Comparison of uniform rainfall method and alternative block method in estimating runoff collecting system in order to control flood in urban areas with ASSA software (case study: Chehel Bazeh Golestan Basin)

Due to an increase in impermeable areas in cities, the runoff volume increases dramatically, causing overflow of runoff and frequent problems in the city. In the present study, the 10th district of the municipality of Mashhad that is known as Chehel Bazeh Golestan was evaluated. In this regard, Arc Map and ASSA software were used for modeling. The basin was modelled according to the existing conditions by using AutoCAD and Arc Map. The geometric parameters of the basin including area of the sub-basins, length of the canals, slope and other characteristics were calculated by using Arc Map. For rainfall distribution, uniform rainfall pattern and alternative block method were used. For the return period of 2 and 5 years, the adequacy of system for runoff was investigated. Results showed that rainfall pattern plays an important role in the basin runoff. The alternative block method for the return period of 2 years, resulted in 33% increase in runoff relative to the uniform rainfall method. In the alternative block method, the outflow runoff of the basin was 29% more than the uniform rainfall method, and the outflow volume increased by 17%. In general, in urban areas due to variation in land use and different basins, uniform rainfall method, due to consideration of geometric parameters of basins, offers better results than alternative block method.