Street Edge Alternatives Project

The landscape elements serve an important role in both providing an aesthetic benefit as well as contributing to the management of rainfall. Trees will help to restore more of the evaporation and transpiration that was present before development.

The other vegetation in the drainage swales will also help to filter and slow the flow of storm water. Over 100 deciduous and evergreen trees and 1100 shrubs were added as part of the project.

Design involvement

The landscape architect was involved more during the design than a typical project. The most apparent example of this is a sidewalk design that not only serves but attracts pedestrians.

There also was an emphasis on retaining existing large-scale trees and relocating vegetation to meet homeowner needs and project goals. The swales and surrounding areas are artfully graded and planted with native wetland and upland plant species. Granite boulders and various sizes of washed river rock provide both function and beauty.

Landscape design

The landscape design complements the drainage system function and focuses on native and salmon-friendly plantings. The system is unique in its use of grading, soil engineering, plant selection and layout as components that function together -- much as they do in a natural ecosystem.

Natural systems approach

Returning drainage and vegetation in the area to a natural systems approach is an important element of this demonstration project.

For example, native soil from excavations was mixed with organic compost to provide rich topsoil and reduce water and fertilizer needs.

As another example, clay was the preferred liner material for swales. This helps to ensure vegetation can survive in the summer months by allowing moisture to move up through the soil. A liner fabric would be less effective in this role.

The cost of this project was $850,000. This included an extensive design and communications budget due to the need to work closely with residents on the design. Future projects will cost less than traditional street improvements.

Benefits

The project helps creeks by reducing stormwater at the source. As Seattle has developed, stormwater running off impervious surfaces has had a major impact on our creeks and wildlife.

SEA Streets provides an example of the environmental benefits that can be realized with natural systems instead of traditional systems, especially in areas which are installing new infrastructure.

Residents of this neighborhood enjoy walking along SEA Streets because it is a natural, soft-edged environment, in contrast to the hard edges of traditional linear streets. Also, more tree cover helps reduce summer heat while absorbing air pollutants and rainfall.

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.