Atlas de l'évolution morphologique et du budget sédimentaire du littoral du Languedoc-Roussillon Thème 2 -Contrat d'étude SIMILAR
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theme 2 Bilans sedimentaires
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The sandy coasts are very dynamic environments at all time and space scales. Today, they constitute an area of strategic importance for humanity because of its strong economic and demographic weight. This thesis aims to study the morphodynamics of the shoreface of the Languedoc-Roussillon region (Mediterranean Sea) at a multi-decadal to secular scale. This work is possible by the use of topo-bathymetric airborne LiDAR able to map large areas. The results of this thesis show that there exist a wide variety of beach and bars states. The organization of these states is mainly controlled by sedimentary characteristics, wave conditions and geological context. The sediment budget of the shoreface was studied at secular scale and indicates an overall deficit of more than 26 million m3 of sediment. Sand bar dynamic was analyzed at a multi-decadal scale and shows the presence and involvement of slightly oblique nearshore bars in the process of Net Offshore Migration. Finally, this thesis has also highlighted the strong influence of human activities on the general shoreface behaviour at the regional scale.
Shoreline changes of the littoral of the Rhone delta is in relation to the sand carried on by the river and the waves conditions. This paper analyses the river-littoral system in the central part of the delta which is caracterised by a river sedimentary source (Petit Rhone) and the east side of the littoral cell (Beauduc Golf). In the Petit Rhone, the evolution of the bed, in erosion, is analysed by several longitudinal profiles (1969-89-95). On the beaches of Beauduc Golf, the sediment budget is realised on the concept of the littoral cell. Quantification (m³) is determined by intergrated (i) surface of shoreline change (aerial photographies 1963-87-95) with (ii) the closure depth. Nevertheless, the results remain theorical because of the determination of the closure depth. In the medium bed river incison is directly related to the rythm and the intensity of hydrologies events. The littoral longshore sediment transport may be bi-directional in relation to wave climate. Consequently the contribution of the sand river in the littoral cell is difficult to determine. In the short term, the exceptional river flood of 1993-1994 is able to reduce the incision of the bed in the river and it may also reduce the erosion on the beaches. But this reduction can be awarded to coastal protections too.
The sandy microtidal wave-dominated littoral of Sète, with its barred shoreface, shows that the morphological evolution of the sedimentary bars can be synthesized according to two conceptual models, in reaction to hydrodynamic variability. (1) The model of "oscillation around a position of equilibrium" (O.P.E) defines the usual mode of behaviour of the bars, with an alternation of shoreward and seaward movements. (2) The model of "Net Offshore Migration" (N.O.M) points to the tendency to a retreat of the bars under the effect of paroxysmal events (storms with a 20 to 50 yrs return time) being a prelude to their degeneration. A few years after, the standard pattern is restored. These results are compared with those described in the literature.
Le golfe du Lion est une marge continentale à construction deltaïque soumise à l'impact des apports sédimentaires (1) du Rhône, un des plus gros fleuves méditerranéens, à caractère saisonnier ; (2) ainsi que d'une série de fleuves côtiers à caractère torrentiel. Des zones de dépôts temporaires du matériel fin se développent en face de ces fleuves vers 30 m de profondeur à la limite d'action des houles de tempête. La dynamique de ces zones soumise à l'impact des évènements extrêmes joue un rôle important dans (1) la dissémination des sédiments fins et des contaminants associés depuis les sources (bassins versants) jusque vers le large, et (2) la qualité des eaux dans la zone littorale. Une station de suivi automatique haute fréquence et à long terme a été installée sur le fleuve Têt, exemple de fleuve côtier méditerranéen à caractère torrentiel et sur son prodelta afin de suivre la variabilité, l'impact des évènements hydro-climatiques sur le devenir des sédiments fins dans la zone littorale. Les crues engendrées par les entrées maritimes sont les principales sources de sédiments fins à la zone littorale. Les tempêtes qui accompagnent ces crues créent une redistribution de ce matériel vers la vasière circa-littorale vers 50 m de profondeur et la limite du plateau où se situent les canyons sous-marins. La succession de ces évènements extrêmes au cours de l'année détermine la conservation du matériel fin dans la zone littorale ou son exportation vers le large. Une forte variabilité interannuelle existe également dans le golfe du Lion entre des années humides avec de fortes crues et tempêtes apportant et redistribuant de grandes quantités de matériel sédimentaire sur le plateau et des années sèches caractérisées par des courants intenses à l'origine de plongées d'eaux denses dévalant et érodant le plateau jusqu'au canyons sous-marins. Les évènements hydro-climatiques extrêmes (crues, tempêtes, plongées d'eaux denses) sont les acteurs principaux qui façonnent les fonds sédimentaires du golfe du Lion. L'impact des activités humaines et du changement climatique sur la fréquence de ces évènements extrêmes semble jouer un rôle important sur la variabilité de la qualité du milieu littoral et du devenir des sédiments fins et contaminants.
