Swale Maintenance Guidance

Many private developments and properties constructed in the City had requirements during project permitting to manage and control stormwater onsite and/or the improved frontage of the property.  Most private property drainage systems consist of catch basins and pipes, or sloped surfaces which direct stormwater runoff into bio-infiltration swales (swales).  Depending on the size, most swales contain a drywell which has a grate 4-6” above the bottom of the swale and collects overflow during heavy rain or snowmelt events.  Drywells are considered a Class-V injection well and discharge stormwater into the ground.

What is a swale?

Bio-Infiltration swales, or simply, “swales” are the most common stormwater facility in Spokane Valley, with over 7,000 privately-owned swales within the city. Swales are typically grassed or vegetation-covered depression areas that are used to store and treat stormwater runoff. The vegetation and soil in the swale help to filter pollutants from stormwater prior to infiltrating deeper into the ground. Sometimes, swales will be used in series with other swales and can border roadways.   They can range greatly in size, such as multi-acre areas treating large commercial parking lots, to small grassy areas bordering single-family homes.  

There are several swales design configurations depending on the contributing hard surfaces that provide runoff to the swale and site constraints like roadway or sidewalk locations.   Overall, swale maintenance is uniform across all swales, but some configurations may be less noticeable to the public and may seem like a landscape feature of their neighborhood.

Who is responsible for swale maintenance?

The property owner is responsible for maintenance of swales and all other drainage structures located on private property.   Additionally, it’s usually the responsibility of the adjacent property owner to maintain swales next to their property that receive road stormwater runoff and are in border/stormwater easements according to SVMC 22.150.100. The city does maintain several swales built in conjunction with City arterial roadway projects.  

If you are unsure who is responsible for swale maintenance, you can contact stormwater utility staff at 50-720-5000 for confirmation of who is responsible.

Swale inspection and maintenance

The first step in understanding if maintenance is required is to perform an inspection of the swale.  It’s best to perform these quick inspections annually so that any maintenance needs can be addressed as soon as possible.

The best time to inspect your swale is between April and October, preferably after a large rain event to see how well stormwater is infiltrating through the swale or if there are any blockages in the system causing flooding.

Swales can be inspected by property owners using this table which outlines the components of the swale, potential defects, conditions when maintenance is needed, and the expected results when maintenance is performed. Most maintenance items can be completed the property owner or landscape company. Common maintenance needs include:

• Trash, debris, and sediment removal
• Lowering of sod at swale inlets or in the swale bottom as sod or sediment build up over time to restore swale storage capacity.
• Restoring swale to original design if they have been manipulated or removed
• Replacement of the bioretention soil media when swale is compacted and not infiltrating

If you have any questions about how to properly maintain your swale or drainage facilities, please contact the stormwater utility staff at 509-720-5000.