2:00 PM - 2:15 PM
[HCG22-02] Process of coastal erosion by the 2011 Tohoku-oki tsunami on the Osuka Coast of Hachinohe City, Aomori Prefecture, Japan, based on topographic analysis
Keywords:erosion, dune, ground penetrating radar, tsunami, the 2011 Tohoku-oki tsunami
Tsunamis cause significant sedimentation and erosion. In the case of the 2011 Tohoku-oki tsunami, the distribution and characteristics of the deposits have been well clarified (e.g., Goto et al., 2021). Erosion by the 2011 Tohoku-oki tsunami has also been studied (e.g., Tanaka et al., 2012). However, tsunami erosion has not been well studied due to its limited area of formation and less preservation potential than sedimentary records. Thus, the process and regional differences of tsunami erosion have not been fully understood.
In this study, we focused on the Osuka Coast in Hachinohe City, Aomori Prefecture, Japan. It has been protected as part of a national park, so there have been no major reconstruction projects after the 2011 Tohoku-oki tsunami.
The tsunami overtopped the 7 m-high dunes and inundated the coastal lowlands. Kamada (2011), who conducted a survey immediately after the tsunami, considered that the tsunami overcame the dune and deposited beach and dune sands on the landward side of the dune and that the backwash caused large-scale erosion landforms due to the quick seaward discharge. However, there are still many unknowns about the erosion process. Therefore, the purpose of this study was set to elucidate the erosion process by the 2011 Tohoku-oki tsunami on the Osuka coast.
The first step in this study was to examine time-series topographic changes using a digital elevation models (DEMs). Since there were only a few available DEM data for the target area, we created a DEM from past aerial photographs. In addition, excavation and ground penetrating radar (GPR) surveys were conducted in 2021 and 2022.
A topographic change analysis using DEMs revealed that the ‘valley’ formed by the 2011 Tohoku-oki tsunami was formed by the breakup of the dune and that the erosion depth reached up to 5 m from the dune top. In general, tsunami erosion is common in areas with steep slopes or large coastal lowlands (Tanaka et al., 2012; Takamura et al., 2015). However, the small coastal lowland and gentle slope of the Osuka coast make it difficult for generating a strong backwash.
We then examined the causes of this erosion. First, DEMs show that the coastal lowlands extended between the high dunes and landward slopes before the tsunami and that the entire lowlands were widely inundated during the tsunami. The GPR survey of the subsurface structures in the area, where erosion formed ‘valleys’ and was slightly buried by sand, revealed a prominent reflection surface at a depth of about 2 m below the ground surface. Comparison with a DEM of the immediate post-tsunami topography indicates that this is close to the surface immediately after the 2011 tsunami. The GPR reflection surface indicates the existence of a boundary surface between layers with different properties. Therefore, the reflection surface suggests the boundary created by the post-tsunami deposition of sand on the surface exposed by the tsunami erosion.
Next, to determine the cause of the strong reflection surface observed by the GPR, excavation was conducted on the transect of the GPR survey. The underwater surface was confirmed at a depth of nearly 2 m, and no further excavation was possible with the hand auger because of a solid layer. This suggests the existence of an impermeable solid surface at a depth of about 2 m.
Meanwhile, there are no large rivers flowing out to the sea on the Osuka coast, but there are some small streams that flow in from the landward hills. These streams cannot be confirmed on the surfaces of coastal lowland, dune and beach and are thus considered to flow underground in this area toward the sea. Based on this, we re-examined the DEM and 3D topographic data and found that the ‘valleys’ created by the tsunami are located just seaward of where the stream flows into the Osuka coast.
