We were contracted to replace flashboards and perform apron scour repairs at a hydroelectric dam along the Rock River in the Upper Midwest. Our client is a privately owned entity that owns and operates 80+ hydroelectric facilities throughout the United States.
The project’s significant goals involved increasing the dam’s retainage capacity, protecting the existing spillway structure, and providing an aesthetically pleasing spillway for the community's residents. This project was scheduled for the summer months when flow rates are at their lowest, mainly due to the volatility of the Rock River and the need for a slow-moving current to keep our divers safe.
Developing a Safe and Efficient Work Environment through Innovation
To perform this work safely and efficiently, our team devised means and methods to divert the water away from the spillway crest, exposing the flashboards and providing a dewatered work area for crew members. A temporary cofferdam system composed of large steel A-frames and thick fabric membranes was rented to accomplish this task. Portadam®, a New Jersey-based company, was initially skeptical if their product would be effective in this application as the cofferdam system would be deployed in the fast-moving current near the spillway crest.
The project was ready to mobilize after we completed an initial site visit to verify water depths. Then, a 5-man dive team mobilized to remove debris before dewatering the work area in the spillway.
Brennan's ADCI-certified commercial diver is preparing to remove debris from the work area before dewatering the spillway.
Risk Mitigation: Safeguarding A-Frame Installation in Underwater Environments
The first step in the project was to load each of the 250 LB, 7 FT tall A-frames onto inner tubes for transport to the work area. Each set of frames is only a few feet wide and must be linked together to serve as one structure.
Our commercial diver navigates the spillway through the 7-FT tall A-frames that serve as the bones of the cofferdam.
After assembly, tarp-like membranes were attached near the base of the frames to slightly reduce the initial, sudden head pressure exerted on the structure. This specific portion of the installation process was Portadam’s main area of concern, as once the final outer membrane was laid over the frames, the entire structure would be subject to approximately 5–6 FT of head pressure. If the structure started sliding towards the flashboards, it could potentially be overturned and sent over the spillway.
Before installation, Brennan team members connect sections of the modified outer membrane.
We revised installation procedures for deploying the outer membrane to mitigate the overall risk. Each 50 FT x 33 FT membrane was linked to cover the frame width, secured to the top portion of each frame, and then slowly unrolled roughly 27 FT upstream before submerging. Sandbags were used to hold the tarps in place when submerged, and the work area containing the old flashboards was dewatered within minutes.
Expert Engineering for Efficient Dam Crest Restoration
The dam’s crest was previously extended vertically via a steel framework anchored directly to the spillway concrete. Treated tongue and groove boards, plywood, and angle iron were used to span the width between frames. We initially removed all the deteriorated lumber and angle iron. New tongue and groove boards were cut to length and inserted into angle iron slots, which were welded to the steel framework.
Arial photograph of our Portadam® installation, including our own membrane modifications to mitigate risk from water pressure.
Uneven frames were adjusted and secured to recreate the level crest across the spillway. Rolls of ethylene propylene diene monomer (EPDM) rubber roofing were then installed over the tongue-and-groove boards to prevent leakage and deterioration. Sheets of treated plywood were cut to fit so they could also slide into the angle iron slots. Finally, full-length pieces of angle iron, bolted through the lumber, were placed across the spillway crest to seal the entire flashboard assembly from the top.
Our Safety-First Approach to Efficient Spillway Restoration and Client Satisfaction
We dewatered each work area, removed and replaced flashboards, and dismantled the Portadam in 3 phases as we moved across the 220 FT spillway width. This was a repetitive and tedious process; however, the on-site crew executed the work safely and efficiently.
Final Results: Our hydro dam spillway repairs, post-construction.
The client was extremely pleased with the final product and the innovative techniques applied to increase crew members’ safety and reduce costs. We will surely apply what we learned while overcoming challenges throughout this project to future projects across the company.
About J.F. Brennan Company
J.F. Brennan Company, Inc. (Brennan) is a fourth-generation, family-owned marine construction firm specializing in environmental remediation, dam construction, commercial dive, harbor management, and submarine cable services. Working closely with public and private owners of water-based infrastructure since 1919, Brennan operates nationwide throughout coastal and inland waterways.
