Narrow Results

Erosion of the nourished beach in Bethany Beach in Delaware is examined using available beach profile, wave and tide data during September 2007 to September 2010. The volume of the placed sand with the median diameter of 0.31 mm was about 500 m³/m along the curved shoreline of 1.8 km length. The placed sand volume along this shoreline decreased to approximately 30% for the duration of 2.5 years. The nourished beach was attacked by two severe storms in May 2008 and November 2009. The eroded sand volume above the mean sea level (MSL) was about 70 m³/m for each of the two storms and emergency repairs were necessary. The recovery after the first storm was about 8 m³/m above MSL and much smaller than the eroded volume probably because the large placed sand volume resulted in the relatively steep eroded profile below MSL. The numerical cross-shore model with multiple cross-shore lines is used to compute the cross-shore and longshore sediment transport. The decrease of the placed sand volume is found to be caused partly by the increase of the longshore sand transport in the downdrift direction. The beach erosion by the two storms is shown to be caused by the offshore sand transport and the alongshore gradient of the longshore sand transport rate. The small recovery after the first storm is difficult to reproduce without increasing the onshore bed load transport rate by a factor of 2.

Shore protection projects require the prediction of coastal storm damage and economic loss but the damage processes are not well understood. An exploratory experiment consisting of 11 tests was conducted in a wave flume with a sand beach to examine the movement of 10 wooden blocks (floatable objects) placed on the foreshore and berm as well as on short and long pilings. The still water level was varied to create accretional and erosional profile changes. The cross-shore wave transformation on the beach and the wave overtopping and overwash of the berm were measured in 101 runs of irregular waves where each run lasted 400s. The initial block elevation above the sand surface had little effect on the hydrodynamics, sediment transport, and profile evolution in this experiment with widely-spaced blocks. The block floating and sliding on the sand surface and the block falling from the pilings depended on the swash hydrodynamics and block clearance above the foreshore and berm whose profile varied during each test. A simple probabilistic model is developed to estimate the immersion, sliding, and floating probabilities for the blocks in the swash zone. The predicted probabilities are compared with the observed cross-shore variation of the block response on or above the accretional and erosional beach profiles. The accurate prediction of the block response is shown to require the accurate prediction of the beach profile change.

The characteristics of currents, waves and turbulent velocities in and above submerged vegetation and the resulting wave attenuation are examined in a series of laboratory experiments and using simple theoretical models based on the mixing layer analogy for vegetated flows, the time-averaged momentum equation including the horizontal drag force, and linear wave theory.

The sand concentration, free surface elevations and velocities measured above equilibrium terraced and barred beaches are compared with the time-averaged and time-dependent models based on the depth-integrated continuity equation of suspended sand and the continuity, momentum and energy equations for finite-amplitude shallow-water waves.

Mudflat profiles were measured monthly along two cross-shore lines exceeding 1,000 m in the vicinity of a river mouth for four years. Assuming the measured mudflat profile evolution as the superposition of annual accretion rate, seasonal variation, and episodic variation, the annual accretion rate was 4.2cm/year and 1.6cm/year on the left and right profiles, the seasonal variation was in the range -5 to 5 cm for both profiles, and the maximum episodic variation due to storm action was 12.1cm and 22.0 cm for the left and right profiles, respectively. The comparison between calculated depth-averaged tidal currents and wave orbital velocities indicate that wind waves may play an important role in mud suspension and transport and that the combined effects of tides and wind waves will need to be taken into account for the sediment transport.

Cobble beaches and revetments are poorly understood partly because no simple model is available for the prediction of irregular breaking waves on porous beaches and structures. A numerical model based on time-averaged continuity, momentum and energy equations is developed to predict the mean and standard deviation of the free surface elevation and horizontal fluid velocities above and inside a porous beach or structure. The time-averaged model is compared with the corresponding time-dependent model which was verified using three tests for irregular wave runup on a 1/3 slope with a thick porous layer. The cross-shore variations of the mean and standard deviation of the free surface elevation and fluid velocities computed by the two models are shown to be in agreement for the three tests. The time-averaged model reduces the computation time by a factor of 10^-3 and requires only the offshore wave data which is normally available.

Tidal flats play an important roll as a habitat for the rich ecosystems. Global warming would raise the sea levels about 1 m in the next 100 years. The Intertidal areas decrease or deform with raising the sea levels. Therefore, future variation of inner bay environment has received considerable attention, because the purification abilities of tidelands depend on the ecosystems…

A computationally-efficient model based on time-averaged continuity, momentum and energy equations coupled with a probabilistic runup model is developed to predict the mean and standard deviation of the free surface elevation and horizontal fluid velocities above and inside a porous layer. The developed model is calibrated and verified using laboratory experiments on irregular breaking wave transmission over submerged porous breakwaters, irregular wave runup on permeable slopes, and irregular wave seepage and overtopping on wide permeable crests. Furthermore, the developed model coupled with a model for sand suspension and transport is used to predict irregular wave transformation and sediment transport from outside the surf zone to the lower swash zone on sand breaches. The coupled model is compared with laboratory experiments in which detailed measurements were made of the free surface elevations, velocities and sand concentrations. A bedload model is developed and added to this numerical model for the prediction of beach profile evolution including berm and dune erosion.

