Proper construction methods can prevent soil runoff and other water quality problems

The San Juan Islands Conservation District actively promotes sustainable land use practices. Our Natural Resources Planner is trained in LID (Low Impact Development) design techniques and is available for individual site visits to help landowners conserve their natural resources.

Contact us if you are interested in a site visit to assess your soil and water resources. A follow-up report will contain information on the soils of your site, an aerial photo, LID fact sheets and site specific information such as vegetation management plan, a raingarden handbook and native plant lists.

LID Resource Links:

Low Impact Development Approaches Save Money and Protect Island Watersheds!

  • Preserving forested or natural areas can save up to $10 per square foot or $435,000 per acre over conventional landscape solutions.
  • Balancing cut and fill on site can save up to $100 per cubic yard in haul costs.
  • Using raingardens and bioretention areas can save up to $4,800 per residential lot over conventional engineered solutions (Sherwood Gap Creek, 2000).
  • Creating narrow streets (24 feet wide) versus wide streets (32 feet wide) can save up to $30 per linear foot in street costs.
  • The use of bioretention areas can save up to $4,000 per residential lot over standard stormwater management pond costs (Somerset, MD, 2005).
  • Bioretention of runoff can save up to 75 percent of stormwater fees per residential lot (Kensington Estates, WA, 2001).
  • Shade trees on the south side of buildings can save up to $47 per tree per year in energy costs (Peper, 2007).
  • Green roofs can retain more than 75 percent of rainfall annually, reducing downstream stormwater management costs (ASLA Green Roof, 2007)
  • Recycling construction waste can save tens of thousands of dollars in haul costs, dump fees, and material costs (Stapleton, 2006).

Effects of Polluted Stormwater

  • Stormwater drains are not connected to the sanitary sewer systems
  • Individual human activity, not industrial dumping, is the primary cause of pollution in rivers, wetlands, and lakes and in Puget Sound.
  • Biodegradable soap is not a safe addition to stormwater drains and should be kept from running into the stormwater drainage system.
  • Wash your car in an area where the soapy runoff will be absorbed by the ground or take your car to a commercial car wash. Soapy water should not be allowed to flow into the street or into a drainage ditch.
  • Impervious bricks or pavers contribute to excessive runoff. Use pervious bricks or pavers help to reduce stormwater pollution in the environment.
  • Sediment is pollution and should be prevented from entering the stormwater drainage system.
  • Grass clippings and leaves in stormwater are regarded as pollution and should be kept out of the stormwater drainage system.


Control the Flow

Bioretention with Underdrain

Facilities are landscaped shallow depressions that capture and filter stormwater runoff. As stormwater passes down through the planting soil, pollutants are filtered, absorbed and biodegraded by the soil and plants. Because they are not contained within an impermeable structure, they may allow for infiltration. For sites not passing the infiltration feasibility an impermeable liner may be needed to prevent incidental infiltration.

Planter Boxes

Provide bioretention treatment control measures that are completely contained within an impermeable structure with an underdrain (they do not infiltrate). They are similar to bioretention facilities with underdrains except they are situated at or above ground and are bound by impermeable walls. Planter boxes may be placed adjacent to buildings, structures or sidewalks.


Facilities that are designed for partial infiltration of runoff and partial biotreatment. These are similar to bioretention devices with underdrains but they include a raised underdrain above a gravel sump designed to facilitate infiltration and denitrification. These facilities can be used in areas where there are no hazards associated with infiltration, but infiltration measurements show low infiltration rates or high depths of fill.

Vegetated Swales

Open, shallow channels with dense, low‐lying vegetation covering the side slopes and bottom that collect and slowly convey runoff to downstream discharge points. An effective vegetated swale achieves uniform sheet flow through the densely vegetated area for a period of several minutes. The vegetation in the swale can vary depending on its location and is the choice of the designer. Most swales are grass‐lined.

Filter Strips (to be used as part of a treatment train)

Vegetated areas designed to treat sheet flow runoff from adjacent impervious surfaces such as parking lots and roadways, or intensive landscaped areas such as golf courses. While some assimilation of dissolved constituents may occur, filter strips are generally more effective in trapping sediment and particulate‐bound metals, nutrients, and pesticides. Filter strips are more effective when the runoff passes through the vegetation and thatch layer in the form of shallow, uniform flow. Filter strips are primarily used to pre-treat runoff before it flows to an infiltration BMP or another biofiltration BMP.

How to: Calculate the Design Volume

Infiltration facilities shall be sized to capture and infiltrate the design capture volume (V design) based on the runoff produced from a 0.75‐inch (0.0625 ft) storm event.

V design (cu ft) = 0.0625 x Catchment Area (sq ft)

Where: Catchment Area = (Impervious Area x 0.9) + [(Pervious Area + Undeveloped Area) x 0.1]
For catchment areas given in acres, multiply the above equation by 43,560 sq. ft./acre.

Rain Gardens

A natural way to reduce watershed pollution from your home is to build a rain garden which slows stormwater flow so that plants can absorb and digest extra nutrients and many other pollutants.

What’s the Problem?Raingarden1

Rain on the Islands runs as stormwater through local watersheds collecting nutrients and pollutants that enter the Salish Sea. Stormwater flows slowly through forests and healthy soils, allowing plants and microbes to digest the pollutants. But when storm water runs off impervious surfaces like pavement or hard earth, any pollutants it picks up quickly collect in streams where they can have toxic effects. Monitoring programs around the Islands show nutrient levels higher than background levels, which could cause excess algae growth that consumes the dissolved oxygen that other organisms need to survive.

How Rain Gardens Help

A rain garden slows water from your home runoff in the same way that forests slow down watershed runoff. Layers of soil disperse the water and let it seep into the ground below instead of washing across the surface causing erosion. Native plants that don’t require fertilizer further slow this water down by trapping its flow through the soil with roots, and sucking the water into their leaves to evaporate back into the air. Their roots also help develop microbial communities which are well known to digest many man-made chemicals.

Rain Garden Handbook