Impacts Of Natural And Social Conditions On Urban Drainage Systems In Cities With Different Spatial Patterns

The rapid growth of population and variation of water utilization level have led tothe change of wastewater discharge, while the high uncertainty of precipitation causedby climate change has brought shock load to the urban drainage system. As thesenatural and social factors vary, how to control the pollution load, manage the stormrunoff, and improve the water quality, have become the challenges for the planning anddesign of urban drainage system. In view of this, based on the layout planning modelWasLaM already developed, the paper discussed natural and social factors influencingurban drainage system in cities with different spatial patterns from the aspects of systemscheme selection and facility optimization.The research included three parts. First, based on quantifying multiple objectivesof system economic cost, impact on the receiving water and capability to handle thestorm water, the paper developed the W_V2model which expanded WasLaM modeland supplemented urban runoff simulation. W_V2model could help planning separatedsewer system and combined sewer system, solving the problem how to choose a betterurban drainage system scheme and design the systems’ layout. The output of the W_V2includes the recommendation of urban drainage system type, either combined system orseparate system, and also the optimum schemes including the system layout, keyhydraulic parameter of the pipe system, the location of the wastewater treatment plants,and the corresponding service areas and scales. Second, the paper chose Chengde city asa representative spatial pattern for urban drainage system planning under differentconditions when population density, water utilization level and precipitation intensity.Various design standards were also considered. The paper discussed how these naturaland social factors influenced the urban drainage system, and the systems’ adaptivecapability to the city development. Third, the paper chose5cities that were Chengde,Datong, Kunshan, Huzhou and Zhenjiang with different spatial patterns. Theperformances of urban drainage systems under the conditions of different populationdensity, water utilization level and precipitation intensity were compared, so thatcharacteristics of impacts caused by these natural and social factors on urban drainagesystems of cities with different spatial patterns were identified for the sake of supportingbetter system planning. The results showed precipitation is the major influential factor for the economicperformance and the internal flooding load, while population density and waterutilization level are the major influential factors for the pollution load discharging to theoutside of the city. The main factor affecting the choice between separated andcombined system is precipitation, although the intensity thresholds were different underdifferent spatial patterns. Based on all uncertainties of natural and social conditions, aseparated system scheme, a facility layout with locally decentralized WWTPs, and afacility scale graduation of “one large plus several small WWTPs”, should beconsidered as the best conservative choice for the similar cities to Datong, Chengde andKunshan. On the contrary, the similar cities to Huzhou and Zhenjiang could randomlychoose either a separated or combined system, a layout of evenly decentralizeddistribution of WWTPs with scales about the same.