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Ho Chi Minh City, Vietnam

Tamura T.,Geological Survey of Japan | Tamura T.,University of Sheffield | Saito Y.,Geological Survey of Japan | Bateman M.D.,University of Sheffield | And 3 more authors.
Marine Geology

Empirical understanding of decadal- to centennial-scale deltaic shoreline changes in the past is essential for understanding the fate of coasts in the coming decades and centuries. We tested the effectiveness of quartz optically-stimulated luminescence (OSL) dating of beach ridges to constrain shoreline changes of the Mekong River delta over the last 3500. years. Forty-seven OSL ages have been analyzed from the Tra Vinh delta plain, central Mekong River delta. The Tra Vinh beach ridges are recurved and branching, showing the hierarchy of shoreline changes, which include discontinuous shifts c. 5. km seawards caused by the emergence and elongation of a delta-front bar/island, and subsequent downdrift accretion of spits. The spit accretion is interrupted by changes of shoreline orientation. Each of the discontinuous shifts resulted in a cluster of beach ridges, which is referred to as ridgeset. The beach ridge sediments were found to have excellent luminescence properties resulting in low age uncertainties of c. 5%. The OSL chronology agrees well with shoreline changes over recent decades and with radiocarbon ages of tidal flat sediment underlying the beach ridges. The chronology clearly illustrates the coastal progradation from 3500. years (relative to AD 2010) onwards. Two main types of beach ridge are present: recurved and trunk ridges. OSL ages of recurved ridges are consistently younger both downdrift and seawards, documenting decadal- to centennial-scale shoreline migration especially over the last 1500. years. Trunk ridges in contrast have less systematic ages because a trunk ridge is formed where the sediment is likely to have been reworked by waves. Ages of updrift trunk ridges characteristically show relatively young ages in each ridgeset, representing the extensive erosion and resedimentation of the updrift coast. Major changes in shoreline orientation occurred around 400-500. years, suggesting strengthening of the northeasterly winter monsoon at the beginning of the Little Ice Age. The rate of progradation of the Tra Vinh delta plain is described in two ways: frequency of the discontinuous shift, and growth rate of the delta plain area. Both suggest the coast has expanded regularly over the last 1000. years. A decrease in sand supply to the coast in the last few decades due to river dam construction and fluvial sand dredging is inferred, possibly affecting the behavior of the modern and future shorelines, which can be compared with the less human-influenced, centennial- to decadal-scale past shoreline changes reconstructed in this study. © 2012 Elsevier B.V. Source

Tamura T.,Geological Survey of Japan | Horaguchi K.,Niigata University | Saito Y.,Geological Survey of Japan | Nguyen V.L.,HCMC Institute of Resources Geography | And 4 more authors.

A mesotidal beach on the Mekong River delta coast exhibits unique patterns of river sediment discharge and experiences reversals of ocean wave directions in response to the summer and winter monsoons. We analyzed long-term changes of the shoreline since 1936 from past topographic maps and satellite images, and short-term changes in geomorphology and sedimentology by undertaking repeated field surveys between November 2005 and February 2008 along six shore-normal beach transects. The shoreline of the Mekong River delta coast has changed asymmetrically over the last 70 years in response to net southwestward sediment transport related to dry northeasterly winter monsoons. During the summer rainy season, large volumes of sediments are discharged from the river; at this time, wave direction is reversed in response to the relatively weak southwesterly summer monsoon. Mud and very fine sand in the surface sediments of the northeastern (updrift) beach transects tend to be removed during winter, suggesting that the sediment supplied from the river during summer is temporarily deposited near the river mouth and later transported southwestward during the winter monsoon. The relief of intertidal bars on the beach increased during winter in response to higher waves. However, previous studies have suggested that higher waves flatten bars. We hypothesize that the increase in relief that we observed is related to a wide and shallow subtidal delta-front platform that attenuates ocean waves at all times other than at high tide. The beach profile varies longshore: the accreting to stable beach dips seaward at a gradient of about 1/80 with up to three longshore intertidal bars, classified here as either slip-face bars or low-amplitude ridges. In contrast, the eroding beach profile is much steeper (1/20-1/40) and linear without intertidal bars, and is similar in form to a low-tide terrace. Because there is little variation of wave size and grain size within the study area, we consider that the spatial variation of the beach profile is related to the availability of sediment accumulated on the beach. © 2009 Elsevier B.V. All rights reserved. Source

