Water infiltration and water buffer

Infiltrate water

Infiltration is the sinking of water into the soil. In built-up areas, the large percentage of pavement impedes natural infiltration. Sidewalks, streets, buildings and parking lots do not let rainwater through. Almost everywhere, the rainwater is therefore drained via the sewer to the treatment plant or to nearby surface water. But when it rains hard, the sewage system cannot handle the amount of rainwater and the street is flooded. It may also happen that the treatment plant cannot cope with the volumes of water and the rainwater is discharged together with the sewage water into the surface water, which then becomes polluted.

Infiltration crates form an underground storage space for rainwater. From these crates, the stored rainwater slowly sinks into the soil, towards the groundwater.

Infiltration crates do not take up any space above ground and often have a larger storage capacity than above ground rainwater storage. Depending on the size of the plastic crates and the possibilities in the subsurface, a considerable amount of precipitation water can be stored and then slowly sink ('infiltrate') into the soil.

There are infiltration crates in various sizes: from a few crates for individual houses to connected facilities for entire city districts. The crates can be used, for example, in gardens and under roads, sports fields and parking squares. This makes double land use possible. Infiltration crates can only be used in areas where the groundwater level is not high, otherwise the crates will already be in the groundwater. Then the addition of these crates is not effective.

The big advantage of using these crates is that little space is taken up above ground, while a lot of rainwater storage is achieved. For this, space is needed underground (which is sometimes difficult in a city, with existing pipes, wires, pipes, etc.). The crates are wrapped with geotextile to prevent soil material from ending up in the crates.

Holding and storing water

Thanks to climate change, extreme cloudbursts are becoming more common. Our current sewerage system is not designed for this. As a result, the urban drainage system for rainwater is overloaded, so that water remains on the street and can run into the basements. However, adapting the sewerage system to climate extremes would be very costly, which is why it is better to store rainwater locally.

The capacity of our current sewer system is not designed for extreme downpours. In the old city center, the sewer is often also a mixed system in which the waste water is led to the treatment plant together with the rainwater. In case of overload, in a mixed system the raw wastewater is dumped together with rainwater on the surface water. However, adapting the sewer system to climate extremes would require very large and expensive sewers that are unused most of the time and for which there is little space left in the ground.

That is why it is better to retain the rainwater locally. In the Netherlands, the policy is 'hold, store, remove'. The rainwater is first retained locally as much as possible, then stored in surface water or other storage facilities, and only when this is not possible is the rainwater drained.

By temporarily retaining rainwater locally, damage can be limited. The peak of the discharge of rainwater towards the sewer is thus shifted: the rainwater only flows slowly towards the sewer pipe when it is empty again.

Rainwater can be retained by constructing wadis, ditches, above-ground water buffers, green roofs, rainwater ponds, underground storage facilities, water plazas, or rainwater utilization installations.

Rainwater can also be infiltrated directly, where soil conditions allow, by maximizing softening and greening. But to limit damage during heavy cloudbursts, this must always be combined with other water retention facilities, especially in places where infiltration is not possible due to high groundwater levels.

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