It wasn't just water: The hidden force inside Japan's 2011 tsunami changed everything
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As the tsunami moved across the land, it picked up large amounts of clay and silt and became much denser, forming what researchers call a debritic head (a mud-rich front that behaves more like slurry than clear water). Mud is heavier than water, and when this sediment-rich moving mass hit buildings, the force was far greater than standard flood models (that assume clear water) predict.
The researchers also found that this fast-moving tide of debris was eroding the ground for at least 2 kilometers inland, meaning it was continually picking up sediment.
"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," wrote the study authors in their paper.
Improving tsunami hazard assessments
The team shows how the mud-carrying wave likely exerted more powerful destructive forces. As a result, they believe debritic heads should be taken into account when developing tsunami hazard assessments.
"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."