Effects of heavy rainfall and outer river level on urban waterlogging in hilly areas of South China: a case study of Nanhu Port, Changsha City

With the renewal of urban construction, disasters caused by heavy rainfall caused by urban water-logging risk is more , more high, seriously affecting the safety of people's lives and property, caus-ing the "city to see the sea" spectacle. Most of the hilly areas in southern China belong to the sub-tropical monsoon climate , heavy rains are usually concentrated from May to August. During this period, there is a significant positive correlation between external river level and heavy rain-fall. This study comprehensively evaluates the influence of external river level change on the over-flow capacity of urban drainage system by superimposing different external river level and dif-ferent rainfall intensity. It provides the basis for the operation, construction and reconstruction of urban drainage system. The results showed that the total water volume of the region increased by 54.47%-111.17% as the outer river level increased from 25.50 to 38.34 m. When the outer river level was higher than 36.15 m, the maximum variation range of urban waterlogging water was 26.13%. As the rainfall return period changed from one in 100 y to one in 10 y, the regional total water volume decreased by 77.78%-152.09%. Therefore, the main reason of urban waterlogging risk lies in heavy rainfall rather than external river level, but the design of urban drainage pipe network should con-sider the influence of external river level in combination with actual development and surrounding environment.