Birch Lake Dam Low Level Outlet Abandonment

The Washburn County Highway Department retained Ayres to design repairs for the Birch Lake Dam in accordance with 2016 Wisconsin Department of Natural Resources Municipal Dam Grant guidelines. The original plan for the repairs was to repair or replace the existing low level gate, which had been inoperable for several years, to make it operable and to fill in a low spot on the embankment crest of the dam.

However, concurrently with this project, Ayres completed a dam failure analysis (DFA) for the dam that indicated the low level outlet was not needed for spillway capacity and that the embankment in its existing condition provided adequate freeboard. Therefore, the design was modified to abandon the low level outlet by fully grouting the upstream and downstream portions of the penstock and the gate. This change in design saved Washburn County an estimated $100,000 to $200,000 in construction costs and eliminated the need for future low level gate maintenance. These cost savings were a direct result of completing an updated DFA.

As part of the design, a professional dive documented the dimensions of the existing low level gate, gate dimensions and type, rough opening, thimble dimensions, stem braces, and other hardware controlling the low level outlet and concrete dimensions surrounding the low level outlet gate.

Final design information included comparing the existing penstock empty compared to fully grouted (evaluating settlement and sliding potential), specifications on the grout for penstock abandonment, and details for adding survey monuments for future inspection reference points.

Construction documents showed the low level penstock, how it was abandoned, and what materials were needed for the repair. Ayres also provided grant acquisition assistance, bidding assistance, construction observation, and grant closeout documentation.

• Ellen Faulkner, Water Resources Engineer
• Pete Haug, Manager - Water Resources

• Dams & Flood Control