Monsoon-influenced variations in morphology and sediment of a mesotidal beach on the Mekong River delta coast

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.