Hydraulic Permeability of the Subsurface
Rain should infiltrate into the ground rather than end up in the sewerage system. In the city, however, tarmac and concrete often thwart this plan. Percolation facilities help to counterbalance this. They come as green swales, large lawns or grass block pavers in car parks.
However, in the process of smoothing the way for rainwater into the ground, the structure of such facilities is only part of the equation. For example, rainwater traverses sand or coarse rock quite easily, which is not the case for boulder marl. If you are planning a percolation facility, you must also know what is going on below the surface.
Berlin has determined the water permeability of its subsurface up to a depth of five metres for this purpose. For most of the west and all along the glacial valley, all layers up to this depth were assigned a high to medium hydraulic permeability. Half of Berlin is therefore presumably well suited for the percolation of precipitation.
Determining the exact nature of the subsurface is extremely complex. The city was able to analyse the data from 160,000 boreholes for this purpose. Sometimes, however, just a few metres away from your borehole, there may be sediments less permeable to water, rather than sand, or the other way around. The maps therefore primarily serve as an overview to determine the feasibility and assist in the implementation of potential measures for the decentralised percolation of rainwater. They do not exempt project owners from their obligation to carry out onsite investigations.