is a small community of about 4,600 people that is on an east-facing slope 400
feet above Puget Sound. The downtown area lies at the foot of this slope and is
prone to seasonal flooding and ponding. The causes of these issues are high
groundwater, an undersized storm drain system, and sheet flow from roadways.
This runoff allows surface pollutants to enter Puget Sound which ultimately
degrades the natural habitat.
County received a grant from the WA Department of Ecology and funding from the
Department of Public Works to address these issues. The funds were used to
build an environmentally friendly, state-of-the-art stormwater treatment
facility that would also serve as a park with community meeting space. The
County worked with engineers from Parametrix to evaluate more than 30
potential retrofit options. Because the drainage basin selected drains directly
to the Puget Sound, only water-quality treatment was required (no flow
control). The goal for the project was to achieve full compliance with current
stormwater volume treatment requirements for new development (91% treatment standard),
as well as meeting enhanced water quality treatment objectives (i.e., removal
of total suspended solids and dissolved metals).
0.5-acre parcel located just uphill from the beach outfall was selected as the
site of the new water quality treatment facility. Because of the small size of
the available parcel, the relatively large contributing drainage area, and the
high level of water-quality treatment desired, a conventional pond, vault, or
even sand-filter design would not meet treatment goals. This pushed the design
team toward a more innovative approach, which integrated both treatment
requirements and preferred ”green” stormwater solutions. Therefore, using
Filterra Bioscape soon became the preferred treatment
method. Filterra Bioscape is similar to bioretention in its function and
application but has been optimized for high volume/flow treatment and high
pollutant removal. Its small footprint allows it to be used on highly developed
sites. The Filterra Bioscape configuration is very flexible and can be designed
to integrate seamlessly with site amenities and aesthetics.
runoff enters the facility via conveyance piping. Water pressure forces the
untreated stormwater up into 18 distribution channels that are positioned to
uniformly and simultaneously deliver the water into all four Filterra Bioscape
vaults that are located around the perimeter of the park. The Filterra Bioscape
vaults contain Filterra media (a specific gradation of washed aggregate and
organic material), mulch and plants that aid in the treatment process by uptake
of pollutants and by mediating microbial degradation of other pollutants. These
plants also provide aesthetic elements to the overall stormwater park design.
water is collected in an underdrain system and subsequently discharged to the
outfall at the beach. The system is designed to treat flows well exceeding
2,000 gallons per minute, equating to around 100 million gallons of polluted
runoff treated annually. It is estimated that more than 100,000 pounds of
contaminated suspended solids will be cleansed from the upstream stormwater
runoff over the next 10 to 20 years. Additionally, the project was awarded
“Project of the Year” in 2016 by the Washington Chapter of APWA, and the City
of Manchester has a beautiful park in the center of downtown for the community
to enjoy year-round.