Adequacy Evaluation of Existing Run-off Detention Pond in Ab-o-Bargh Basin for Green Space Irrigation and Propose Appropriate Location for Development Plans

Population growth, urbanization, and industrialization of human communities have adverse effects on basin’s hydrology. Increasing the impervious area increases the amount of surface flow coefficient, runoff and flood volumes. According to earth surface changes, one of the best human activities is the use of surface runoff for different purposes in urban area. The main purpose of this study is to investigate the use of surface runoff for irrigation of green spaces. Due to the high volume of water demand in 9th district of Mashhad, in this study, we propose to utilize the existing detention ponds (and design new detention ponds) for irrigation of the green spaces during the flood events.

The case study is located in ab-o-bargh basin with about 770 hectares area. In order to determine the dimensions and length of the canals, maps from the fava organization are employed. According to field visit as well as digit elevation, the study area is divided into 88 sub-basins. Infiltration coefficient was selected based on the soil texture of the basin (residential, street, etc.). Manning coefficient was determined as 0.015 according to the software help for the channels and its coverage for each sub-basins. The effective catchment width with a suitable approximation obtained from division of the basin area over the channel length. For calculating the average slope of the basin, the most accurate method is the use of slope pattern algorithm in GIS. Channel slope was calculated similarly with the start and end elevations of each channel. Horton and keirpich methods were used to estimate the infiltration and concentration time, respectively. Regarding the return period and rainfall pattern, the IDF curve of Mashhad was used. Two-year return period was considered for simulating flood distribution in the study area. The first step for designing flood’s design period is to select an appropriate rainfall return period. In this study, average daily water demand for irrigation of green space is equal to 3.936 m3 per day. The volume of existing reservoirs is about 1305 m3. For modeling the study area, at first, in AutoCAD software, all sub-basins and channels are defined and introduced to model separately. According to the characteristics of each layer, the physiographic data of each layers such as area, basin width, average slope, digit elevation, manning's coefficient was defined in the model. For simulating different features of the study are, ASSA software was employed. Then, the required physiographic and topography data was imported from GIS to this software. After that, precipitation data, horton coefficients, and the other metrological and hydrological data was introduced into the model.

After running the model, the discharge and the total runoff volume of flood are equal to 6.55 m3/s and 17531 m3, respectively. The total numbers and volume of available tanks are equal to seven and 1305 m3, respectively. Two tanks have 9*9*4 m3, two tanks have 6*6*3 m3 and three tanks have 7*7*3 m3 dimensions. The dimensions of the proposed detention ponds are three tanks with 9*9*4 m3 and three tanks with 7*7*3 m3 dimensions with a total volume of 1,413 m3. Results showed that without detention ponds, the runoff volume and peak was equal to 17,531 m3 and 6.55 m3/s, respectively. Using the available detention ponds in the study area, the total volume of runoff is decreased from 17531 m3 to 16298 m3, demonstrated 7% decreases in flood volume. Peak of runoff is also decreased to 5.71 m3/s, indicated 13% decreases. Moreover, by adding the proposed detention ponds, the amount of outflow runoff is decreased to 14,900 m3, which is demonstrated 9% decreases in comparison to using the available detention ponds and 15% decreases in comparison with no detention pond state.