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Video Sensing of Nearshore Bathymetry Evolution with Error Estimate

LEGOS (Université de Toulouse/CNRS/CNES/IRD), 31400 Toulouse, France
Faculty of Civil Engineering, Thuyloi University, 175 Tay Son, Dong Da, Hanoi, Vietnam
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2019, 7(7), 233;
Received: 24 May 2019 / Revised: 28 June 2019 / Accepted: 3 July 2019 / Published: 19 July 2019
(This article belongs to the Special Issue Application of Remote Sensing Methods to Monitor Coastal Zones)
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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. View Full-Text
Keywords: depth inversion; low tide terrace; morphological evolution; error quantification; wind waves depth inversion; low tide terrace; morphological evolution; error quantification; wind waves

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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).

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Thuan, D.H.; Almar, R.; Marchesiello, P.; Viet, N.T. Video Sensing of Nearshore Bathymetry Evolution with Error Estimate. J. Mar. Sci. Eng. 2019, 7, 233.

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