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In the description and modelling of coastal profile change, the assumption is made that at a certain water depth cross-shore sediment transport becomes negligible. This seaward boundary condition is often referred to as `closure depth'. Although the assumption of closure at deep water is not valid for large time scales (e.g., during large-scale coastal retreat), analysis of profile measurements from time periods up to 30 years showed that the hypothesis of a seaward boundary of sediment transport is a useful concept.
Since this depth of closure is dependent on the movement of sediment by waves, the position of the depth of closure along the cross-shore profile is a function of density, size and shape of the grains and, furthermore, of wave characteristics and time. Changes in sediment characteristics along the depth of closure give therefore information on variations in hydrodynamic conditions on the measured time scales. Figure 1 schematically indicates the cross-shore profile and the limit of initiation of motion (depth of closure) as function of grain size and density. Large, heavy grains are harder to transport than small, light grains and, for equal wave conditions, the initiation of motion occurs at a more shallow water depth, higher on the profile. Similarly, heavier grains have a more shallow closure depth than light grains of the same diameter.
Earlier investigations of selective transport processes of heavy minerals under shoaling waves showed that selective transport processes are rather rule than exception and start directly at the initiation of motion of sediments. Therefore, measurements of changes in sediment gradation provide information on the initiation of motion.
Measurements of sediment characteristics in the field to determine hydrodynamic conditions have been conducted previously. However, these measurements lacked high spatial resolution and were too time consuming for regular measurement campaigns. With the techniques developed at NGD, high-resolution in-situ measurements can be conducted and this makes an approach of describing hydrodynamic conditions from sediment characteristics feasible.
Figure
1: The seaward limit of significant sediment transport is dependent on grain
size and density.
The MEDUSA detector system was used to measure sediment composition (grain size and density) in-situ at sea, off the coast of the Dutch barrier island Ameland and in a laboratory experiment. The results of both assessments show a similar behavior of sediment sorting on the coast. The interpreted depth of closure based on these sediment distributions proved to be similar to the depth of closure measured and predicted by `classical' methods [1].
