drainage icon Drainage
Reduce impervious (hard) surfaces by narrowing the road; create more space for plants and soil to absorb rain water; control flooding and move stormwater away from the roadway.


drainage area

Solutions for Steeper Slopes

To address the steep slope on NW 110th Street, the project designers used a series of stair-step swales to direct stormwater runoff through the Cascade and create spaces to slow, treat and infiltrate runoff.

Large, round sedimentation catchbasins, which can be seen at the previous tour stop, are located at each end of the swales. These store excess runoff, releasing water into the system at a slower rate. Meanwhile sediments picked up by the runoff have time to settle to the bottom of the catchbasin, which helps clean the water.

Another device used by designers, shown at left, is a "weir wall" that separates each swale cell into two levels. Stormwater accumulates on the upstream side of each weir wall, and eventually flows downstream through the notch in the top of the wall. This design helps engineers control the timing and volume of runoff on steep slopes, to create a slow, steady release of runoff downstream.