DJ&A was tasked with completing dam assessments for 53 remote, earthen dams located across the Beaverhead-Deerlodge National Forest in southwest Montana. Assessments included review of existing historical dam data, site visits to evaluate conditions of the existing dams and appurtenances, breach analysis, hazard classifications and final reporting. One of the main goals of this project was to identify high priority dam rehabilitation projects and recognize deficiencies to be addressed by the dam owners.
The majority of the dams were remote and accessed with ATVs or by foot. In many cases, engineers backpacked into isolated alpine dam sites located at elevations over 8,500’ above sea level. Field assessment included: measurements of dam embankment, spillways, intakes, outtakes, and other appurtenances as well as documentation of deficiencies. Data was collected at all downgradient structures with potential risks of inundation by a dam-breach flood.
Breach inundation maps were created to exhibit simulated dam-breach flooding extents based upon photogrammetry and topographic maps and hydraulic modeling using US Army Corps of Engineers HEC-HMS and HEC-RAS programs. Analysis led to downstream hazard classification reports. Reports included description of conditions of dams and appurtenant hydraulic structures along with dam breach-modeling results, stream cross sections, analysis of potential downstream hazards, calculations, and breach inundation maps. Finally, the dams were classified as High, Significant, or Low hazard.
- Field assessments for 53 remote dams
- Hydraulic modeling and analysis of downstream hazards
- Creation of breach inundation maps
- Creation of hydrographs
- Production of hazard classification reports
Due to the large number of dams requiring site visits and the short weather window available, the dam site visits were split evenly amongst two teams of engineers. This allowed all field work to be completed in the fall following the peak summer recreational season, yet prior to heavy snowfall. In preparation of each site visit, a GIS desktop review of topographic mapping and aerial imagery was conducted to assess the terrain of the downstream reach and develop awareness of features that may be affected by inundation.