Study On Wave Transformation And Overtopping At Vertical Seawalls On Coral-reef Topography

Coral reefs,formed by thousands of years of sedimentation of coral skeletons and mixed with biological residues,are abundant features in tropical and subtropical oceans,as well as in the South China Sea.Because of the unique geomorphic feature of coral reefs,the hydrodynamic processes on them are distinct from that on sandy beaches.In this thesis,wave transformation and overtopping of vertical seawalls on twodimensional coral reef torgraphy were studied.The major contents are summarized as follows:(1)A new definition of wave groupiness factor was proposed based on waveheight history of wave trains.This new definition takes into account the dual roles of both zero-downcrossing wave height and wave period in the spatial and temporal evolution of the wave groupiness.The new groupiness factor is less sensitive to the operational definition and can describe the groupiness degree of not only linear wave trains but also highly nonlinear ones.The variation of the wave groupiness across a laboratory fringing reef was investigated.Results showed that in the nonlinear shoaling zone,the groupiness became more pronounced.Decreasing of the groupiness was triggered by wave breaking in the vicinity of the breakpoint.The groupiness decreased rapidly shortly after breaking,whereas it increased in the inner surf zone.Infragravity wave modulation of short-wave water depth was the main source of groupiness after breaking.(2)Laboratory experiments with irregular waves were carried out to study the wave transformation and wave-induced setup on reef flat.It was found that stability of hydrodynamic conditions on reef flat cannot be reached until 200 times of peak period.Irregular wave trains have more complex breaking process than that of regular wave trains,and the larger waves in the same wave train break on the fore-reef slope in plunging manner,while smaller waves break on the reef flat in spilling manner or do not break.Wave height decreases and wave mean period increases after wave breaking.The maximum set-up on reef flat increases with the increase of peak period,decreases with the increase of the water depth on reef flat,and increases linearly with the increase of significant wave height.The formula proposed by Gourlay for estimating set-up on reef flat induced by regular waves was extended to irregular wave conditions.(3)Random wave overtopping at vertical seawalls on coral reefs was studied by conducting physical experiments.Accuracies of prediction formulas for overtopping rates of vertical seawalls were tested.Results showed that all the formulas cannot give accurate results for vertical seawalls on coral reefs,induced by the special wave hydrodynamics of coral reefs.It was found that the overtopping rate at vertical seawalls on coral reefs is influenced by wave height,waterdepth,and freebroad.A new formula for vertical seawalls on coral reefs was proposed.(4)An artificial neural network(ANN)model for predicting overtopping rates at impermeable vertical seawalls on coral reefs was developed.A subset of the experimental data was used to calibrate a nonhydrostatic wave model.Then the calibrated nonhydrostatic wave model was run to provide a comprehensive numerical database for training of the ANN model.The database covered a wide range of hydrologic conditions,geometrical morphologies of reefs,and heights of seawalls.The advantage of the new ANN model is featured by its particular applicability to the coral reef.It can provide reliable prediction of overtopping rates using the parameters of deep-water waves as inputs instead of the wave parameters at the toe of the structure.The convenience in obtaining wave parameters in deep-waters makes the ANN model a highly practical predictor that is useful in the preliminary design stage of vertical seawalls and in real-time forecast of wave-induced flooding in the coral reef environments.