periodic cleaning and modification of slope is recommended.