Urban Rainwater Utilization and its Role in Mitigating Urban Waterlogging Problems-A Case Study in Nanjing, China

With the acceleration of the urbanization process, urban waterlogging problems are becoming more and more serious in Nanjing, China. In order to mitigate the urban waterlogging problems, it is necessary to reduce surface runoff from the source by rainwater harvesting and utilization. An urban residential district with an area of 0.58 km(2) in Nanjing was selected as the study area. Based on a large-scale topographic map data and the long term rainfall data (1951-2008), the types of underlying surfaces were classified. The potentiality of collectable rainwater and the possibility of runoff volume reduction were calculated. The results showed that exploitation of rainwater harvesting from rooftops and other underlying surfaces has high potential. The annual collectable rainwater is approximately 372,284 m(3), 314,034 m(3) and 275,180 m(3) under different cumulative frequency of rainfall at 20 %, 50 % and 75 %, respectively. The total capacity of cisterns under assumptions of return period of rainfall and rainfall duration with 5 years and 20 min is 11,022 m(3). The cistern's capacity which is used for roof rainwater harvesting is 4,083 m(3), the cistern capacity for per unit roof area (1 m(2)) is 0.0267 m(3). The results of the feasibility analysis of setting up above-ground cisterns showed that 55 % of the total roof areas in the study area are available for setting up cisterns. In the three building types, 16 % of the commercial building's roof areas and 77 % of that of the residential and the "others" buildings are available for setting up cisterns. Urban waterlogging problems can be effectively reduced through rainwater harvesting by 13.9 %, 30.2 % and 57.7 % of runoff volume reduction in three cases of the maximum daily rainfall (207.2 mm), the average annual maximum daily rainfall (95.5 mm) and the critical rainfall of rainstorm (50 mm).