Abstract
Although coastal morphology results essentially from underwater sediment transports, the evolution of underwater beach profiles along the diverse coastlines of the world is still poorly documented. Bathymetry inversion from shore-based video cameras set forth a more systematic evaluation and is becoming more commonly used. However, there are limitations to this profiling method that are insufficiently assessed, undermining confidence in operational applications. In this paper, we investigate the daily evolution of a low tide terrace (LTT) in Nha Trang beach, Vietnam, under strong seasonal forcing: from weak wind waves during summer monsoon to moderate waves during winter monsoon. A new error estimation for depth inversion is presented based on tidal evaluation. The method compares video-based estimate and direct measurement of tidal amplitudes to provide a quality criterion. It reveals three types of errors, the main one being a deep water error associated with physical limits—loss of celerity-bathymetry relationship in deep water. This error is dependent on wave period and thus has a strong seasonal pattern in Vietnam. It is generally detrimental to depth inversion where wind waves are dominant (in summer here). On the contrary, the second error type is larger for larger waves and is located at breakpoint, altering wave detection. The last error type is due to nonlinear effects and wave setup in shallow water. After removing the faulty data, we finally present the first reliable three-year time-series of a beach profile in Nha Trang, Vietnam. A main result is the overall stability demonstrated for the LTT beach, with rapid exchange of sediment between the terrace and the upper beach during typhoons, monsoon events or seasonal cycles. These tropical environments may provide faster beach recovery compared with mid-latitude configurations.
Highlights
Low tide terrace (LTT) beaches are a common type of sandy beach but is one of the least documented [1]
LTT are intermediate beaches associated with low incoming wave energy [4,10]; theoretically, it can transform into a transverse bar and rip state (TBR) when wave energy increases and into the lowest energy beach state, which is a reflective beach, when energy decreases
We present the three-year morphological evolution of a low tide terrace sandy beach in Nha Trang, Vietnam, based on bathymetry inversion from a shore-based camera system
Summary
Low tide terrace (LTT) beaches are a common type of sandy beach but is one of the least documented [1]. It can be seen both in low-tide (microtidal) [2,3] and high-tide (macro- and meso-tidal) regimes [4,5]. The terrace formation is due to a coupling process between the effects of tides and waves. Waves plunge heavily on the outer terrace, but at high tide, they may pass across the terrace unbroken to surge up the beach face, producing reflection [7,8,9].
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