Hydraulic hazard exposure of humans swept away in a whitewater river
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Despite many deaths annually worldwide due to floods, no strategy exists to mechanistically map hydraulic hazards people face when entrained in a river. Previous work determined water depth–velocity product thresholds for human instability from standing or walking positions. Because whitewater rivers attract diverse recreation that risks entraining people into hazardous flow, this study takes the next step by predicting the hazard pattern facing people swept away. The study site was the 12.2-km bedrock–alluvial upper South Yuba River in the Sierra Nevada Mountains. A novel algorithm was developed and applied to two-dimensional hydrodynamic model outputs to delineate three hydraulic hazard categories associated with conditions for which people may be unable to save themselves: emergent unsavable and steep emergent surfaces, submerged unsavable surfaces, and hydraulic jumps. Model results were used to quantify exposure of both an upright and supine entrained person to collision and body entrapment hazards. Hazard exposure was expressed with two metrics: passage proximity (how closely a body approached a hazard) and reaction time (time available to respond to and avoid a hazard). Hazard exposure maps were produced for multiple discharges, and the areal distributions of exposure were synthesized for the river segment. Analyses revealed that the maximum hazard exposure occurred at an intermediate discharge. Additionally, longitudinal profiles of the results indicated both discharge-dependent and discharge-independent hazards. Relative to the upright body, the supine body was overall exposed to less dangerous channel regions in passage down the river, but experienced more abrupt encounters with the danger that did occur.
KeywordsHydraulic hazards River rapids Floods Hydraulic jumps Whitewater
No external grant was provided directly for this work. Indirect support was received from the USDA National Institute of Food and Agriculture, Hatch Project Number #CA-D-LAW-7034-H. Precursor data were collected for different purposes with an award from the Instream Flow Assessment Program of the Public Interest Energy Research Program of the California Energy Commission. The Instream Flow Assessment Program was administered by the Center of Aquatic Biology and Aquaculture of the University of California, Davis. This project involved a large collaborative effort that was only possible by gracious contributions of effort and resources by many people, including relicensing stakeholders, their consultants, our paid project staff, and UC Davis student volunteers. Helpful reviews and feedback of the final technical report with the 2D model and other precursor study components leading up the work herein were provided by Professor Allen James (University of South Carolina), Dudley Reiser (R2 Resource Consultants, Inc), Michael Barclay (HDR/DTA), Thomas Studley (PG&E), and Dr. Joshua Wyrick (Lafayette College). Lastly, Daniel Brasuell provided assistance in locating the rapids present along the study segment.
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