Abstract
Shoreface nourishments are commonly applied for coastal maintenance, but their behaviour is not well understood. Bathymetric data of 19 shoreface nourishments located at alongshore uniform sections of the Dutch coast were therefore analyzed and used to validate an efficient method for predicting the erosion of shoreface nourishments. Data shows that considerable cross-shore profile change takes place at a shoreface nourishment, while an impact at the adjacent coast is hard to distinguish. The considered shoreface nourishments provide a long-term (3 to ∼30 years) cross-shore supply of sediment to the beach, but with small impact on the local shoreline shape. An efficient modelling approach is presented using a lookup table filled with computed initial erosion–sedimentation rates for a range of potential environmental conditions at a single post-construction bathymetry. Cross-shore transport contributed the majority of the losses from the initial nourishment region. This transport was driven partly by water-level setup driven currents (e.g., rip currents) and increased velocity asymmetry of the waves due to the geometrical change at the shoreface nourishment. Most erosion of the nourishment takes place during energetic wave conditions ( H m 0 ≥ 3 m) as milder waves are propagated over the nourishment without breaking. A data-model comparison shows that this approach can be used to accurately assess the erosion rates of shoreface nourishments in the first years after construction.
Highlights
The preservation of sandy coastlines around the world requires regular maintenance with ‘soft measures’ using sand to mitigate potential erosion from natural and anthropogenic causes [1,2,3,4,5,6]
Bathymetric surveys at shoreface nourishments show that shoreface nourishments are quite persistent compared to beach nourishments
Morphological data of 19 sub-tidal sand nourishments at the Dutch coast and numerical modelling with XBeach were used for this purpose
Summary
The preservation of sandy coastlines around the world requires regular maintenance with ‘soft measures’ using sand to mitigate potential erosion from natural and anthropogenic causes [1,2,3,4,5,6] Over time, these sand nourishments will disappear, but the sand will still be beneficial for the sediment balance of the coastal cell. The relative contribution of alongshore and cross-shore processes could, not be quantified, as 2DH models were hindered by artificial flattening of the bars (e.g., [14]), while a stable sub-tidal bar could only be maintained in cross-shore profile models ([15,16]) This is a problem since answering the questions on the driving processes at shoreface nourishments will require a method, which can compute both alongshore and cross-shore profile change while keeping the natural profile (with sub-tidal bar) in place
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