The purpose of this Project is to remove sediment from Walnut and Grayson Creeks that has accumulated since the last desilt operation in 2006. This work is part of periodic maintenance of these channels as required by the U.S. Army Corps of Engineers. In their current state, the hydraulic capacity of both channels is reduced from the design capacity due to siltation. The hydraulic function of these channels is critical to the operations of the District facilities in the area, and this Project will help regain the hydraulic capacity of both channels. During project planning, four different alternatives were considered: 1) do nothing, 2) raise the levees and build floodwalls, 3) desilt the channels indiscriminately, or 4) selective desilting, which was the option chosen for this Project.
The proposed desilt locations are as follows:
• Grayson Creek from Chilpancingo Parkway to Imhoff Drive
• Walnut Creek from approximately 1,200 feet downstream of Diamond Boulevard to
approximately 1,300 feet downstream of Concord Avenue
These reaches were chosen due to their cost-effectiveness and ability to provide the most flood risk reduction in high-priority areas of Concord, Pleasant Hill, Pacheco, and Vine Hill while minimizing impacts to Environmentally Sensitive Areas (ESAs) such as wetlands and marshes. The channels in these reaches were divided into regions for desilting, termed sediment bars (see Figure 2), which represent areas that are most beneficial to hydraulic capacity and least impactful to ESAs.
In order to minimize environmental impacts, ESAs were identified along Walnut and Grayson Creeks via a thorough set of procedures. Using LiDAR data from a 2018 drone survey, the lowest points of the channel were identified and highlighted for investigation as potential ESAs. Aerial photos on Google Maps (taken during the dry season in October 2017) were used to locate the banks of the channel and vegetation patches (greenery). The assumption was that areas of greenery must have available water and could contain ESAs. A 2-foot buffer was established around the low flow channel to minimize impacts to the flowing channel. In addition, District
staff performed site investigations in September 2019 to evaluate channel conditions. Following the ESA delineation by biological consultant Nomad Ecology, the “Ordinary High Water Mark” (OHWM) was used to determine the extent of the low flow channel around which the 2-ft buffer was redrawn. The OHWM is a technical term for the physical indicators of the typical water level. Open water/freshwater marshes were also included as ESAs. The flowing channel will be avoided completely. Other ESAs will be avoided to the extent feasible.
The vertical limit of sediment removal will be to the original designed geometry of the channels, derived from the as-built plans (see Figure 3). There is approximately 2 to 7 feet of sediment to be removed on each of the sediment bars contributing to about 172,300 cubic yards (cy);129,800 cy from Walnut Creek and 42,500 cy from Grayson Creek. The sediment removal activities will take place within District right-of-way.
Some large sediment bars were divided into smaller segments. The sediment bars were then divided into Rank 1 and Rank 2 sediment bars based on flood risk reduction. The Project currently has enough funding to desilt both Rank 1 and 2 sediment bars (known as the Large Desilt), with the potential of reducing the Project to exclude Rank 2 sediment bars if desilting costs come in high. The analysis in this document assumes the Large Desilt and therefore covers the greatest impact. The extent of the Large Desilt includes all the locations noted above. In contrast, the Small Desilt includes the following: Grayson Creek from 2nd Avenue South to SR- 4, and Walnut Creek from approximately 1,700 feet downstream of Diamond Boulevard to
approximately 300 feet downstream of Concord Avenue.
The current plan for the excavated material from Walnut Creek entails transporting it to the nearby Marathon Refinery and stockpiling the sediment in an upland location. The sediment from Grayson Creek will be disposed at the nearest permitted landfill. If Marathon Refinery cannot accept the Walnut Creek sediment, and another user cannot be identified, it will be properly managed and disposed of at an appropriate permitted landfill.
Nine bridges cross the Project area: Imhoff Drive, State Route 4, Interstate 680, Pacheco Boulevard, Center Avenue, 2nd Avenue South, Chilpancingo Parkway, Concord Avenue, and the Iron Horse Regional Trail. Trucks carrying sediment will be using surrounding city roads to access the identified sediment receiving sites. A staging area for trucks will not be necessary since they will be travelling in and out of the Project site. Other vehicles and equipment will be staged in the work area on levee access roads adjacent to the creeks.
Differences between the two creek locations will be accommodated through modifications to the sediment removal process and types of equipment used. Work along Grayson Creek, which has a rather narrow channel, is likely to move slower as a long armed excavator is planned to remove sediment from the top of bank. Along Walnut Creek and in some areas of Grayson Creek, an excavator will likely be positioned on sediment bars directly via access ramps where sediment bars are wide enough to allow equipment to operate. In these areas, temporary earthen ramps for equipment will be constructed on the banks, and removed following project completion. Soil for the access ramps will be collected from the sediment bars to be desilted via a long armed excavator positioned at the top of bank. In total, 16 access points (three for Walnut Creek, 13 for Grayson Creek) and 14 temporary access ramps (three for Walnut Creek, 11 for Grayson Creek) are anticipated. The quantity of material to be used for an access ramp is anticipated to be 340 cy, and the ramp will extend from the top of the trapezoidal channel into the sediment bars. The dimensions of the ramps will be approximately 15 feet wide by 170 feet long with 1V:3H side slopes, and some additional fill possibly needed for flaring the ramps at their ends to avoid wetlands. The access ramps will terminate near the toe of the creek bank, above both the OHWM and the water level of the dry season. Some adjustments may be needed due to field conditions; however, any impacts would be similar to those already discussed in this document. Anticipated locations of access ramps are shown on Figure 2. A biologist will go into the field before desilting starts and flag wetland areas so they can be avoided during construction of the temporary ramps as much as possible. Temporary wetland crossing mats also may be installed to facilitate additional access for the movement of equipment if needed.
Access to Walnut Creek north of Concord Avenue is expected to be from underneath the Concord Avenue and Iron Horse Regional Trail bridges, with construction vehicles and equipment driving down the creek along designated haul routes that will be established for the Project. Between 1 to 3 feet of sediment will be removed beneath the bridges in order to provide sufficient clearance.
Work is proposed to occur from April to October of two consecutive years expected to start in 2022 such that only one side of the channel is desilted each year minimizing impacts to the ESAs. The contractor is anticipated to work sequentially on the channels, desilting each of them separately, with a single crew using all the equipment to complete work on one side before proceeding with the next. The estimated duration of Project work is a total of 192 days spent over two seasons (96 days per season). All ESAs in the vicinity will be separated from the work by a temporary fence or flagging, with an additional 2-foot berm buffering the creek channel from excavation. After desilting, the disturbed areas at all Project sites will be reseeded with a mixture of native plants. Prior to the rainy season, cuts will be made intermittently in the berm so that the flow of water will naturally erode the barrier following desilting activities. This will prevent ponding of water in desilted areas. The District is the Project applicant; however, the Project will be assigned to Contra Costa County Public Works Department’s (CCCPWD) Design/Construction Division for Project implementation.