Daniel T. Cox

Identification of intense, intermittent coherent motions under shoaling and breaking waves

Laboratory measurements of the instantaneous horizontal and vertical turbulent velocities, u′ and w′, induced by regular waves spilling on a rough, impermeable slope were analyzed to elucidate the nature of turbulence generated in the bottom boundary layer and by wave breaking. The analyses focused on the instantaneous turbulent events such as the absolute shear stress |τ′| = |−u′w′| and turbulent kinetic energy k′ = (u′2 + w′2)/2. Below trough level inside the surf zone, the horizontal and vertical velocity records showed intense, intermittent turbulent events that did not occur with the passing of each wave, and the instantaneous quantities of |τ′| and k′ could not be explained in terms of the phase-averaged quantities. The intermittent turbulent events extended into the bottom boundary layer inside the surf zone, and in this region the infrequent but intense turbulence generated by wave breaking was an order of magnitude larger than the turbulence generated locally at the boundary. Two techniques were used to analyze the turbulent motions. A quadrant analysis technique showed that although the large turbulent motions did not occur with each wave, they were phase-dependent near trough level and less so near the bottom. A conditional sampling technique quantified the intensity and duration of the turbulent motions, indicating that coherent events (intense events) occurred for ∼10% (2%) of the record and accounted for approximately 50% (20%) of the turbulent motion. It is likely that the exactitude of these statistics will differ depending on breaker type. Nevertheless, these statistics indicated that large turbulent motions are infrequent but contribute significantly to the turbulence intensity and possibly the suspension of bottom sediments.

Bottom shear stress in the surf zone

To investigate the bottom shear stress in the surf zone, detailed laboratory measurements were made of the free surface elevations and velocities for the case of regular waves spilling on a rough, impermeable 1:35 slope. The velocity profiles were measured at several vertical lines in the cross-shore direction to include the shoaling region seaward of breaking, the break point, the transition region, and the inner surf zone. Each vertical line included measuring points at a fraction of the grain height above the rough, fixed bottom. A logarithmic layer was found to exist in the bottom boundary layer for most of the phases over a wave period seaward of the break point and in the surf zone. A regression analysis was used at each phase to estimate the shear velocity and bottom roughness from the phase-averaged horizontal velocities in the lower portion of the bottom boundary layer. The bottom friction factor was estimated from a quadratic friction equation based on the measured horizontal velocity above the bottom boundary layer together with the estimated shear velocity. The quadratic friction equation with the fitted friction factor was shown to predict the temporal variation of the bottom shear stress within a factor of 2. The bottom roughness estimated from the grain size assuming rough turbulent flow was shown to agree qualitatively with the measured values. The cross-shore variation of the friction factor estimated from an empirical formula developed for nonbreaking waves was shown to agree within a factor of 2 of the measured values.

Biophysical feedback mediates effects of invasive grasses on coastal dune shape

Vegetation at the aquatic-terrestrial interface can alter landscape features through its growth and interactions with sediment and fluids. Even similar species may impart different effects due to variation in their interactions and feedbacks with the environment. Consequently, replacement of one engineering species by another can cause significant change in the physical environment. Here we investigate the species-specific ecological mechanisms influencing the geomorphology of U.S. Pacific Northwest coastal dunes. Over the last century, this system changed from open, shifting sand dunes with sparse vegetation (including native beach grass, Elymus mollis), to densely vegetated continuous foredune ridges resulting from the introduction and subsequent invasions of two nonnative grass species (Ammophila arenaria and Ammophila breviligulata), each of which is associated with different dune shapes and sediment supply rates along the coast. Here we propose a biophysical feedback responsible for differences in dune shape, and we investigate two, non-mutually exclusive ecological mechanisms for these differences: (1) species differ in their ability to capture sand and (2) species differ in their growth habit in response to sand deposition. To investigate sand capture, we used a moveable bed wind tunnel experiment and found that increasing tiller density increased sand capture efficiency and that, under different experimental densities, the native grass had higher sand capture efficiency compared to the Ammophila congeners. However, the greater densities of nonnative grasses under field conditions suggest that they have greater potential to capture more sand overall. We used a mesocosm experiment to look at plant growth responses to sand deposition and found that, in response to increasing sand supply rates, A. arenaria produced higher-density vertical tillers (characteristic of higher sand capture efficiency), while A. breviligulata and E. mollis responded with lower-density lateral tiller growth (characteristic of lower sand capture efficiency). Combined, these experiments provide evidence for a species-specific effect on coastal dune shape. Understanding how dominant ecosystem engineers, especially nonnative ones, differ in their interactions with abiotic factors is necessary to better parameterize coastal vulnerability models and inform management practices related to both coastal protection ecosystem services and ecosystem restoration.

