Observations of tidal flow, waves and drag within a fringing coastal mangrove forest in the Mekong delta

In recent years attention has focused on the ability of mangroves to protect shorelines against damage from the combined hydrodynamic forces of waves and tides, owing to the presence of roots (pneumatophores) and tree trunks enhancing vegetative drag. However, field measurements within these dynamic environments are limited. We report on field observations from the seaward side of Cù Lao Dung Island (Soc Trang Province) in the Mekong Delta, Vietnam. The island encompasses two contrasting environments from a sandy, prograding flat with gentle topographic slope on the southwest side to a steep, eroding and muddy fringe region on the northeast side. The data capture the flow transitions from mudflat across the fringing region to the forest interior. We observe a rotation of the obliquely incident flows to an orientation perpendicular to the vegetated/unvegetated boundary. The balances governing the large scale flow are assessed and indicate the relative importance of friction, winds and depth-averaged pressure forces. We find drag coefficients of 10-30 times greater than the usual values associated with bottom friction, with values particularly elevated in the regions of dense pneumatophores that are important during the early stages of the tidal cycle.

The field observations are used in the set-up of a simple one-dimensional process model. The model predicts the movement of the tide across the vegetated flat, associated sediment transport and evolution of the across flat profile. Preliminary results indicate that mangrove profiles may evolve towards a close to linear shape in contrast to systems with temperate species or no vegetation.