Rohnert Park wins $6 million grant to help build flood-control basin

Recent flooding along Rohnert Park’s east side highlights the need for better flood-control along Copeland Creek, and help is on the way.

Rohnert Park has won a $6 million FEMA grant to mitigate flood problems and help recharge the area’s groundwater.

The Federal Emergency Management Agency’s 2020 Building Resilient Infrastructure and Communities grant will partially fund the construction of the Copeland Creek detention basin, a flood-control project the city has planned since at least 2007.

The multi-purpose basin along Copeland Creek east of Sonoma State University is expected to detain stormwater flows and mitigate the damage potential of 10-year flood events. Capturing storm water in the basin will allow a slower recharge of the groundwater and create habitat for fish passage and salmonid habitat creation, according to the city.

“The timing could actually not be better,” said Councilmember Susan Hollingsworth Adams, “because we had huge flooding issues with this last set of storms. This will help the flooding issues our residents were experiencing.”

The $6 million in grant funding is almost half of the total project cost of $12.4 million. The rest of the funding will come from developer fees and contributions from developers, the city said.

The schedule for construction will depend on when FEMA approves a start date, although it is expected to be sometime in 2022. It should take about three years before the basin will be put into use.

Rohnert Park sits within the Laguna de Santa Rosa watershed and the eastern edge of the city is at the bottom edge of an alluvial fan created from water flowing down from the hillsides of the Fairfield Osborn Preserve, depositing sediment there.

During rainy seasons, stormwater and sediment rush into Rohnert Park and cause the eastern part of town and the city channels to flood.

The FEMA grant program shifts the federal focus from reactive disaster spending to proactive investment so communities are better prepared for when floods, hurricanes or wildfires happen, according to the agency. Including Rohnert Park’s award, FEMA selected projects throughout the country totaling $377 million.

In its application, Rohnert Park noted that recent floods in the area have damaged more than 100 county transportation lines and 2,100 business and residential properties.

The detention basin will collect and temporarily store stormwater while releasing a lesser amount that lowers the risk of flooding downstream.

In 2007, the city began planning for such a project in conjunction with the Sonoma County Water Agency, now Sonoma Water, which was studying storm water retention and groundwater recharge in the region.

That study helped complete the environmental analysis and nearly all of the construction design for what eventually became this project. Costs put the project lower on the priority list.

Because of the area’s adobe soil, which doesn’t absorb water as well as other types of soil, construction along Copeland Creek will require a significant amount of engineering fill and soil protection below ground to prevent overflow toward the city’s urban areas, according to the city.

The design should allow for a controlled flow through the recharge area, slowing storm water flow and allowing more water to percolate through the gravel of the creek in order to increase groundwater recharge.

It is also expected to create a flood plain south of Copeland Creek that will create low-flow areas for fish to hide and rest as they make their way upstream to spawn, and areas that can be planted and contoured as wetlands.

You can reach Staff Writer Lori A. Carter at 707-521-5470 or lori.carter@pressdemocrat.com. On Twitter @loriacarter.