城市排水 flow to the river, it is related to 水利工程

When the water level is high in the rivers sourrounding the city, the water in the city will be accumulated. The worst case is that the water in the sourrounding river pouring to the city.

Can the original author calarify the case in Beijing? Is the 城市排水 system to small, or the water level in the sourrounding rivers is too high?