Stormwater infrastructure improved in Cottonwood

Published on Wed, Jan 8, 2014 by Ian Ferguson

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Stormwater improvements seven years in the making are now complete in the Cottonwood neighborhood in Birch Bay.

The project, funded by Birch Bay Watershed and Aquatic Resources Management District (BBWARM), was designed to address longstanding drainage problems in the area, including flooding caused by a failing pipe and poor water quality. 

Contractors installed a new stormwater conveyance system under Cedar Avenue, a bioswale that will filter out pollutants with the help of living plants and a separate near-shore drainage system along Birch Bay Drive. 

The project was first envisioned in 2006 as a priority capital improvement project in Birch Bay’s comprehensive stormwater management plan.

The new system increases flow capacity for the 145-acre drainage area above Cedar Avenue, including the creek flowing through Halverson Park, and treats stormwater flows from 11 acres of residential development. With the new system in place, failing portions of the old stormwater system were removed and a drainage confluence that formed a hazardous sinkhole was eliminated.

Reichhardt & Ebe Engineering of Lynden and Whatcom County Public Works completed engineering design for the project. Len Honcoop Gravel, Inc. of Lynden was the contractor with a construction contract of $468,000.

Ingrid Enschede, program specialist for Whatcom County Public Works, said time will tell how effective the improvements will be, but project managers are optimistic.

“We fully anticipate the drainage and flooding problems caused by the failing pipe to be addressed,” she said. “As for improved water quality, the bioswale will help, but only in the percentage of the watershed that flows through it. We still need residents to help improve water quality in Birch Bay by doing what they can to keep pollutants out of the waterway.”

The bioswale stormwater treatment facility was an important part of the project because stormwater from the Cottonwood neighborhood had the highest levels of fecal coliform bacteria in Birch Bay, and could create public health risks if left untreated.

The bioswale will treat stormwater flows from the Seaview Drive and Maple Crest Avenue area. 

Bioswales are engineered drainage features that use soil and plants to naturally filter pollutants from stormwater, Enschede explained. The bioswale has an impervious liner with six cells containing drain rock and a special soil mix separated by a filter layer. It was seeded with a mix of grasses and other plants that will work with the soil to remove pollutants like fecal coliform bacteria, nitrogen and phosphorous. Clean water will soak into the drain rock then flow out through an under drain to the new pipe under Cedar Avenue.

“I would invite the public to take a walk and look at the bioswale,” Enschede said. “It’s really a pretty cool feature. It’s a complex, engineered structure designed to filter water that looks like a landscaped rain garden.”

The plants in the bioswale will take about a year to grow, at which point stormwater outflow will be redirected to flow through it. The vegetation will be mowed regularly to prevent other species of plants from moving in. 

A similar bioswale was installed in the Lake Whatcom watershed in 2010, and measurements of the quality of water coming out of it have been encouraging, although the data is limited to 13 storm events.

Located on Lahti Drive near Lake Whatcom, the bioswale has reduced fecal coliform bacteria amounts by an average of 70 percent and phosphorous amounts by an average of 56 percent.
 
“The data is preliminary, but it’s promising,” Enschede said. “The project in the Cottonwood neighborhood features a similar bioswale, so our expectation is that we’ll see similar results.”

Whatcom County Public Works, BBWARM, and the Whatcom County Marine Resources Committee have been working with the Cottonwood community over the past year to raise awareness about water quality problems and ask for residents’ help keeping the water clean. Bacteria levels decreased in 2013, but they are still significantly higher than state health standards. 

Residents all around the Birch Bay and Drayton Harbor watersheds can help keep stormwater clean by picking up pet waste, making sure septic systems and side sewer connections are in good repair and keeping food away from wild animals like raccoons.

Next on the list for BBWARM is a similar project on Beachway Drive just northwest of Cottonwood Drive. Engineers are in the design phase of the Beachway Drive and Fern Park stormwater improvement project, and if permitting goes smoothly in the spring, construction will begin this summer.

A slideshow of the Cottonwood neighborhood stormwater improvements project, water quality data, and more information are available at bbwarm.whatcomcounty.org.