Infiltration calculations determine how much rainwater a plot must be able to store and how quickly that water can sink into the soil, so that an infiltration facility is sized correctly. They form the technical basis for disconnecting rainwater and are compulsory in many municipalities when applying for a building permit.
What is calculated?
An infiltration calculation brings supply and demand together:
- The connected surface area — the area of roofs and paving on which rain falls and which drains to the facility
- The design storm — a set amount of rainfall that must be stored, often expressed in millimetres (for example a storm of a certain return period)
- The soil permeability — how quickly water infiltrates, expressed by the permeability coefficient (k-value)
- The groundwater level — determines the available storage space below ground level
From calculation to facility
Based on this data, the required storage volume follows, for example the number of infiltration crates or the size of a gravel pit or swale. The rule of thumb is: the larger the paved surface and the worse the soil infiltrates, the more storage volume is needed.
Application
- Permit application — municipalities often require a calculation proving that the storm is managed on your own plot
- Garden and site design — sizing infiltration facilities when disconnecting
- New-build projects — basis for the water management plan
Points to note
- If in doubt about the soil, carry out an infiltration test to determine the actual k-value
- Calculate using the locally applicable design requirements; these differ per municipality and water authority
Related terms
- Infiltration
- Rainwater drainage
- Groundwater level
- Climate-adaptive
- Permeability coefficient
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