The Lake Bronson Dam creates the centerpiece of a state park in Kittson County in western Minnesota. The dam is a strong focal point of Lake Bronson State Park and is owned by the Minnesota Department of Natural Resources.
The structure needs rehabilitation to address geotechnical, hydrologic, and operational deficiencies. As part of the effort to prioritize and better define the benefits of various remediation options, Ayres developed an SRH-2D model of the downstream river valley, including the already flood-prone community of Lake Bronson. The project team’s decision to expand the study to include a 2D model followed an initial one-dimensional dam-break analysis using the HEC-RAS model. This decision was driven by the need for detail in the downstream developed areas, as well as the flat topography and diffuse drainage patterns of western Minnesota.
The SRH-2D model extended from the tailwater of the dam to a point approximately 4 miles downstream. Using bare-earth lidar elevation data, Ayres generated a model mesh containing approximately 126,000 elements with a minimum node spacing of approximately 4 feet and a maximum spacing of approximately 65 feet. The dam, its impoundment, and the breach process were modeled in HEC-RAS, delivering an upstream breach hydrograph to the SRH-2D model reach.
The objective of the modeling was not to establish a deterministic inflow design flood (IDF), but to provide state decision makers with information on the relative risks associated with a range of natural floods, flood-induced dam failures, and dam failures arising from other potential vulnerabilities in the dam. Natural floods ranging from the current spillway capacity to the 1,000 year event were modeled, as well as dam failures at the current maximum capacity and an earlier deterministic estimate of the IDF.
Ayres worked as a subconsultant to RJH Consultants on this project. Services included creating a preliminary design; finalizing the IDF evaluation; developing a preliminary qualitative risk register; performing preliminary hydraulic, structural, and geotechnical analyses; performing an independent technical review; and providing final design.