Otoa Tsukano

Understanding the characteristics of water-related accidents in rivers flowing through lowland areas

Naoyuki Inukai

 Accidents in lowland rivers often occur because these rivers are close to residential areas and easy to access. Because the channel surface looks gentle and calm, risks are easily underestimated, yet localized deep pools and complex flow can rapidly lead to serious drowning. This study examined three cases in the Asahi River (Okayama), Shiroishi River (Miyagi), and Inunaki River (Fukuoka). By integrating topography (rapid depth increase), visibility (riverbed recognizability), and hydraulics (e.g., recirculating flow), we organized the accident process from entry to loss of self-recovery ability and discussed directions for hazard identification and prevention.
 External conditions on accident days were summarized using weather charts, AMeDAS rainfall, and water-level records. Field surveys included UAV imagery and interviews, depth measurements to locate abrupt deepening, dye tracing to visualize flow paths, ADCP observations to capture velocity distributions and recirculation, and bed-material sampling. For the Shiroishi River, two-dimensional flow and advection–diffusion simulations were also conducted to reproduce recirculation and transport pathways.
 Across all three cases, common factors were identified: (1) a shallow, walkable zone followed by sudden deepening within a short distance, (2) turbidity and surface reflection that cause people to misjudge depth as a gradual slope, and (3) combined flow-direction changes, rapid velocity increases, and recirculation that prevent returning to the bank after balance is lost. These results suggest an accident chain in which easy entry encourages forward movement, abrupt deepening triggers loss of footing, and complex flow hinders recovery. Effective prevention requires evaluating overlapping hazard zones and combining site measures (signs, boundary markings, access control, rescue routes) with behavioral rules, such as avoiding water deeper than knee level and recognizing that areas just beyond visible shallows are most dangerous.