LAUSR

ABSTRACT Do, T.K.A.; de Vries, S., and Stive, M.J.F., 2018. Beach evolution adjacent to a seasonally varying tidal inlet in central Vietnam. Cua Dai Inlet is a typical, seasonally varying tidal inlet in central Vietnam. Since 1995 the northern adjacent coast, known as Cua Dai Beach, has experienced serious erosion. The decadal scale behavior of this inlet appears to reflect a nonperiodic cyclic process. Inlet channel shifting from north to south has welded the abandoned ebb-tidal delta with Cua Dai Beach, leading to accretion but subsequently triggering erosion. Although erosion of Cua Dai Beach was exacerbated by decrease of sediment supply from the estuary and ebb-tidal delta and by coastal developments, the channel shifting to the south, and the ebb shoal development were important primary controlling mechanisms. This study aims to quantify the main erosional processes in and near the Cua Dai coastal inlet and adjacent beaches since 1995. First, satellite data were used to detect shoreline change trends and to estimate volume changes. Second, alongshore, wave-driven sediment transports were estimated using numerical models. Observed shoreline changes indicate that, during the period from 2000 to 2010, erosion rates at the northern side of the inlet were on average 12 m/y. Close to the inlet, erosion rates were larger, up to 19 m/y. At the same time, the southern coast of the inlet was found to accrete with a mean rate of 11 m/y. Calculated alongshore sediment transport rates explain the observed erosion and accretion patterns. The overall system lost a significant sediment volume, which is estimated to amount to 243,000–310,000 m3/y. A logical conclusion is that the effects of the shifting of the inlet channel to the south caused erosion of the northern adjacent coast, whereas human interventions in the river catchment, the estuary, and along the coast contributed importantly to the overall sediment deficit of the inlet system and its beaches and to the shifting erosion pattern toward the north.