Screening Tool for Predicting Drowning Potential at Low-Head Dams

Recirculating currents downstream from low-head dams can trap recreational river users where they are continuously and repeatedly impacted by the high-velocity flow coming over the dam and eventually drown. Case studies and previous laboratory research have shown that the tailwater level is a primary determinant of the danger, with medium tailwater depths common to a wide range of flows creating the nearly inescapable submerged hydraulic jump. This study investigates a set of low-head dams at which fatalities occurred to evaluate the effectiveness of a previously developed spreadsheet tool for identifying the dangerous flow range. The study considered 58 fatal incidents at 29 low-head dams across the United States and also describes site visits to six low-head dams in Utah where there was no history of fatalities. Discharge information was available for all sites from nearby gaging stations. Downstream channel slopes were estimated from the National Hydrography Dataset (NHD), and tailwater depths were estimated using the Manning equation with a uniform assumption of n = 0.030. With these settings, the algorithm made correct predictions in 75% of the cases. For the successfully modeled cases, flow-duration curves were developed and used to estimate the frequency of the predicted submerged hydraulic jumps. These dams were determined to have submerged hydraulic jumps for an average of 343 days per year, or 94% of the time. Similar frequencies were obtained using streamflow estimates obtained from the National Water Model (NWM) and the GEO Global Water Sustainability Initiative (GEOGLOWS) model (GEO stands for the Group on Earth Observations). Sensitivity of predicted results was tested using the channel roughness defined by the Manning’s n value, the downstream channel slope, and a streambed elevation adjustment factor to account for aggradation or degradation at the dam toe. Prediction success improved with increased channel roughness, decreased channel slope, or aggradation of the downstream channel above the dam toe. Accuracy decreased slightly with a negative streambed adjustment (degradation), decreased channel roughness, and increased channel slope. Further testing is recommended using field-derived values for inputs wherever possible.