Interactions of sand and clay or stone in surf and swash zones
Date
2021
Authors
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Publisher
University of Delaware
Abstract
The erosion processes of consolidated cohesive sediment under irregular breaking waves were formulated to predict the profile evolution of a cohesive sediment beach with a layer of sand. The cohesive sediment is eroded by turbulence generated by wave energy dissipation caused by wave breaking and bottom friction. Sand released from the eroded sediment is transported onshore or offshore by wave action. The cohesive sediment erosion rate is increased by a thin mobile layer of sand and decreased by a thick sand layer. The complicated interactions of waves, sand, and cohesive bottom were simplified and incorporated into the existing cross-shore numerical model CSHORE. The numerical model was compared with available flume experiment data of till erosion under normally incident waves and available wave basin experiment data of soft cliff erosion under obliquely incident waves. First, the measured till erosion rates of the order of 0.05 cm/h were predicted by the numerical model including a dimensionless function to account for the abrasive and protective effects of the sand layer on the cohesive sediment bottom. The calibrated model was used to simulate the temporal change of the till erosion rate for the duration of 100 h. The scale effect was examined in hypothetical prototype tests of the same till with a length ratio of 1/4. The prototype 200-h simulations predicted much larger till erosion near the shoreline and sand deposition in the surf zone. ☐ Second, the measured cliff recession rates under oblique breaking waves for cliffs built of wet sand and a sand/clay mixture (90.8% sand) were reproduced by the numerical model, which was modified to account for sand loss associated with the alongshore gradient of longshore sand transport. The computed cliff recession rates depend on the incident wave angle and the alongshore gradient. The sand loss is affected by the beach material (concrete or sand) when the incident irregular waves break on the beach and cause sand transport seaward of the cliff toe with small depth. The effect of sediment cohesion on cliff erosion was examined using the numerical model to assess its predictive capability of cohesive sediment containing sand. For cohesive sediment with weak resistance against wave action, the cliff recession rate was limited by the rate of sand removal by longshore and cross-shore sand transport. The recession rate decreased when the resistance of the cliff material exceeded a critical value. These findings will need to be verified using actual field data. ☐ The sand and clay interaction was investigated above numerically using available data. The sand and stone interaction was studied experimentally for the following engineering problem. ☐ Irregular wave overwash may cause landward migration and crest lowering of a barrier beach during a storm. A rock structure may be constructed to protect the barrier beach, but available data of the interaction between the sand and rock are very limited. Sand can be transported over, through, and under the rock structure by overwashing waves on the barrier beach during a storm. Three hydraulic model tests were conducted in a small-scale experiment to investigate wave overwash and erosion process of a narrow sand barrier with no structure, a rock mound, and a rock cover. The rock mound consisting of three layers of stable stones reduced the landward migration and crest lowering of the sand barrier, but sand transport through the porous structure was appreciable. The rock cover consisting of a single layer of stable stones was not effective in reducing the barrier deformation because of the stone settlement and spreading that resulted in the exposure of underlying sand to direct wave action. The exposed sand among the stones was eroded and transported mostly landward. The numerical model CSHORE was calibrated for the rock structure placed directly on the eroding sand barrier with no filter. The mean and standard deviation of the free surface elevation and cross-shore velocity were predicted within errors of about 20% except for small transmitted waves. The bed load parameter was increased by a factor of five to reproduce the degree of the profile deformation of the sand barrier without and with the rock mound and cover. The numerical model allows stone settlement but neglects stone spreading and sand exposure. The stone spreading and sand exposure will need to be examined in more detail and should be incorporated into the model CSHORE.
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Keywords
Barrier beach, Bluff erosion, Cohesive and cohesionless sediment, Sediment transport, Wave overwash