Wave Staff Research Articles

Cross-shore wind-induced changes to field-scale overturning wave shape

The shape of depth-limited breaking-wave overturns is important for turbulence injection, bubble entrainment and sediment suspension. Overturning wave shape depends on a nonlinearity parameter$H/h$, where$H$is the wave height, and$h$is the water depth. Cross-shore wind direction (offshore/onshore) and magnitude affect laboratory shoaling wave shape and breakpoint location$X_{{bp}}$, but wind effects on overturning wave shape are largely unstudied. We perform field-scale experiments at the Surf Ranch wave basin with fixed bathymetry and$\approx 2.25$m shoaling solitons with small height variations propagating at$C=6.7\ \mathrm {m}\ \mathrm {s}^{-1}$. Observed non-dimensional cross-wave wind$U_w$was onshore and offshore, varying realistically ($-1.2 < U_{w}/C < 0.7$). Georectified images, a wave staff, and lidar are used to estimate$X_{{bp}}$,$H/h$, overturn area$A$and aspect ratio for 22 waves. The non-dimensionalized$X_{{bp}}$was inversely related to$U_{w}/C$. The non-dimensional overturn area and aspect ratio also were inversely related to$U_{w}/C$, with smaller and narrower overturns for increasing onshore wind. No overturning shape dependence on the weakly varying$H/h$was seen. The overturning shape variation was as large as prior laboratory experiments with strong$H/h$variations without wind. An idealized potential air flow simulation on steep shoaling soliton shape has strong surface pressure variations, potentially inducing overturning shape changes. Through wave-overturning impacts on turbulence and sediment suspension, coastal wind variations could be relevant for near-shore morphology.

Observations of Wave Height Amplification Behind an Oyster Castle Breakwater System in a High-Energy Environment: Gandys Beach, NJ

This study reports findings of an analysis of modifications to the incident wave field caused by constructed Oyster Castle® breakwater systems at Gandys Beach Preserve in Downe Township, NJ. The Nature Conservancy’s (TNC) Gandys Beach Preserve is a one-mile stretch of beach located along with intertidal mud flats, sandy beaches, tidal creeks, and salt marshes. Gandys Beach can be classified as a high-energy environment, with open water fetches exceeding 30 miles, and a tidal range on the order of 2 m. The Gandys Beach living shoreline project was designed and constructed by TNC in partnership with the United States Fish and Wildlife Service (USFWS) to investigate the effectiveness of various natural and nature-based features (NNBFs) in protecting and enhancing salt marshes and beaches in high (er) energy environments. Many of the NNBF techniques used at Gandys Beach, such as Oyster Castle® block breakwaters (Oyster Castles), had only been implemented at smaller scales in New Jersey prior to the project. Stevens was contracted by USFWS/TNC to evaluate the impact of the breakwater systems on incident waves. Four breakwater sections along the beach were selected to monitor wave attenuation. Month-long deployments of wave staffs and pressure sensors occurred in the summer and winter of 2019. Analysis of the data indicates that when crests of the Oyster Castles are exposed the breakwater system effectively attenuates waves. However, when the structures are submerged, wave height attenuation decreases, and under certain conditions wave heights behind Oyster Castles can be amplified more than 80%. These results are troubling, especially in areas experiencing sea level rise where the frequency of submergence will likely increase in the future. Due to the complex nature of Gandys Beach, exact mechanisms causing this amplification remain uncertain. Furthermore, transmission coefficients (Kt) above 1 are not typically modeled in existing empirical equations. Seabrook and Hall (Coast. Eng. Proc., 1998, 1 (26), 2000) is the only studied empirical formula that indicated an amplification of the Hs as observed during these field deployments and therefore was used to model Kt. However, poor agreement between the modeled and observed Kt was found and a better predictive tool is needed.

