The image above left shows temporary ponding of water near the project site and along paradise creek. The stormwater runoff is from a large parking lot and the water flows through grass swales. The soil was already saturated from rain the day before (there was .32" of rain in the 48 hours preceding the photograph). The image on the right shows the pond at the end of the storm while the image on the right shows that most of the standing water was gone 24 hours later. However, there were winds of 20 miles per hour average for about 8 hours after the photo on the left was taken. Also, the relative humidity dropped below 60% during the period of high wind. Therefore, evaporation probably played a factor in the reduction of the standing water. Nevertheless, this example illustrates that infiltration is certainly a viable practice for the project site.

The table below defines the infiltration rate for various soil textures. Notice that Silt Loam has a rate of .5 inches per hour. This is probably too high given the amount of clay in the Latahco soil type. Use .2 inches per hour in your calculations for a more accurate rate.

Soil Infiltration Rate, Inches/Hour

Note: Rates based on full cover. These figures decrease with time and percent of cover. Derived from USDA information. (SCC 0812 § 1, 1990.) 

The Street Edge Alternatives project is a City of Seattle demonstration and prototype intended to document the effectiveness and costs of using an ecological infrastructure in contrast to conventional storm sewer approaches.

The image above left shows the existing condition. The paved area and the right-of-way is very wide. This is amplified by the ample building setback requirement.

The second image illustrates the new narrowed street with its curvilinear alignment. Notice that the sidewalks are narrow but that there is no curb. The combined width of the road and walks allows fire vehicles to access in an emergency.

The image at left, top, shows the finished project just after construction was completed. Enlarge to image to see the depth and extent of the infiltration areas, since these are obscured in the images that show mature vegetation.

In the second image notice that on this block there is a sidewalk on only one side of the street. The street sheet flows to the right where the swales and infiltration beds are placed. This changes the conventional practice of crowning a road and draining to a curb, gutter and catch basin. Sheet flow grading responds particularly well to sloping sites.

When residential lots slope steeply toward the street, then retaining walls or steepened slope are required to create the space needed for the swales and infiltration beds.

Along streets that are very steep the bed of the infiltration swale must be made more level to reduce velocity, encourage infiltration, and improve water quality. Drop structures are used to make periodic vertical changes in elevation. In the image above, notice the trapezoidal notch in the concrete drop structure. This is a weir. It allows for low flows and permits easy measurement of the gallons per minute (or cubic feet per second) flowing through the weir.

This image shows the plunge pool below the drop structure. The deepened and rock lined pool dissipates the energy in the water and prevents erosion.

The SEA Streets project was enormously successful. It improved water quality and reduced the total volume of stormwater leaving the neighborhood by 99 percent. This reduction has the important benefits of reducing downstream channelization and sedimentation that impacts the local salmon stream. The SEA street completely solved the problem of historic basement flooding in the adjacent residences.

This holistic approach is not only to be better for the environment, but is also less expensive than conventional piped stormwater drainage systems.

Low impact development is the phrase applied to development that controls runoff and other development impacts on the parcel rather than in neighborhood or municipal scale measures, such as detention basins. Some of the principles of this approach are listed below.

• Implement natural drainage including a natural drainage system at various levels of the development so that it can mimic nature.

• Use bioswales along street edges to slow, filter, and infiltrate stormwater.

• Utilize smaller block scale features such as rain gardens, disconnected downspouts, roof gardens, multi-functional open spaces that act as infiltration and storage areas, and grass swales.

• Plant a lot of trees

• Implement the low impact development in ways that work with site specific concerns, like snow storage, to minimize maintenance and ensure performance.

The Siskiyou street demonstration project was constructed by the city of Portland, Oregon and consists of a pair of stormwater curb extensions that retrofit an existing residential street. Located on NE Siskiyou Street between 35th Place & 36th Avenue in Portland, the landscaped curb extensions are within the parking zones on each side of the street just above storm drain inlets.

This well documented project designed by Kevin Perry, ASLA was installed in October, 2003. Total project cost, including management, design, and construction was $20,000 of which $3,000 is attributed to ancillary street and sidewalk repairs costs. These would not be needed for similar projects or new construction. Total cost for the stormwater curb extensions was $17,000 or $1.83 per square foot of impervious area managed.

The curb extensions converted approximately 590 square feet of pavement to landscape. The size of the catchment is a little over 9,000 square feet and was considered fairly representative of conditions in the surrounding neighborhood. The infiltration area is approximately 5% of the drainage area, while the City of Portland’s 2004 Stormwater Management Manual (SWMM) specifies 6% for surface infiltration.

The configuration of the local combined sewer allowed for placement of a flow monitor. There is also a rain gage near the project. The project is maintained by neighborhood residents. Cities like Portland that have some combined sanitary and storm sewers are motivated to remove the stormwater runoff from the combined sewer since the stormwater causes the wastewater treatment plants to overflow during large storms. This causes inadequately treated sewage to be discharged into the local river causing environmental and human health hazards, not to mention the EPA fines for violations of the discharge permits.

