HYDROLOGIC RESPONSES OF URBAN WATERSHED: THE CASE STUDY OF GONDAR TOWN

Abstract

Urbanization changes the hydrologic behavior by increasing the proportional area under impervious surface and level of imperviousness. An increase in imperviousness is associated with increase in runoff magnitude and decreased time to peak. These phenomena have adverse impacts on urban storm water structures and receiv ing streams. Hydrologic and hydraulic data are essential to sustainable design of storm water structures and for planning of mitigation measures. The objective of this study is to estimate the hydrologic response to watershed characteristics of Gondar town. The total area of Gondar town is 10.64 sq.kms. About 80% of this area (8.78 sq.kms) drains to Keha stream: tributary of Megech River. Therefore the scope of the study is geographically limited to Keha watershed. The EPA SWMM5.1 which is a distributed rainfall runoff model was used in runoff simulation. SWMM parameters are broadly sub catchment parameters and drainage parameters. Sub catchment parameters comprised physical and hydrological parameters. ASTER 30m×30m DEM was used to extracted physical parameters by terrain processing on ArcGIS/HECGeoHMS. Hydrological parameters are soil properties and percent of impervious. Percent of impervious were determined from land use land cover map prepared from RS image through supervised image classification on ERDAS IMAGIN 2010. Runoff coefficient was determined matching recommended runoff coefficients for LULC categories recommended in literatures. Drainage parameters were determined from data of field survey, road map and contour map. Continuous simulation of Time series of daily rainfall data (from 1996 to 2011 GC) was used. The model continuous simulation result shows that the capacity of the existing lined channel is adequate but the velocity in stream channels is higher than permissible values recommended by literatures. The same simulation result provides annual peak flows and their exceeedance frequency on the outlet link of the study watershed (along Keha stream). Simulation by replacing projected % of impervious for undeveloped and less developed sub catchment indicates that peak flows of Keha stream increases by 0.94 %, 1.71%, 2.88% and 3.87% when % impervious increases by 10%, 20%, 30% and 40% respectively; and the relation between impervious and annual flow is linear which indicates the flow estimation is good. Other simulation results from three scenarios; existing, future development (uncontrolled) and developed controlled by porous pavement proves that application of porous pavement reduces runoff to a certain level.