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What is Stormwater?

Stormwater is runoff during and following precipitation and snowmelt events, which includes surface runoff, and interflow. An interflow, for our area, is water traveling laterally or horizontally through the aeration zone (the ground between the Earth’s surface and the water table) that discharges to the Spokane River during or immediately after a precipitation event.

Stormwater has great potential to transport many pollutants to the aquifer and surface waters in and around the City of Spokane. In order to prevent this, a number of facilities are used to help clean stormwater before it is discharged into the ground or rivers.

Pollution Prevention

1. Bioinfiltration Grassy Swales
   

   The purpose of grassy swales is to remove pollutants from stormwater prior to infiltrating into the ground, helping to protect the Spokane Valley-Rathdrum Prairie Aquifer. It is important to maintain grass swales because they act as a biofiltration system for treating runoff.

   Here are some tips to help with the longevity of your grassy swale. To view, please click on the word maintenance.

   Keep the inlets maintained by removing sediment and debris that accumulate at the inlet of swales. Also keep the grass and sediment low at the entry of the inlet; to allow the water to enter the swale and percolate.
   
   Remove accumulated trash and debris.
   
   Maintain the grass and remove the glass clippings from mowing. Remove dead grass, rototill and re-seed or re-sod.
   
   Use fertilizer as directed and sparingly.
   
   Do not dispose of chemicals in swales.
   
   Do not replace the grass with rocks.
   
   Do not over water the grass. This slows or stops water movement through the soils and will destroy the vegetation at the base of the swale.
   
   
   Here are some examples of failing or bad inlets or swales: (click on image for a larger view)
   Here are some examples of good/functioning swales: (click on image for a larger view)
   
   
2. Catch Basins
   

   Catch basins are structures designed to remove debris and other pollutants from stormwater before it is discharged into water bodies. Instead of allowing stormwater to flow directly into the sewer system, the catch basin traps debris (see diagram) while the water flows into the connecting sewer main when the level in the catch basin rises above the exit pipe level.

   

   Catch basins eventually become clogged with the debris they trap and must be cleaned by City crews. The cleaner that the public can help keep the streets, the more likely catch basins will be to stay clean. As a result, the quality of the stormwater will increase, leading to environmental benefits.

3. Drywells

   A drywell is an underground structure that disposes stormwater runoff by dissipating it into the ground. They are passive structures that allow water to flow through them under the influence of gravity. Drywells are small pits lined with geotextile fabric or weed cloth and filled with 11/2” to 3” gravel. The soils surrounding the drywell are sufficiently permeable to allow adequate infiltration of the runoff. Drywells are best used to collect runoff from areas with little or no sediment or pollutants.