Study On The Influence Of Urbanization On Flood Characteristics Of Small And Medium Rivers In Mountain Area

With the development of urbanization,the conditions of the underlying surface of the urban middle and small river basins have undergone significant changes,and the changes of meteorological environment and the intensification of human activities in the region have changed the conditions of the river basin.Flood peak increases,the total runoff increases,the floods are reduced,the runoff coefficient is getting bigger and so on to the city river flood safety has brought a huge impact,due to the mountainous special climate environment and topography conditions,so that impact is more prominent.The traditional water conservancy reasoning formula and the municipal rainstorm formula have different irrationality in the simulation of the middle and small river flood runoff after urbanization.Therefore,this paper uses SWMM to simulate the change of the production and distribution of the river basin after the urbanization and the design flow of the export section,On the characteristics of floods in small and medium rivers in mountainous areas,and provide the basis for engineering design and related research.In this paper,the Longzhou Bay weir river in Banan District of Chongqing City is the object of study.After the urbanization,the conditions of the underlying surface and the topography have changed greatly.The proportion of impervious area increased from 20% to 65%,and the flooding characteristics occurred in the basin Significant changes.In this paper,SWMM model is used to simulate the change of production and production under the conditions of 5a,20 a,50a and 100 a before and after the urbanization,and the rainfall duration is 120 min,and the variation of flood characteristics in the basin before and after urbanization is obtained.Through the model simulation results show that:(1)Under different rainfall scenarios and recurrence conditions,the variation trend of the cumulative rainfall runoff coefficient is: at the beginning of rainfall,the runoff coefficient is zero,and when the rainfall occurs to a certain moment,the accumulated rainfall gradually increases from zero and increases The slope is getting bigger and bigger,after a period of time to reach the peak,and then slowly lower.(2)The effect of rainfall recurrence on rainfall runoff coefficient is relatively small.The difference of runoff coefficient in the same subsurface is different under different reproduction periods,but the change of land surface condition leads to the change of the underlying surface condition,the impact of the basin is high,and the runoff peak and runoff are all larger than those in the natural watershed.(3)When the SWMM model simulates the production of the runoff,the effect of the underlying surface element on its results is very obvious.Among them,the characteristic width,water permeability and water impermeability coefficient N of the sub-catchment area indicate that the Manning coefficient of the sub-catchment area has a great influence on the calculation results of the model,and the slope,impermeable area ratio,the sensitivity of the Horton infiltration condition to the results is relatively low.The author also calculates the flood runoff in the study area by using the formula of water conservancy reasoning and the formula of urban torrential rain,and compares the flood value of SWMM simulated urban area before and after urbanization,and obtains the following conclusions:(1)The model of water conservancy department reasoning adopts the annual maximum method to obtain the corresponding torrential floods for the corresponding recurrence frequency,while the municipal heavy rain intensity formula adopts the non-annual maximum value method,which is the number of samples.Compared with the reasoning formula,the recurrence period of municipal heavy rain intensity formula is low,and this paper also summarizes the relationship between them.(2)The reasoning formula is compared with the SWMM model after urbanization.The reasoning formula is mainly applied to the calculation and analysis of the floods in the middle and small watershed under natural conditions.When the underlying surface changes greatly,the calculated river channel Design flow is smaller than the actual value,affecting the safety of engineering design.(3)Comparing the calculated values of the municipal heavy rain intensity formula with the SWMM simulated urban area before and after the urbanization,and it can be seen that it is mainly applicable to the calculation of the rainfall runoff in the small watershed under the low reproduction period.Flood value is more than the actual large.