Large-scale laboratory observations of turbulence on a fixed barred beach

The details of a large-scale laboratory experiment to study the turbulence generated by waves breaking on a fixed barred beach are presented. The data set includes comprehensive measurements of free surface displacement and fluid velocity for one random and one regular wave case. Observations of the time-averaged turbulent kinetic energy per unit mass, , show that the turbulence generated by wave breaking was greatest at the bar crest and did not fully dissipate prior to reaching the bed. This indicates that, even in a time-averaged sense, wave breaking turbulence may be important for near-bed processes. Onshore of the bar, turbulence was generally confined to the upper part of the water column and had dissipated once the waves reformed (approximately 1.5 wavelengths onshore of the bar crest). The turbulent structure was the same in the random and regular wave cases; however, the magnitude of was much less in the random wave case, despite similar offshore wave conditions. Additionally, three methods were used to separate the wave-induced and turbulent components of velocity:. ensemble averaging, high-pass filtering and a differencing method proposed by Trowbridge (1998 J. Atmos. Ocean. Technol. 15 290–8). The magnitude of varied by as much as a factor of 5 among these methods, but qualitatively, the cross-shore and vertical structure were independent of the method used. The differencing method agreed closely with ensemble averaging in terms of the magnitude and structure of time-averaged quantities and in the signature of the time-dependent turbulent kinetic energy. Given this agreement, the differencing method appears to be the most suitable for application to random waves, such as those observed in the field.

VERTICAL VARIATIONS OF FLUID VELOCITIES AND SHEAR STRESS IN SURF ZONES

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.

Large‐scale laboratory observations of wave breaking turbulence over an evolving beach

[1] Wave breaking turbulence over an evolving beach was observed in a large-scale laboratory flume, as part of the CROss-Shore Sediment Transport EXperiment (CROSSTEX). The data set included comprehensive measurements of water surface elevation, fluid velocity, and morphology for irregular waves under erosive and accretive wave conditions. For the both conditions, the beach reached a quasi-equilibrium state, defined as when the bar shape was stable. Wave breaking characteristics, such as wave heights, average rate of energy dissipation by bores, and surf similarity parameter, were investigated in response to morphodynamics of the bar. Turbulent kinetic energy (TKE) was estimated using the method by Shaw and Trowbridge (2001). As the beach evolved, a less amount of TKE was observed at the trough for the erosive case, while more TKE was observed at the trough for the accretive case. It was also found that the temporal variation of the time-averaged TKE were closely associated with the average rate of energy dissipation by bores. Comparing with the bar trough, the vertical distribution of nondimensionalized time-averaged TKE and turbulence dissipation rate at the bar crest showed a large increase near the bottom, probably due to a strong cross-shore RMS velocity. Finally, in the quasi-equilibrium state, time-averaged TKE, and turbulence dissipation rate at the bar trough were smaller than those inside the surf zone. Inside the surf zone, significant turbulence intensities were observed due to a second breaking on a shallow water depth.

