BIA Dam Safety Hydrology and Hydraulics

As part of the IDIQ contract with the Bureau of Indian Affairs (BIA) Dam Safety Security and Emergency Management, WBK Engineering subcontracted to contribute to the Comprehensive Review (CR) of eleven dams located on Reservation Lands in Arizona, New Mexico, South Dakota, and Wisconsin for the U.S. Department of the Interior, and the BIA. These dams range in height from 20’ to 50’, have watershed areas ranging from less than 1 square mile to over 100 square miles, and reservoir capacities ranging from 35 acre-feet (AF) to 16,500 AF.

In general, a CR provides a periodic, in-depth assessment of the safety of the dam and its appurtenant structures. WBK’s role in the CR process for each dam was to complete a Hydrologic Hazard Report (HHR), a Dam Failure Inundation Study (DFIS), and a Dam Failure Consequences Memorandum. Each of these studies contributes to and informs the overall CR. WBK completed all of these assessments on-time and on-budget.

The HHR was developed to estimate rainfall-runoff inflow hydrographs into the dam reservoir for various frequency and PMP storm events. The resulting flood hydrographs were then routed through the reservoir and dam to identify the potential hydrologic hazards of the dam. The HHR provides a summary of the hydrologic analysis including flood precipitation, runoff derivation, and reservoir routing. Hydrologic modeling was completed in HEC-HMS and geospatial data processing was completed in ESRI ArcMap.

The purpose of the DFIS was to estimate the impacts to the region downstream of the dam under various flood scenarios including breach of the dam. The analysis was used to create downstream inundation maps. The studies included routing reservoir inflow hydrographs, developing dam breach parameters, and assessing downstream inundation for multiple scenarios. Inundation scenarios included a fair weather (sunny-day) failure, a PMF non-dam-failure, a PMF failure, and a Non- dam-failure advisory flood. Both breach routing and downstream hydraulic analysis were completed concurrently within an unsteady 2-dimensional (2D) HEC-RAS model. Inundation mapping was completed in ESRI Arc Map. In some cases routing extended for over 100 miles downstream of the dam.

The Consequences Memorandum presents estimates of population at risk (PAR) and consequences in terms of loss of life in the event of failure of the dam in multiple scenarios. The population at risk was determined by identifying residential structures through aerial photography and applying a PAR per structure based on U.S. Census data. Where appropriate, visitor statistics for recreational areas and AADT of inundated roadways also contributed to PAR estimates. Loss of life estimates were derived by applying the U.S. Bureau of Reclamation Consequence Estimating Methodology (RCEM) which determines a fatality rate as a factor of flood depth x velocity and warning times. WBK also completed additional analysis at the request of the BIA to test the sensitivity of loss of life estimates to population spatial aggregation and to test of the sensitivity of loss of life estimates to different RCEM fatality factor curves.