Ad Reniers

Morphodynamic modeling of an embayed beach under wave group forcing

[1] The morphodynamic response of the nearshore zone of an embayed beach induced by wave groups is examined with a numerical model. The model utilizes the nonlinear shallow water equations to phase resolve the mean and infragravity motions in combination with an advection-diffusion equation for the sediment transport. The sediment transport associated with the short-wave asymmetry is accounted for by means of a time-integrated contribution of the wave nonlinearity using stream function theory. The two-dimensional (2-D) computations consider wave group energy made up of directionally spread, short waves with a zero mean approach angle with respect to the shore normal, incident on an initially alongshore uniform barred beach. Prior to the 2-D computations, the model is calibrated with prototype flume measurements of waves, currents, and bed level changes during erosive and accretive conditions. The most prominent feature of the 2-D model computations is the development of an alongshore quasi-periodic bathymetry of shoals cut by rip channels. Without directional spreading, the smallest alongshore separation of the rip channels is obtained, and the beach response is self-organizing in nature. Introducing a small amount of directional spreading (less than 2°) results in a strong increase in the alongshore length scales as the beach response changes from self-organizing to being quasi-forced. A further increase in directional spreading leads again to smaller length scales. The hypothesized correlation between the observed rip spacing and wave group forced edge waves over the initially alongshore uniform bathymetry is not found. However, there is a correlation between the alongshore length scales of the wave group-induced quasi-steady flow circulations and the eventual alongshore spacing of the rip channels. This suggests that the scouring associated with the quasi-steady flow induced by the initial wave groups triggers the development of rip channels via a positive feedback mechanism in which the small scour holes start attracting more and more discharge.

Modeling cross-shore sandbar behavior on the timescale of weeks

We compare predictions of a coupled, wave-averaged, cross-shore waves-currents-bathymetric evolution model to observations of onshore and offshore nearshore sandbar migration. The observations span a 10- and 44-day period with onshore/offshore bar migration at Duck, North Carolina, and at Hasaki, Kashima Coast, Japan, respectively, a 3.5-month period of onshore bar migration at Duck, and a 22-day period of offshore bar migration at Egmond, Netherlands. With best fit parameter values the modeled temporal evolution of the cross-shore bed profiles agrees well with the observations. Model skill, defined as 1 minus the ratio of prediction to no-change error variances, ranges from 0.50 at Egmond to 0.88 for the prolonged onshore bar migration at Duck. Localized (in time and space) reductions in model skill coincide with alongshore variations in the observed morphology. Consistent with earlier observations, simulated offshore bar migration takes place during storms when large waves break on the bar and is due to the feedback between waves, undertow, suspended sediment transport, and the sandbar. Simulated onshore bar migration is predicted for energetic, weakly to nonbreaking conditions and is due to the feedback between near-bed wave skewness, bedload transport, and the sandbar, with negligible to small effects of bound infragravity waves and near-bed streaming. Under small waves and conditions, when breaking and nonbreaking conditions alternate with the tide, the sandbar is predicted to remain stationary. The intersite differences in the optimum parameter values are, at least partly, induced by insensitivity to parameter variations, parameter interdependence, and errors in the offshore wave forcing.

A laboratory study of longshore currents over barred and non-barred beaches

Abstract A detailed description is given of the results of laboratory experiments on wave-driven longshore currents on both barred and non-barred beaches. The objective is to examine the cross-shore distribution of the longshore current velocities for purely wave-driven currents, with emphasis on the position of maximum current velocity with respect to areas where wave energy is dissipated. Unidirectional obliquely incident waves, both regular and random, were used. The measurements were performed in a large wave basin with a pump recirculation system to create spatially homogeneous longshore currents. The experiments yielded information on wave transformation, set-up of the mean water level and the cross-shore distribution of wave-driven longshore current velocity. A number of cases are presented and compared with each other. The measurements show that in the case of purely wave-driven longshore currents, the maximum current velocities occur close to areas where wave breaking is most intense. The effect of mixing, bottom friction and wave rollers on the longshore current velocity profile are examined in more detail with help of numerical modelling. Existing model equations, based on the assumption of alongshore uniformity, are used. The results for the mean longshore current profile on a barred beach are in close agreement with the measurements.

Shoaling and shoreline dissipation of low‐frequency waves

The growth rate, shoreline reflection, and dissipation of low?frequency waves are investigated using data obtained from physical experiments in the Delft University of Technology research flume and by parameter variation using the numerical model Delft3D?SurfBeat. The growth rate of the shoaling incoming long wave varies with depth with an exponent between 0.25 and 2.5. The exponent depends on a dimensionless normalized bed slope parameter ?, which distinguishes between a mild?slope regime and a steep?slope regime. This dependency on ? alone is valid if the forcing short waves are not in shallow water; that is, the forcing is off?resonant. The ? parameter also controls the reflection coefficient at the shoreline because for small values of ?, long waves are shown to break. In this mild?slope regime the dissipation due to breaking of the long waves in the vicinity of the shoreline is much higher than the dissipation due to bottom friction, confirming the findings of Thomson et al. (2006) and Henderson et al. (2006). The energy transfer from low frequencies to higher frequencies is partly due to triad interactions between low? and high?frequency waves but with decreasing depth is increasingly dominated by long?wave self?self interactions, which cause the long?wave front to steepen up and eventually break. The role of the breaking process in the near?shore evolution of the long waves is experimentally confirmed by observations of monochromatic free long waves propagating on a plane sloping beach, which shows strikingly similar characteristics, including the steepening and breaking.

