| Popis: |
Onshore wind is the primary driver of sediment transport allowing the construction of coastal dunes. In contrast, offshore winds result in the seaward export of sediment inducing a loss of the terrestrial beach sedimentary budget. Many parameters limit aeolian sand transport such as moisture, beach slope, beach length, vegetation and sediment characteristics. Although the impact of grain size on wind transport is well known, few studies have focused on its temporal variability. The temporal evolution of grain size characteristics is particularly important in microtidal environments where the relatively small tidal range minimises the mixing of the beach sand and winds have a strong impact on grain size sorting, resulting in the coarsening of the beach grain size. Leucate beach (SE, France) is a wave-dominated microtidal environment, subject to a strong offshore wind (72 % of the annual time and 17.5 % over 10m/s) which made this site suitable to this study. During the 19 months of meteorological surveys, 5 field measurements campaigns of 1 to 3 days were conducted. For this purpose, wind processes (intensity and direction); aeolian sand transport (24 runs), morphological variations of the beach-dune system and also many sub-surface sediment samples were collected.The results show a large temporal variability in beach grain size ranging from medium to very coarse sand in relation to wind and wave conditions. Aeolian processes produce a coarser beach grain size at days/months scale, whereas short marine storms (day) induce a mixing and finer beach grain size, resulting in very different aeolian sediment transport values for similar incident wind conditions. For example, with a wind speed of 10 to 14 m/s the measured sediment flux ranged from10 kg/m/h (coarse beach grain size) to 50 to 150 kg/m/h (medium beach grain size). Morphological variations of the upper beach surface are not significant when the sand is coarse but can cause lower by the upper beach surface by 0.5 m when the sand is composed of medium-sized. The time scale of the temporal beach grain size variations is closely related to the frequency and intensity of marine and wind storms. This study quantifies the effect of the beach grain size variability on the aeolian sand transport and thus the morphological changes of the beach. We conclude that because of their importance in the temporal variability of sediment size and the inherited sedimentological framework of the beach, it is crucial to take into account marine and aeolian processes to refine the predictions of wind transport rates. This confirms the need in a microtidal environment, to obtain beach grain size temporal data to better understand the aeolian sediment transport rates affecting a study site and not underestimated its impact when calculating transport rates with empirical formulas and numeric models. |