Bottom Stress in the Inner Surf and Swash Zone

A physical model study was conducted in a narrow wave flume to verify a previously developed numerical model for predicting the hydrodynamic response to irregular waves on a rough impermeable slope. In the physical model study, one case was run using a narrow banded TMA spectrum and a rough impermeable composite slope. The composite slope consisted of a 1:10 slope fronted by a 1:35 slope. The incident and reflected waves were resolved at 13 locations along the 1:35 slope using a three gage array. In addition, the free surface was measured along the 1:10 slope at 15 locations. A vertical stack of three capacitance sunup gages was used to measure the waterline oscillation along the 1:10 slope. Also, velocities were measured using a laser Doppler velocimeter (LDV) at 11 horizontal locations along the 1:10 slope at several elevations. A logarithmic profile was shown to exist for many phases of the onshore and offshore flows in the surf zone. Inside the swash zone, the logarithmic profile was more clearly established during the offshore flow. The measured and computed hydrodynamic responses were compared to evaluate the capability of the model in predicting the flow on a rough slope in the swash zone. The incident wave time series at two locations was specified at the seaward boundary of the numerical model. The measured and computed time series of both the reflected wave time series at the seaward boundary and the water line oscillation on the slope were plotted. The ability of the model to predict the detailed time varying quantities as well as the general wave characteristics of wave reflection, sunup, and flow velocity on the rough impermeable slope indicate that the model could be used to predict the swash zone hydrodynamics. The model showed a strong sensitivity to changes in the friction factor along the 1:10 slope. Improvements in the estimation of the bottom friction factor are needed to better predict the bed stress in the swash zone.

Laboratory observations of green water overtopping a fixed deck

A small-scale laboratory experiment was conducted to quantify a transient wave overtopping a horizontal, deck fixed above the free surface. Detailed free surface and velocity measurements were made for two cases with and without the deck structure to quantify the effect of the deck on the wave kinematics. The study showed that the structure increased the free surface above the leading edge of the deck by 20%. The velocity profile at the leading edge was fairly uniform, and the maximum horizontal velocity was similar to the maximum crest velocity measured without the deck. Immediately below the deck, the maximum velocity was 2.5 times greater than the corresponding velocity without the deck and 2.1 times greater than the maximum crest velocity without the deck. On the deck, the wave collapsed into a thin bore with velocities that exceeded 2.4 times the maximum crest velocity measured without the deck.

Going with the flow or against the grain? The promise of vegetation for protecting beaches, dunes, and barrier islands from erosion

Coastlines have traditionally been engineered to maintain structural stability and to protect property from storm-related damage, but their ability to endure will be challenged over the next century. The use of vegetation to reduce erosion on ocean-facing mainland and barrier island shorelines – including the sand dunes and beaches on these islands – could be part of a more flexible strategy. Although there is growing enthusiasm for using vegetation for this purpose, empirical data supporting this approach are lacking. Here, we identify the potential roles of vegetation in coastal protection, including the capture of sediment, ecological succession, and the building of islands, dunes, and beaches; the development of wave-resistant soils by increasing effective grain size and sedimentary cohesion; the ability of aboveground architecture to attenuate waves and impede through-flow; the capability of roots to bind sediments subjected to wave action; and the alteration of coastline resiliency by plant structur...

Experimental Setup for a Large-Scale Bridge Superstructure Model Subjected to Waves

To gain a better understanding of the wave forces that led to the failure of numerous causeway-type coastal highway bridges along the U.S. Gulf coast, a series of experiments was conducted on a realistic, 1:5 scale reinforced concrete bridge superstructure. The experimental setup is unique compared to other wave-in-deck studies in that the stiffness of the horizontal support system can be varied to represent different dynamic properties of the bridge system. The bridge specimen is subjected to a wide range of regular and random wave conditions at multiple water levels. In addition to measuring pressures and forces, the experiments measure the dynamic response of the bridge specimen using strain gauges, displacement sensors, and accelerometers. This paper presents the innovative experimental setup, and a preliminary analysis of the data showing the effect of wave height, wave period, and water level, on the forces experienced by the bridge superstructure.