著者

SASAKI Natsuki SUGAI Toshihiko

出版者

The Association of Japanese Geographers

雑誌

Geographical review of Japan series B (ISSN:18834396)

巻号頁・発行日

vol.87, no.2, pp.103-114, 2015 (Released:2015-07-04)

参考文献数

41

被引用文献数

2 7

---

This study examines geomorphological controls on the distribution and development of wetlands in the Hachimantai Volcanic Group, northeastern Japan. The study area is dissected by various sizes of landslides and has many wetlands of varied origin. Among 526 wetlands in the study area, 195 are on landslides and account for 63.9% of the total area of wetlands. Wetlands outside of landslides tend to be found in clusters on undissected volcanic surfaces. Many of these are small wetlands in nivation hollows that are supplied by meteoric water from snow, and some of them are ponds in the craters of Hachimantai volcano and large peat bogs on lava flows. Wetlands inside landslides are widely scattered in large or deep depressions along landslide scarps and in small depressions among pressure ridges. Many of these are ponds supplied mainly by groundwater. Large landslides tend to have ponds. On the larger landslides, depressions created by landslide processes provide favorable conditions for the development of wetlands, and their size is constrained by the microtopography of the landslide surface. When drainage channels that dissect landslide bodies breach these closed depressions, the wetlands there progress rapidly from ponds to peat bogs. Landslide activities can create wetlands of various ages, and the dissection of landslide bodies controls the developmental stages of these wetlands. Therefore, a large landslide may contain coexisting wetlands of various ages and types.

著者

NAGUMO Naoko SUGAI Toshihiko KUBO Sumiko

出版者

The Association of Japanese Geographers

雑誌

Geographical review of Japan series B (ISSN:18834396)

巻号頁・発行日

vol.87, no.2, pp.115-121, 2015 (Released:2015-07-04)

参考文献数

21

被引用文献数

3 5

---

Tributaries of Lake Tonle Sap in the Lower Mekong Basin are strongly influenced by seasonal changes of water level in Lake Tonle Sap and discharge of the Mekong River. The aim of this research was to gain a better understanding of the fluvial geomorphology of the Stung Sen River, a tributary of Lake Tonle Sap. We used stereopairs of aerial photographs and satellite images to identify the microtopography of the floodplain and riverbed, and field surveys to observe bankside topography and deposits. We recognized four types of channel bar in the lower Stung Seng River: lateral bars (type A), point bars (type B), concave-bank benches (type C), and diagonal and island bars (type D). Type A appears to have a complementary relationship with type D. In some instances types A and D bars transition to type B bars and, in rare instances, into type C bars. These changes are probably related to channel sinuosity and changes in the volume of transported sediment. Sediment transport and construction of the channel bars appears to be controlled by shifts of the flow regime of the Stung Sen River related to differences in the rate of water level rise in the river compared to those in Lake Tonle Sap. The riverine environment differs greatly from that of the floodplain, where sediment is deposited from suspension during periods of inundation.

著者

SUGAI Toshihiko Elena Yu. NOVENKO

出版者

The Association of Japanese Geographers

雑誌

Geographical review of Japan series B (ISSN:18834396)

巻号頁・発行日

vol.87, no.2, pp.80-81, 2015 (Released:2015-07-04)

著者

YOSHIDA Hidetsugu SUGAI Toshihiko

出版者

The Association of Japanese Geographers

雑誌

Geographical review of Japan series B (ISSN:18834396)

巻号頁・発行日

vol.83, no.1, pp.64-72, 2010-09-30 (Released:2010-10-10)

参考文献数

20

被引用文献数

3 4

---

Debris avalanche hummocks are created by catastrophic volcanic sector collapses. The paucity of quantitative data on their orientation relative to the debris avalanche direction has caused conflicting views among researchers. We evaluated hummock orientation in the Zenkoji debris avalanche deposits at Usu volcano, Japan. Our results show that hummocks within the upper part of the main depositional area, the axial band leading directly from the source to the central extremity of the debris avalanche deposits, tend to be aligned perpendicular to the presumed flow direction. However, hummock alignment varied systematically along the longitudinal path. Although hummocks close to the source were oriented perpendicular to the flow direction, we noted a gradual transition to a parallel orientation with increasing distance from the source. At the same time, hummocks had arisen on compression-dominated features of the debris avalanche surfaces in the marginal areas. These observations are consistent with the following scenario of the debris avalanche movement. To begin with, the origin is explainable for some large-scale hummocks near the source as below. At the moment of collapse, a small number of normal faults develop perpendicular to the landsliding direction, generating a series of ridges and grabens. Then, the alignment of the other hummocks was given rise to by an extensional regime as the runout distance increased and the lateral constraints on the erosional path near the volcano disappeared. As the longitudinal velocity increased further with respect to lateral velocity, hummocks became more strongly aligned at such distances from the proximal area. On the other hand, more-scattered orientations of hummocks have been observed in the marginal areas. This is probably because they were originated from the sediments pushed out of the main flow instead of being transported by the most direct way from the source.