| With the development of urbanization,the underlying surface condition of small watershed in mountainous city has changed a great deal,which directly causes runoff and confluence of small watershed has changed a lot.At the same time,following the conventional theory of runoff and confluence,two reasons should take responsibility for the change of the runoff yield of the small watershed.one is the water-impermeable rate,another is the slope.Therefore,the paper is aimed at learning about the specific change of runoff and outlet flow of small watershed of mountainous city after urbanization and analyzing the influence of urbanization on the characteristics of runoff and confluence of small watershed of mountainous city by using the model to simulate through which it can provide the basis for engineering design and related research.The part B of Longzhou Bay New District in the Banan of Chongqing is chose as the object in this paper,and it mainly aims to figure out the change of the characteristics of the runoff after the urbanization.In this paper,the SWMM model is used to simulate the runoff effect of mountain city under the conditions of 120 min rainfall duration,5 a,20 a,50 a,100 a respectively,The variation rule of runoff and confluence characteristics in small watershed of mountainous city is also analyzed.The results obtained from the model simulation are as follows:(1)The swmm model was used to simulate the rainstorm runoff in mountainous cities.The results showed that the runoff coefficient of the watershed increased with the increase of rainfall intensity.In the process of rainfall,the total discharge value of the research area increases from zero,and the rate of increase is faster and faster.After a period of time to peak,then slowly decreased,the rate of decline is slower and slower.The time to decrease the total traffic is significantly longer than the time when the total traffic increases.(2)In the process of simulating and calculating the runoff and confluence of small watershed in mountainous cities by SWMM model,it can be seen that the change of underlying surface factor has a very obvious effect on the model results.To be more specific,the area parameters of sub-catchment area,the proportion of impermeable water rate and the water storage parameters of impermeable depressions have great influence on the calculation results of surface runoff.while there are three factors have great effect on the water storage parameters of impermeable depressions,calculation results of surface runoff,Slope parameters and Manning roughness coefficient.The results of the change of the water-impermeable rate and the change of the slope are also simulated,and the following conclusions are obtained:(1)The increase of water permeability in the underlying surface of the city increases the runoff yield,the runoff coefficient,the total flow of the outlet,and the peak time ahead of time.However,with the increase of impermeable water rate,the increase rate of runoff coefficient and total discharge of discharge becomes smaller.At the same time,the influence of impermeable water rate on runoff coefficient and discharge is more sensitive than that of heavy rain under the condition of light rain intensity.(2)The influence of slope on runoff coefficient is relatively small.However,if the region has powerful hysteresis capacity,the influence of slope change on runoff coefficient is more obvious.The slope has a relatively large influence on the discharge amount.When the rainfall intensity is light,the total discharge amount increases with the increase of slope.However,when the rainfall intensity is heavy,the discharge amount will decrease with the increase of slope.The peak time will advance with the increase of slope.But under the pouring condition,the change of slope has more influence on the peak time than that of light rain. |
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