‘Lego’ cells buried beneath West Central sidewalks as part of stormwater project - Storm Water Jobs

‘Lego’ cells buried beneath West Central sidewalks as part of stormwater project


STORMWATER AND LEGOS - That’s the hope of Spokane’s public works department, which has spent the summer burying a network of plastic chambers on block corners throughout the historic neighborhood. When finished, the structures – called Silva Cells – will trap tens of thousands of gallons of stormwater runoff beneath the sidewalks and prevent it from entering Spokane’s combined sewer system. When that system is inundated, untreated sewage flows into the Spokane River, occurrences that must be limited under discharge agreements between the city and environmental regulators. It’s the city’s first experience with the technology, which was developed by an urban landscaping firm called DeepRoot based in San Francisco. The appearance of the plastic framing, which is buried and covered with wheelchair-accessible sidewalks, grasses, shrubs and trees, has prompted more than a few analogies from the city’s engineering team. “Legos,” said Kyle Twohig, director of engineering for the public works department. “Glorified milk crates,” said Lindsay Duvall, the inspector for the project with the city’s engineering department. The modular building blocks arrive in stacks of boxes ordered from DeepRoot. Crews from Halme Construction, the contractor who won a $3.4 million bid to complete the work, chop up existing sidewalks and dig several feet down, where the plastic pieces are put together and filled with a type of soil designed to promote filtration of pollutants in water running down storm drains. The chambers, which are topped with a plastic deck that is covered with concrete, allow the soil to remain loose, promoting the growth of root systems in the 96 trees that crews are planting above the stormwater chambers. The chamber is not completely filled with soil, allowing space for stormwater. That water flows into large concrete chambers and is dispersed throughout the Silva Cell using another plastic pipe slitted at 6-inch intervals to allow the release of captured rainwater. “It is really helping us both with our stormwater treatment, as well as giving us a neighborhood aesthetic,” Twohig said. The approach is an alternative to the many massive subterranean tanks that are being built around town to hold stormwater runoff during heavy rains, but the end goal is the same: Less water in the pipes means fewer pollutant discharges into the river. “This was the basin, of all the basins in our system, that penciled out the best for what we call a green solution,” Twohig said. “It would have taken multiple tanks, and we couldn’t easily locate a site where we needed the tanks.” The work is taking runoff in the West Central neighborhood out of the combined system. Rather than heavy runoff filling up the pipes and causing discharge into the river, any backups in the system would occur in the gutters and along curbs in the neighborhood while the underground chambers catch up to the heavy rainfall or snowmelt. The system has been designed to withstand a 10-year storm, which means backups should only happen about once a decade. “It would really just run down to the next cell, or the next storm drain,” Twohig said. To date, the city has spent close to $70 million on the tank system, according to payment vouchers with firms that include Halme, Garco Construction, Clearwater Construction and others who have signed contracts for the projects. Another $70 million is expected to be spent before the tanks are completed, with part of the construction being funded by green bonds the city sold in 2014 as well as other sources, including grants from the state Department of Ecology. Halme is under city orders to limit construction in the neighborhood to corridors, and not to work on more than five chamber locations at any one time, Duvall said. While neighbors said they’ve been pleased with the look of the replaced sidewalks and the speed of construction in most cases, Duvall acknowledged there had been some signs of discontent.

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