The city traffic engineers considered the low-traffic residential setting to be ideal for a demonstration project. The street is 28 feet wide with a longitudinal slope of 2%. The addition of two 7-foot wide curb extensions created an acceptable lane configuration at the street intersection. Water lines were the only subsurface utilities within the project area and did not present obstacles. The project did not eliminate on-street parking. Adjacent property owners can park in front of their houses on SE 35th Place.

Landscaped curb extensions help filter pollutants from stormwater runoff, reduce stormwater flow, recharge ground water, and increase the aesthetic appeal of the streetscape. The image above is the simple plan for the infiltration area. The four bays trap sediment and provide time for the water to percolate into the soil.

In the plan view (top left), water enters the infiltration area at curb cut locations and is retained with check dams. Any water that does not soak into the ground will then overflow at curb cuts at the end of the basin. In this project the space available for the curb extensions – length of curb unbroken by driveways or near a fire hydrant – was considered representative of conditions in other areas.

Check dams slow the flow of water and promote the infiltration of stormwater.

(Snell, 2007) Check Dam)

Plants such as rushes, sedges, and Oregon grape “green” the neighborhood while providing direct stormwater benefits.

The section, above left, shows the construction requirements of the infiltration bed. Notice the maximum depth of the water stored.

Soil infiltration rate is a critical issue but in this case an infiltration test was not required before construction. Adequate documentation of the characteristics of the local soils already existed. The Natural Resources Conservation Service (NRCS) soil survey for Multnomah County classifies the soils as 51A-Urban Land and well-drained Multnomah soils. The surface horizon typically is dark brown silt loam about 25” thick. Soil below this depth is gravelly silt loam and gravelly sand to a depth of approximately 60”.

The second image above is a table that shows the infiltration rates for various soil types.

(Snell, 2007) Soil profiles showing texture and horizons)

The image above shows a geotextile fabric. It is used to separate soil types. In this project it was placed on top of the existing subgrade to prevent the permeable backfill from sinking into the natural soil. This practice is no longer recommended since clay particles can migrate to the geotextile fabric and clog the openings. This dramatically slows the infiltration into the subsoil.

The image below shows the curb extension on the north side of the street. The two curb inlets are shown and the excavation is complete.

(Snell, 2007)

After installation, use and analysis the performance of the infiltration beds can be assessed.

The physical volume of the infiltration basin is large enough to store the initial peak of the 25-Year storm without infiltration. That translates into a minimum 60% reduction in peak flow, and would provide flooding protection for the majority of the local basements regardless of the infiltration rate.

The combination of the facility volume and infiltration into the subsoil was sufficient to capture a vast majority of each design storm. Infiltration rates at this facility were excellent, even during saturated conditions.

(Snell, 2007) 25 year storm simulation test

Some interior check dams were eroded by the highest flow rates and required repair after the initial tests. A new check dam design is recommended to reduce maintenance. A rock clad check dam is shown below. An improvement to the constructed system includes an outlet dam or weir incorporated into each basin to maximize the retention volume.

The data shows that the curb extensions on Siskiyou are capturing and infiltrating a large proportion of the runoff that drains to them.

The plants grew vigorously during the first year and little weeding was required.

During the first year, the vegetated fore bays filled twice with sediment and debris (to a depth of 4-6 inches). City staff removed the sediment by hand with a shovel and rakes. The sediment removal typically required 30 minutes per fore-bay.

Although planted infiltration basins like this one are usually built for water quality improvement this example demonstrates significant stormwater rate and volume reductions.

Infiltration or bioretention is the permanent capture of a volume of water within a planted bed. These systems reduce the total amount of water that runs off the site, as illustrated in by the Siskiyou project above. This is in contrast to detention systems that only slow the rate at which water runs off.

The image at upper left is at the Glencoe Elementary School, SE Belmont, Portland, OR. In the image foreground there is a cobble inlet that serves to direct water from two streets into the garden. The cobble area serves as a sediment trap. The next sections store water. An overflow is shown to accept water above the design storm. However, enough water is infiltrated and stored to slow runoff which reduces downstream damage. There is no permanent pond (wet pond) in this case.

The image, above right, shows a bioswale that receives the runoff from the school parking lot. The bioswale traps sediment and conducts the water to the infiltration garden. The swale is intended to clean the water but logs across the swale also retain some water. Often a sequence of bioswales, retention and detention measures make up a stormwater management plan.

In the image below you can see the shallow ponding in the project during a storm and just after the project was planted.

(www.werf.org)

The Portland State University project takes a slightly different approach with a more urban character. Notice in these images that curb cuts allow water to flow into long narrow infiltration beds. The planted beds are set back from the curb to allow passengers in parked cars to exit without stepping into the plantings.

The planting are architectural in character and there are few species. Both of these features are consistent with the urban context of the project. Notice in the third image that curb cuts also drain water from the sidewalk into the planted beds.

Water Quality Improvement with Bioretention Basins