Due to urbanization and climate change the high-density-short-duration storms have occurred frequently in recent years in Zhenjiang. The drainage system in this historical city is undersized to convey these runoffs. In June of 2015 two successive heavy storms hit the city causing severe property damage and one life. The main reasons of those urban flooding are high intensity of rainfall, low capacity of conveyance system, low lying areas, and back water effect from high downstream boundary conditions of receiving water levels.
There are three tributaries of rivers flowing through the major urban area of Zhenjiang City. They are Yunliang River, Guyun River, and Hongqiaogang River. These three rivers discharge flows into Jinshan Lake which is an embayment of Yangzi River. The water levels in Jinshan Lake are controlled by ship locks and gates at east end and west end. The normal water level in the lake is 5.8 m and the maximum water level is 7.5 m.
The purposes of this study include: (1) To establish the relation of urban flooding with the water levels in Jinshan Lake. (2) To find out reasonable water levels in Jinshan Lake before rain start to fall and to minimize the damage of urban flooding caused by backwater effect. (3) To use the results of this study for flood warning within the decision support system of urban flooding.
To achieve the above goals, the drainage system and three rivers were all built using EPA SWMM. The hydrological and hydraulic characteristics of the drainage system and rivers were simulated at 10 years, 20 years, 30 years, and 50 years designed storm event respectively. Downstream boundary water levels in Jinshan Lake was set at 4, 4.5, 5, 5.5, 6 and 6.5 m respectivel. Hence a total of 24 scenarios with different combinations of design storm frequency and downstream water level were simulated by SWMM Model.
The systems are evaluated based on three performance criteria: the total flooding volume (V), the numbers of flooding manholes (N), and the average flooding duration (D). The simulation results show that
(1) Under the same design storm, V, N, and D are closely related to the downstream water levels in the lake as an exponential function. The correlation coefficient is greater than 0.99. On V, a downstream water level of 4.5 m is a critical point. Above 4.5 m, the influence of downstream water level is significant.
At 4 – 4.5 m, V is not sensitive to the downstream water levels. On N, a boundary water level of 4 m is a critical point. Below 4 m, N is not sensitive to the downstream water levels; however D is still sensitive. Hence we suggest a downstream water level of 4 – 4.4 m in Jinshan Lake should be maintained before rainstorms, so the risk of urban flooding could be reduced.
(2) The simulation results show that if the downstream water level is between 4 and 5.5 m, urban flooding will occur when daily rainfall exceeds 85 mm. If the downstream water level is at 6m, urban flooding will occur when daily rainfall exceeds 65 mm. If downstream water level is at 6.5m, urban flooding will occur when daily rainfall exceeds 30 mm. From the above analysis we conclude that the influence of downstream boundary condition of water levels is very important to urban flooding.