2:45 PM - 3:00 PM
[MIS16-05] Formation processes and conditions of a tsunami-related erosional feature and its significance
Keywords:Tohoku-oki tsunami, sediment transport, numerical simulation
Tsunamis and storm surges induce erosion and transport of sediments on sandy beaches, resulting in changes of coastal geomorphology with diverse mode and size, ranging from small-scale scour on beach ridge to total disappearance of sand bar. Erosional features are considered as one of the proxies of extreme waves, and some previous studies discussed the history of past extreme waves based on the formation age of erosional scarps detected within coastal sediment sequences.
Improved understandings on characteristics, formation and burial processes and sedimentary structure through observation and analysis of modern examples are needed to establish criteria for identifying subsurface erosional feature and to clarify the relevance to the trigger wave events. During the 2011 Tohoku-oki tsunami in Sendai Plain, breaches, embayments and shore-parallel ditches were commonly formed on sandy beaches, and they have been associated with tsunami overwash of the engineered dikes and drainage due to backwashes. Although the formation of the shore-parallel ditches was prominent near elevated engineered structures such as protection walls, comparison of pre- and pos-tsunami high-resolution elevation data showed that shore-parallel ditches were formed on some natural topographies away from engineered structures. In addition, satellite imageries showed that these ditches were ponded for a long time until reconstruction works modify the coastal topographies. These observations imply that erosional features like shore-parallel ditches can be formed on natural topography of beach ridges and preserved as a trace of tsunami inundation.
In this presentation, pre- and post-tsunami topographic characteristics and hydraulic properties of the erosional features will be examined using the examples from the Tohoku-oki tsunami. Formation processes and conditions of the erosional features are analyzed based on numerical simulations of tsunami-induced sediment transport, and significance of erosional features for tsunami deposit research and tsunami size estimation will be discussed.
Improved understandings on characteristics, formation and burial processes and sedimentary structure through observation and analysis of modern examples are needed to establish criteria for identifying subsurface erosional feature and to clarify the relevance to the trigger wave events. During the 2011 Tohoku-oki tsunami in Sendai Plain, breaches, embayments and shore-parallel ditches were commonly formed on sandy beaches, and they have been associated with tsunami overwash of the engineered dikes and drainage due to backwashes. Although the formation of the shore-parallel ditches was prominent near elevated engineered structures such as protection walls, comparison of pre- and pos-tsunami high-resolution elevation data showed that shore-parallel ditches were formed on some natural topographies away from engineered structures. In addition, satellite imageries showed that these ditches were ponded for a long time until reconstruction works modify the coastal topographies. These observations imply that erosional features like shore-parallel ditches can be formed on natural topography of beach ridges and preserved as a trace of tsunami inundation.
In this presentation, pre- and post-tsunami topographic characteristics and hydraulic properties of the erosional features will be examined using the examples from the Tohoku-oki tsunami. Formation processes and conditions of the erosional features are analyzed based on numerical simulations of tsunami-induced sediment transport, and significance of erosional features for tsunami deposit research and tsunami size estimation will be discussed.