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Facilities - Infiltration basin

Infiltration basin

Infiltration Basin with water
A properly maintained and functioning infiltration basin shortly after a rain storm.
Dry Infiltration Basin
A dry infiltration basin. Unlike a detention pond, an infiltration basin has an underground structure that helps water soak into the ground.

A stormwater infiltration basin holds runoff and lets it soak into the ground. The basins are open facilities with grass or sand bases. They can either drain rapidly or act as permanent ponds where water levels rise and fall with stormwater flows.

Infiltration facilities should be designed to handle all runoff from a typical storm, but could overflow in a larger one. Since the facility is designed to soak water into the ground, anything that can clog the base will reduce performance and be a concern.

Generally, infiltration basins are managed like detention ponds but with greater emphasis on maintaining the ability to infiltrate stormwater. Some infiltration basins incorporate other types of facilities, such as biofiltration swalestreatment wetlands or presettling ponds.

Virtual tour

Click on the image below to get inside of an infiltration basin and learn about common components and maintenance tasks.

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Maintenance needs commonly associated with infiltration basins

Man tripping vegetation in swale.



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Pet waste 

inlet of a stormwater facility

Inlets and outlets 


combined stormdrain catch basin and curb inlet.

Catch basin 

riprap in a stormwater facility


Maintenance is needed if you see these signs

Problem Infiltration Basin
Problem: The infiltration basin below still has water more than three days after the rain storm indicating an issue that slows infiltration.
Problem Detention Pond
Problem: The detention pond above is over- grown with vegetation.

Bare, exposed soil

Slopes that are deteriorating

Sediment that restricts flow or clogs inlet and outlet pipes

Sediment buildup; the facility is not draining or conveying runoff

Unhealthy or dead vegetation

Blackberries or other problem weeds

Overgrown vegetation

Holes in berms or slopes

Leaves, trash and other debris

Water surface is discolored or has a sheen

Water stands in infiltration basin or detention ponds longer than 72 hours after rain stops

Bottom of slopes show signs of seepage and leaking

Trees, often alders, growing on the slopes



Tips for fixing problems and general maintenance


Remove weeds such as blackberries and English ivy, then check for them on a regular basis and remove new vines.  Contact Clark County Vegetation Management for questions about controlling weeds at (360) 397-6140 or email 

Remove cattails before they start to dominate a facility.

Remove all unplanned trees or saplings that block parts of the facility or hinder maintenance.

Make sure banks, slopes and areas designed for vegetation are planted with native or easy-to maintain species. Avoid trees near the pond and on berms.

Plant at appropriate times during the year so vegetation can get established.

Replace vegetation damaged or removed during maintenance.

Check access roads and fencing, if the facility has them. They should be free of overgrown vegetation and other materials so that the facility is easily accessible for maintenance.


Control erosion by reseeding areas where soil is exposed, especially on slopes around a facility.

Fill in eroded areas and cover them with sod, mulch or other erosion control materials.


Removing sediment is best done in July and August before winter rains set in.


Pick up leaves before rains begin.

Inspect and remove debris regularly, particularly after storms.

Remove any material clogging drains, outfalls and channels.

Avoid using fertilizers, herbicides or pesticides in or near the facility. Chemicals pollute the water and can cause unwanted plant growth.

Identify sources of leaks or spills and contain them as quickly as possible.


Check inlets and any open or confined standing water for mosquito larvae (see photos below). If mosquitoes are a concern, contact Clark County Mosquito Control District for information. The 24-hour service request line is (360) 397-8430.

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Check roads and fencing in the facility. They should be maintained to allow easy access.