Debritic head formation during the Tōhoku-oki 2011 tsunami reveals enhanced risk in mud-rich coastlines

Tsunamis pose a major hazard, exaggerated by large floating debris within the flow. However, understanding of how finer sediment (sand, silt and clay) influences the tsunami flow remains incomplete, introducing uncertainty in predictive models of their impact. Typically, a turbulent and dilute tsunami wave is assumed, but the differing conditions at the flow front have yet to be quantified. Here, videos of the 2011 Tōhoku-oki tsunami in the Sendai Plain, Japan, were analysed to document trends in the properties of the tsunami flow front. Results reveal rapid temporal and abrupt spatial changes in velocity and the development of a steep gradient to the flow front (∼25–59°). Deposits reveal the tsunami flow had a high mud content and caused almost continuous erosion for at least 2 km inland. This evidence shows that a highly cohesive flow with a dense debritic head formed in the mid-shore region, transforming from an initially turbulent flow through the entrainment of cohesive material. The altered hydrodynamics and the greater force exerted by a dense debritic head highlight the need to incorporate debritic heads into tsunami hazard assessments on mud-rich coastlines, where the hazard will be enhanced.