Development And Application Of Urban Sewerage System Model In Mountainous City

Due to the urbanization process, the underlying surface of the city has beenchanged enormously; the increased impervious surface area ration has changed theconditions of rainfall production and convergence thus arouse a series of detrimentaleffects, such as a lack of capacity of water drainage system in urban city, city inundationand the intensity of flood control risk. The surface slope is large; the speed of therainfall production and convergence is fast and the destructive power of rainwater andflood runoff is tremendous. The data shows that during the24years from1980to2003,there were in total31heavy rainfalls in Chongqing which has led to different degrees offlooding, resulting in a lot of infrastructure damage, casualties and huge economiclosses in many districts of Chongqing. Therefore the storm runoff model in mountaincity, based on hydrology, hydraulics and the watershed special data is of greatsignificance to achieve the goal of making an analog experiment on the processing ofurban storm runoff, to locate the city inundation spot and predict the time of the waterwithdrawal as well as manage and utilize the water volume and quality of the stormrunoff.Looking at the models home and abroad, few storm runoff models are developedspecially targeted at the complex topography and physiognomy in mountain city. Hence,the author hope that trough this research, a storm runoff model for mountain city can bedeveloped that is suitable for China’s national conditions and mountainous cities. Theresearch is funded by State Major Special Project Fund for Water Pollution Control andTreatment Technology（Issue No:2008ZX07315-001). Urban Sewerage system model isdeveloped on the GIS platform combined with American software SWMM. This modelhas been applied for analyzing the storm runoff in Panxi River Catchment and HuxiRiver Catchment in Chongqing. Results are as following:Sensitivity analysis shows that, the most remarkable parameters to the output ofthis model is the width, gradient, and imperviousness area rate of the runoff. Then theoptimal ranges of parameters are gotten by thrusting reverser Nash-Sutcliffe Ens value.Such as, Dstore-perv：5.1~6.9mm;Dstore-Imperv：0.90~1.00mm;Manning-Perv：0.38~0.42;Manning-Imperv：0.018~0.023. Contrast to parameter values in flat areaurban model, the ground with same landuse in mountainous city has less ability tostagnant water because of its large slope. Permeable ground could undermine the impact of slope on its ability to control water by its overdue buffering capacity. Unlikely, thestagnant ability of impervious ground declines sharply with the increasing slope.This model has been applied for analyzing the storm runoff in Panxi RiverCatchment and Huxi River Catchment in Chongqing. The credibility of the model isverified by four independent rainfall events that Nash-Sutcliffe coefficient, Ens is above0.7, the simulated series can reflect the actual runoff process well. Besides, the accuracyof the model is largely affected by the rainfall intensity. Best fit parameters are asfollows: Dstore-perv：6mm;Dstore-Imperv：1mm;Manning-Perv：0.4;Manning-Imperv：0.02. Panxi River model was developed based on the Huxi River model, Thecredibility of the model is verified by two independent rainfall events that in7thJune2010rainfall event, the Ens is0.56and the and the relative error is11.8%, compared thesimulated result with the actual measurements4thJuly2010, the Ens is0.76, and therelative error is8.82%.The drainage capacity assessment was conducted in Panxi River catchment usingthe typical rainstorm reproduction of once a year, once in twenty years, once in fiftyyears, and once-in-a-century rainstorms for rainfall duration of60minutes,90minutesand120minutes. In the peak time of overall drainage load, there were21pipes200%overloaded under the60-min-long rain (return period=20a); whereas,27pipes (returnperiod=50a), and28pipes (return period=100a). By contrast, there were23pipes (returnperiod=20a),31pipes (return period=50a), and32pipes (return period=100a)200%overloaded under90-min-long rain; and there were31pipes (return period=20a),35pipes (return period=50a), and37pipes (return period=100a)200%overloaded under120-min-long rain. Overloaded drains located mainly in central commercial center andsoutheast of the basin. Thus, the drainage system is reliable for the storm sewer pipeswere used to be full flow. In addition, the flood discharge peak time occurred earlierwith the increase of the return period, its response time was growth slowly with theextension of the duration of rainfall.The drainage capacity assessment which based on pipe flow showed that60minutes rainfall would cause more pipes overload than120minutes rainfall underhigher storm intensity (return period=50a or100a); Rainfall duration had a greaterimpact on the flood discharge load under lower storm intensity(return period=1a or5a).likewise, the peak of overall drainage load occurred earlier under the same rainfall.Long rainfall duration will weaken the impact of rainfall intensity which will diminishthe pipes networks. As to the Key nodes, there were37waterlogging points under120minutes rainwith100a return period. Among these points,10would discharge rapidly within10minutes,15located at permeable surface and12located in buildings and roads.Waterlogging points should be renovated by sustainable measures for rainwaterutilization and sustainable development.