عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Population growth, urbanization, and industrialization of human communities have negative effects on basin’s hydrology. Besides, expanding the impervious area increases the amount of surface flow coefficient, runoff and flood volumes. According to land use changes, one of the best management 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 the 9th district of Mashhad City, Iran, this study proposed the utilization of the existing detention ponds as well as 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 Mashhad, Iran. In order to determine the dimensions and length of the canals, the maps were obtained from the Information Technology Organization of Iran (ITO). According to field visits as well as analyzing the digital elevation maps, the study area could be divided into 88 sub-basins. Infiltration coefficient was selected based on the land use of the basin (residential, street, etc.). Manning coefficient was determined as 0.015 according to the software help of GIS for the channels and its coverage for each sub-basin. The effective catchment width with a suitable approximation was obtained from dividing of the basin area to the channel length. For calculating the average slope of the basin, the most accurate method was the use of slope pattern algorithm in GIS software. 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. The IDF curve of Mashhad was used to determine the return period and rainfall pattern. Two-year return period was considered for simulating flood distribution in the studied area. The prerequisite for determination of design flood period is to select an appropriate rainfall return period. In this regard, the average daily water demand determined for irrigation of green space was equal to 3.936 m3 per day. The volume of the existing reservoirs was calculated about 1305 m3. As the first step for modeling the studied area, all sub-basins and channels were defined in AutoCAD software and introduced to model, separately. In the second step according to the characteristics of each layer, the physiographic data such as area, basin width, average slope, digit elevation, manning's coefficient were defined in the model for each layer. Afterward as the third step, ASSA software was employed for simulating different features of the studied area. Then in the fourth step, the required physiographic and topography data were imported from GIS to this software. After that, precipitation data, Horton coefficients, and the other metrological and hydrological data were introduced into the model.
After running the model, the discharge and the total runoff volume of flood were estimated equal to 6.55 m3/s and 17531 m3, respectively. The total numbers and volume of available tanks were equal to seven and 1305 m3, respectively. Among the existed tanks, two tanks had 9*9*4 m3, two tanks had 6*6*3 m3 and the other three tanks had 7*7*3 m3 dimensions. The dimensions of the proposed detention ponds were 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 of the model showed that without detention ponds, the runoff volume and the peak were equal to 17,531 m3 and 6.55 m3/s, respectively. Using the available detention ponds in the studied area, leaded the total volume of runoff to be decreased from 17531 to 16298 m3, which demonstrated 7% decreases in the flood volume. Peak of runoff was also decreased to 5.71 m3/s, indicated 13% decreases. Moreover, by adding the proposed detention ponds, the amount of outflow runoff was decreased to 14,900 m3, which demonstrated 9% decreases in comparison with using the available detention ponds and 15% decreases in comparison with no detention pond state. Eventually, regarding the results obtained in the current study, it could be acclaimed that use of run-off detection ponds can reduce peak flow and outflow run-off volume from the basin, as well as economic benefits for irrigating green space.