A Navier-Stokes solver (NEWFLUME) is used to evaluate velocity, local, convective and total accelerations in the swash zone for three different types of breaking, surging, plunging and nearly spilling waves which are obtained by altering the beach slope holding the forcing conditions constant. The hydrodynamic results have been presented for a level right above the bed which is more important for sediment transport issues regarding the three breakers. Also, depth-averaged velocity and acceleration parameters are shown. The results show that regardless of the breaker types, moving from the inner surf to swash zone, the flow velocities magnitude increases and the largest onshore-directed velocity occurs at the beginning of uprush, also the greatest offshore velocities take place at the end of backwash phase. It has been found that the convective accelerations are mainly onshore-directed or nearly zero and reach their highest values at the early uprush and at the end of backwash phases in the onshore direction. We also find that in the surging case the local accelerations are offshore-directed during a swash event except at the beginning of swash cycle when the local acceleration value decreases during the time and reaches its maximum offshore-directed value at the mid-swash time. Just outside and at the seaward edge of the swash zone, local accelerations are shoreward-directed for the plunging case and near zero but shoreward-directed for the spilling wave.

A numerical model based on the time-averaged continuity, cross-shore momentum, longshore momentum and energy equations is developed to predict the crossshore variations of the mean and standard deviation of the free surface elevation and depth-averaged cross-shore and longshore velocities under obliquely-incident irregular breaking waves. The suspended sediment volume per unit horizontal area is estimated using the computed energy dissipation rates due to wave breaking and bottom friction. The longshore suspended sediment transport rate is estimated as the product of the longshore current and suspended sediment volume. The developed model is compared with limited field and laboratory data. The calibrated model is in fair agreement with the data. The longshore suspended sediment transport rate is shown to be approximately proportional to the square of the longshore current. The developed model appears promising but will need to be evaluated using extensive data sets.

Cross-shore sediment transport on beaches has been investigated extensively, however the prediction of beach erosion and accretion accurately even for the idealized case of alongshore uniformity, normally-incident waves, and uniform sediment is still not possible. In order to improve our predictive capabilities, sediment transport models are getting more sophisticated but less transparent. Many are employing sophisticated and computationally intensive wave and hydrodynamic components while still relying on simple empirical formulations for sediment transport. Moreover, the roles of bedload and suspended load are not clear judging from the successful prediction of the onshore bar migration at Duck, North Carolina by Hoefel and Elgar (2003) using the skewed acceleration effect on bedload and by Henderson et al. (2004) using a suspended sediment model. An attempt is made here to synthesize and simplify existing cross-shore sediment transport models with the aim of developing a simple and robust model that is suited for engineering applications. This paper presents a simple probabilistic formulation for bed and suspended sand transport, and examines its effectiveness in predicting the beach profile evolution in the small-scale laboratory setting.

Multiple sand bars occur as rhythmic features on the intertidal zone at Okoshiki Beach in Ariake Bay, Japan. Cross-shore profiles of intertidal sand bars were measured monthly for 2 years from July 2003 to July 2005. Each measured profile is separated into a mean profile and a bar geometry using quadratic polynomial fitting. Superimposed on the mild foreshore (1/250) are at least seven low height (0.1-0.6m) ridges with an average wavelength of 40 m. The sand bar morphology appears to be a permanent feature in both form and position. The calculated depth-averaged tidal currents indicate that sediment movement occurs less than 4% of the tidal period.

The effect of the finite beach length on the long term beach renourishment strategy is analysed using a standard diffusion equation with open boundary conditions. Computed results indicate the importance of the beach length due to longshore sediment losses. In addition, a small scale experiment on berm erosion was done for two profiles (flat berm and tilted berm). The results were compared with a time-averaged cross-shore process-based model. A fairly good agreement was found between the measured and computed profiles but the model will need to be improved by including the bottom slope effect and alongshore variability.

Experiments were conducted in a wave flume to investigate wave seepage and overtopping of permeable stone slopes with wide crests. The numerical model based on the time-averaged continuity, momentum and energy equations is extended to include the landward water flux due to wave seepage and overtopping. The measured wave runup distributions are fitted to the Weibull distribution whose shape parameter increases with the increase of the wave overtopping probability. The wave overtopping rate normalized by the wave-induced water flux at the still water shoreline is shown to depend on the wave overtopping probability and the horizontal number of stones above the maximum wave setup. A simple formula for the seepage rate is proposed by analyzing the seepage flow driven by the wave setup on the seaward slope. The extended numerical model is shown to be in agreement with the measurements of the free surface elevation, crossshore velocity, wave runup, and seepage and overtopping rates but will need to be evaluated using more extensive data sets.