Surf zone surface retention on a rip‐channeled beach

The retention of floating matter within the surf zone on a rip-channeled beach is examined with a combination of detailed field observations obtained during the Rip Current Experiment and a three-dimensional (3-D) wave and flow model. The acoustic Doppler current profiler–observed hourly vertical cross-shore velocity structure variability over a period of 3 days with normally incident swell is well reproduced by the computations, although the strong vertical attenuation of the subsurface rip current velocities at the most offshore location outside the surf zone in 4 m water depth is not well predicted. Corresponding mean alongshore velocities are less well predicted with errors on the order of 10 cm/s for the most offshore sensors. Model calculations of very low frequency motions (VLFs) with O(10) min timescales typically explain over 60% of the observed variability, both inside and outside of the surf zone. The model calculations also match the mean rip-current surface flow field inferred from GPS-equipped drifter trajectories. Seeding the surf zone with a large number of equally spaced virtual drifters, the computed instantaneous surface velocity fields are used to calculate the hourly drifter trajectories. Collecting the hourly drifter exits, good agreement with the observed surf zone retention is obtained provided that both Stokes drift and VLF motions are accounted for in the modeling of the computed drifter trajectories. Without Stokes drift, the estimated number of virtual drifter exits is O(80)%, almost an order of magnitude larger than the O(20)% of observed exits during the drifter deployments. Conversely, when excluding the VLF motions instead, the number of calculated drifter exits is less than 5%, thus significantly underestimating the number of observed exits.

Modeling of very low frequency motions during RIPEX

Numerical computations are used to explain the presence of very low frequency motions (VLFs), with frequencies less than 0.004 Hz, in the rip current velocity signals observed during the Rip-current field Experiment (RIPEX) field experiment. Observations show that the VLF motions are most intense within the surfzone and then quickly taper off in the offshore direction. By comparing computed VLF intensity (URMS,vlf) distributions in both the cross-shore and alongshore direction with observations in a qualitative sense, the most important contributions to the VLF dynamics are established. VLF motions at neighboring rip-channels are seen to interact in the computations, with stronger surfzone intensity for increasing bathymetric variation. The intermittent forcing by spatially varying wave groups is essential in obtaining the correct URMS,vlf distribution in the cross-shore direction, suggesting this is the predominant mechanism responsible for the generation of the VLF motions observed during RIPEX. Computations also suggest that VLF motions can occasionally propagate offshore but are mostly confined to the surfzone corresponding to surfzone eddies. A quantitative comparison shows good correspondence between model computations and measurements of URMS,vlf with a model skill of O(0.7), with generally increased (decreased) URMS,vlf during mean low (high) water levels.

Surf zone diffusivity on a rip‐channeled beach

Absolute and relative diffusivity are measured on a rip-channeled beach using 30 position-tracking drifters released in clusters (4–12 drifters) deployed on 7 days with different wave forcing and tidal elevations at Sand City, Monterey Bay, California. Diffusivity and dispersion were found to be larger on days with rip current flow patterns and larger waves. Rip currents cause material to diffuse quickly for t 0.9]. Two independent methods are used to measure the small-scale turbulent diffusion contribution (kxy), which are found significantly correlated (R2 = 0.95) with each other and calculated surf zone wave breaking induced turbulent eddy viscosity. Here kxy is small relative to the total dispersion (Ke/kxy = 3–30), indicating that the shear flow is the primary mechanism responsible for dispersion in a rip current system.

Spatial and temporal variation in indicator microbe sampling is influential in beach management decisions.

Fecal indicator microbes, such as enterococci, are often used to assess potential health risks caused by pathogens at recreational beaches. Microbe levels often vary based on collection time and sampling location. The primary goal of this study was to assess how spatial and temporal variations in sample collection, which are driven by environmental parameters, impact enterococci measurements and beach management decisions. A secondary goal was to assess whether enterococci levels can be predictive of the presence of Staphylococcus aureus, a skin pathogen. Over a ten-day period, hydrometeorologic data, hydrodynamic data, bather densities, enterococci levels, and S. aureus levels including methicillin-resistant S. aureus (MRSA) were measured in both water and sand. Samples were collected hourly for both water and sediment at knee-depth, and every 6 h for water at waist-depth, supratidal sand, intertidal sand, and waterline sand. Results showed that solar radiation, tides, and rainfall events were major environmental factors that impacted enterococci levels. S. aureus levels were associated with bathing load, but did not correlate with enterococci levels or any other measured parameters. The results imply that frequencies of advisories depend heavily upon sample collection policies due to spatial and temporal variation of enterococci levels in response to environmental parameters. Thus, sampling at different times of the day and at different depths can significantly impact beach management decisions. Additionally, the lack of correlation between S. aureus and enterococci suggests that use of fecal indicators may not accurately assess risk for some pathogens.

Rip?current pulses tied to Lagrangian coherent structures

The trapping and ejection of surfzone floating material is examined by unveiling Lagrangian Coherent Structures (LCSs) hidden in the pulsating rip?current surface velocity field produced by a three?dimensional numerical model resolving wave?group induced Very Low Frequency motions (VLFs). LCSs explain the typically observed patchiness of flotsam within the surf zone and the streaky distribution outside of the surf zone. The ejection of surfzone material occurs when filament?like LCSs separate form the main rip?current circulation corresponding to a situation where eddies temporarily extend the rip current beyond the surf zone and subsequently detach. The LCSs support the idea that VLFs form the dominant exchange mechanism of surfzone floating material with the inner shelf.

A Guide to Modeling Coastal Morphology

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