Low Impact Development Of The Sponge Cities-The Research Of The Typical Mountain Runoff Effect

Recently, the regional urbanization has been accelerated with the urban-rural integration and most of city construction was in the traditional mode due to the unscientific planning and the low design standard of drainage. In this situation, the distribution of land use was essentially changed, that is the sharply increasing of the impervious area. Then, the hydrologic process was also sharply changed in urban. The total amount of runoff (the runoff coefficient) and the peak of the discharge would increase and the confluence time of runoff would decrease in urban in the same precipitation process. The essentially hydrologic changing in above resulted in frequent waterlogging in large and middling cities and the phenomenon of watching the sea in city was reported. The waterlogging would threat the life safety and cause more huge economic loss because the population, constructions and economy were congregated in urban. The above runoff effect would be more abnormal in rapid development city of mountainous area because it was not only facing waterlogging, but also the flash floods. The developed experience has proved that the traditional mode could not satisfy sustainable development. On the contrary, the sponge mode is advocated as a new one by more and more governments, which is based on the ecosystem and emphasizing the natural work, harmonious with nature. The control percentage of the runoff is key to evaluate the effect of the sponge mode, so it is crucial to analyze the runoff effect in different precipitation process.Fenghuang City was selected as the study region in this paper. The runoff effect of different land use scenarios was analyzed and compared using SWMM model in the natural mode, the traditional mode and the sponge mode, In this research, parameter calibration was referred to the recommended values of the model, the observed river discharge was used to the model verify. The pattern of design rainstorms is CHM and 1y, 2y,5y, 10y,20y are chosen as the return period of design rainstorms. The runoff effect in the different design rainstorms in three modes were analyzed, the related results were as following.(1)The runoff effect analysis. The runoff coefficient would increase with the urbanization increasing in the same design rainstorms; The runoff quantity increased, the peak discharge increased and the peak current time became earlier compared with the natural mode, due to the increasing of impervious area. The permeable area in sponge mode would be superior to natural development model and more superior to the traditional mode. The same orderliness in different design rainstorms among three modes was illuminated that the total control quantity of runoff is list in sponge mode, less in the natural mode and most one in the traditional mode; This result illustrates that kinds of LID measures would be fitting, applicable and practical, of which the mainly effect is the reducing of runoff and peak discharge. And these corresponding measures would be more significant in medium and small rainfall events, when the return period is more than 20 years, the related effect would become weak.(2) The runoff effect evaluation in sponge mode. First of all the total control percentage of runoff (less than 83%) was simulated and calculated through the model under sponge mode,which satisfies the total control percentage of runoff (more than 75%) in aim of the sponge city contruction.Then using KPI to evaluate the runoff and runoff time,the results satisfied the aims and requirements of sponge city contruction. The drainage waterlogging evaluation in sponge mode, the maximum depth of water points is 0.13m in Fenghuang, which is less than 0.15of the outdoor drainage standard (the return period is 20y), The computed two indicators through the model in sponge mode were both satisfied the aims and requirements of sponge city contruction, which proved the reliability and feasibility of the sponge city project,which further enrich the scope of application of SWMM Model.At all, Fenghuang City was selected as the study region in this paper, innovativing to propose the refinement method based on coupling GIS and SWMM, which determined the unit of sub-basin, then convertedd from the urban road to drainage network.For urban surface conditions and development plan, designing the planning of different types of land distribution, using multiple LID, the detailed analysis and comparison study of rainfall runoff effects in different designs..This research would supplement the content of urban rainstorms runoff and supply the technology to the sponge city project.