Lam-Dao N.,HCMC Institute of Resources Geography | Le-Toan T.,CNRS Center for the Study of the Biosphere from Space | Hoang-Phi P.,HCMC Institute of Resources Geography
34th Asian Conference on Remote Sensing 2013, ACRS 2013

In recent years, the changes in cultural practices have impacts on remote sensing methods developed for rice crop monitoring in the Mekong Delta, Vietnam. The paper presents research results of rice monitoring using radar remote sensing data such as ENVISAT-ASAR and TerraSAR-X collected at different rice-growing stages in 2007 and 2010-2011, respectively. These data were used for rice mapping and estimating the production of rice growing areas in An Giang province using a statistical model. The results are quite good and prove to be a potential tool for rice production estimation in the Mekong Delta, Vietnam. In addition, a current research project on rice crop monitoring in the Mekong Delta for Asia-RiCE is introduced. Source

Nguyen L.-D.,HCMC Institute of Resources Geography | Thuy L.-T.,CNRS Center for the Study of the Biosphere from Space | Claudia K.,German Remote Sensing Data Center | Viet P.-B.,HCMC Institute of Resources Geography
31st Asian Conference on Remote Sensing 2010, ACRS 2010

The Mekong Delta is one of the most endangered regions in the world under the effects of global warming. Such effects include ocean warming, a rise in sea level, heat waves and periods of unusually warm weather, intense precipitations, typhoons, high tides and storm surges. Those effects result in coastal flooding, river flooding, inland flooding, salt water intrusion, coastal erosion, coastal sedimentation and cause degradation of biodiversity, spread of disease, changes in the population and habitat of plants and animals (e.g. birds and fish). Studies need to be conducted to quantify the changes observed by satellites in land use / land cover, in coastline, river bank, in flood extent and duration, and in cultural practices. The role of Earth Observation data is significant to provide both large view on the Mekong delta and high resolution observations in the regions where significant impacts of global change are being observed. Several sources of remote sensing data dating back from few decades can be used to quantify the changes. The paper presents the preliminary results of remote sensing applications in the Mekong Delta for change detection such as land use / land cover and inundation (WISDOM project); mangrove and rice/agriculture (Planet Action project); coastal line and river bank erosion and the further research works (WISDOM and RICEMAN projects). Source

Lam-Dao N.,HCMC Institute of Resources Geography | Hoang-Phi P.,HCMC Institute of Resources Geography | Huth J.,German Aerospace Center | Cao-Van P.,Cuu Long Rice Research Institute
32nd Asian Conference on Remote Sensing 2011, ACRS 2011

Food security has currently become a key global issue due to rapid population growth in many parts of Asia, as well as the effects of climate change. For this reason, there is a need to develop a spatio-temporal monitoring system that can accurately assess rice area planted and rice production. Changes in rice cultivation systems have been observed in various countries of the world, especially in the Mekong Delta, Vietnam. The changes in cultural practices have impacts on remote sensing methods developed for rice monitoring, in particular, methods using new generation radar data. The objective of the study was to estimate the rice yield using time-series Synthetic Aperture Radar (SAR) imagery. Field data collection and in situ measurement of rice crop parameters were conducted in An Giang province, Mekong Delta in 2010. The average values of the radar backscattering coefficients that corresponded to the sampling fields were extracted from the TerraSAR-X StripMap (TSX SM) images taken during a crop season. The temporal rice backscatter behaviour was analysed for HH (Horizontal transmit and Horizontal receive), VV (Vertical transmit and Vertical receive), and polarisation ratio data. For rice yield estimation, the predictive model based on multiple linear regression analysis (Lam-Dao, N. et al., 2009a) between in situ measured yields and polarisation ratios attained good correlation and thus proved to be a potential tool for estimating rice production in the study area. Source

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