Background
The Whetstone River is a tributary of the Minnesota River near Ortonville, MN. The river was diverted into Big Stone Lake in the 1930s to increase the lake’s levels during droughts. The river now flows into the lake just upstream from Big Stone Lake Dam.
The Upper Minnesota River Watershed District retained HEI to restore perennial flows to the historic Whetstone Channel. The Whetstone River Restoration Project will restore the hydrologic connection to the Whetstone River, where the historic Whetstone River channel previously drained into the Minnesota River. The flow will be restored to 9,000 feet of the historic Whetstone River channel between Big Stone Lake and the confluence with the Minnesota River. This project will also reestablish the downstream floodplain and floodplain wetlands along the lower Whetstone River to the confluence of the Minnesota River. The restoration will improve the ecological integrity of the Whetstone River and the water quality in Big Stone Lake.
The Project
HEI is working on completing the final plans with a multidisciplinary team. The project will include placing weir structures within the Whetstone River to direct water into the restored channel. Levees will also be placed on adjacent segments of the restored channel area to create a natural floodplain and protect nearby farming activities from flooding. The channel restoration will make it necessary to make changes to nearby roadways both in Minnesota and South Dakota, which includes constructing a new bridge.
As part of the restoration project, HEI was tasked with preparing a joint Federal Environmental Assessment (EA) and state Environmental Assessment Worksheet (EAW), as well as various federal, state, and local permits. These environmental review documents are currently underway and will be published for public comment soon.
To complete our future permitting tasks, our environmental scientists have laid out all local and federal permitting schedules to ensure that all of our environmental permitting obligations will be met on time. We will then review the project plans, certifications, and various permitting items that need to be delivered to agencies for approval.
A hydrologic model that was developed for this project.
Our team developed an Unsteady HEC-RAS model for the Minnesota River from Big Stone Lake to Highway 75 as well as the Whetstone River to the United States Geological Survey (USGS) gage at the City of Big Stone. The hydrologic inputs were based on gage frequency and volume analysis. The model geometry used both surveyed data through the project area and LiDAR. This model was updated to reflect the 30% design and add 2D flow areas to accurately model overland flows near the stream. This helped determine how the new design met the District's goal of diverting discharge through the restored channel.
Our team quantified the downstream impacts of the project with an HEC-HMS model. The Minnesota River reach that we modeled is a series of pools. The area has few riverine flow areas during large events. These pools have control structures at each outlet. However, during spring and summer events, the gates are generally open with little to no operation during an ongoing flood event. All major tributaries to the Minnesota River between Big Stone Lake and Montevideo have USGS gage monitoring stations and recent flooding events cover a wide range of hydrologic frequencies and conditions. For this reason, we developed a new HEC-HMS model based on historic gage data.
Client Benefits
- Restored channel designs to meet District goals and return Whetstone River to its original state.
- Reviewed project plans and certifications to ensure permitting agency compliance.
- Handling state and federal environmental review documentation and future permitting obligations to ensure project schedules are met.
- Reduced flood elevations in Big Stone Lake.
- Improved the water quality within Big Stone Lake and restored the ecological integrity of the Whetstone River.