Experimental Investigation of a Moored, Circular Pipe Breakwater

Floating breakwaters are commonly used for shoreline protection in coastal areas and may offer a relatively cost-effective method for embankment protection in irrigation reservoirs. This study explored the potential use of a moored cylindrical floating breakwater design constructed from corrugated irrigation pipe and included a preliminary investigation of the performance of a floating breakwater that was subjected to regular waves of varying height and period in a laboratory wave tank. Experiments were carried out at the USDA Agricultural Research Service (ARS), National Sedimentation Laboratory in Oxford, Mississippi. The model breakwater was made of a 17.8-cm outer diameter, high-density polyethylene (HDPE) corrugated pipe section, filled with water and restrained under two mooring configurations using steel mooring lines attached to the floor of the flume, either vertically or at an angle on each side of the breakwater. The draft of the breakwater was varied between approximately 87% and 143% of the outer diameter by adjusting the tension of the mooring lines. Additional controlled experiments were performed using the same pipe section under fixed conditions. Waves were measured using capacitance-type wave staffs located both upwave and downwave of the breakwater, and mooring forces were measured using force gauges. Experimental results indicated that the floating breakwater arrangements studied show potential for usefulness in field application, as wave heights were reduced by as much as 60% in some cases. Cable-moored models performed best when fully submerged relatively close to the still water surface. Mooring line slackness reduced the effectiveness of the model in wave attenuation but also reduced the amount of force incurred by the cable mooring system. The cylindrical model best attenuated shorter waves of low to moderate steepness.

Directional Surface Wave Spectra from Point Measurements of Height and Slope

AbstractWe describe here a method for recovering directional ocean surface wave spectra obtained from height and slope measurements made over a small area, the iterative deconvolution method (IDM). We show that IDM is a more reliable method for estimating directional wave spectra than more common spectral estimation techniques by comparing it with the widely used maximum entropy method (MEM). IDM is based on the observation that pitch–roll buoys produce directional spectra that are the true spectra convolved with an angular windowing function and are therefore much broader than the true spectra. We test IDM against simulated data and find that it does a better job of retrieving the known input spectra than does MEM, which often produces false double peaks or incorrect angular widths. We compare IDM recoveries to spectra obtained using a nonstandard processing technique, the wavelet directional method (WDM) on data from a compact array of wave staffs on Lake Ontario. We find that IDM produces directional wave spectra very nearly identical to those obtained using WDM, verifying both techniques. Finally, we processed standard NDBC buoy directional spectra and showed that IDM recovers ocean wave spectra that narrow in the Strait of Juan de Fuca and that follow a changing wind in the expected manner. Neither of these phenomena are reliably obtained using MEM due to its tendency to produce false bimodal peaks and peaks that are too narrow.

A New Inverse Phase Speed Spectrum of Nonlinear Gravity Wind Waves

AbstractThe rear face of the wave spectrum is described by an equilibrium and a saturation subrange. Although accurate information about these ranges are highly relevant for wave modeling and many practical applications, there have been inconsistencies between results originating from temporal and spatial measurements. These discrepancies have been explained by the Doppler shift and the harmonics of nonlinear waves. We present high‐frequency wave measurements from the Baltic Sea gathered with R/V Aranda using a wave staff array, which provided directional frequency‐wavenumber data. In addition to the traditional wavenumber and frequency spectra, F(k) and S(ω), we also define a new spectrum that is a function of the inverse phase speed. We denote this spectrum Q(ν), where ν=kω−1. The properties of this Q‐spectrum were studied using data from four different sites. A strongly forced fetch‐limited case showed an equilibrium‐to‐saturation transition in the Q‐spectrum, with less variations in the equilibrium constants compared to the frequency spectra. The transition to a saturation regime happened around Uν=3 in all spectra where an equilibrium range was identified. Most duration‐limited spectra had no equilibrium range in the inverse phase speed domain. The absence of an equilibrium range was consistent with the wavenumber domain, but the frequency spectra still showed an apparent equilibrium subrange extending to ωU/g=5. The consistency of the saturation ranges between the Q‐spectrum and the wavenumber spectrum indicate a weak Doppler shift effect. We deduced that the main factor distorting the frequency spectra was wave nonlinearities.

