SQRA Methods Bringing Clarity to a Spillway Design Flow Decision

The Minnesota Department of Natural Resources owns, operates, and regulates the Lake Bronson Dam in northwestern Minnesota. This facility is estimated to have inadequate spillway capacity, and the dam owner has decided to address this issue by replacing the existing gated primary spillway with a new labyrinth weir structure. We teamed with Freese and Nichols to design a new labyrinth spillway structure, with Barr focusing on the hydrologic and hydraulic modeling, and the determination of the design flow. After developing a 2D HEC-RAS model, we modeled a range of flood events, both with and without the dam breaching, and estimated consequences. By integrating the consequences over the range of modeled flood and failure probabilities for a set of increased conveyance capacities to consider for the new primary spillway, our team semi-quantitatively estimated risks for both the non-breach and the breach condition. The results highlighted a clear design point for the primary spillway where the incremental risks were not further reduced substantially by building a larger spillway and the overall risk magnitudes were in the acceptable range.