From the above, the following factors are considered to have caused large-scale erosion of the dunes on the Osuka coast. First, due to the unique topography of the Osuka coast, seawater that entered behind the dune by the tsunami pooled in the coastal lowlands without being rapidly discharged to the sea due to the strong backwash. The seawater was then slowly but continuously discharged into the sea, and the flow was concentrated in limited areas. As a result, even though the area of the Osuka coast is small and cannot pool as much water as strand plains, the relatively small amount of water may have caused significant erosion. The relationship between the location of the groundwater flow paths and streams identified by GPR reflection and excavation and the location of the valleys suggests that the presence of groundwater and other terrestrial water may have been a major factor in defining the location where tsunami erosion occurred.
In this study, we focused on the Osuka Coast in Hachinohe City, Aomori Prefecture, Japan. It has been protected as part of a national park, so there have been no major reconstruction projects after the 2011 Tohoku-oki tsunami.
The tsunami overtopped the 7 m-high dunes and inundated the coastal lowlands. Kamada (2011), who conducted a survey immediately after the tsunami, considered that the tsunami overcame the dune and deposited beach and dune sands on the landward side of the dune and that the backwash caused large-scale erosion landforms due to the quick seaward discharge. However, there are still many unknowns about the erosion process. Therefore, the purpose of this study was set to elucidate the erosion process by the 2011 Tohoku-oki tsunami on the Osuka coast.
The first step in this study was to examine time-series topographic changes using a digital elevation models (DEMs). Since there were only a few available DEM data for the target area, we created a DEM from past aerial photographs. In addition, excavation and ground penetrating radar (GPR) surveys were conducted in 2021 and 2022.
A topographic change analysis using DEMs revealed that the ‘valley’ formed by the 2011 Tohoku-oki tsunami was formed by the breakup of the dune and that the erosion depth reached up to 5 m from the dune top. In general, tsunami erosion is common in areas with steep slopes or large coastal lowlands (Tanaka et al., 2012; Takamura et al., 2015). However, the small coastal lowland and gentle slope of the Osuka coast make it difficult for generating a strong backwash.
We then examined the causes of this erosion. First, DEMs show that the coastal lowlands extended between the high dunes and landward slopes before the tsunami and that the entire lowlands were widely inundated during the tsunami. The GPR survey of the subsurface structures in the area, where erosion formed ‘valleys’ and was slightly buried by sand, revealed a prominent reflection surface at a depth of about 2 m below the ground surface. Comparison with a DEM of the immediate post-tsunami topography indicates that this is close to the surface immediately after the 2011 tsunami. The GPR reflection surface indicates the existence of a boundary surface between layers with different properties. Therefore, the reflection surface suggests the boundary created by the post-tsunami deposition of sand on the surface exposed by the tsunami erosion.
Next, to determine the cause of the strong reflection surface observed by the GPR, excavation was conducted on the transect of the GPR survey. The underwater surface was confirmed at a depth of nearly 2 m, and no further excavation was possible with the hand auger because of a solid layer. This suggests the existence of an impermeable solid surface at a depth of about 2 m.
Meanwhile, there are no large rivers flowing out to the sea on the Osuka coast, but there are some small streams that flow in from the landward hills. These streams cannot be confirmed on the surfaces of coastal lowland, dune and beach and are thus considered to flow underground in this area toward the sea. Based on this, we re-examined the DEM and 3D topographic data and found that the ‘valleys’ created by the tsunami are located just seaward of where the stream flows into the Osuka coast.
From the above, the following factors are considered to have caused large-scale erosion of the dunes on the Osuka coast. First, due to the unique topography of the Osuka coast, seawater that entered behind the dune by the tsunami pooled in the coastal lowlands without being rapidly discharged to the sea due to the strong backwash. The seawater was then slowly but continuously discharged into the sea, and the flow was concentrated in limited areas. As a result, even though the area of the Osuka coast is small and cannot pool as much water as strand plains, the relatively small amount of water may have caused significant erosion. The relationship between the location of the groundwater flow paths and streams identified by GPR reflection and excavation and the location of the valleys suggests that the presence of groundwater and other terrestrial water may have been a major factor in defining the location where tsunami erosion occurred.