The Impacts of Urbanization and Impervious Surfaces on Runoff of Sardaabrud Basin, Kalardasht, using HEC-HMS Rainfall-Runoff Model

Urbanization development and residential constructions cause permanent changes in agricultural land use features. Therefore the outcome of these changes is an increase in impervious areas in cost of farm lands and natural resources ecosystems، which in turn results in devastating urban runoff. To reduce the cause of urbanization it is inevitable to control، conduct and manage urban flooding. Therefore، the aim of the present study is to assess the impacts of urbanization development on the watershed runoff of Sardaabrud basin، Kalardasht، using HEC-HMS hydrological model. Using the most recent satellite images the residential land areas in the sub-basin were determined. Results show that residential area in downstream of sub-basin (S2) has remarkable growth in compared to upstream sub-basin (S1). Residential area in S1 sub basin includes 0. 3 percent of total area، while in S2 sub-basin it took 8 percent of the total area. Four flooding events were employed to analyze the sensitivity of the model. Results showed that initial loss has high sensitivity compared to the other parameters. Based on this finding the model was validated. The RMSE، t test and correlation coefficient (r) values were acceptable for peak flow in model simulation. Finally، in order to determine the impacts of urban area on watershed runoff، the thunderstorm characteristics were estimated based on intensity – duration. For this purpose the frequency analysis of rainfall data was employed using extreme value type 1 and its curves for 2، 5، 10، 25، 50، 100 years return period. The results show that 25 years flood for S2 sub-basin is almost 4 times of S1 sub-basin. The potential runoff producing of the S2 sub-basin has increased compared to the S1 sub-basin، which is due to development of residential areas.