Rainwater infiltration: the correct way to divert rainwater in the garden

For those who have their own house and garden, rainwater drainage is an inevitable issue. The amount of precipitation must always flow off in a controlled and trouble-free manner so that there is no flooding. Urban sewers are one option, but that regularly costs a lot of money. Alternatively, you can infiltrate your own garden. Read here what different infiltration systems are available.

The importance of water quality

Not all rainwater is allowed to seep into the garden without restrictions. This is only permissible if the amount of pollutants it contains is not too high. For this reason, the contamination of the rainwater must be ascertained prior to setting up an infiltration facility. It is not about the actual pollution, but much more about the potential pollution hazard to which the water is exposed. The following categories apply:

• harmlessly loaded
• tolerably loaded
• unacceptably burdened

Harmless polluted water

The rainwater is considered unpolluted if it comes from non-metallic roof surfaces and terrace areas in residential areas as well as comparable commercial areas. Harmlessly polluted water can seep away through an overgrown soil layer without further cleaning measures. But be careful: Within certain water protection zones, no rainwater at all may seep away. Even harmless water is not exempt from this ban. You should therefore find out in good time whether your garden is in such an area.
Tip: You can usually get information about whether your garden is in a water protection area from the waterworks. The Office for Environmental Protection is also a suitable point of contact.

Tolerably polluted water

Water that is only exposed to a slight risk of pollution is still considered to be tolerable. This is usually the case with private property if it comes from the following areas:

• Sidewalks
• Courtyard areas
• Garage driveways on which vehicle washing is prohibited
• metallic roof surfaces

Precipitation water that is tolerably contaminated may seep away if it is subjected to a suitable pre-treatment beforehand. Infiltration is also possible if the polluted water goes through cleaning processes in the infiltration system. Infiltration through an overgrown topsoil layer usually fulfills these requirements.

Intolerably polluted water

If rainwater comes from areas that are likely to be heavily polluted, it must not seep into the garden under any circumstances. It has to be collected separately and then passed through the sewage system to a sewage treatment plant. Fortunately, there is seldom such a high risk of pollution on residential properties.
Note: Rainwater must not be allowed to seep away on properties with contaminated sites and soil contamination. The pollutants in the soil could get into the groundwater through water seepage.

Infiltration capacity of the soil

The composition of the subsoil determines how well water can seep away. If the proportion of gravel and sand is high, the garden is well suited for seepage. A loamy soil, on the other hand, is not sufficiently permeable to water. Since not every garden is the same, the water permeability should be checked. You are completely on the safe side with a geological survey by a soil surveyor. The infiltration capacity of the soil largely determines the suitable infiltration system.

Infiltration in new buildings

In some federal states, infiltration is mandatory for new buildings. There is then no freedom of choice, it is only a matter of choosing the appropriate type of infiltration. Therefore, find out about the current legal status in your state before starting construction.

Infiltration in old buildings

A rain infiltration system can also be implemented retrospectively in existing buildings. There can be several reasons for this:

• upcoming changes to the sewer pipes
• planned redesign of the garden, if necessary Pond system
• Saving of disposal fees for rainwater
• Environmental aspects

What infiltration systems are there?

If you want to let the rainwater seep into your garden, you have various options to choose from:

• Area infiltration
• Hollow infiltration
• Shaft infiltration
• Infiltration infiltration
• Hollow infiltration infiltration

Area infiltration

In the case of surface infiltration, the water seeps away directly on the water-permeable surface on which the rain falls. In addition, the water from adjacent areas can also be absorbed if their insurance capacity is insufficient.

• for little-used courtyards, terraces and garden paths
• technical effort is low
• good cleaning effect if the area is overgrown
• The space requirement is usually high
• especially if the soil is poorly permeable

Hollow infiltration

Rainwater infiltration is also possible via shallow terrain depressions, so-called. Infiltration troughs, possible. The hollows are specifically created in the garden for this purpose. The water to seep away is first introduced into it, where it gradually seeps into the ground through the overgrown floor of the hollow. This is ideal for heavy downpours, when the water cannot seep away so quickly.

