Rain gardens like the one here at Ballard Corners Park are being considered by Seattle Public Utilities to decrease the number of sewage overflows in Ballard.
City starts tests to install natural drainage in Ballard
Seattle Public Utilities hosted the first of three public meetings with the Ballard community to present their plans to install bio-retention facilities to help manage the storm water runoff in a 10-block radius of the neighborhood.
The pilot program, the Ballard Natural Drainage Project, will install facilities at blocks between Northwest 68th and 85th streets and 28th and 22nd avenues northwest.
“Essentially, we’re looking at taking a planting strip and basically amending the soil," Shanti Colwell of Seattle Public Utilities said.
The facilities will be within the existing planting areas and in some places they will “bump” out into the existing roadway for a short length, said Colwell.
The bio-retention facilities will contain special soils designed to effectively infiltrate and filter storm water off the road and will accommodate up to six inches of ponding to help store and treat as much runoff as possible.
The facilities will be densely vegetated to help build soil structure and enhance pollutant removal while providing attractive landscaping, according to the city. The thick vegetation serves to reduce the speed of the water flow to allow sediments and pollutant to settle out.
The technique has already been used in North Seattle and High Point in West Seattle. Now Seattle Public Utilities is moving the project into Ballard, which has a combined basin.
"This means that all the water that falls in the streets gets combined into one pipe with all the waste water from homes and goes out to Westpoint for treatment," Colwell said.
She added that in larger events when rain keeps coming down there may be too much water, causing an overflow of diluted sewage that then flows out to Salmon Bay.
Colwell explained that in Ballard’s basin there are about 11 overflows a year and Seattle Public Utilities hopes the pilot project will lessen those occurrences.
The city is required by EPA to control the number of overflows and has a few strategies to prevent or decrease the problem.
One strategy would be using the larger basins, which are like large storage tank underground facilities that have a capacity to hold large amounts of water when there is too much for the pipe system to handle, it would then release water slowly, said Colwell.
Another option is using system retrofits. The city is looking at existing systems and figuring out how it can maximize capacity.
Lastly, Seattle Public Utilities is looking at rain technologies (i.e. rain gardens.)
All these technologies manage the water so it can be released slowly or does not enter the storm or plant system at all and reduces the number of overflows, Colwell said.
The bio-retention facilities would be shallow depressions that are filled with the soil mix and plants and would capture and infiltrate the storm water.
One design includes working with an existing planting strip in Ballard. Colwell said many are wide enough to allow Seattle Public Utilities to cut into them and create curb cuts to let water run through.
Another would be creating bump outs, which would cause water to run down the curb line of the street, enter the facility where the water will be ponded and will then infiltrate it.
Colwell said they are estimating an infiltration of 90 to 95 percent of street run off with these systems and are hoping to manage 80 percent of the total.
As of now Seattle Utilities is testing a number of different areas to determine the infiltration rate of the soil, Karen York of Seattle Public Utilities said.
"It'll be good information to analyze where we will locate the bio-soils," she said.
Some people at the hearing were concerned about where the testing would take place. York explained the city will test in 30 parking strips, where a three foot by four foot hole, two feet deep will be dug, with a maximum of two to three on a street at one time.
Crews have been filling the holes with water and are measuring how fast infiltration is in that area's soil.
Another concern heard from residents was the maintenance, puddling and parking of the bio-soil facilities.
York explained that the designers are taking these concerns into consideration and will not eliminate parking on an entire street. She said the city is looking for alternatives and are evaluating the feasibility of using alleys and will work with the community before they make any final decisions.
Seattle Public Utilities plans to continue with the infiltration testing from now until early August, and in early fall hold a second public meeting where more information will be provided on results of the infiltration testing.
Residents will be able to comment on test findings before the second public meeting, said York.
For more information contact Karen York, Seattle Public Utilities Project Manager at email@example.com.