5:15 PM - 6:45 PM
[MIS20-P10] Field survey of tsunami-related erosional topography in a coastal plain and numerical modeling of associated flow parameters
Tsunami deposits, in particular they exist shallow surface, are poorly preserved due to natural or anthropogenic disturbances and erosion. It results in local variation of layer thicknesses and even discontinuous distribution of the layer. Incomplete paleotsunami records have a significant impact on assessment of the extent and magnitudes of the tsunami source and recurrence intervals of past events.
Objective of the present study is to obtain new information from tsunami-related erosional topography, which is an alternative paleotsunami trace that compliments tsunami deposit. To this end, we conduct ground-penetrating radar (GPR) surveys and numerical modeling to estimate the flow parameters. The erosional topography studied here is buried scours in the lee side of a beach ridge and breaches that cut across a ridge, which can be associated with past tsunami events. Despite its potential as a paleotsunami record, erosional topography has not received much attention in previous researches. Formation age and causative flow parameters of erosional topography, will provide a new insight into the timing and magnitude of paleotsunamis.
In this study, structure and sediments of shallow subsurface were examined at paddy fields near Minami-Naganuma and at Chochingama Wetland in the Sendai Plain. In the Chochingama Wetland, significant erosion and deposition of sediments occurred at the time of the 2011 Tohoku-oki tsunami. Note that sandy layers found from the Sendai Plain have been correlated with the 1611 Keicho-Oshu, 1454 Kyotoku, 869 Jogan tsunamis, and other earlier tsunamis which occurred in the 4th-5th century, 1st century, and 3rd-4th century BC.
Comparison of the pre- and post-2011 DEMs revealed that the elevation of the Chochingama Wetland has been decreased by more than 60 cm due to the Tohoku-oki tsunami (Tomita et al, 2013). The GPR profile taken along a shore-perpendicular transect showed seaward-downlap reflections, consistent with beach deposits. Within the core retrieved along the transect, sands of the beach ridge is gradually changed o soil, which is then overlain by bluish-gray silt. Based on the results of diatom analysis, this silt layer is considered to have been deposited in a wetland near the sea. In this presentation, possible relevance to the past tsunami events will be examined to explain the environmental change from a beach ridge to wetland.
According to old topographic map and aerial photographs, (1) a small coastal pond and (2) an elongated, shore-perpendicular pond were existed in the Minami-Naganuma area. The GPR profile taken along the transects that cut across the pond (1) showed a landward-dipping reflection that cuts the radar facies of foreshore and backshore. A convex-down reflection with a width of ca. 40 m and a depth of 1.0 m was also observed. The GPR profile along the lines that intersect the pond (2) also showed a convex-down reflection with a width of ca. 80 m and a depth of 0.5 m depth. Radiocarbon dating of the sediment samples suggested that the convex-down reflection at the pond (1) formed after 915 AD and before 1169 AD. Although no historical tsunami is known during this period, these convex-down reflections can be associated with erosional topography formed by unrecorded tsunamis or storms.
In this presentation, plausible origin of the erosional topography observed at Chochingama Wetland and Minami-Naganuma is examined by using numerical simulation of storm and tsunami, such as the Delft3D and TUNAMI-STM models. Numerical experiments with varying flow parameters are performed to clarify the relationship between the type and size of external force and the dimension of erosional topography. Flow parameters of the paleotsunami or storm are then evaluated by applying the observed data to the modeled relationship between flow parameters and erosional topography.
Objective of the present study is to obtain new information from tsunami-related erosional topography, which is an alternative paleotsunami trace that compliments tsunami deposit. To this end, we conduct ground-penetrating radar (GPR) surveys and numerical modeling to estimate the flow parameters. The erosional topography studied here is buried scours in the lee side of a beach ridge and breaches that cut across a ridge, which can be associated with past tsunami events. Despite its potential as a paleotsunami record, erosional topography has not received much attention in previous researches. Formation age and causative flow parameters of erosional topography, will provide a new insight into the timing and magnitude of paleotsunamis.
In this study, structure and sediments of shallow subsurface were examined at paddy fields near Minami-Naganuma and at Chochingama Wetland in the Sendai Plain. In the Chochingama Wetland, significant erosion and deposition of sediments occurred at the time of the 2011 Tohoku-oki tsunami. Note that sandy layers found from the Sendai Plain have been correlated with the 1611 Keicho-Oshu, 1454 Kyotoku, 869 Jogan tsunamis, and other earlier tsunamis which occurred in the 4th-5th century, 1st century, and 3rd-4th century BC.
Comparison of the pre- and post-2011 DEMs revealed that the elevation of the Chochingama Wetland has been decreased by more than 60 cm due to the Tohoku-oki tsunami (Tomita et al, 2013). The GPR profile taken along a shore-perpendicular transect showed seaward-downlap reflections, consistent with beach deposits. Within the core retrieved along the transect, sands of the beach ridge is gradually changed o soil, which is then overlain by bluish-gray silt. Based on the results of diatom analysis, this silt layer is considered to have been deposited in a wetland near the sea. In this presentation, possible relevance to the past tsunami events will be examined to explain the environmental change from a beach ridge to wetland.
According to old topographic map and aerial photographs, (1) a small coastal pond and (2) an elongated, shore-perpendicular pond were existed in the Minami-Naganuma area. The GPR profile taken along the transects that cut across the pond (1) showed a landward-dipping reflection that cuts the radar facies of foreshore and backshore. A convex-down reflection with a width of ca. 40 m and a depth of 1.0 m was also observed. The GPR profile along the lines that intersect the pond (2) also showed a convex-down reflection with a width of ca. 80 m and a depth of 0.5 m depth. Radiocarbon dating of the sediment samples suggested that the convex-down reflection at the pond (1) formed after 915 AD and before 1169 AD. Although no historical tsunami is known during this period, these convex-down reflections can be associated with erosional topography formed by unrecorded tsunamis or storms.
In this presentation, plausible origin of the erosional topography observed at Chochingama Wetland and Minami-Naganuma is examined by using numerical simulation of storm and tsunami, such as the Delft3D and TUNAMI-STM models. Numerical experiments with varying flow parameters are performed to clarify the relationship between the type and size of external force and the dimension of erosional topography. Flow parameters of the paleotsunami or storm are then evaluated by applying the observed data to the modeled relationship between flow parameters and erosional topography.