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Abstract: The largest plain in the North Vietnam has formed by the redundant sediment of the Red River system. Sediment supply is not equally distributed, causing erosion in some places. The paper analyzes the evolvement and physical mechanism of the erosion. The overlay of five recent topographical maps (1930, 1965, 1985, 1995, and 2001) shows that sediment redundantly deposits at some big river mouths (Ba Lat, Lach, and Day), leading to rapid accretion (up to 100m/y). Typical mechanism of delta propagation is forming and connecting sand bars in front of the mouths. Erosion coasts are distributed either between the river mouths (Hai Hau) or nearby them (Giao Long, Giao Phong, and Nghia Phuc). The evolvement of erosion is caused by wave-induced longshore southwestward sediment transport. Meanwhile sediment from the river mouths is not directed to deposit nearshore. The development of sand bars can intensively reduce the erosion rate nearby river mouths. Erosion in Hai Hau is accelerated by sea level rise and upstream dams. Sea dike stability is seriously threatened by erosion-induced lowering of beach profiles, sea level rise, typhoon, and storm surge. [Copyright &y& Elsevier]

Abstract: The Red River (China/Vietnam, A=155,000km²) is a typical humid tropics river originating from the mountainous area of Yunnan Province in China. Based on information on daily discharge (Q) and suspended particulate matter (SPM) concentration between 1960–2008 for the SonTay gauging station (outlet of the River and entry to the Delta) provided by the National Institute IMHE-MONRE, the mean annual SPM flux was estimated at 90 Mt/yr, corresponding to a sediment yield of 600t/km²/yr. The temporal variability of annual SPM fluxes (ranging from 24 to 200Mt/yr) is strongly related to the interannual hydrological conditions. However, some years of high water flow were not associated with high sediment fluxes, especially after 1989 when the HoaBinh dam came into operation. Therefore, the median discharge pre- (3389m3/s) and post 1989 (3495m3/s) are similar indicating there was little or no change between both periods. Sediment rating curves (power law-type; SPM=aQb) were fitted for both periods (1960–1989; 1990–2008). The analysis of the pre- and post-1989 sediment rating parameters (a, b) suggests a downshift of b-parameter values after 1989, attributed to a decrease of the sediment supply due to the commissioning of the HoaBinh dam. A single sediment rating curve derived from 1960–1989 data was used to simulate the annual variability of former sediment delivery, generating excellent cumulative flux estimates (error ~1%). In contrast, applying the same rating curve to the 1990–2008 data resulted in systematic and substantial (up to 109%) overestimation. This suggests that the HoaBinh dam reduces annual SPM delivery to the delta by half, implying important metal/metalloid storage behind the HoaBinh dam. [ABSTRACT FROM AUTHOR]

Abstract: In this study we applied 40Ar/39Ar dating methods to muscovite grains in sands from the modern Red and Yangtze Rivers in order to constrain palaeo and modern erosion patterns and sediment transport processes. Micas in the headwaters of the Red River and its southern Da River tributary are dominantly Cenozoic, while the northern Lo River tributary shows a typical 160–220Ma population. The lower reaches of the Red River comprise 53% of such Triassic grains, indicating that the Lo River is the most important net contributor to the modern delta. However, this contrasts with zircon U–Pb data indicating that the upper reaches of the Red River dominate that mineral group. We suggest that the mismatch reflects the rapid transport of mica relative to zircon in the river and the fact that the Red River is not in equilibrium. We hypothesize that the zircons were eroded >8ka under a regime of stronger monsoon that enhanced erosion in the northern drainage basin, while modern erosion is focused in the south, especially in the Song Chay Massif. Micas from the upper Yangtze River are resolvably older than those in the Red River (230–250Ma), so that this method could be used to test for drainage capture in palaeo-delta sediments. Micas in the modern Yangtze delta are generally much younger than those in the upper reaches and indicate that erosion in the Longmen Shan (Sichuan) and neighbouring regions is more important than the upper Yangtze in supplying sediment, reflecting the stronger monsoon rains in those areas. [Copyright &y& Elsevier]

Abstract: 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 70years 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. [Copyright &y& Elsevier]

