Toshihiko Sugai

River terrace development by concurrent fluvial processes and climatic changes

Abstract Along the Usui River in central Japan, three well-developed terrace levels are observed. Each terrace is subdivided into two segments: a filltop terrace along the upper reach and a strath terrace along the lower one. Three valley fillings with coarse gravel beds in the upper reach correspond to the last three glacial ages. The positions of these valleysfills migrated upstream from the older terrace to the younger one. To clarify the response of fluvial systems to climatic change, terrace sediments were interpreted from the viewpoint of paleohydraulics. Using the grain diameter of terrace sediments, paleo-tractive forces were evaluated based on the correlation between tractive force and grain diameter of the sediment on the present river bed. The valley filling was caused by the diminution in tractive force related to the decrease in heavy rain produced by typhoons during the glacial ages. During the interglacial ages, the reverse responses occurred due to the increased incidence of typhoons. From another aspect of the development of river landforms, the decrease in the liver channel gradient during the interglacial ages caused the transportational and/or depositional area of coarse gravels to migrate upstream in the subsequent glacial ages.

Location of a pre-Angkor capital city in relation to geomorphological features of lower reach of the Stung Sen River, central Cambodia

The Sambor Prei Kuk archaeological site in lower reach of the Stung Sen River, central Cambodia, is the site of the capital city of the pre- Angkor state of Chenla. The location of Sambor Prei Kuk is discussed in relation to the geomorphological characteristics of lower reach of the Stung Sen River and the requirements of the inhabitants. The uplands were divided into upland I, which is characterized by hills, and uplands II and III, both of which have flat surfaces, with upland III lower than upland II. The present river follows a meandering course within a conspicuous meander scroll zone, and deposits along the channel are repeatedly eroded and redeposited. Back marsh areas have gradually filled with suspended flood water sediments during the last 4600 years at an accumulation rate of 0.6 mm/yr. The floodplain and lake plain were divided into five zones to evaluate the monsoonal flood risk and accessibility to upland, considering uplands distribution. Sambor Prei Kuk on upland II with the port town adjacent the river is situated on the place where water transport is feasible and the risk of monsoonal flood is low, which means the people accommodated to the highly different dry and monsoonal environment.

Geomorphology and Tectonic Setting of the Sanriku Coast, Northeastern Japan, and Introduction of Recent Studies on the Formation of Alluvial Plains and Holocene Crustal Movements Along the Coast

The 2011 Tohoku-Oki earthquake dramatically changed the coastal environment along the Pacific coast of northeastern Japan. In particular, coseismic subsidence of up to 1.3 m was recorded along the Sanriku Coast . However, well-preserved Middle to Late Pleistocene marine terraces along the northern Sanriku Coast have been interpreted to indicate uplift since the Late Quaternary. This discrepancy between long-term uplift and short-term subsidence has been attributed to coseismic uplift during an unidentified megathrust earthquake. To clarify the interplay of uplift and subsidence, we introduce the tectonic setting of the Sanriku Coast and recent studies on the geomorphology and geology along the coast, which focus on crustal movements during the Holocene along the southern Sanriku Coast. The results of recent studies indicate that, contrary to the previous view that the northern Sanriku Coast has experienced long-term uplift, the southern Sanriku Coast has been subsiding since the 10 ka (since the latest Pleistocene).

Simple DEM‐Based Methods to Delineate Channel Networks for Hydrogeomorphological Mapping

To delineate channel networks from DEMs regardless of landform type, this article proposes a new method using slope-weighted flow accumulation. To validate the method, SRTM-3, a global DEM dataset with a resolution of approximately 90 m, was used for analysis of the Loess Plateau, China. Channel networks delineated with and without slope-weighted flow accumulation were derived in both uplands and hilly lands for comparison. In the weighted flow accumulation method, the thresholds for delineating the channels were defined by detecting a turning point in the frequency distribution of the weighted flow accumulation function or by visual similarity with drainage channels extracted from topographic maps. The channel networks delineated with weighting showed closer correlation with a topographic map than the channel networks without weighting, despite the differences in thresholds. Moreover, the channel networks delineated with weighting represented the differences between landform types, while the channel networks without weighting did not. Weighting on the basis of the slope angle shows promise as a general channel delineation method which reflects the actual topography due to its hydrogeomorphological functions.