A numerical model based on time-averaged continuity, momentum and energy equations is applied to irregular wave transformation over a deforming submerged breakwater. Experiments were also conducted to investigate the accuracy of the numerical model and the relation between the function of a wide submerged breakwater and its deformation caused by wave attack. The numerical model predicts the measured cross-shore variations of the mean and standard deviation of the free surface elevation and horizontal velocity well. The results of the experiments show that the considerable deformation of the submerged breakwater has little influence on wave transmission and reflection.

The time-dependent cross-shore sediment transport model CBREAK is used to investigate the sediment dynamics on a steep foreshore slope (approximately 1:6) in the swash zone on an equilibrium beach. The computed resutls show that the forcing mechanisms and characteristics of suspended sediment dynamics change drastically from the surf zone to the swash zone at the downrush limit. Contrary to the sediment suspension in the surf zone dominated by intermittent high suspension events under irregular breaking waves, the sediment suspension in the swash zone is dominated by the bottom friction due to the uprush and downrush of individual waves. In addition, the sediment concentration and suspension rate are sensitive to small water depth in the swash zone. The computed results are qualitatively realistic but need to be verified against measurements.

A synthesis of existing models and formulas is made to compute the virtual performance of rubble mound structures in shallow water under combined storm surge and breaking waves for sequences of hurricanes. The computed results for ten 500-yr simulations are presented for a typical structure as an example. The crest height and armor weight of the structure are designed against the peak of a 100-yr storm. The structure designed conventionally is exposed to approximately 350 storms for each 500-yr simulation. The computed wave overtopping rate and volume during the entire duration of each storm are analyzed to assess the severity of flooding hazards. The computed progression of damage to the armor layer is caused episodically by several major storms but slows down as the structure ages. The computed results are also used to quantify the equivalent duration of the peak of a storm which yields the same overtopping water volume and damage increment as those computed for the entire storm duration.

A laboratory experiment was conducted in a wave flume to observe and measure sand suspension events under shoaling waves on a rippled bed and under breaking waves and bores on an equilibrium terraced beach consisting of fine sand. The same irregular waves were generated for 37 tests to measure the three-dimensional velocities and concentrations at several elevations above the local bed at five cross-shore locations. The measured alongshore velocity is shown to be about 20% of the cross-shore velocity and an effective proxy for detecting three-dimensional vortices and turbulence near the rippled bed and inside the surf zone. The spectra of the alongshore velocities inside the surf zone were dominated by low frequency components associated with intermittent irregular wave breaking. The measured sand concentrations were dominated by intermittent suspension events accompanied by large fluctuating alongshore velocities. Intermittent suspension events occurred under the steep fronts of breaking waves with large fluid accelerations. Strong plunging breakers occurring intermittently caused very intense suspension events lasting at least several seconds. Moderate suspension events were also observed under uprushing bores. The time-averaged sand fluxes on the equilibrium beach calculated from the measured time series were relatively small and not accurate enough to explain the equilibrium profile.

Wave runup and overtopping on inclined coastal structures and wave runup on beaches are reviewed together to examine the ranges of wave runup processes occurring on slopes of different inclinations. Laboratory experiments on regular wave runup and overtopping on coastal structures are reviewed first to provide historical perspective. More recent laboratory experiments on irregular wave runup and overtopping on coastal structures are summarized to show the improved quantitative understanding due to the improved capabilities for irregular wave experiments. Field experiments on wave runup on beaches are then reviewed to discuss the possible dominance and causes of low-frequency shoreline oscillations on gently sloping beaches. The recent development of time-dependent numerical models is reviewed to indicate the rapid progress of the numerical capabilities of predicting irregular wave runup on inclined coastal structures and beaches. This review indicates that the improved quantitative understanding of irregular wave runup and overtopping on inclined coastal structures and irregular wave runup on beaches has essentially been limited to normally incident waves on coastal structures and beaches of alongshore uniformity. Future experimental and numerical studies are suggested in this review.

Simple formulas for the cross-shore and longshore transport rates of suspended sand and bedload on beaches are proposed by synthesizing available data and formulas. A combined wave and current model based on the time-averaged continuity, momentum, and energy equations for water is improved and used to provide hydrodynamic input to the proposed sand transport model. The model is compared with spilling and plunging wave tests conducted in a large wave basin using fine sand. The numeric model predicts the measured longshore suspended sand and total transport rates within a factor of about 2. The longshore bedload transport rate is predicted to be small. The predicted cross-shore sand transport rates are relatively small on the quasi-equilibrium beaches in these tests. The computed beach profile change under 10-h wave action is less than about 5 cm. The proposed sediment transport model will need to be verified using additional data but no bedload data is available in surf zones and reliable suspended load data is scarce.