• suitable for roof surfaces and terraces
• also for paths and courtyards
• good cleaning effect
• The hollow is about 30 cm deep
• is about 10 to 20 percent of the total area
• can be easily integrated optically in the garden
• variable planting possible
• complex implementation in hillside gardens

Tip: By installing cascades, this type of rainwater infiltration can also work well in gardens with a sloping surface.

Shaft infiltration

Manhole infiltration is another way of infiltrating rainwater. The water is fed directly into the shaft, where it can seep through the walls and bottom of the shaft. The polluted water is not cleaned here, so that this type of infiltration is now only approved in exceptional cases and only for unpolluted water.

• low space requirement
• only a few square meters
• extends more than 1 m in depth
• ideal when the deeper layer is only permeable
• Areas above can be used as required
• Rainwater is fed underground
• underground water storage
• Infiltration through walls and floor
• upstream sludge trap

This infiltration system is difficult to maintain. If the fines lead to a blockage, the repair is associated with high costs.

Infiltration infiltration

Infiltration ditches that are filled with gravel or coarse gravel are called trenches. The water is discharged directly into the trench. The rainwater is temporarily stored in the underground part of the trench until it can seep away completely. This method lacks the cleaning effect of an overgrown topsoil.

• little space required
• requires about 10-20% of the connected area
• suitable for roof surfaces
• also for paths and courtyards
• can be built in depth
• poorly permeable layers can be overcome in this way
• Length, width and depth are interdependent
• Areas above the trench can be used as desired
• Intermediate storage in the trench
• Infiltration through the floor and walls of the trench

The digging of the trench is labor-intensive and accordingly takes a lot of time. Fine substances in the water can also lead to clogging. Unfortunately, the trench does not offer any preventive maintenance.
Tip: If the area above the trench is to be planted, only plants with shallow roots should be selected. Otherwise there is a risk of root penetration.

Hollow infiltration infiltration

Hollow infiltration infiltration is a combination of hollow infiltration and infiltration infiltration. The hollow is above the gravel-filled trench. The rainwater first flows into the basin and then seeps into the trench. It passes through an overgrown layer of topsoil and is cleaned in this way.

• requires about 5-15% of the connected area
• The hollow and the trench serve as intermediate storage
• Rainwater is fed in above ground
• good cleaning effect
• can be optically integrated well into the garden

Approval and funding

A test by the competent authority is required for rainwater to seep into the garden. The Office for Environmental Protection is usually the right place to go. Corresponding forms are also available there. Occasionally, federal states and municipalities encourage the establishment of an infiltration system. It is worth making an inquiry about this.

Planning and execution

Area infiltration and hollow infiltration are among the simpler infiltration methods. As a garden owner, you can do the planning and implementation yourself. However, it must be ensured that the soil conditions are suitable for this. Otherwise damage can occur due to the overflow of the system, which can also affect the neighboring garden. Other infiltration systems are better off with experts. At least the planning and the calculation should be left to them.

Instructions for an infiltration basin

A drainage basin can also be created relatively easily by garden owners themselves. The construction is explained step by step below.

1. First calculate the size of the recess and mark the dimensions.

2. Remove any existing vegetation or surface reinforcement.

3. Remove the topsoil, about 70 to 80 cm deep. First store it next to it.

4. Form a hollow from the trench with harmonious transitions to the rest of the garden. Use the excavated topsoil for this. The deepest point of the hollow should lead to an area that can be flooded from time to time.

5. Sow lawn seeds.

6. Make the supply line to the hollow as soon as the lawn seeds have come up.

Tip: If your garden is very loamy, the hollow should be dug 15-20 cm deeper so that it can temporarily store more water. When forming the hollow, plenty of sand should also be added, which will increase the soil permeability.