The city of Mountlake Terrace, Washington needed a new stormwater retention facility to provide stormwater treatment and downstream flood control. The facility would treat runoff from an area of about 13 acres in the city’s downtown core and protect the water quality in the Lake Ballinger watershed.
There were a number of challenges designing the system. The system needed to capture a large volume of water (approx.80,000 cubic feet) and because the system was being built in an urban environment, there was limited footprint to provide temporary storage. Another challenge was the system would need to be very deep, requiring about 15’ of cover. Fortunately there were suitable soils in the area for infiltration.
To address these issues the engineers on the project, Perteet, proposed the creation of a facility below the Mountlake Terrace Library parking lot utilizing a CDS pre-treatment system in front of a large perforated CMP retention system from Contech Engineered Solutions.
is a hydrodynamic separator that screens, separates and traps debris, sediment and oil from stormwater runoff. CDS is an ideal choice for pretreatment, as it prevents downstream solid occlusion and lengthens the longevity of CMP retention systems. The CDS was selected for its treatable flow capability and WA Dept. of Ecology Approval.
After treatment with the CDS, runoff is directed to a perforated CMP system made of 800 linear feet of 120” diameter, perforated Aluminized Steel Type 2
CMP that allows the runoff to slowly infiltrate the surrounding soil. CMP was selected for its ability to accommodate the deep bury, the relatively small footprint, and cost effectiveness.