Storm Damage Repair and Restoration of Stockponds D2, D5, H6, K3, and M1 within the Los Vaqueros Watershed - FEMA 4683

Storm Damage Repair and Restoration of Stockponds D2, D5, H6, K3, and M1 within the Los Vaqueros Watershed - FEMA 4683

CCWD proposes to restore the dam embankments and habitats of Ponds M1, D2, K3, D5, and H6, all of which were damaged by the 2023 winter storms. CCWD will also improve the existing earthen spillways or culvert spillways at each of these ponds to meet the 25-year storm event. The purpose of this project is to restore the damaged pond habitats and enhance spillways so that these habitats are more resilient to future storm events. See Figure 1 for a map of pond locations at the Los Vaqueros Watershed. Prior to the start of construction, ponds holding water will need to have water drained using screened pumps prior to construction. Drained pond water will be placed in the intermittent streams located downstream of each pond. Low permeability soil (clay) that is similar to the existing dam embankments will be imported to each of the pond sites and compacted to restore breached embankments. The imported fill will need to be benched into the existing pond embankments at the site of erosion scars. This will require an approximate maximum depth of excavation of 3 feet below grade under the dams. Below is a description of existing conditions and proposed repairs at each of the five ponds. Pond M1: During the 2023 storm event, Pond M1’s earthen spillway failed to direct overflow water away from the pond dam face due to sedimentation from an adjacent steep hillside that raised the elevation of the spillway over time. As a result, water overtopped the dam face and formed an erosion scar. As an emergency measure following the 2023 storm event, CCWD recontoured the earthen spillway and laid visqueen and sandbags over the erosion scar. CCWD proposes to: • Repair the erosion scar with approximately 70 cubic yards of imported engineered fill and recompact the area. • Fill in the existing earthen spillway with approximately 600 cubic yards of imported engineered fill • Construct a new earthen spillway further away from the steep hillside to prevent future sedimentation issues. This will be a trapezoidal spillway that has a 3-foot wide base and a height of 3 feet. The spillway will be lined with geotextile fabric and riprap (140 cubic yards) to prevent future erosion. Pond K3: Pond K3 is located adjacent to a Watershed gravel access road. During the storms, the Pond’s earthen spillway located adjacent to the road eroded and overtopping of the dam caused a dam breach to form on the dam embankment. Pond K3 still retains water. CCWD proposes to: • Repair the dam breach with approximately 55 cubic yards or imported engineered fill. • Reconstruct the earthen spillway as a riprap-lined weir that spans across most of the dam. The riprap-lined weir will be approximately 35 feet wide by 50 feet long (300 cubic yards) and have a layer of geotextile fabric underneath. Pond D2: The storm events resulted in overtopping of Pond D2’s dam embankment and caused dam failure and significant erosion. The dam embankment failure occurred at the Pond’s two 20-inch side-by-side culverts which acted as the Pond’s spillways. These culverts were washed downstream along with displaced soil and rock when the dam failed. Currently, Pond D2 is retaining water, but has an open breach within the dam embankment. CCWD proposes to: • Repair the breach within the dam embankment with 200 cubic yards of imported engineered fill. • Replace and upsize culvert spillways to two 42-inch side-by-side culvert spillways that meet the 25-year storm. • Construct riprap aprons around the culvert inlets and outlets to help prevent erosion. The riprap apron at the inlets will be approximately 17 feet by 33 feet (50 cubic yards) and at the outlets riprap will be 25 feet by 25 feet (55 cubic yard). Pond D5: Pond D5 has an existing 36-inch culvert spillway that became clogged and failed to engage during the 2023 storms. Dam failure and significant erosion occurred at the pond, with the existing culvert remaining in place. Currently, the pond is not able to retain water due to the damage. CCWD proposes to: • Import approximately 410 cubic yards of fill to restore the pond embankment. • Installing a new 42-inch culvert spillway in the location of the dam breach. The existing 36-inch culvert spillway will remain in place. • A riprap apron will be installed at the culvert inlet and outlet. The riprap apron at the inlet will be approximately 17 feet by 33 feet (50 cubic yards) and at the outlet it will be approximately 12 feet by 14 feet (20 cubic yards). Pond H6: Pond H6 has an existing 36-inch culvert spillway. While the pond’s embankment did not breach during the storm, erosion at the culvert spillway’s outfall resulted in severe erosion of the pond’s dam face. CCWD will: • Restore the eroded dam face with approximately 275 cubic yards of imported fill. • Replace and upsize the existing culvert with a 42-inch culvert spillway that is approximately 100 feet long to discharge downslope of the dam. • A riprap apron will be installed at the culvert inlet and outlet. The riprap apron at the inlet will be at the inlet will be approximately 17 feet by 33 feet (50 cubic yards) and at the outlet it will be approximately 18 feet by 18 feet (40 cubic yard). CCWD will also explore opportunies to provide additional species/habitat benefit at each of the ponds.

Contra Costa

Unincorporated

Contra Costa