Long Lake Dam contains an overflow spillway consisting of eight bays that channel spill water into a plunge pool at the dam’s base. During spill events, the plunge pool became supersaturated with TDG. Elevated levels cause fish to become saturated with high dissolved oxygen, resulting in “stunned” fish that float to the surface and become easy prey. To reduce fish mortality and decrease the overall environmental impacts of energy production, our team developed alternatives to diminish excess TDG.
McMillen’s design team used computational fluid dynamic analysis, including physical model construction and testing, to select a final design option which included installing two deflectors and removing a rock outcropping below two of the spillway bays. In addition to the final design, our team developed a construction approach, means, and methods, including access, staging, and cost estimates to support construction planning and permitting efforts.
McMillen’s self-performed construction tasks included:
- Modifications to the spillway including spillway toe repairs
- Plunge pool depth reduction
- Removal of a rock outcropping in the plunge pool using controlled blasting and heavy equipment only 15 feet from the face of the dam and less than 100 feet from the operational powerhouse
- Placement of 5,000 cubic yards of reinforced concrete using approximately 7,000 dowels
- Installation of 1,300 yards of grout to support grouted riprap in the plunge pool
- Construction and removal of a 1-mile temporary access road and a staging area
- Placement of temporary construction signage
- Temporary cofferdam construction and dewatering
- Reshaping the construction disturbance area with excess fill materials to create a more natural streambank
- Revegetation of the construction site and impact areas
- Planting of 3,000 bushes and trees and 4 acres of hydroseeding
All construction was performed with environmental protection in mind. Our team emphasized a “no-net waste effort” by avoiding potential waste, like concrete debris from demolition activities, instead using the debris to help reshape the flow channel below the dam. A floating turbidity curtain was installed to prevent suspended sediment from seeping through the cofferdam or erosion from the sides of the access road. All temporary and permanent flow points entering the river from uphill construction zones were fitted with various mitigation measures including the construction of temporary sediment retention basins, silt fences, and straw wattles to minimize sediment transport. The Washington State Department of Ecology hailed our environmental practices, stating that this project was the most environmentally conscious site they had ever visited.
Our team coordinated with twelve agencies to secure numerous permits. Early in the process, our team collaborated with the client to analyze potential complications that might impact permitting. Defining permits early in the process allowed us to quickly adapt if issues arose—we could swiftly contact each of the agencies affected, modify the permit, obtain approvals, and continue work without delay. Our managers also performed a Readiness Review Process to methodically identify risks and challenges early. We coordinated these reviews with the client, inspection team, and operations personnel, and any identified challenges were addressed and incorporated into the work plan. This approach led to a smooth, transparent process with no surprises.
Because our team was forward-thinking, we were able to deliver the Long Lake Dam project ahead of schedule. Work was completed within a single low-flow river season, resulting in environmental benefits from lower exposure to ongoing sediment transport and reduced potential conflicts with regulatory requirements. These efforts helped the project come in under budget.
