Field observation of wave overtopping at sea dike using shore-based video images

Wave overtopping at a sea dike was observed using video images during a storm in July 2018 at Rizhao Coast,China.A shore-based video monitoring system was mounted to collect coastal images with a sample frequency of 1 Hz in the beginning 10 min of each hour during daylight.A method to extract the frequency,location,width and duration of individual overtopping events based on the shore-based video monitoring system was developed.A total of 6252 individual overtopping events were detected over a 360m long sea dike during the storm of 10 h in a safe and labor-saving way,enabling a detailed analysis of the temporal and spatial variation of wave overtopping.The temporal variation of overtopping duration,frequency and volume is basically in sync with the change of tidal level(R=0.87,0.82 and 0.76,respectively).The increase of wave height increases the overtopping frequency significantly.We found a high correlation between the hourly observed data and the predicted results of two commonly used formulae.Unlike the previous field measurements of overtopping that were limited in a single location,significant spatial variation of overtopping in the alongshore direction is found.The overtopping volume varies with a factor of 6 within the range of several wave lengths.It is further revealed that the spatial variation of overtopping is highly correlated with the alongshore variation of surf zone width with a correlation coefficient of 0.895.The present study suggests the feasibility of shore-based video monitoring technique to capture the main features of wave overtopping at coastal dikes,providing new possibilities to monitor wave overtopping in the field and to improve prediction tools.

Numerical test of scale relations for modelling coastal sandbar migration and inspiration to physical model design

Physical model experiments on sandbar migration are widely conducted in the wave flume to reveal its hydro-sediment dynamic mechanisms.Selecting an appropriate scale relation is an important part of designing a laboratory experiment.However,evaluating the scale effect is complicated since it is impractical to design wave flume experiments based on strict geometry scales derived from the prototype beach or other laboratory beaches.Here,a process-based numerical model is used to test the Dean and Shields similitudes by comparing sandbar migration and sediment transport in different undistorted model scales using natural sediments.The numerical model provides excellent predictions for both offshore sandbar migration on a real-world beach and onshore sandbar migration in the wave flume.Consistent with previous studies,sandbar offshore migration and suspended sediment transport are similar when the Dean similitude is fulfilled.Sandbar onshore migration and bedload transport can be well reproduced only if the Shields similitude is achieved.Although previous laboratory experiments of sandbar migration generally used the Dean similitude,it is found that the Shields similitude is effective and indispensable for modelling sandbar onshore migration,which will be underestimated if the Dean similitude is only considered.Each scale relation cannot be used simultaneously for modelling both onshore and offshore sandbar migration in physical models.