3:45 PM - 4:00 PM
[HDS08-07] To what extent can the internal structure of lowland microgeomorphology be understood non-destructively? --- Application cases of GPR survey
Keywords:landform classification, microgeomorphology, GPR survey, beach levee, natural levee, tombolo
1. Introduction
Landform classification information is useful to understand the history of land in the area and to estimate the risk potential of natural disasters in the area in advance. In landform classification, we use topographic maps, aerial photographs, and digital elevation models (DEMs) to interpret and classify areas with similar features such as shape and origin as the same landform type. However, from aerial photographs and DEMs, basically only the shape of the ground surface can be interpreted, and information on subsurface structures that are useful for understanding the origin and formation order cannot be obtained.
In this study, we investigated whether the internal structure of lowland microgeomorphology such as beach levee and natural levee can be captured by ground penetrating radar (GPR) survey, and whether it is possible to identify the difference of landform type from the internal structure.
2. Study outline
The survey area was beach levees and natural levee on the flood plain and the strand plain in the lower reaches of the Kikugawa River in Kakegawa City, Shizuoka Prefecture. Among them, the survey results of the natural levee (Naka site) distributed on the side of the hill composed of siltstone of the bedrock between the Kikugawa, Shimo-ogasa and Kameso rivers were examined. This natural levee is V-shaped with axes extending mainly in the north-south and east-west directions. Although it is classified as a natural levee in the landform classification map of the Geospatial Information Authority of Japan, there is a possibility that the geomorphology formed between a hill and an isolated hill is a tombolo (land-tied sandbar).
3. Result & Discussion
In the GPR profile along the natural levee extending in the north-south direction, a horizontal layer structure was detected up to 2 – 5 m underground basically. In the northern part of the survey line, some reflection structures tilted to the north were observed. In the GPR profile across the natural levee extending in the east-west direction, an almost horizontal and slightly convex layer structure along the transverse shape of the terrain was detected up to 2 – 4 m underground. In the southern part of natural levee, a weak reflection with a northward slope was confirmed under the horizontal layer.
If the landform of the Naka site is a natural levee, the longitudinal section of the part extending in the north-south direction is expected to have a layered structure that tilts to the south, which is the downflow direction of the river. However, such a structure could not be confirmed in the GPR profile. In the cross-section of the part extending in the east-west direction, a layer structure that tilts to the north side opposite to the river is expected. However, the GPR profile basically shows a layered structure along the cross-sectional shape of the terrain.
On the other hand, if the landform is a tombolo, a layer structure along the cross-section of the terrain in the east-west direction is expected, and GPR profile shows such a structure.
From the above, it is suggested that the landform of the Naka site may be tombolo. However, in the profile of the landform extending in the east-west direction, there is a structure that suggest natural levee, therefore, there is a possibility that the tombolo and the natural levee are compounded according to the change of sedimentary environment.
4. Summary
We detected the internal structure of the landform considered to be a natural levee by landform classification, and indicated the possibility that the landform is a tombolo or a composite landform of a natural levee and a tombolo. This suggests that a shallow subsurface structure information by the non-destructive survey may lead to support during landform classification work.
Acknowledgements: This work was supported by JSPS KAKENHI Grand Number JP18H00765. The GPR devices used for the survey was borrowed from Prof. Yasuhiro Suzuki of Nagoya University.
Landform classification information is useful to understand the history of land in the area and to estimate the risk potential of natural disasters in the area in advance. In landform classification, we use topographic maps, aerial photographs, and digital elevation models (DEMs) to interpret and classify areas with similar features such as shape and origin as the same landform type. However, from aerial photographs and DEMs, basically only the shape of the ground surface can be interpreted, and information on subsurface structures that are useful for understanding the origin and formation order cannot be obtained.
In this study, we investigated whether the internal structure of lowland microgeomorphology such as beach levee and natural levee can be captured by ground penetrating radar (GPR) survey, and whether it is possible to identify the difference of landform type from the internal structure.
2. Study outline
The survey area was beach levees and natural levee on the flood plain and the strand plain in the lower reaches of the Kikugawa River in Kakegawa City, Shizuoka Prefecture. Among them, the survey results of the natural levee (Naka site) distributed on the side of the hill composed of siltstone of the bedrock between the Kikugawa, Shimo-ogasa and Kameso rivers were examined. This natural levee is V-shaped with axes extending mainly in the north-south and east-west directions. Although it is classified as a natural levee in the landform classification map of the Geospatial Information Authority of Japan, there is a possibility that the geomorphology formed between a hill and an isolated hill is a tombolo (land-tied sandbar).
3. Result & Discussion
In the GPR profile along the natural levee extending in the north-south direction, a horizontal layer structure was detected up to 2 – 5 m underground basically. In the northern part of the survey line, some reflection structures tilted to the north were observed. In the GPR profile across the natural levee extending in the east-west direction, an almost horizontal and slightly convex layer structure along the transverse shape of the terrain was detected up to 2 – 4 m underground. In the southern part of natural levee, a weak reflection with a northward slope was confirmed under the horizontal layer.
If the landform of the Naka site is a natural levee, the longitudinal section of the part extending in the north-south direction is expected to have a layered structure that tilts to the south, which is the downflow direction of the river. However, such a structure could not be confirmed in the GPR profile. In the cross-section of the part extending in the east-west direction, a layer structure that tilts to the north side opposite to the river is expected. However, the GPR profile basically shows a layered structure along the cross-sectional shape of the terrain.
On the other hand, if the landform is a tombolo, a layer structure along the cross-section of the terrain in the east-west direction is expected, and GPR profile shows such a structure.
From the above, it is suggested that the landform of the Naka site may be tombolo. However, in the profile of the landform extending in the east-west direction, there is a structure that suggest natural levee, therefore, there is a possibility that the tombolo and the natural levee are compounded according to the change of sedimentary environment.
4. Summary
We detected the internal structure of the landform considered to be a natural levee by landform classification, and indicated the possibility that the landform is a tombolo or a composite landform of a natural levee and a tombolo. This suggests that a shallow subsurface structure information by the non-destructive survey may lead to support during landform classification work.
Acknowledgements: This work was supported by JSPS KAKENHI Grand Number JP18H00765. The GPR devices used for the survey was borrowed from Prof. Yasuhiro Suzuki of Nagoya University.