A photograph of a rain garden at Western Seminary near a curb in the parking lot. There are several plants on the curb, and to the right of them are several parked vehicles.
Basins collect rainwater to control flooding and prevent erosion. They are an attractive way to manage the rain on your property and can add interest to your landscape. They are most suitable for large, open spaces. 
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What Is a Basin?

A basin is a shallow landscaped area designed to capture, store, and filter rainwater. Basins can either be lined or unlined and designed for total or partial drainage, depending on the soil type. They can be lined where drainage (infiltration) isn’t allowed or isn’t possible. Basins should be integrated into the overall site design. 

How Do Basins Work?

In general, basins work by collecting rainwater and slowly releasing it into the ground or into the City’s sewer or stormwater pipes. The slow release of water can help prevent erosion, flooding, or sewer backups.  

In unlined basins, water soaks into the soil at a predictable and steady rate (called the infiltration rate). In addition, most basins require a solution such as an overflow pipe to prevent overflows by removing the extra water that the soil can't absorb.  

Summary of Basin Design Requirements

These site and design requirements can help you decide if a lined or unlined basin might be appropriate for your project.  Consult the Stormwater Management Manual for full information.

Basins can be round, like a pond, or long and narrow, like a swale. The shape depends on the space available and the designer's preference.

To stay safe and avoid damaging buildings or other structures, unlined basins must be:  

  • At least 5 feet from neighboring property lines.
  • At least 10 feet from foundations.

In addition, basins are not suitable for steep areas. The maximum side slope for all basins is 3:1 (3 horizontal to 1 vertical), or 4:1 next to pedestrian areas.  

The City's Stormwater Management Manual separates basins into two categories based on the complexity of the project and oversight needed. These categories are called the Simplified Approach and the Performance Approach. Find more information on these approaches in the manual.  

A property owner does not need to hire a designer when using the Simplified Approach, but must meet the following design requirements:  

  • Width (top) – at least 9 feet.
  • Width (bottom) – at least 2 feet.
  • Slope – no more than 6 percent (a check dam is required every 10 feet if the slope is greater than 4 percent).
  • Ponding depth – no more than 12 inches.
  • Freeboard (the vertical space between the highest surface of water and the ground level) – 2 inches.
  • Must have an underdrain and overflow pipe, unless otherwise approved.
  • Must have a soil infiltration rate of at least 2 inches per hour.

Consult the Stormwater Management Manual for complete construction requirements.

Property owners using the Performance Approach will work with professional designers so that the basin can be sized to meet applicable water quality, flow control, and disposal requirements. These basins may also be lined, which removes the setback restrictions.  

It is very important to remember to:

  • Avoid compacting the native soils. Keep equipment out of the excavation area and do not store materials there.
  • Prevent runoff and sediment from filling the basin during construction.

Environmental Services typically requires site visits during construction. More site visit information will be provided after permitting.

When to Call a Professional

A designer is not required when following the Simplified Approach.  

A professional designer is needed for building:  

  • Lined basins.
  • Large basins that collect water from an area exceeding 10,000 square feet.  
  • A basin large enough to avoid the requirement for a piped overflow.
  • Any basin if the infiltration rate is less than 2 inches per hour. A professional designer can also help downsize an existing basin if the infiltration rate is greater than 2 inches per hour.  

Costs and Permits

A permit is required to build a basin.  

Permit applications will be reviewed by at least two City bureaus. The Bureau of Development Services will require a site development permit, and the Bureau of Environmental Services will review the stormwater component. See the Stormwater Management Manual for permit application requirements and process.  

Maintenance Requirements

Maintenance for basins is similar to other landscape maintenance. Property owners should inspect basins regularly.

  • Water the plants deeply once a week during dry months (May to October) to encourage root growth and keep plants strong, especially while plants are getting established during the first two summers.  
  • Pull weeds by hand before they become established (avoid chemical weed killers).  
  • Remove sediment and debris, watch for erosion, and replace plants as needed.  
  • Regularly check gutters, downspouts, and inlet pipes to ensure they are free and clear of debris and rainwater can flow freely into the basin.
  • Once a year, layer compost or mulch 2 inches deep to suppress weeds and feed plants.  
  • Thin and prune plants as needed. Divide dense plantings every two to five years.  
  • Remove leaves in the fall. Leaf build-up will reduce capacity and smother plantings.  

Find additional maintenance information and common problems and solutions. With adequate maintenance, basins can last indefinitely.  

Stormwater Management Manual

Consult the City’s Stormwater Management Manual for the complete set of requirements on how to safely site, build, and maintain basins or other stormwater management solution on your property.

Clean River Rewards Eligibility  

Residential and commercial property owners who install qualified stormwater management solutions may be eligible for discounts on the stormwater charges of their sewer, stormwater, and water bill through the Clean River Rewards program.  

To be eligible for the discount, a basin must meet the safety and space requirements outlined in the Stormwater Management Manual. Visit Clean River Rewards for more information.

Find More Resources

Plants are not required in basins but may be added if desired. When choosing plants for your basin, see the stormwater manual’s plant list for suitable native and non-native plants:

Sources for the required soil blend can be found in the City's soil vendor list:

Permission is required for alternatives blends.


Private Property Drainage Inquiries

Environmental Services
phone number503-823-5858For questions from home and business owners about safe ways to manage the rain on their property or solutions to drainage problems.

Systems Development

Environmental Services
phone number503-823-7761Questions about sewer connections, stormwater management, and drainage reserves at the land use or building permit stage.