In Bellingham, Washington – as in many communities in the Pacific Northwest and across the United States – the number of projects constructed utilizing green or sustainable practices is increasing rapidly. Often these projects utilize emerging technologies and construction practices and, without the economies of scale, these green features can come with a cost premium. The goal of this project is to take on the challenge of being green and sustainable while also meeting affordable housing requirements. The Matthei Place project’s goal is to build 14 permanently affordable single family attached homes and 21 parking places on ½ of a city block in Bellingham’s south side.
The primary challenge for this project was to achieve a level of density that is much higher than the surrounding development. The site for the project was ½ of a city block that was designated for use as affordable housing by the Bellingham City Council following the construction of a new fire station on the other ½ of the block. The original size of the site dedicated to the project was approximately 37,000 sf which included part of a vacated right of way. In order to maximize the size of the site to allow the 14 units and the required parking to be provided while maintaining some open space, additional land was needed.
When the fire station was built, about 5,000 sf of land on the project’s ½ of the block was used to provide stormwater treatment and detention/flow control in a stormwater “wet pond”. A “wet pond” provides dead storage for stormwater treatment and live storage controlled by an outlet structure for stormwater detention.
“At the level of density necessary to build this type of project, dedicating that much space to a detention pond is just not an efficient use of land,” said the project’s Civil Engineer. The engineer and the owner approached the City of Bellingham with an offer that if the project proved to provide the detention and stormwater treatment function afforded by the existing “wet pond”, the City would have to allow the land containing this pond to be added to the project.
To meet the project’s green goals and to take full advantage of the infiltration capacity of the project’s sandy loam soils, a pervious concrete pavement system was proposed for all new parking areas.
The City of Bellingham agreed to surplus the land for the stormwater wet pond to the project in exchange for an agreement to manage the fire station’s stormwater for both water quality and flow control indefinitely. The addition of this land to the project site took the total project site to just about an acre - approximately 42,000 sf. This additional area allowed the architect to lay out the project with more open space and allowed the construction of a bike barn to encourage non-car transportation.
To provide the required treatment for the off-site stormwater flows from the fire station, two CatchBasin StormFilters were selected. To meet the detention requirements for the off-site flows and for the roof water from the on-site homes, a 60-in. diameter corrugated metal pipe vault system was used. The new 12,000 sf on-site parking lot did not require any additional treatment or flow control as it was designed to directly infiltrate.
The off-site flows are treated by two CatchBasin StormFilters. One is a single cartridge unit located off-site in the back parking lot of the fire station. The second unit is located on-site and provides treatment for the large apron in front of the fire station and a portion of Harris Avenue.
Following treatment in the CatchBasin StormFilters, the stormwater flows are directed to an on-site vault system of approximately 250 lf of 60-in. diameter corrugated metal pipe in three parallel pipes with a header. The flow from the roofs of the 14 homes is also directed to the vault where it combines with the off-site flows and is detained prior to being released to the city’s stormwater system and ultimately into Padden Creek.
In summary, the use of the CatchBasin StormFilters and the corrugated metal vault system was part of a system that incorporated the Low Impact Development strategy of pervious pavement which allowed this project to meet its goals of affordability and sustainability. The efficient use of land in the dense urban areas is important to allow development to occur where services exist and avoid sprawl.
The Stormwater Management StormFilter is a passive media filtration system that removes TSS, soluble metals, oil and grease and total nutrients from stormwater. Siphon-actuated filter cartridges – which trap and absorb pollutants – feature a surface cleaning system that prevents blinding and extends maintenance intervals.
Contech CMP retention systems allow captured stormwater to percolate into the subsoil and offer efficient and economical groundwater recharge. Perforated CMP is installed and typically enclosed with a high quality, soil compatible geotextile. Standard pipe-wall perforations provide approximately 2.5% open area.