Dispersing water

If you have a large, vegetated lot, you may want to explore surface dispersion systems to slow and process stormwater. In these systems, stormwater is forced to flow as a sheet (“sheet flow”) across the surface and is dispersed by the vegetation and topography. The combination of spreading out the flow and slowing it down enhances infiltration of the stormwater into the native soils.

All surface dispersion systems have specific design requirements, which vary by jurisdiction. However, broadly, all dispersion Best Management Practices (BMPs) have the following three components:

  • Contributing areas – where the stormwater comes from, such as a driveway or roof.
  • A “flow path” – the route water takes as it flows from contributing area to the dispersion area.
  • The dispersion area – the area or piece of land where the stormwater will be directed to for dispersion.

The simplest dispersion systems use unaltered native vegetated areas to accept stormwater runoff generated from impervious or compacted surfaces (roofs, roads, driveways, and paved areas). However, forested or heavily vegetated landscapes with soils that have not been compacted by vehicles or heavy equipment are also often well suited to process stormwater runoff. Other dispersion systems include additional components to gently direct and spread stormwater flow onto landscape areas designated for stormwater accumulation and infiltration.

Here you will learn the basics about detention system options. 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.

Creating a dispersion plan

You may want to determine which design is right for your property by creating a dispersion plan. Here is an example:

dispersion plan

You can base your dispersion plan on a site plan of your property. View the Fact Sheet Understanding your Site Conditions for more information.

Downspout dispersion 

There are three different roof downspout dispersion methods that are site specific:

  • Downspout splashblock
  • Downspout dispersion trench
  • Dispersion trench with notched grade board

Downspout dispersion using a splashblock is the simplest to implement of the three options listed above. Splashblocks require adequate vegetation, well-draining soils, proper flow paths, room for effective dispersion, and ground that slopes gently away from the foundation. Splashblock dispersion requires a vegetated flow path that infiltrates the water before reaching a structure, waterway, or property line. The distance is dependent on the volume of water anticipated from a significant rainfall event and the permeability of the soil.

downspout splashblock

If an ideal distance is not attainable, a dispersion trench can be added. Depending on the site, the trench may also require a notched board to spread out the concentrated flow across the face of the vegetated flow path.

dispersion trench with notched board

If the vegetated flow path is shorter than 25 feet, a perforated stub-out connection may be necessary. This will allow for some infiltration, storage, and flow reduction prior to the dispersion area.

perforated stub-out connection

Sheet flow dispersion 

Sheet flow dispersion systems have a transition zone and a vegetated buffer or dispersion area. The transition zone is primarily a two-foot-wide area of material such as drain rock, which is next to the impervious surface (e.g., driveway). The transition zone helps prevent erosion and channeling and aids the even flow to the designated vegetative buffer. The vegetated buffer size is related to the size of the impervious surface and the permeability of the soils; the larger the driveway, the larger the vegetated buffer. With moderately well-drained soils, a driveway up to 20 feet wide requires a 10-foot vegetated buffer. An additional 10 feet of vegetated buffer width is needed for every additional 20 feet of impervious surface width or fraction thereof. A 30-foot-wide driveway, for example, would require a vegetated buffer with a width of 20 feet.

sheet flow dispersion from driveway

Concentrated flow dispersion 

Stormwater generated from impervious surfaces on sloped properties tends to flow along concentrated or channelized pathways. Therefore, specifically adapted concentrated flow dispersion BMPs include additional features like a rock pad or a dispersion trench to disperse concentrated flows. The rock pads or dispersion trenches are types of level spreaders – they slow down the water and spread it out over a wider area. The addition of a notched board, placed in a level manner across the slope, can also be added to ensure even distribution of the flow into the vegetated area. Downstream of the level spreader, there needs to be an unobstructed vegetated flow path of at least 25 feet free of rigid structures, steep slopes, water bodies, or other impervious surfaces to ensure proper dispersion. Additionally, for sites with septic systems, the discharge point must be ten feet downgradient of the drain field and primary reserve areas. The steepness of the property determines the length of the flow path and what type of level spreader is necessary.

concentrated flow from driveway

Comparison of dispersion options

FeaturesDownspout DispersionSheet Flow DispersionConcentrated Dispersion
PurposeDisperse roof runoff water at bottom of roof gutter downspout.Dispersion system designed to manage stormwater off gently sloped impervious surfaces.Dispersion system designed to manage stormwater off steeper-sloped impervious surfaces where water concentrates and does not sheet flow.
DesignWater is directed away from structures to dispersion areas. Various dispersion techniques can be used, depending on site conditions.Transition zone next to impervious surface that leads to a dispersion area.Concentrated flow from an impervious surface is spread out before entering a dispersion area.
SoilsUncompacted, well-draining soil is essential.Uncompacted, well-draining soil is essential.Uncompacted, well-draining soil is essential.
VegetationSpecifically sized vegetated flow path requirements.Specifically sized vegetated buffer requirements.Specifically sized vegetated flow path requirements.
Maintenance* Monitor flow path for erosion or sediment accumulation.
* Maintain healthy vegetation.
* Monitor transition zone and vegetated buffer for erosion or sediment accumulation.
*Maintain healthy vegetation.
* Monitor flow path for erosion or sediment accumulation.
* Maintain healthy vegetation.

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