Article is confusing: Is the 城市排水 system too small, or the surro

Is Beijing flood problem from the 城市排水 system too small? Is the water level too high surrounding the Beijing  was too high so the water accumulated in the city? From the reports, it seems that the water in the surrounding rivers pouring to the city.

Please clarify so we understand what was the real problem.

As an engineer, you should know that the water in the city flows to the river, and the river flows to the sea, right?