Wave attenuation in coastal mangroves in the Red River Delta, Vietnam

Wave attenuation was studied in a coastal mangrove system in the Red River Delta, Vietnam on the coast north of Do Son. From sea towards land the study area consisted of a bare mudflat, covered by a sandy layer with embryonic cheniers, abruptly changing into a muddy tidal flat overgrown with mangroves. Three instrumented tripods (A–C) placed in a cross-shore profile, were used to measure current velocity and water level, at the open tidal flat, at the beginning of the mangrove vegetation, and in the mangrove vegetation, respectively. Measurements were conducted in the wet season in July and August 2000. The elevation of the area was surveyed using a levelling instrument. Over the bare sandy surface of the mudflat, the incoming waves are reduced in height (and energy density) due to bottom friction. This reduction decreases with increasing water depth. In the mangrove vegetation, the bottom friction exerted by the clay particles is very low. However, the dense network of trunks, branches and above ground roots of the mangrove vegetation causes a much higher drag force. For the mangrove vegetation which mainly consists of Kandelia candel, the drag force can be approached by the function CD = 0.6e0.15A (with A being the projected cross-sectional area of the under water obstacles at a certain water depth). For the same muddy surface without mangroves the function would be CD = 0.6.