Masatomo Umitsu

Late quaternary sedimentary environments and landforms in the Ganges Delta

Abstract Late Quaternary sediments, deposited since the lowest stand of sea-level during the last glacial maximum, in the Ganges Delta and its surrounding region were classified into five units according to their sedimentary facies. These are the lowest unit of sandy gravels, the lower unit of sand with a few gravels, and the middle, upper and uppermost units consisting mainly of sand and silt with occasional peat layers. The contact between the lower and middle units is fairly sharp, and the upper part of the lower unit is oxidized in some places. This suggests a period of subaerial exposure after the deposition of the lower unit. During the low stand of sea-level, about the time of the last glacial maximum, rivers in the Ganges Delta and its surrounding region dissected the surfaces of the region, and deposited the lowest unit on the valley floors. The lower unit was deposited over the lowest unit during an early stage of the post-glacial transgression. Between ca. 12,000 yr BP and 10,000 yr BP, the surface of the delta was slightly dissected and the top of the lower unit was weathered and oxidized. Following this period, the middle and upper units of alluvial or deltaic sand and silt with occasional peat were deposited. The coastline in the early Holocene retreated towards the central part of the present Ganges Delta. In the middle and late Holocene, the silt and clay with occasional peat layers of the uppermost unit indicate that the lowland gradually became marshy and poorly drained as the rate of transgression became slower.

Holocene palaeoecology and formation of the Shoalhaven River deltaic-estuarine plains, southeast Australia

The Shoalhaven River is one of the largest rivers on the south coast of New South Wales, and the deltaic-estuarine plains associated with its mouth represent a mature stage of infill of a barrier estuary. The stratigraphy of sediments from more than 60 drillholes from the plains indicates that the area has infilled during the mid-Holocene as a result of input of marine sands associated with a sand barrier on the high-energy coast, and fluvial mud and sands from the Shoalhaven River. Molluscan assemblages indicate that marine influence was initially widespread throughout the eastern and southern parts of the embayment, and that most of the plains infilled under estuarine conditions. Prominent levees across the plains surface are interpreted to be part of a birdsfoot delta distributary complex, and individual floodbasins accreted progressively as they were encapsulated by distributary extension. These former tidal environments mean that there are extensive potential acid sulphate soil conditions beneath the plains. The upper occurrence ofNotospisula in drillholes records the cessation of tidal influence, and diatoms from the top of one drillhole record the transition from brackish conditions to freshwater alluvial sedimentation. Radiocarbon dating provides a chronology of mid-Holocene ecological changes on the plains.

Seismic or hydrodynamic control of rapid late-holocene sea-level rises in Southern coastal Oregon, USA?

Intertidal stratigraphy has been instrumental in demonstrating the hazard posed by great earthquakes at the Cascadia subduction zone, but inferring an earthquake history from interbedded sequences of peat and mud is complicated by many factors that influence sedimentation and relative sea-level change on both tectonic and nontectonic coasts. Rapid-to-sudden rises in relative sea level marked by sharp contacts between intertidal peat and overlying mud or sand may reflect coseismic coastal subsidence and tsunami deposition or, alternatively, nonseismic hydrodynamic changes in estuaries. Reconnaissance coring at 16 sites in the marshes fringing a narrow, protected tidal inlet of Coos Bay, supplemented by diatom and 14C analyses at four sites, reveals a stratigraphic record too fragmentary and ambiguous to distinguish seismic from hydrodynamic causes for more than three of the 10 rises in relative sea-level identified. Only three sharp contacts have the wide extent and evidence of substantial (>0.5 m) submergence that distinguish them from similar contacts produced by nonseismic processes. Correlation with stratigraphic sequences at other estuaries shows that the fringing marshes suddenly subsided and were partially buried by tsunami sand during a great plate-boundary earthquake about 300 years ago. Similar contacts were produced by earthquakes about 1500–1800 years ago, and perhaps about 2400–2700 years ago. Other earthquakes with substantially less subsidence may also have occurred, but evidence is too ambiguous to reconstruct a more complete history.