Characteristics of extreme monsoon floods and local land use in the Lower Mekong Basin, Cambodia

Rivers in the Lower Mekong Basin, Cambodia, receive additional water and waterborne sediments annually during the monsoon. In the 2011 monsoon season, the region was inundated by extreme flood events. This chapter identifies inundation characteristics and explains fluvial landforms developed over a long-term flood history near Phnom Penh and Kampong Thom in Cambodia, based on land classification and field surveys. Understanding the development of fluvial plains and promoting land use in accordance with river behavior can minimize flood risk and allow local populations to live sustainably in flood-prone areas.

Distribution of Liquefaction Sites and Coastal Alluvium in Japan

In the Tohoku and Kanto regions, the 2011 off the Pacific coast of Tohoku Earthquake (2011 Tohoku earthquake) caused widespread sediment liquefaction along more than 600 km of the Pacific coast facing the Japan Trench. Liquefaction sites tended to be concentrated in reclaimed lands on the north side of Tokyo Bay, especially in sandy fill overlying thick alluvium, and along large rivers in alluvial lowlands. In Japan, alluvial deposits along the lower reaches of rivers often form coastal prisms (CPs) between the river profiles of the present and the last glacial. A basal gravel body (BG) generally forms the lower boundary of the CP. Within a thick CP, the period and duration of seismic shear waves (S-waves) are lengthened, and multiple S-wave reflections from the BG and the slow S-wave velocity increase the internal water pressure, leading to liquefaction of sand layers in the CP. The distributions of both liquefaction induced by the 2011 Tohoku earthquake and historic liquefaction sites correspond closely to the distribution of thick CP deposits. Large subduction-zone earthquakes have repeatedly caused liquefaction within CPs greater than 30 m thick, with liquefaction occurring as far inland as the upstream edge of the CP. In the long CPs in the Kanto region, along the Kinu, Ara, and Naka (Furutone) rivers, liquefaction induced by the 2011 Tohoku earthquake was particularly widespread. Mega-earthquakes can also cause liquefaction in sediment-filled basins inland of the CPs; the Tohoku earthquake liquefied the thick late Quaternary alluvium in the Koriyama and Aizu-Wakamatsu basins.

Late Quaternary Landform Development of the Kanto Plain

The Kanto Plain is the tectonically active setting of the Tokyo metropolitan area. It is characterized by widespread uplands, consisting of Pleistocene fluvial, marine sediment, tephra layers, and narrow lowlands filled with thick Holocene alluvium. This landscape owes its origin to the interaction of basin-forming movements, eustasy, erosion, and sedimentation during the Quaternary Period. Marine transgressions in marine isotope stages (MIS) 11, 9, and 5.5 repeatedly overlapped the land, producing Paleo Tokyo Bay, with its wide mouth facing the Pacific Ocean to the east. In contrast, the Holocene (MIS 1) marine transgression was limited to the river valleys incised since MIS 4, and instead of Paleo Tokyo Bay, Modern Tokyo Bay has appeared. Modern Tokyo Bay, which opens to the south, resulted from the combination of: (1) a lower sea level in MIS 1 than in MIS 5.5; (2) tectonic uplift of the Kanto Plain, especially the east side; and (3) the accumulation of fluvial sediment and tephra since MIS 5.4.

Natural Disaster and Coastal Geomorphology

Asia and the Pacific regions are exposed to severe natural disasters because many densely populated cities are sited on coastal plains. The geomorphic features in coastal areas have a key role in defining the risk levels and the vulnerability to natural disasters of coastal regions. Rapid land-use changes in recent decades have forced a transformation in risk assessment to approaches that analyze and evaluate the relationship between social structures and natural disasters. Disaster resilience should incorporate more insight into social structures and more local community activities as the next advance in disaster mitigation. The 2011 East Japan (Tohoku Japan) earthquake has presented many lessons for consideration by regional land-use planners regarding the relationship between geomorphology and tsunami damage, and the appropriate activities of local communities. In this introduction chapter, the author illustrates the relationship between a natural disaster and coastal geomorphology by using the response of the lower Abukuma River basin to the tsunami that followed the 2011 East Japan earthquake. The damage caused by the earthquake and tsunami took different forms on each type of coastal landform and demanded specific responses from the relevant social structures, including evacuation activities. Future regional planning for disaster mitigation should incorporate scientific knowledge of coastal geomorphology.