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The goal of the work presented in a two-companion paper is to pave the way for reliably assessing the risks of damage to buildings on the shore, induced by the detonation of unexploded historical ordnance (UXO) of large weights in variable shallow water environments with a water depth less than 50 m. The risk assessment is quantified through the seismic magnitude on the Richter scale, induced by the detonation of charges of different weights (between 80- and 680-kg TNT-equivalent). This metric is investigated experimentally using a coupled seismo-acoustic approach within the framework of a UXO clearance (countermining) campaign in the Mediterranean Sea. Analysis of real acoustic and seismic data shows that, compared to a charge detonation in water, a similar detonation on the seabed generates seismic signals of lower frequencies and higher amplitudes that propagate in the seabed. The larger the charge weight, the higher the seismic amplitude. Besides the explosion-coast distance, the ground properties also affect the signals. The sediments favor a longer signal duration and the presence of late dispersive and very low-frequency signals with a large amplitude, whereas the rocky grounds better preserve the high-frequency energy propagation. For the local environment considered in this study, a charge detonation on the seafloor generates seismic events of higher magnitudes compared to a detonation in water. However, these magnitudes are likely low enough to prevent any large damage in the nearby inland infrastructures. [ABSTRACT FROM AUTHOR]

Garlan, T.; Mathias, X.; Brenon E.; Favretto-Cristini, N.; Deschamps, A.; Beucler, E., Guyomard P., and Morio, O., 2018. Circular sedimentary figures of anthropic origin in a sediment stability context. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 411–415. Coconut Creek (Florida), ISSN 0749-0208. The French POSA project studies the seismic energy released by bomb and mine blasting. This project is based on seismological and acoustical measurements recording during blasting actions in order to study seismic wave propagation nucleated by controlled sources (location and explosive energy). These measurements are compared to numerical modeling of the acoustic and seismic wave propagation based on the most faithful 3D representation of the seabed sediments and underlying rocks. This project first focused on the 3D environment representation based on specific surveys and on two blasting experiments which consist in exploding bombs of different loads at two distinct environmental places. The in-situ measurements reveal several hundred circular figures with very specific characteristics which clearly differentiate them from other sedimentary, biological and geological figures encountered on the seabed. They are found within the whole studied area and their diameter is from 10 to 130 m. About 30 circles by kilometer square are observed without correlation with the depth which varies from 10 to 150m. These sedimentary structures indicate that they have been generated by the explosion of bombs, but at different periods. The oldest ones most probably result from bombs dropped in 1940 and 1944 during the landing of the Allied forces in the region of Toulon. These original anthropic sedimentary figures and their differences compared with other circular structures observed on the seabed are described. [ABSTRACT FROM AUTHOR]

A mine burial field experiment was carried out on two sandy seafloors between January and April 2004 in the Bay of Brest, France. Burial recording mines (BRMs) were used to measure burial and mine orientation at 15-rain intervals. Sonar and bottom photographs were also used to characterize sediment morphology and mine burial. These observations are compared with the predictions of mine burial using the following three models: a momentary liquefaction model, a current-induced scour model, and a wave-induced scour model. Analysis combines mine burial data, sediment data, seabed observations, and hydrodynamic measurements. At the first site, "Rascas," the seabed dynamics are dominated by tides and river runoff. Almost no mine burial was measured during the experiment which is in agreement with predictions of mine burial models (current-induced scour and liquefaction). Dynamics at the second site, "Bertheaume," are driven by tides and ocean waves. A long storm lone week) and several swell events were experienced and significant mine burial was observed in conjunction with high significant waveheights. Mine burial models suggest that burial at "Bertheaume" was dominated by wave-induced scour rather than current-induced scour or momentary liquefaction. [ABSTRACT FROM AUTHOR]

Side-scan sonar investigations in the eastern part of the macrotidal Bay of Seine have revealed the presence of numerous rippled scour depressions (RSDs) at water depths of 5-9 m. The sediments in these depressions consist essentially of coarse-grained shell hash derived from underlying Holocene sediments dated at roughly 6,500 yearsBP, and arranged in large wave-generated ripples. The shallow marine area where these features occur consists of a wave-generated ravinement surface produced during the marine flooding of the late Holocene transgression. It can be shown that, during the last 20 years at least, erosion of the muddy sand and sandy seabed has exposed underlying relict sediments. These consist of stiff clays, silts and a layer of shell debris which, when exposed, cover the bottom of large scour depressions which appear to be in equilibrium with the local hydrodynamic regime. Morphological and hydrodynamic data suggest that the RSDs are generated by strong cross-shore bottom currents flowing parallel to the features in the direction of the prevailing waves, and probably associated with storm-induced downwelling events. [ABSTRACT FROM AUTHOR]

Based on experiments, a trawling-induced erosion dynamics was included in a 3D realistic and pre-validated hydro-sedimentary model so as to assess its contribution to the overall regional sediment dynamics in the specific case of the Bay of Biscay continental shelf. The effect of trawling was investigated throughout a 5-year period in terms of horizontal/vertical fluxes and changes in seabed mud content, and compared to the natural variability of sediment fluxes at different spatio-temporal scales. Trawling is shown in the model to significantly contribute to sediment re-suspension, and changes in the seabed at the scale of the shelf. The contribution of trawling to re-suspension is >80% during the high fishing season at many locations on the outer shelf and over the Grande-Vasière area (a large and intensively trawled muddy zone located in the middle of the shelf). The contribution of trawling is significantly weaker in winter and at annual scale, particularly over zones with (muddy-) sand facies (between 1‰ and 1%), but remains high, and reaches 20% of the annual erosion over muddy areas (e.g. northern part of the Grande-Vasière). This significant trawling-induced erosion dynamics results in high sediment fluxes which increase natural poleward dynamics from a few percent up to 40%, and contributes to the natural offshore export of sediment on the outer shelf (e.g. +35% of mud fluxes along the 130 m isobath). It is noteworthy that trawling-induced fluxes are larger in winter although the corresponding re-suspension is lower than in summer: this is due to stronger near-bottom flows occurring at this period (largely influenced by energetic wind conditions). In contrast, on the inner shelf, trawling-induced sediment fluxes are rather oriented towards the coast. Finally, simulation results reveal a trawling signature on changes in the seabed. A significant decrease (between 10% and >50%) in mud content is simulated over trawling hotspots, especially over the internal part of the Grande-Vasière and on mud patches located in front of the Gironde estuary. Conversely, trawling causes or intensifies mud deposition on both sides of these preferentially trawled areas. • Trawling-induced sediment dynamics is significant over mid and outer shelf regions. • Trawling increases natural off-shelf export of fine sediment by 30%. • Trawling induces seabed nature changes over and around fishing zones. [ABSTRACT FROM AUTHOR]