Forward-looking planning
The drainage-effective areas below the backwater level that have a gradient towards the building, such as garage entrances, building entrances or excavated areas leading to basement flats, should be kept as small as possible. Rainwater from small areas (approximately 5 m²) such as basement access points and similar areas may be allowed to infiltrate. If this is not possible, such areas may be drained via backwater valves in accordance with DIN EN 13564-1 where a natural gradient exists, provided that suitable measures, e.g. thresholds at cellar entrances, prevent low-lying rooms from being flooded by rainwater whilst the backwater valve is closed.
Forward-thinking architecture and landscape planning can mitigate the risk of building flooding through appropriate site design. However, it is often unavoidable that areas with effective drainage below the backwater level have a gradient towards buildings. In such cases, these areas – such as garage entrances, building entrances or excavated areas leading to basement dwellings – should be kept as small as possible.
DIN 1986-100
In general, according to DIN 1986-100, Section 13.1.3: “Drainage outlets for rainwater from areas below the backwater level may only be connected to the public sewerage system separately from domestic wastewater and in a backflow-free manner.”
Rainwater accumulating below the backwater level can be divided into three categories:
- Small areas such as basement entrances
- Larger areas posing no risk to buildings or property
- Larger areas posing a risk to buildings or property
Design rainfall intensities
Areas below the backwater level | Design | Flooding assessment |
| Small areas, such as basement access points | r(5,100) | r(5,100) |
| Larger areas, without risk to buildings and property | r(5.2) | r(5.30) |
| Larger areas, with risk to buildings and property | r(5,100) | r(5,100) |
r(5,100) = The maximum rainfall expected at a specific location once in 100 years over a 5-minute period.
Flood assessment
In general, a flood risk assessment in accordance with DIN 1986-100 must be carried out in all three cases. Different levels of safety result from the different risk assessments.
For large areas below the backwater level, the following must be used:
- Lifting stations in accordance with DIN EN 12050-2
- Lifting stations in accordance with DIN EN 12050-1 for wet-installed systems, but without faecal separation
- Backwater pumping stations, provided they are approved for the area of application
- Backwater lifting stations, provided they are approved for the area of application
