Debritic head formation during the Tōhoku-oki 2011 tsunami reveals enhanced risk

Tsunamis pose a major hazard to coastal communities, costing thousands of lives and destroying infrastructure across extensive coastal areas. Whilst the role of large floating debris in amplifying tsunami impacts is well recognised, the influence of finer sediment (sand, silt and clay) on the tsunami flow dynamics and hazard remains poorly understood. Current hazard assessments assume a turbulent, dilute tsunami flow with sediment concentrations below 5%, yet predictive models cannot resolve the internal variability within the flow during inundation. Such variation is evident in other environmental flows, such as subaqueous gravity currents, where a denser component at the base or front of the flow develops over time, markedly altering the flow behaviour. To observe whether similar processes can occur during tsunamis, we analysed helicopter footage of the Tōhoku-oki 2011 tsunami in the Sendai Plain, Japan, focusing on the evolution of the flow front during inundation at two study sites situated 1 km and 1.9 km inland. Using georeferenced video frames and pre-tsunami satellite imagery, we quantified spatial-temporal variations in the flow front velocity over 20-second intervals. Flow front gradients were also estimated where the flow front was observed to overtop large polytunnels. Results revealed rapid temporal and abrupt spatial changes in velocity, with variations of up to 8 ms^-1 across the 20-second periods at both sites. Such fluctuating velocities are indicative of the pulsed surging typical of high-concentration debris flows, contrasting with the more uniform velocities of turbulent flow fronts. Furthermore, the front developed a steep gradient (~25-59°), which can only be maintained in a cohesive, debris flow, being incompatible with a dilute flow that is typically assumed. This state was observed to develop from an initially dilute, turbulent flow in the nearshore that progressively transitioned to a darker, more viscous and debris-laden state further inland. Sedimentary evidence revealed a transition from sand-dominated deposits in the nearshore to mud-rich deposits in the mid- and far-shore, with sustained erosion for at least 2 km inland. The evidence shows that continuous erosion and entrainment of mud-rich substrates (rice paddies, canals) markedly increased the cohesivity of the flow front into a debritic head, which rapidly transformed from the initially dilute, turbulent state. Beyond ~2 km inland, as erosion ceased, the slowing debritic head was likely overtaken by a trailing, more fluidal flow, analogous to similar processes in subaqueous gravity currents. In the mid-shore region, the enhanced viscosity (1000-10000x higher) and density of a debritic head will alter the flow hydrodynamics and exert a greater force on infrastructure, cf. the dilute flow front previously assumed. Future numerical modelling will aim to quantify the change in hazard in similar coastlines. These findings challenge prevailing assumptions and highlight the need to incorporate debritic heads into tsunami hazard assessments on mud-rich coastlines, where the hazard will be enhanced.