A local and near-natural percolation of rainwater is generally preferable to rainwater drainage systems, due to legal, ecological and economic reasons and those related to water-management (Printed Matter of the House of Representatives of Berlin 18/0212 and 18/0447). Percolation describes the technical process of introducing precipitation into the subsurface using appropriate systems. Whether precipitation can directly percolate locally largely depends on the geological and hydrogeological conditions, such as the subsurface layers, hydraulic permeability and groundwater conditions. When planning percolation facilities, influencing factors that need to be considered, other than the properties of the subsurface, include the potential pollution of the soil or contaminated sites, properties of adjacent areas, the availability of areas and requirements imposed by water authorities and legal regulations.
An accurate knowledge of both the local geological and hydrogeological conditions and the hydraulic permeability of the percolation area is therefore a prerequisite for planning percolation facilities. Hydraulic permeability is the property of the subsurface to conduct water within its porous space, which depends primarily on the grain size of the loose sediments, their distribution, the structure and thus the effective pore volume of the subsurface. Loose sediments with high sand contents have a much higher permeability than clayey, silty sediments, e.g. those consisting of boulder marl. To characterise the hydraulic permeability of the subsurface, the saturated hydraulic conductivity (kf value, permeability coefficient) is used as the soil characteristic value.
Percolation facilities are subject to hydraulic requirements of the subsurface, for example, ranging from kf= 1*10-3 m/s and kf= 1*10-6 m/s, according to A-138 DWA regulations. Depending on the design of the percolation facility, differences in thickness of the subsurface layers are relevant for designing the facilities. In the case of surface percolation, the property of the topsoil (0.3 to 0.5 m below ground level) is decisive. For swales or deep beds, the decisive horizon is located between 0.6 m and 1.0 m below ground level. Significantly greater depths (1.0 m to 2.5 m or deeper), however, need to be considered in the design of swale-infiltration trench systems.
Obtaining direct measurements of hydraulic permeability and saturated hydraulic conductivity is laborious; therefore, only few measurements are available for Berlin. Hydraulic permeability is often determined using soil type and bulk density. Berlin’s data on soil-scientific characteristics that is essential for determining the hydraulic permeability of the subsurface is either only partially available as part of a concept map or not comprehensive. It should also be noted that deeper layers that are not covered in the soil maps are also relevant for percolation facilities. The Engineer’s Geological Map of Berlin 1 : 5,000 (SenStadtWohn 2017a) presents the geological structure of loose rock generally to a depth of 10 m. However, topsoil and landfills with a thickness of less than 5 m are not presented. As an addition to the Engineer’s Geological Map, which currently covers only about half of the Berlin area, a map of the percolation capacity of loose rock at the surface was produced in the 1990s for a selected small number of sheet lines (SenStadt 1990).
Approx. 160,000 boreholes are recorded in the geological database of the State of Berlin. They contain information on the geological structure of the subsurface, such as the stratigraphy (chronological sequence), petrography (rock composition) and genesis (formation), as well as some soil-physical characteristic values. The method used to develop the percolation potential map for the Hanseatic City of Hamburg (Stadt Hamburg 2018) was adapted to Berlin conditions to form the method employed here. Information on existing boreholes was assessed to derive and present the hydraulic permeability of the subsurface. For this purpose, the individual rock layers and petrographic analyses were classified and combined into rock classes, whereby distinguishing between a hydraulic permeability of "high to medium" and "medium to low". The area-based hydraulic permeability of the subsurface is presented in a map displaying the thickness of the topmost layer with a high to medium hydraulic permeability from the surface to a depth of 5.0 m below ground level (cf. Map 02.22.1). An additional map displays the thickness of the layer with a high to medium hydraulic permeability between a depth of 1.0 to 5.0 m below ground level, as this area alone is relevant for the design of percolation systems (swale-infiltration trench systems) in some cases (cf. Map 02.22.2).
The map on the hydraulic permeability of the subsurface primarily serves as an overview of potential measures for the decentralised percolation of rainwater, in terms of their planning implementation and feasibility. On the one hand, it is intended as a strategic instrument for managing city-wide processes. On the other hand, it may serve as a source of information for its "users", e.g. administrators, planners and building owners, and offer advice on percolation measures.
The map on the hydraulic permeability of the subsurface does not exempt individual projects from the obligation to provide evidence for the hydraulic site conditions for percolation facilities by means of probing and on-site boreholes.