Ripple filtering and ridge enhancement applied to morphodynamical tracking of sand dunes

International audienceA 3D global semi-automatic approach based on bathymetric triangulated irregular network (TIN) is developed to extract the overall sand dunes pattern inside a given area. This method takes advantage of a new purpose-built anisotropic filter able to enhance the dominant features of the bathymetric mesh surfaces while smoothing out their ripples. The salient ridge and valley lines of the sand dunes are automatically extracted as 3D parametric curves trough a saliency indicator. This new approach is tested on a fourteen years long time series of thirty MBES bathymetric datasets acquired by the Continental Shelf Service of the FPS Economy of Belgium on a monitoring area located on the northern part of the Middelkerke bank (Flemish banks, Belgian part of the North Sea). Based on the 3D vectors resulting from this approach, 3D augmented representation can right now assist geoscientists in tracking dunes morphology and dynamic but calculation methods should be specifically developed to assess the magnitude and direction of the dunes movement and evaluate the volume of sand involved in the dune dynamics

Control of the repeatability of high frequency multibeam echosounder backscatter by using natural reference areas

The increased use of backscatter measurements in time series for environmental monitoring necessitates the comparability of individual results. With the current lack of pre-calibrated multibeam echosounder systems for absolute backscatter measurement, a pragmatic solution is the use of natural reference areas for ensuring regular assessment of the backscatter measurement repeatability. This method mainly relies on the assumption of a sufficiently stable reference area regarding its backscatter signature. The aptitude of a natural area to provide a stable and uniform backscatter response must be carefully considered and demonstrated by a sufficiently long time-series of measurements. Furthermore, this approach requires a strict control of the acquisition and processing parameters. If all these conditions are met, stability check and relative calibration of a system are possible by comparison with the averaged backscatter values for the area. Based on a common multibeam echosounder and sampling campaign completed by available bathymetric and backscatter time series, the suitability as a backscatter reference area of three different candidates was evaluated. Two among them, Carré Renard and Kwinte, prove to be excellent choices, while the third one, Western Solent, lacks sufficient data over time, but remains a valuable candidate. The case studies and the available backscatter data on these areas prove the applicability of this method. The expansion of the number of commonly used reference areas and the growth of the number of multibeam echosounder controlled thereon could greatly contribute to the further development of quantitative applications based on multibeam echosounder backscatter measurements

Comparison of mechanical disturbance in soft sediments due to tickler-chain SumWing trawl versus electro-fitted PulseWing trawl

This study was part-funded by the EU FP 7 project BENTHIS (grant no. 312088). It does not necessarily reflect the views of the European Commission and does not anticipate the Commission’s future policy in this area. We are grateful for the logistic support of VLIZ, the fishermen of TX43 and TX29 and crew members of RV ISIS and RV Simon Stevin during the sea trials and NIOZ for the use of their box corer. ADR and LRT were partly supported by the project “Impact assessment pulsvisserij”. We are indebted to the skippers and Eddy Buyvoets for drawing the net plans of the trawls. We thank John Aldridge for his insights in sediment transport in relation to natural dynamics; Bavo De Witte for conducting the particle size analysis; Daniel Benden for assisting SPI analyses; Miriam Levenson for English-language editing and Julie Bremner and Stefan Bolam for their critical review. We also wish to thank 3 anonymous reviewers for their constructive comments on earlier drafts of this manuscript.Peer reviewedPostprin

Do tidal sand waves always regenerate after dredging?

Tidal sand waves are rhythmic bedforms found on sandy continental shelves that pose a threat to offshore activities. While emphasis is placed on studying their natural morphodynamic evolution, little is known about if and how fast sand waves recover after dredging. This work presents an analysis of multibeam echosounder data collected at three former sand extraction sites on the Belgian continental shelf. At one of the sites, sand waves seemed to reappear approximately 5 years after dredging had stopped, which did not happen at the other two sites during the measurement period (5 and 9 years). The lack of recovery in those sites is likely the result of larger depths and smaller local sediment availability compared with the site where recovery occurred. Furthermore, these data reveal that in the latter site sand wave recovery was established mainly through local sediment redistribution. • Tidal sand waves are isolated from bathymetric data of the Belgian continental shelf. • At only one of the three sites, sand waves seemed to regenerate after dredging. • Possible explanations are differences in water depth and local sediment availability. • The regenerating tidal sand waves do so as a result of local redistribution of sand

Osculatory surfaces applied to systematic errors estimation in repeated MBES surveys

International audienc

Toward Reliable Volumetric Monitoring of Sandbanks

International audienceThis work forms part of a study addressing volume monitoring over submarine sandbanks through a temporal sequence of DTM models computed from repeat MBES surveys. To perform reliable temporal geomorphometry, the main issue is to get an estimate of the systematic errors of each MBES survey. To this end, while supposing that the evolving shapes still exhibit some partial redundancy, this paper proposes to apply a new trend surface extractor to the first-order time difference between two successive DTMs