SUBSURFACE DAM FEASIBILITY ANALYSIS AND FLOOD PROTECTION AGAINST OVER TOPPING & EROSIONS OF RIVER BANK

Abstract

This study focused mainly on river training and water utilization option in Derek Wuha Catchment of North Wolo zone of Amhara National Regional State. In the study, a GIS based semi-distributed watershed model (HEC­ HMS), HEC-GeoHMS and arcHYDRO softwares were used to establish rainfall-runoff transformations of the ungauged basin while HEC-RAS was utilized for flood depth estimation and bank stability analysis input parameter generation in the catchments. From the hydrological analysis the peak discharge of the catchments was found to be 103 . 7m 3 /sec with 850.9x10 3 m 3 volume of water yield for 100 year return period of 24 hour rainfall at about 31.14km 2 areas of the catchments. For the discharge of 103.7m 3 /sec which is obtained from hydrology model, the corresponding flow depth was generated using HEC-RAS and about 326m of the left river channel [at nursery site starting at 2872m from bridge] is incapable of safely passing the discharge without over topping . Furthermore, the bridge near the outlet of the catchment was simulated with different flood magnitudes and it was observed that the capacity of the bridge could not safely pass a 25 year return period discharge. The river bed and bank material was tested in soil laboratory at Kombolcha Soil and construction materials testing laboratory to determine various engineering properties of the soils and it was found that 31.6% of the bed material is voids at the subsurface dam proposed site. Based on this water holding capacity of the river bed materials, if subsurface barrier or dam is constructed across the river with a depth of 8m, a water requirement of the local residents can be fulfilled at least for 6months of the year without any recharge. The river bank just downstream of bridge location was analyzed using Bank Stability and Toe Erosion Model with the protection work incorporated and it responds to conditionally stable safety factor.