Rainwater infiltration: Properly draining rainwater in the garden

If you have your own house and garden, rainwater drainage is an unavoidable issue. The amount of precipitation must always flow off in a controlled and trouble-free manner so that there is no flooding. City sewers are one option, but they regularly cost a lot of money. Alternatively, there is a seepage in your own garden. Read here which different infiltration systems are available.

The importance of water quality

Not all rainwater is allowed to seep away unrestrictedly in the garden. 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 determined before setting up an infiltration facility. It is not about the actual pollution, but much more about the potential risk of pollution to which the water is exposed. The following categories apply:

• safely charged
• tolerably burdened
• unacceptably burdened

Harmlessly contaminated water

Rainwater is considered unpolluted if it comes from non-metallic roof areas and terrace areas in residential areas and comparable commercial areas. Harmlessly contaminated water may seep through an overgrown layer of soil without further cleaning measures. But beware: Within certain water protection zones, no rainwater may be seeped away at all. Even safe water is not exempt from this ban. Therefore, find out in good time whether your garden is located in such an area.

Tolerable polluted water

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

• sidewalks
• courtyard areas
• Garage entrances where vehicle washing is prohibited
• metallic roof surfaces

Tolerably polluted rainwater may seep away if it has undergone suitable pre-treatment beforehand. Infiltration is also possible if the contaminated water undergoes cleaning processes in the infiltration system. Infiltration through an overgrown topsoil layer usually meets these requirements.

Unacceptably polluted water

If rainwater comes from areas where heavy pollution is expected, it must not seep into the garden under any circumstances. It must be collected separately and then sent to a sewage treatment plant via the sewage system. Fortunately, such a high risk of pollution on residential property is rare.

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 infiltration. 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 on the safe side with a geological investigation by a soil surveyor. The infiltration capacity of the soil largely determines the appropriate infiltration system.

seepage in new buildings

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

seepage in old buildings

A rain seepage system can also be retrofitted to existing buildings. There can be several reasons for this:

• upcoming changes to the sewer lines
• 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 away in your garden, you have a number of options to choose from:

• surface infiltration
• swale infiltration
• shaft infiltration
• trench infiltration
• Hollow trench seepage

surface infiltration

In the case of surface infiltration, the water infiltrates directly onto the water-permeable surface on which the rain falls. In addition, the water from adjacent areas can also be included if their insurance capacity is not sufficient.

• for little-used courtyards, terraces and garden paths
• technical effort is low
• Good cleaning effect if the surface is overgrown
• Space requirements are usually high
• especially if the soil is poorly drained

swale infiltration

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

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

shaft infiltration

Manhole infiltration is another way to infiltrate 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 only approved in exceptional cases and only for unpolluted water.

• low space requirement
• only a few square meters
• reaches more than 1 m in depth
• ideal if only the deeper layer is permeable
• Areas above can be used as desired
• Rainwater is channeled underground
• underground water storage
• seepage through walls and floor
• upstream sludge trap

This infiltration system is poorly maintained. If the fines cause a blockage, it is expensive to fix.

trench infiltration

Infiltration ditches filled with crushed stone or coarse gravel are called infiltration trenches. The water is discharged directly into the ditch. 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 dependent on each other
• Areas above the trench can be used as desired
• Temporary storage in the ditch
• Seepage through the floor and walls of the trench

The digging of the trench is labor intensive and accordingly takes a lot of time. Fines in the water can also lead to clogging. Unfortunately, the moat does not offer an opportunity for preventive maintenance.

Hollow trench seepage

The trough-trench infiltration is a combination of trough infiltration and trough infiltration. The trough is located above the gravel-filled trench. The rainwater first flows into the Mulde and then seeps into the Rigolengraben. It runs through an overgrown topsoil layer and is thus cleaned.

• requires about 5-15% of the connected area
• Mulde and ditch serve as intermediate storage
• Rainwater is fed in above ground
• good cleaning effect
• can be visually integrated into the garden

permits and grants

An inspection by the competent authority is required for the infiltration of rainwater in the garden. The Office for Environmental Protection is usually the right contact point for this. Corresponding forms are also available there. Occasionally, states and municipalities promote the establishment of an infiltration system. It is worth making an inquiry about this.

Planning and execution

Area infiltration and trough infiltration are among the simpler infiltration methods. As a garden owner, you can do the planning and implementation on your own. However, it must be ensured that the soil conditions are suitable for this. Otherwise, the overflow of the system can cause damage, which can also affect the neighboring garden. Other seepage systems are better off with professionals. At the very least, they should be left to plan and calculate.

Instructions for a seepage trough

An infiltration trough can be created relatively easily by garden owners themselves. The construction is explained step by step below.

1. First calculate the size of the trough and mark out the dimensions.
2. Remove any vegetation or surface reinforcement that may be present.
3. Remove the topsoil about 70 to 80 cm deep. First, place it on the side next to it.
4. Shape the ditch into a hollow with harmonious transitions to the rest of the garden. Use the excavated topsoil for this. The deepest point of the trough should lead to an area that can be flooded from time to time.
5. Sow lawn seed.
6. Establish the supply line to the trough as soon as the lawn seed has sprouted.

Learn more about irrigation