Water storage in sewage systems

In areas with a lot of hard surfaces, like cities with streets and houses, the water runs off immediately to sewers or surface water. As there is hardly any water storage in the soil in areas with hard surfaces, you see a quick rising of the surface water level when it rains. When the sewers are used to capacity the streets will flood.

The process of water storage in sewage systems.

An area is 2 ha. The soil is clay with 40% pores, 1 meter deep over a hard rock surface. The pores are half full with water.

1. How many liters of water can be stored? So how many millimeters of rain can be stored here?
2. What if the amount of water you calculated in 1 fell on a hard surface and it couldn’t be drained? How high would the water layer be?