City of Kent plans to dredge Mill Creek to reduce flooding

Kent city officials have big plans under way to dredge Mill Creek to reduce flooding along James Street, 76th Avenue South and Kennebeck Avenue.

“We have sediment buildup that is causing flooding in the Kent Valley,” Matt Knox, city environmental conservation analyst, said in a report last month to the City Council. “We have worked on a number of ways to reduce flooding and we are still working on it. This project we believe will take this to the next step. We will get a big lift to reduce this chronic valley flooding.”

The city plans to hire a contractor to dredge three miles of the creek, from about Smith Street to just north of South 212th Street. But city staff is going through a complex and lengthy permitting process before the dredging starts. Construction might not start until 2020.

“Very extensive environmental documentation is required,” Knox said.

Kent will need to get permits from the state Department of Fish and Wildlife, the U.S. Army Corps of Engineers and the state Department of Ecology. The project also will need to meet requirements of the State Environmental Policy Act (SEPA), and might require a state Environmental Impact Statement (EIS).

City staff expects to find out in the next few weeks whether a EIS is required. If so, it could take up to two years to complete, said Mike Mactutis, city environmental engineering manager, in an email. Completion of federal and state permits and mitigation requirements follow that.

“We have already met with the permitting agencies to get feedback on the city’s proposal and they are very hesitant to allow such a comprehensive reworking of a creek system until they are convinced the city is doing everything it feasibly can to prevent sediment from moving into the creek system again in the future,” Mactutis said. “Based on a schedule with an EIS and permits, construction could begin in 2020.”

City staff doesn’t have a cost estimate yet for the project because mitigation requirements have not been finalized, Mactutis said.

“We are working on partnering on funding this project with King County Drainage District No. 1, which has ownership of a substantial portion of the creek systems in the valley,” Mactutis said. “The city’s portion of the funding will come from its stormwater utility plus any grants that can be obtained. Grants for local storm drainage improvements are typically not readily available, however.”

City officials are in talks with the Muckleshoot Tribe to help determine if areas along the creek need to be surveyed for potential impact on historic sites.

“We are working to complete documentation, and also to make sure no cultural resources are in the way,” Knox said.

Mill Creek runs through the heart of Kent’s industrial area and has caused flooding for years on James Street near Central Avenue, on Kennebeck Avenue and along 76th Avenue South.

“Dredging used to occur regularly prior to the 1990s, but not much recently because of the increased environmental permitting required,” Knox said.

Sediment builds up in the creek and then reed canary grass grows on top of the deposits and it slows the flow of water through the valley.

“Along 76th Avenue grass is growing in the creek and there is a buildup of sediment,” Knox said. “The street goes under water repeatedly in the rainy season and causes damage to the road with heavy vehicles traveling on top of the water-logged roadways.”

Several large Kent businesses are along 76th Avenue South, including Blue Origin, the Jeff Bezos-owned company that builds engines for eventual space travel rockets.

When the dredging starts, crews will pull sediment off the bottom of the channel. Crews will set up a 25-foot work zone for excavators to remove the sediment and dump trucks to haul it away. Crews also will temporarily pump water out of the creek in the work area and remove fish.

The King County Drainage District helps maintain 14 miles of creek in Kent and Renton. Crews remove brush along creek banks as well as downed logs or beaver dams.

The drainage district has 30-foot easements through much of the valley, which the city will use to get equipment set up to perform dredging work to clean out the creek system.

The city will need to buy easements at a few properties.

Kent must also replace wetlands removed from the creek during the dredging with additional wetlands in the area. Knox said about 4 acres of wetlands will be removed, so the city must find 2 to 4 acres for every acre removed.

Knox said the city purchased 20 acres, land known as the Little property, in 2010 that included a lot of wetlands but not enough to meet all of the wetland mitigation requirements.

The Corps of Engineers asked the city to look at ways to reduce sediment. Kent hired a consultant who found spots up in Mill Creek Canyon where erosion occurs and contributes to sediment in valley. Those are areas where the city can create clean-out vaults alongside creek to overflow the creek into the vault, and when sediment builds up, crews can easily clean out those areas without dredging.

The city’s other efforts to reduce Mill Creek flooding over the last several years included culvert cleaning in 2010 and 2011 to remove sediment as well as a project during the South 228th Street bypass in 2011 to drain water into the city’s Green River Natural Resources Area.

Kent had one of its worse floods in 1996 when water covered the East Valley Highway, also known as Central Avenue.

“We have done a number of projects, so the same flood would not cause this much damage,” said Knox as he displayed to the council a photo of the 1996 flood. “But this is the next step with the Mill Creek project to reduce this so we are ready for next large flood.”

That was good news to Council President Bill Boyce.

“We look forward to seeing the flooding reduced,” Boyce said after the report by Knox. “We won’t get rid of all flooding but we will reduce it, right?”

“This will be a significant step,” Knox replied.