Bioretention for Storm Water Management

What the heck is a Bioretention Facility?

As we have mentioned in a post or two already, much of the work we do for new developments and construction in Seattle revolves around stormwater management requirements. One tool for meeting the need to manage heavy rainfall is the installation of bioretention facilities. That is a technical term for ponding reservoirs that temporarily hold rainwater, keeping it from immediately flowing into the gutters. This term includes built or poured concrete planters, and the deeply mulched rain gardens we see in more and more of Seattle’s front yards.

What these facilities all have in common is a collection of water loving plants, a soil mix that provides good drainage, and a thick layer of mulch. The City provides an approved list of plants for bioplanters, and requires at least three different species in each planter. They range from varieties of dogwood to ferns to a number of attractive grasses. Inside the planters, we start with a layer of clear drain rock on the bottom, then add at least 18 inches of “biosoil.” This mix contains an extra quantity of sand to provide some drainage during a deluge, but a good amount of organic material to help hold water for the health of the plants. On top of the soil we place a layer of mulch that is at least three inches deep. The mulch helps to keep down weeds and provides an extra source of nutrients for the plants.

Designed to Infiltrate

Preference goes to designs that allow water to infiltrate into the surrounding soil in order to mimic the pre-development hydrolic flow. However, site conditions may dictate a non-infiltrating design. Non-infiltrating facilities include a concrete bottom or impervious membrane and an underdrain to slowly release the reserved water.  Every facility must have an overflow drain leading to an approved discharge point, which often is the sewer system.

How Bioplanters Help

Bioplanters are designed to hold water in order to compensate for the hard surfaces on a property that prevent water from soaking into the ground where it falls. Storm water runoff is a major source of pollution when it runs off too quickly. Motor oil and other pollutants from our streets flow through the drain system and into our waterways with the runoff. Sewer systems can also become overwhelmed by a heavy rain, causing damage and contamination. The growing number of bioretention facilities in Seattle help mitigate these problems by holding water temporarily and slowly draining it at a more manageable rate. Of course, the plants also use some of that water to grow and beautify the surroundings.

At True Scape Design, we have planted many facilities that others put in place. We also have built them ourselves with the same block material that we use in retaining walls. From the design and permitting perspective, we have helped builders and homeowners meet the city’s requirements for stormwater management on hundreds of projects.

Resources

The City of Seattle Stormwater Manual holds requirements and design specifications for every kind of stormwater management.

The City also hosts a Stormwater page with links to the code, a training presentation, forms, documents, design checklists, and tips.