Collaborative Research: RAPID: Quantifying the fluvial geomorphic response of the Blue Earth River to a catastrophic avulsive dam failure, Rapidan Dam, Minnesota

After three days (17–20 cm) of rainfall flooded the Blue Earth River, the river avulsed around the western edge of the ~114 year old Rapidan Dam in the early morning of 24 June 2024. Rapid incision and lateral migration shifted the bank ~100 m westward in three days. This event, which the investigators have termed an “avulsive dam failure”, provides a brief time window to study the fundamental coupled fluvial and hillslope processes that result from rapid base-level fall, including knickpoint development and retreat coupled with lateral erosion alongside mass-wasting processes associated with sudden valley incision. Climate and land-use change combine to generate larger floods whose erosion then destabilizes the surrounding landscape, thereby producing cascading hazards. The investigators hypothesize that rapid incision and channel migration should immediately follow avulsive dam failure, similarly to basin integration following spillover when a new hydrogeomorphic system is established after breaching a sill. This is followed by relaxation of the river’s longitudinal profile as knickpoints evolve and the river adjusts to a new local base level. The investigators will capture and analyze these geomorphic phenomena through time with repeated collection of unmanned aerial systems (UAS) imagery, supplemented by community-collected data. They will quantify volumes of erosion and deposition, flow velocities, and channel-migration rates using data products derived from structure-from-motion photogrammetry and aerial footage. Aging dam infrastructure across the United States, combined with increased magnitude and frequency of precipitation, will likely drive continued dam failures into the future. The data resulting from this work could inform future management strategies, especially as our national dam infrastructure continues to age. This project will provide high-impact research experiences for both undergraduate and graduate students, while serving to mentor them under a collaborative, multi-institutional environment with broad expertise in geospatial and geoscientific disciplines. Rapid data collection following this avulsive dam failure will help to answer core geomorphic questions about basin integration and subsequent knickzone evolution. Physical models demonstrate that much of the incision from spillover processes occurs during and shortly after the event. Attempts to quantify these changes in a real-world setting must effectively capture these early stages that record the most rapid change within the fluvial system. Additionally, longer-term data will capture knickpoint-retreat rates, including whether the knickpoint remains coherent or diffuses, across stratigraphic units with varied mechanical properties (that is, glacial lake sediments to sandstone bedrock). Over the shorter term, the results of this project may help to explain and predict rapid bluff retreat along the Blue Earth and other rivers, attributed to anthropogenic climate and land-use change. Furthermore, the investigators will quantify downstream sediment dynamics in response to erosion and mobilization of reservoir deposits. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.