This study develops a method to identify the depth of closure using airborne laser bathymetric data, and compares the measured depth of closure with estimated depth of closures. Airborne laser data sets were collected before and after the 2004 hurricane season in southeast Florida. Estimated depth of closures in terms of wave data is compatible to the measured depth of closures. However, the estimated depth of closure is on average larger than was measured, especially in the southern end of the study area, which could be due to geologic influence. Further research is needed to determine if geology plays a critical role in the depth of closure at this study's location.
In this paper, we examine temporal and spatial beach profile volume changes, sediment budget changes, and side-scan sonar images at nourished beaches of northeastern South Carolina. Results of bulk volume change indicate that most sands eroded from the subaerial beach section remain and circulate within the coastal system. The results also indicate more active sediment exchange between the nearshore and offshore zones than expected, indicating that the offshore can be both a sink and a source for the nearshore morphologic change. Our profile volume change results do not show a unidirectional net southerly transport pattern in the study area during the initial postnourishment period. Instead, results show that the net downdrift direction alternates along the shore and that longshore volume drift patterns are often disrupted by the seaward cross-shore transport events that occur at erosional hotspots. Prenourishment erosional patterns, particularly local areas of elevated erosion rates, were re-established after nourishment. Furthermore, those erosional locations often correspond to the offshore locations of paleoriver channel fill sands with low relief, indicating existence of the specific seaward transport pathways along the shore in the study area.
Long-term, net offshore bar migration is a common occurrence on many multiple-barred beaches. The first stage of the process involves the generation of a longshore bar close to the shoreline that oscillates about a mean position for some time, followed by a stage of net offshore migration across the upper shoreface, and finally a stage of decaying bar form through loss of sediment volume at the outer boundary of the upper shoreface. The phenomenon has been previously documented in the Netherlands, the USA, the Canadian Great Lakes, and in New Zealand, but our present understanding of the morphodynamic processes and sediment transport pathways involved in bar decay is limited. In this paper, long-term, net offshore bar migration is investigated at Vejers Beach, located on the North Sea coast of Denmark where offshore bar migration rates are of the order of 45–55ma−1. A wave height transformation model confirmed that the decay of the outer bar results in increased wave heights and undertow speeds at the more landward bar potentially causing this bar to speed up its offshore migration. The causes for outer bar decay were investigated through field measurements of sediment transport at the decaying bar and at a position further seaward on the lower shoreface. The measurements showed that a cross-shore transport convergence exists between the bar and the lower shoreface and that the loss of sediment involved in bar decay is associated with a longshore directed transport by non-surf zone processes. At Vejers, and possibly elsewhere, the net offshore migration of bars and the subsequent loss of sand during bar decay is an important part of the beach and shoreface sediment budget.
Recent research in fluvial geomorphology has emphasized the direct monitoring of channel topography as a useful tool to better understand the interrelationship between river form and process, and in particular to estimate bedload transport rates and reach-scale sediment budgets. Until recently, however, this morphometric approach has been limited to comparison of repeatedly surveyed cross-sections with the associated problems of low frequency sampling and an emphasis on cross-stream as opposed to downstream morphological dynamics. Recent advances in analytical photogrammetry have, however, permitted the acquisition of data at sufficient resolutions to represent macroform (101 m) and microform (10° m) channel geometry, in fully three dimensions. While inherently spatially distributed, the photogrammetric approach to land-surveying is, however, restricted to observable areas of the channel and is thus of limited use in the subaqueous zone, especially if flows are turbid or highly coloured. This paper presents an alternative approach to the study of three-dimensional morphological dynamics of a divided reach of the gravelly River Feshie, Scotland, in which topographic survey of both exposed and submerged areas of the reach was undertaken using the Global Positioning System (GPS). Survey results from two field programmes in 1998 and 1999 are presented in which a Geotronics Geotracer 2000, real-time kinematic GPS receiving unit was used to survey a 200 × 80 m reach. Rapid radial survey methods were employed permitting the collection of 2000 + survey points per day, giving a mean density over the 13 ha reach of 1·1 points per square metre. A detailed assessment of survey errors and DEM quality is presented and a limit of detection of c. 10 cm proposed as an upper threshold for the identification of significant change between DEMs. Preliminary interpretation of channel dynamics in 1998–99 reveals relatively little change to the overall macroscale reach structure. DEM differencing techniques, however, highlight important local changes including the progradation of submerged bar fronts, bar head aggradation and scour along major channel thalwegs and the erosion of fines from sloughs. Copyright © 2000 John Wiley & Sons, Ltd.