Winter rains this week drenched the region, transforming the lazy upper reaches of the Petaluma River into a raging torrent of muddy water that has risen precariously close to homes and businesses along Petaluma Boulevard.
Next year, however, the river is expected to be much more placid even in heavy rains as another flood control project aims to increase water capacity and make the waterway safer.
The Petaluma City Council on Monday approved the construction of the Denman Reach Phase 4 flood control project, using $2.6 million in grants from the Sonoma County Water Agency. As the name implies, the project builds on three other phases of flood control measures on the upper Petaluma River, where threats of inundation have persisted even after a $40 million federal flood control project was completed downstream.
“We’re getting more flood plain storage,” Councilman Mike Healy said. “It’s good stuff.”
The water agency grant will allow the city to purchase a 5.47-acre parcel on Industrial Drive near the auto mall to be used as a flood detention basin, which will capture excess rain water. The city will be able to regulate the influx of water from the basin to the river, limiting the flow of the river during peak flooding events, Public Works Director Dan St. John said.
“This will create a pond upstream to capture localized flood water,” he said. “It adds additional flood control benefit.”
Previous flood control projects upstream have terraced the riverbank to increase water capacity. In addition to helping manage rain runoff, the latest project will also include 1,000 feet of new walking paths around a seasonal wetland that will attract birds and other wildlife, St. John said.
“It’s one of those opportunities to make a good project even better,” he said.
The new paths will link to other phases of the project in the area, creating a continuous walking path along open space from Corona Road almost to Stony Point Road. In addition, roughly 6,000 cubic yards of debris that has accumulated at the base of the Corona Bridge abutments will be removed from the river, allowing water to flow easier at a key bottleneck.
Work is slated to begin in July and finish by December.
The Phase 4 project will likely be the final piece, completing the flood control measures at Denman Reach, St. John said. The city would not have been able to complete the project without the water agency’s help, he said.
The agency approached the city in 2016 with money from a state Department of Water Resources grant that it was not able to use on other projects in the county. Supervisor David Rabbitt directed the funds to the Petaluma River.
“There’s no way the city has that kind of funds for this project,” St. John said.
St. John said the Denman Reach projects are designed to work in conjunction with the Payran flood control project, which the Army Corps of Engineers completed after floods ravaged downstream neighborhoods in the previous decades. The channelization of the river is designed to allow water to move more quickly through the Payran neighborhood, while the projects on the wilder upper reaches of the Petaluma River will allow water to be released more slowly.
Healy said the upper river project will protect the north end of Petaluma, but still doesn’t solve the key need down stream of removing silt from the Turning Basin downtown. The Army Corps of Engineers has not dredged the Turning Basin in more than a decade, and the mud-choked river has become impassable at low tide for commercial and recreational boats. Besides that, it is a flood risk for downstream residents and businesses, Healy said.