Grayson Creek desilting project defense against future flooding

PLEASANT HILL, CA (Sept. 18, 2023) — Sun sparkling off Grayson Creek as it flows near Imhoff Place is a silent reminder that the water source is ever-present, even as drought conditions take a temporary hiatus.

In preparation for future flooding concerns due to heavy rains, as occurred last winter, a $1.5 million desilting effort is underway to alleviate those issues and reestablish the original creek flow.

Started in mid-August and expected to be completed by Oct. 16, the project includes removing and trucking away upwards of 1,600 cubic yards of silt – sometimes more – daily.

Keeping the public safe

About 25,000 cubic yards of material is expected to be removed as part of this project, according to Randy Sanders, senior civil engineer with the Contra Costa County Public Works Department’s Design/Construction Division.

“The benefit of the sediment removal provides much-needed hydraulic capacity within the reach of Grayson that has shown the need for additional freeboard based on some of the storm events that have occurred over the last few years,” said Sanders. “At times, the channel embankments had only a few inches before overtopping may have occurred during these last two seasons.”

Sanders says the desilting is essential.

“Though some of the storms were much greater than a 500-year event and, in most cases, the channels were designed to contain a 100-year storm or a 50-year storm plus freeboard, the restored hydraulic capacity is a necessity for public safety,” he said.

Pedestrians and motorists crossing the bridge along Chilpancingo Parkway, across from CarMax, will see crews working in the area of the creek behind Target. Drivers should expect delays near Imhoff Place along Pacheco Boulevard and Contra Costa Boulevard between Highway 680 and Chilpancingo Parkway as the work progresses and trucks move in and out of the area.

Preserving the natural habitat

Regarding the potential harm to wildlife and vegetation, Sanders noted this project has undergone more than three years of environmental studies and inspections with local agencies such as the state Department of Fish and Wildlife, Regional Water Quality Control Board, Army Corps of Engineers and the Department of Conservation and Development.

Local groups such as the Friends of Pleasant Hill Creeks and the Pacheco Municipal Advisory Committee also provided feedback regarding preservation of the natural habitat.

“Prior to any construction activities this year, a team of biologists surveyed the area for species and vegetation. The impact to avians is minimal as most birds nest in trees and the nesting period generally ends around September 1st,” Sanders said. “The first area of construction performed was in an area without a tree canopy, and the segments to have sediment removed have been surveyed for bird nesting and no observable nests were found.”

Furthermore, most of the vegetation in the creek is considered invasive and non-native. The sediment project will be removing this vegetation and placing it at acceptable upland landfills. Sanders explained that an approved seed mix from local environmental agencies will be hydroseeded before the first rains.

The same technique was used back in 2006, and the work was pretty successful. It is anticipated that the vegetation will be fully established within a year or two.

Crews will place the removed material at a designated upland fill area owned by the John Muir Land Trust, at its request. It will be graded, compacted and hydroseeded.

Sanders noted that the material is largely organic, so it is not suitable for county construction or road projects. But he said the Grayson Creek silt is destined for the $14 million Lower Walnut Creek Restoration Project to sustain plants and animals that call the creek home. Such a project is funded 100% by grants.

More work likely needed in future

The previous desilt undertaking occurred in 2006. Then, crews removed about 12,300 cubic yards of material and gave it to Viano Vineyards in Martinez.

“It is anticipated that heavier rain events will result in increased erosion within the area, making the need to perform another sediment removal in the next decade a possibility,” Sanders reported.

Work on Grayson Creek would happen more often, but Sanders noted that permitting has been much harder to obtain – making the process much more strenuous.

A desilting project of this magnitude comes in response to information provided by the Contra Costa County Flood Control and Water Conservation District, a steward for the Army Corps of Engineers.

As part of the operation and maintenance agreement between the local district and the corps, Contra Costa is required to desilt the channel to the original design capacity about once every decade – subject to the amount of material built up during that time.

The county Board of Supervisors approved the Grayson Creek project on June 27 for $1,477,165, which is provided by Flood Control Zone 3 funds. Estimated construction cost ran as high as $1.63 million.