Modeling of Storm Water Runoff for Improving Floods Resilience, Water Supply, Soil and Ecological Conservation. The Case of Msimbazi Catchment in Dar Es Salaam, Tanzania

Abstract

"In rapidly urbanizing catchments, increase in stormwater runoff quantity may lead to numerous of serious problems such as flash floods, soil erosion and alteration of an ecosystem. Changes in land use usually affect stormwater runoff flow characteristics as much water has to flow on the surfaces rather than infiltrating to the ground due to increase in impervious surfaces. The gap between the hydrologic responses to urbanization and the engineering solutions is wide. Most of the studies related to stormwater management have been more focusing on treating the issues separately leading to insufficient stormwater management systems. In order to have proper and sufficient stormwater management systems, more information is needed to link the abovementioned cases together. In this study, the HEC HMS model was used for rainfall-runoff simulation to quantify the amount of runoff generated with respect to the land use and precipitation data for runoff dentation facilities design. Satellite images for 1998, 2009 and 2018 of the catchment area were selected based on the quality of data and the available resolution. ArcGIS and GIS extension tools were used to extract hydrological characteristics of the catchment; HEC –HEC-HMS was used to simulate the rainfallrunoff process on Msimbazi river watershed which is the major watershed located in Dar es Salaam. The hydrological modeling was accomplished by dividing the watershed into different sub-catchments. To compute infiltration loss SCS CN method; converting excess rainfall to runoff model SCS unit hydrograph, and channel flow routing accomplished by using simple Lag routing method of the HEC-HMS model. The land use processing identified five major land use classes namely; water bodies, forests, developed low intensity which its coverage changed from 61.67% in 1998 to 21.13%, developed medium intensity changed from 25.97% in 1998 to 61.12% in 2018 and developed high intensity changed from 3.82% in 1998 to 19.77% in 2018. The computed peak discharge for the 1998 study year was 355.7m3/s, 402.7m3/s in 2009 and 437.8m3/s in 2018 which is an increase of about 23.08% from 1998 to 2018. Some small sub-basins were merged to form five bigger sub-basins linked with a designed reservoir at each outlet of the five sub-basins. Which means reservoir 1 receives water form subbasin 1, reservoir 2 from sub-basin 2, reservoir 3 from sub-basin 3, reservoir 4 from sub-basin 4 and reservoir 5 from sub-basin 5. The peak inflows for all detention basins were observed in the extreme event of 20 December 2011, where reservoir 1 had peak inflow of 123.6 m3/s, reservoir 2 had 43.5m3/s, reservoir 3 had 37.5 m3/s, reservoir 4 had 60.5 m3/s and reservoir 5 had 19.2 m3/s. A total of 284.5 m3/s which is about 65% of the peak runoff discharge generated in the catchment. The findings of this study will redound to the benefit of the society in Dar es Salaam and other places with similar characteristics since stormwater management has been a growing issue of concern in highly urbanized cities. The detention basins designed in this study will play a great role towards capturing and slowing runoff and extend flow period of the Msimbazi river."