Landforms and floods in the Ganges delta and coastal lowland of Bangladesh

Regional characteristics of the landform changes in the Bengal lowland are closely related to the characteristics of the floods and tidal surges of each region. Due to the remarkable increase of the discharge in the rainy season, most parts of Bangladesh, except Hill Tract, are covered by floods. Flooding conditions and changes of the river course are closely related to the characteristics of the alluvial landforms in the Bengal lowland. Landforms of the southern Ganges delta are characterized as the deltaic and tidal lowlands with low and flat surfaces. Ground height of the region is 2–3 m, but the flooding inundation depth in the deltaic region is not so deep. In the active delta around the mouth of the Meghna (mouth of the Ganges), landforms are extremely unstable due to the strong river flow in the rainy season and the large quantity of sediments. Height of the islands near the mouth of the Meghna is lower than 3 m and, mostly, 1–2 m above mean sea level. Furthermore, sediments of the islands are very...

Delineation of small-scale landforms relative to flood inundation in the western Red River delta, northern Vietnam using remotely sensed data

Abstract Flood susceptibility mapping using geomorphologic approaches is effective for delineating flood extent and various degrees of potential flood-affected areas. This approach is useful where the channel system and floodplain morphology change dynamically and in regions where detailed digital elevation models are not available. The first important step in flood zonation using this approach is detailed geomorphologic mapping, also called landform classification. This study aims to describe landform classification using the rule-based method of Ho et al. (Int J Geoinform 8(4):27–38, 2012) adapted to local characteristics in the western plain of the Red River delta, northern Vietnam. The original classification scheme is generally based on the moist condition classification, local land-surface parameters, and relative position indices derived from multi-temporal Landsat data and a shuttle radar topographic mission digital elevation model (SRTM DEM). This study uses average elevations and the standard deviation of elevations as local land-surface parameters rather than local relief, which was used in the study by Ho et al. (Int J Geoinform 8(4):27–38, 2012). Multi-temporal land cover classification was performed using an integrated method to effectively correct the SRTM DEM. The overall classification is consistent with manual mapping by visual comparison. The quantitative comparison between landform units and past flood-affected areas demonstrates a precise boundary delineation of landform objects using this method. The high agreement between the boundaries of landform units and flood-inundated areas suggests the applicability of this method taking advantage of readily available remotely sensed datasets in alluvial floodplains.

Late Holocene Mangrove Habitat and Evolution of Coastal Lowlands in Southern Thailand.

Holocene landforms and sediments of the coastal lowlands around Satun and Songkhla Lake regions in southern Thailand are studied in relation to mangrove habitat. Landforms of the Satun lowland are net a simple geomorphic surface but the surfaces of Pleistocene and late Holocene. Present mangrove vegetation develops mainly en the Holecene surface in the southern part of Satun lowland. The area is almost the same as that covered with mangrove in the period of Holocene maximum transgression. Based on the fuceis and ages of the sediments in the Satun lowland, sedimentation rate of the lowland since 1000 yr BP is relatively rapid and the mangrove forest expanded rapidly in the period. On the other hand, most mangrove forest has already disappeared in the coastal lowland along Lake Songkhla. The thickness of the Holocene sediments is less than 5 meters in most. t places, and consists of peat layer and overlying muddy sediments. Based on the radiocarbon ages of the peat layers collected from the southern part of the coastal lowland, mangrove forest started to develop around 6700 yr BP and expanded rapidly in the region. This is because the surface of the Pleistocene sediments is. very flat and shallow. Seawater invaded and expanded in the region very quickly and formed a wide tidal plain along the coast of lagoon. This condition causes rapid expansion of mangrove forest along the Songkhla Lake. Based en these facts, it is clarified that the development of mangrove forests in the study areas is related to the evolution of the Holocene coastal landforms. and deposition of sediments in mid to late Holecence. Furthermore, landform evolution and sedimentation arc related to the landforms of the pre-Holocene surface.

Automated micro-landform classification by combination of satellite images and SRTM DEM

The objective of this study is to produce an automated micro-landform map of an alluvial plain by combining land surface parameters (LSPs) from Shuttle Radar Topographic Mission Digital Elevation Model (SRTM DEM) and land cover characteristics from satellite images (Landsat and ASTER) for further flood hazard assessment. Local relief and average elevation from SRTM DEM are designated as LSPs. In addition, channel network is extracted by using multiple Landsat images and r.param.scale function. Modified Normalized Difference Water Index (MNDWI) by Landsat in rainy season can help to separate non-water areas from water and moist surface by determining thresholds. Land cover features are obtained by unsupervised classification by the ISODATA method. Micro-landform units of automated micro-landform map are also classified based on relative position of objects. A case study is conducted in the alluvial plain of the Thu Bon - Vu Gia river, central Vietnam.