10:45 〜 12:15
[AGE27-P12] Soil Physical Properties and Their Spatial Distribution in the Landslide Site of Lake Juniko, Tsugaru Region
キーワード：地すべり、土壌物理性、津軽十二湖
Soil properties are influenced and changed by climate, topographic features, geology, and time. Landslides, such as collapses and debris flows, disturb the soil and can alter soil properties in the topsoil and they have rich spatial diversity. Lake Juniko (hereafter referred to as "Juniko"), where 33 lakes are distributed, is located in the southwestern part of Aomori Prefecture, as well as an example of a large-scale landslide site in Japan. Although studies and investigations on water quality, geology, and causes of the landslide occurrence have been conducted, few studies have focused on the soil physical properties of this site. Therefore, the objective of this study is to understand the influence and interrelationship of landslides in Juniko on the variation of surface soil physical properties and spatial characteristics and to clarify the relationship among soil, geology, and landslides.
Samples were collected in areas unaffected by the landslide (Bedrock), areas with landslide debris (Debris), and areas at the landslide boundary where materials are usually mixed with debris and bedrock (Complex). The bedrock of the landslide is mainly characterized by acid pumice-tuff and andesite tuff-breccia and volcanic conglomerate. Debris is mainly associated with the later geology type. Finally, a total of 22 points were selected, including 8, 9, and 5 points from “Bedrock”, "Debris", and "Complex", respectively. Disturbed samples, excluding litter, were collected at depths of <10 cm, 0-30 cm, 30-60 cm, and 60-90 cm below the ground surface and from outcrops exposed near the forest trail. Undisturbed samples were also collected from depths <10 cm and from the outcrops. Soil physical properties, including soil particle density, particle size distribution, three-phase distribution, dry density, saturated hydraulic conductivity, and soil color of these samples, were investigated in the laboratory. Besides, the spatial distribution of soil physical properties was estimated using the inverse distance weighting method.
As for “Bedrock”, soil particle density varied little with depth and was greater than about 2.5 g cm-3 (andesite tuff-breccia) or less (pumice-tuff), depending on geology. The particle size distribution showed that the clay content was lower than that of the debris and the complex. Soils derived from the Juniko tuff were sandier (S, LS, etc.) than Sandy Loam (SL), and those derived from andesite tuff-breccia had higher clay content (L, CL, etc.) than SL. Solid fraction and dry bulk density tended to be greater in soils derived from andesite tuff-breccia than in those from the pumice-tuff.
As for “Debris”, soil particle density varied significantly with depth due to the presence of organic matter in the topsoil. Contrary to “Bedrock”, the particle size distribution was low in sand and high in clay, with a large standard deviation compared to the andesite tuff-breccia-derived soils in Bedrock. The solid fraction, porosity, and dry density showed the widest range of values among the three categories due to the influence of organic matter and gravel.
As for “Complex”, soil particle density varied widely with depth due to the influence of organic matter, as in “Debris”. The soil texture was a mixture of S and CL, like a combination of bedrock and debris.
Spatial distribution of soil particle density, especially at depth of 60-90cm from the soil surface, exhibited the differences in the values between ”Bedrock” and the other two (“Debris” and “Complex”). For particle size distribution, the boundaries of three categories were observed depending on sand and clay. No relationship with landslide topography was observed for saturated hydraulic conductivity.
Samples were collected in areas unaffected by the landslide (Bedrock), areas with landslide debris (Debris), and areas at the landslide boundary where materials are usually mixed with debris and bedrock (Complex). The bedrock of the landslide is mainly characterized by acid pumice-tuff and andesite tuff-breccia and volcanic conglomerate. Debris is mainly associated with the later geology type. Finally, a total of 22 points were selected, including 8, 9, and 5 points from “Bedrock”, "Debris", and "Complex", respectively. Disturbed samples, excluding litter, were collected at depths of <10 cm, 0-30 cm, 30-60 cm, and 60-90 cm below the ground surface and from outcrops exposed near the forest trail. Undisturbed samples were also collected from depths <10 cm and from the outcrops. Soil physical properties, including soil particle density, particle size distribution, three-phase distribution, dry density, saturated hydraulic conductivity, and soil color of these samples, were investigated in the laboratory. Besides, the spatial distribution of soil physical properties was estimated using the inverse distance weighting method.
As for “Bedrock”, soil particle density varied little with depth and was greater than about 2.5 g cm-3 (andesite tuff-breccia) or less (pumice-tuff), depending on geology. The particle size distribution showed that the clay content was lower than that of the debris and the complex. Soils derived from the Juniko tuff were sandier (S, LS, etc.) than Sandy Loam (SL), and those derived from andesite tuff-breccia had higher clay content (L, CL, etc.) than SL. Solid fraction and dry bulk density tended to be greater in soils derived from andesite tuff-breccia than in those from the pumice-tuff.
As for “Debris”, soil particle density varied significantly with depth due to the presence of organic matter in the topsoil. Contrary to “Bedrock”, the particle size distribution was low in sand and high in clay, with a large standard deviation compared to the andesite tuff-breccia-derived soils in Bedrock. The solid fraction, porosity, and dry density showed the widest range of values among the three categories due to the influence of organic matter and gravel.
As for “Complex”, soil particle density varied widely with depth due to the influence of organic matter, as in “Debris”. The soil texture was a mixture of S and CL, like a combination of bedrock and debris.
Spatial distribution of soil particle density, especially at depth of 60-90cm from the soil surface, exhibited the differences in the values between ”Bedrock” and the other two (“Debris” and “Complex”). For particle size distribution, the boundaries of three categories were observed depending on sand and clay. No relationship with landslide topography was observed for saturated hydraulic conductivity.