Drainage Improvements

The drainage design represents a unique hybrid, combining hydraulic engineering with soil science and botany to create a more natural system.

Hydraulic engineering required strict control of elevations utilizing various aggregates and soil mixes below grade. Drainage improvements combine contoured swales with traditional drainage infrastructure (culverts, catch basins, flow control structures and slotted pipe) to regulate the flow and discharge of storm water.

Project goals

One of our primary drainage goals was to use surface retention or detention to reduce 2-year, 24-hour storm event (1.68-inches) peak runoff rate and volume to pre-developed conditions.

Since our goal was developed to reduce the impacts of our urban environment on streams inhabited by salmon, we chose to detain stormwater for the entire contributing drainage area (street right-of-way and properties along the east side, total 2.3 acres), not just new or redone impervious area.

Also, we worked to meet the goal of conveying the site’s 25-year, 24-hour storm event, which is the design requirement for all City of Seattle drainage systems.

Methods used

The method we used for achieving this goal was to maximize the stormwater time of concentration and the sites detention volume, without compromising homeowner access and parking needs on the street.

Using a long flow path length and high surface roughness along the flow path increased the basin's time of concentration. All areas within the right-of-way without hard surfaces were used toward detention.

Some additional considerations

Our original hope for retaining flows and allowing infiltration into the native soils throughout the length of the block was not possible because some homes had an existing groundwater intrusion problem. To limit the potential for stormwater to adversely impact the residences of concern, our geotechnical engineers identified some swales that needed an impermeable liner – for example, a six inch depth of natural clay material was used as the preferred material.

Sizing the swales to detain the pre-developed two-year, 24-hr storm event, while not allowing infiltration, resulted in the need for very small outlet orifice diameters. Due to maintenance concerns, we wanted outlet orifice diameters greater than ½-inch. To reduce potential for these small orifices to clog, we also provided a sump in the flow control structure. Working with the site’s flat topography (1% longitudinal slope) and the minimum orifice size restraint, we chose to use only 3 flow control facilities on the block.

We hydraulically connected the swales into three groups, with each group controlled by a flow control structure. Detention volume achieved by swales was 2,500 cubic feet; 37% more volume than would have been required by our drainage ordinance.