Study On The Effects Of Rainwater Detention Tank In Dianchi Multi-level Drainage System

In order to protect the water quality of Dianchi the biggest lake in Southwest China the government has made great effort to build the Lake-surrounding system which is used to protect the Dianchi from pollution by the Initial Rainwater and combined wastewater. As an important instrument to control the non-point source pollution, Rainwater detention tank can make a very obvious effect in this Lake-surrounding system.This paper studied the layout scheme of multi-level rainwater dentention tank and the efficiency of detention tank. The paper provide three layout schemes of multi-level rainwater dentention tank, besides, this paper simulated the flow pattern of water in the dentention tank provided the method to improve the flow and the efficacy of Rainwater detention tank in the pollution reduce.At first, the paper briefly introduced the the production and development of SWMM, and then introduced its modular theory and simulation capability and typical application, at last, the paper introduced the SWMM development method and write a C language program accoding to the data provided by EPA. This paper used SWMM to establish the water drainage model which located in east coast of Dianchi south to Haiyan north to the Luolong River sewage treatment plant. The model covers an area of 60.97 square kilometers.Once every half year of rainfall time series was synthesized, according to the formula of rainfall in Kunming and Chicago rain type. The rainfall depth is 18.51mm; the whole model received a total of 1 million 1128.6 thousand cubic meters of rain. It is found that the overflow place of the whole model is 106 and the water overflowed was 191 thousand m3 if the Rainwater dentention tank was not set. The main sewage pipe which is surrounding the Lake overflowed 127000m3, other main pipe overflowed 64000 m3.In order to control the overflow effectively, three layout scheme of multi-level rainwater dentention tank was put out:(1) put the detention tank near the place where the water overflowed from the sewage model,(2) put all the detention tank on the main sewage pipe which is surrounding the Lake(3)put all the detention tank on the main sewage pipe which is’ t surrounding the Lake. Of three kinds of layout scheme of rainwater dentention tank simulation analysis, if the guarantee 0.5 years of rainfall does not overflow, three schemes required detention tanks with a total volume of: 23.57 million m3, 2.53 million m3, 2.501 million m3 respectively. Corresponding pollutant removal capacity, the second scheme can remove the most pollutants, the third scheme, the first is the worst.In this paper, the flow pattern and flow velocity distribution in the detention tank are simulated and studied, besides a method to improve the flow pattern and the uniformity of the flow velocity is proposed. Simulation shows that under the condition of full flow. By changing the storage pond inlet position and can not effectively reduce circulation phenomenon, nor did it improve the regulation of the uniformity of the velocity of flow in the pool. If the water in inlet is on the middle of the pool, the maximum velocity will be 0.021 m/s, velocity distribution will between-0.003 m/s to 0.021 m/s. If the water inlet is on the side of the pool, the maximum velocity will be 0.042 m/s, velocity distribution will between-0.0017 m/s to 0.0042 m/s. Provided with a perforated plate, storage pool flow velocity uniformity and flow regime can get obvious improvement. If a perforated plate which the bore diameter is 0.1m was set, the flow velocity distribution in the detention tank will between 0.009 m/s to 0.0011 m/s, also the circulating flow phenomenon can not be seen in the streamline map. Setting flow diversion device can also effectively improve the flow pattern and the uniformity of the flow velocity. After the improvement of the water diversion device is arranged, the flow velocity in the detention tank will between 0.004 m/s to 0.015 m/s. Compared the detention tank with non setting flow diversion divice, the circulation area is smaller in the detention tank which the diversion divice is set.