Abstract: The coast between Ngason and Haiphong is largely formed by accretion of the Red River system. In the region, five main surface sediment types (sand, sandy silt, silt, mud and sand at shoals) could be defined, which differ from one another in their sedimentary characteristics. Sand dominates along the shoreline between 0 and 15m water depth. Down to a water depth of about 25–30m, the sediment is dominantly silt. Further offshore the surface sediments are mainly sandy silt and sand of older units (Early-Middle Holocene, Late Pleistocene). Net sediment transport directions are defined by grainsize analysis according to the method of Gao and Collins [Gao, S., Collins, M., 1992. Net sediment transport patterns inferred from grain-size trends, based upon definition of transport vectors. Sedimen. Geol. 80, 47–60, 1992]. At river mouths, directions of sediment transport are variable where the depths are shallower than 5m. From 5 to 10 m water depth, sediments are mainly transported southeastward at the Ba Lat, Lach and Day mouths, northeastward at the Tra Ly mouth and eastward at the Thai Binh mouth. Recently, the Hai Hau erosional shoreline is not supplied with sediment from the Ba Lat mouth and sediments are transported southwestward alongshore in the region shallower than 5m. The region of depths from 10 to 30m is specified by southward sediment transport. [Copyright &y& Elsevier]

Abstract: The coastline of the Red River Delta is characterized by alternating patterns of rapid accretion and severe erosion. The main branch of the Red River, the Ba Lat, is presently expanding seaward with a main depositional area several km downstream and offshore the Ba Lat River mouth. Sediment deposition rates are approximately 6m in the past 50 years. Field measurements were done to determine the processes that regulate marine dispersal and deposition of sediment supplied by the Ba Lat. These measurements reveal that the waters surrounding the Ba Lat delta are strongly stratified with a pronounced southward-flowing surface layer. This southward-flowing surface layer is a coastal current which is generated by river plumes that flow into the coastal zone north of the Ba Lat. However, outflow of turbid river water is not continuous and most sediment enters the coastal zone when the alongshore surface velocities are low. As a consequence, most sediment settles from suspension close to the river mouth. In addition to the southward surface flow, the southward near-bottom currents are also stronger than northward currents. Contrasting with the residual flow near-surface, this southward flow component near-bottom is caused by tidal asymmetry. Because most sediment is supplied by the Ba Lat when wave heights are low, sediment is able to consolidate and therefore the long-term deposition is southward of, but still close to, the Ba Lat mouth. [Copyright &y& Elsevier]

The Nam Dinh Coast is a constituent part of the dynamic coastline, belonging to the Red River Delta. It is bounded to the north and south by the Red and Day Rivers, with Ninh Co and So Rivers located in the middle. This coast has been undergoing severe erosion during the last hundred years. Over this period, vital agricultural area along the coastline has vanished into the sea and families have been forced to abandon their houses due to the erosive action of waves. Numerous studies were conducted to identify the major causes governing the erosion problems, and several hypotheses have been proposed. In the present work, remote sensing data analysis techniques with both optical and synthetic aperture radar (SAR) were employed to examine the hypothesis that land subsidence is contributing to the erosion along the Nam Dinh Coast. The results revealed that the high lateral erosion rates observed at sandy beaches between estuaries are approximately -6 m/yr in the south and -10 m/yr in the north of the So River. Meanwhile, sediments transported from the rivers accumulate mainly around the estuaries, leading to rapid accretions of approximately +93, +4, and +46 m/yr for Day, Ha Lan (So River), and Ba Lat (Red River), respectively. Simultaneously, land is sinking and uplifting at highly uneven rates, with a few sections subsiding 10 times faster than the overall spatial average. Uplifting observed in southern areas ranges from +1.2 to +1.9 mm/yr. In contrast, subsidence was identified in the northern parts, with mean rates varying from -0.1 to -12.3 mm/yr. Based on the Bruun Rule, the combined effects of land subsidence and relative sea level rise (SLR) could explain approximately 66% of the observed rate of erosion in the northern part of Ha Lan. Findings from surface deformation data contribute to a deeper insight into coastal erosion in Nam Dinh. • The erosion mechanism in the Nam Dinh Coast is not yet fully understood. • Combined use of SAR and optical images is applied to detect subsidence and to discuss its effect on coastal erosion. • Land is sinking and uplifting at highly uneven rates, varying from -21 to +2.5 mm/yr. • Combined effects of land subsidence and relative sea level rise contribute to 66% of the erosion. [ABSTRACT FROM AUTHOR]