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.

Storing Stormwater on Steep Slopes

Within the Broadview Green Grid, streets that run east-west have steep downhill slopes. Before construction, rainfall on these slopes was conveyed quickly into ditches along the roadside; there was little time for the roadside soils and grasses to absorb the water. Creating a Natural Drainage Systems project on a steep slope, like NW 107th Street (shown here) offered a new challenge for SPU engineers.

In order to increase the time and space for runoff to be absorbed by the landscape, the project team designed a series of giant swale cells down one side of the street. Each cell is divided by a series of weirs with flow control notches, creating a "staircase" for water flowing down the slope. This image shows the weirs in one swale cell during construction.

If you compare these weirs to those used two years earlier on the 110th Cascade Project (three blocks to the north), you will see that NW 107th Street has more weirs, in order to better control drainage functions. The project team also chose to use rock walls for the outer edges of the swale cells, providing stability and a coherent aesthetic along the whole street.

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