Detaining water

You may want to install a retention system on your property to detain and infiltrate water. This will reduce flooding and erosion and assist in the removal of harmful contaminants from the stormwater.

There are a number of ways to detain and allow stormwater to infiltrate from the ground surface into the native soils below or to an underdrain. Detention systems are designed with flat bottom areas to allow for uniform infiltration.

Here you will learn the basics about four kinds of detention systems. Click an item below to jump to a topic, or scroll down to view them all.

Refer to the WSU Fact Sheet: Options for Managing Surface Water Drainage to learn more.

Bioretention systems 

Bioretention is the process that captures and detains stormwater and allows for slow infiltration to remove contaminants and sediment using soil, plants, and the soil ecosystem. “Bioretention system” is a term used to refer to engineered systems using a specified Bioretention Soil Mix (BSM) designed to accommodate large volumes of stormwater runoff, often in commercial areas. The size, shape, and other characteristics of bioretention systems depend on the contributing area size, precipitation characteristics, the infiltration rate of the native soil, and site constraints such as steep slopes.

bioretention system

Rain gardens

A rain garden is a bioretention system that typically does not need an engineer to design and is suitable for homeowners to install with a basic level of training. It is a shallow, flat bottomed, bowl-shaped landscape feature designed to collect, filter, and infiltrate stormwater runoff from roofs and pavements. Rain gardens require a special soil mix or amendment of the existing soils with compost, plants that can withstand inundation and drought, and several inches of mulch to retain moisture and reduce maintenance. These components work together to remove contaminants, infiltrate water, and provide other ecosystem and aesthetic benefits. Learn about building rain gardens by viewing a demonstration rain garden, built step-by-step, including a time lapse video of the planting.

rain garden

Gravel trenches (French / curtain drains), drywells

Gravel trenches are narrow, stone-filled channels situated over well-draining soil that utilize the empty spaces between stones to act as a temporary reservoir for stormwater as it soaks into the surrounding soil.

These trenches can be open (dispersion trenches) or closed (infiltration trenches). Open infiltration trenches collect stormwater that flows from the surface of impervious areas such as a driveway through a vegetated filter strip that removes sediment, and then flows into the top of the trench. A closed or subsurface infiltration trench handles stormwater that comes through a pipe into a sedimentation basin and eventually into the subsurface of the trench through a perforated pipe. Trenches are typically designed with an overflow berm where excess water can flow out to a vegetated area.

open infiltration trench

French drains and curtain drains are examples of gravel trench systems, however, these terms are also used to describe similar systems that are designed to capture and re-route shallow groundwater.

Drywells are similar to gravel trenches, but function vertically (from downspout for example) rather than horizontally. They are small underground chambers or holes that are filled with stone and allow runoff to infiltrate into the surrounding soils. They are sized according to the amount of water that needs to be managed, and the permeability of the soils. Runoff moves into the drywell through underground piping. Most drywells are buried and have an observation pipe. They are commonly used for runoff from sources that are low in stormwater contaminants.


Comparison of detention options

FeaturesBioretention SystemRain GardenGravel Trench/Drywell
Scale and suitabilityEngineered bioretention system primarily used in larger applications. Size depends on infiltration rates and size of contributing area; engineered to meet water quality and quantity performance goals. Requires specific soil mix to ensure that it will adequately handle the flow and treat pollution.

A type of bioretention system. Suitable for many locations but typically designed for homeowners and residential areas.

Size is dependent on soil-drainage rates, but typically less than 15% of the size of the contributing area. Shape often depends on site constraints, landowner aesthetics and can range from long-skinny, kidney bean shaped, square or round.
Suitable for managing low contaminant runoff from sites. A good choice in locations where other methods won’t fit.

Gravel trench: Linear detention and infiltration system perpendicular to flow.

Drywell: A buried chamber or pit filled with gravel.
Water quantity and quality benefitsProvides known level of contaminant and sediment treatment. Reduces peak flows and total runoff volumes.Provides contaminant and sediment treatment but treatment levels are variable. Reduces peak flows and total runoff volumes.Reduces peak flows and total runoff volumes. Provides some level of containment and sediment treatment.
Soils and mulch
* Uses an engineered mix of sand and compost–the sand and compost must meet a specific gradation.
* Often topped with mulch.
* Uses a “rain garden soil mix” of onsite soils or sand, plus yard-waste compost.
* Topped with mulch.
*Gravel or drain rock.
* No mulch.
VegetationCan be beautifully planted to fit any landscape. Need to select plants that can survive periods of saturation and ideally summer drought (to limit watering).Can be beautifully planted to fit any landscape. Need to select plants that can survive periods of saturation and ideally summer drought (to limit watering).Gravel trench usually not vegetated.

Closed infiltration trenches and drywells can be covered with grass.
Maintenance* Weeding, inflow & overflow cleaning.
*May need irrigation for plant establishment.
*Underdrain (if used) requires maintenance.
* Add mulch as needed.
*Weeding, inflow & overflow cleaning as needed.
* May need irrigation especially during first 2 years for plant establishment.
* Add mulch as needed.
* Remove debris.
* Weeding.
* Mow vegetative filter strip (bag grass clippings).

Refer to the WSU Fact Sheet: Options for Managing Surface Water Drainage for more details.