Hydraulic hazard exposure of humans swept away in a whitewater river

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.