HIGHLY RESOLVED DIRECTIONAL PROPERTIES OF WIND WAVES AND SWELL WITH VARIOUS SCALES

Wave directional spectrum is basic information as the forcing on the coastal process. The directional properties of coastal waves have been widely investigated through observations and the results are applied for design use of coastal structures and implementation of numerical wave model. But these studies focused only on wind waves under moderate sea conditions because the limitations of instruments such as buoys and wave staffs make them unsuitable for observing under various sea conditions including the severe sea. Thus there are few systematic studies for coastal engineering application which needs treatment for various sea state, from modest to severe sea. In this study, we observed the wave directional spectrum with the bottom mounted ADCP (Acoustic Doppler Current Profiler) which can record even under severe conditions. We investigated the directional properties of wind waves and swell with various scales.

Evaluation of the Flow Distortion around the Campbell Scientific CSAT3 Sonic Anemometer Relative to Incident Wind Direction

Abstract In June 2002, a high-frequency air–sea momentum system was deployed in the surf zone for 3 days as part of an experiment to quantify air–sea momentum transfer when the wind and wave direction were at angles. The system obtained measurements in the nearshore via a high-resolution Campbell Scientific CSAT3 3D sonic anemometer and five high-frequency saltwater wave staffs. An advantage of the air–sea momentum system is that direct measurements of the atmospheric turbulent fluctuations can be obtained and applied to the calculation of momentum transfer at the air–sea interface. The Campbell Scientific CSAT3 sonic anemometer was postcalibrated under turbulent wind conditions to determine incident wind direction measurements influenced by the geometry of the instrument. Measurement results are compared to a pre-established benchmark, constant tow speed; and the mean wind speed, incident wind direction, and spectral density characteristics are evaluated to resolve specific instrument orientations in which the measurements are corrupted by the head and probe supports of the sonic anemometer. Calibration testing of the sonic anemometer determined that the mean wind speeds are reduced by 16% over a 40° range for incident wind angles of 160°–200° relative to the head of the anemometer. Tilting the anemometer is found to decrease mean wind speed reduction influenced by the geometry of the anemometer. Variations in the measured wind directions were found to be greater than 1° for incident wind angles between 160° and 200° for 0° and 10° of tilt. Spectral characteristics were highly repeatable for all wind angles except for incident wind angles of 180° for 0° and 10° of tilt.

Shallow Angle Wave Profiling Lidar

A lidar scanning system is described that is primarily designed to measure sea wave shape. The device is capable of measuring real-time spatial profiles over distances of hundreds of meters, and as the lidar must inevitably operate from modest elevations (e.g., from a vessel’s masthead), it is inherently a very shallow angle metrology device. This results in a highly nonuniform distribution of the wave elevation values. The vertical and horizontal resolution is primarily set by the characteristics of the optical system employed and range/data capacity is set by signal-to-noise ratio considerations. Illustrative data are presented as consecutive profiles taken 0.2 s apart for highly trochoidal waves under conditions where the height was recorded to ±0.03 m and horizontal sample separation to ±0.025 m. A comparison is presented with traditional wave staff measurements.

Statistics of Wave Crests From Models vs. Measurements

The analysis phase of the Wave Crest Sensor Intercomparison Study (WACSIS) focussed on the interpretation of the wave data collected by the project during the winter of 1997–98. Many aspects of wave statistics have been studied, but the main emphasis has been on crest height distributions, and recommendations for crest heights to be used in air gap calculations. In this paper, we first describe comparisons of the crest height distributions derived from the sensors (radars, wave staffs, laser) and from simulations based on 3-D second-order irregular wave models. These comparisons permit us to make conclusions on the quality of these models and to qualify the ability of some sensors to measure the crest heights accurately. In the second part, two new parametric models of the crest height distributions are discussed and their superiority to standard parametric models is demonstrated.

Wave Crest Sensor Intercomparison Study: An Overview of WACSIS

The Wave Crest Sensor Intercomparison Study (WACSIS) was designed as a thorough investigation of the statistical distribution of crest heights. Measurements were made in the southern North Sea during the winter of 1997–1998 from the Meetpost Noordwijk in 18 m water depth. The platform was outfitted with several popular wave sensors, including Saab and Marex radars, an EMI laser, a Baylor wave staff and a Vlissingen step gauge. Buoys were moored nearby to obtain directional spectra. Two video cameras viewed the ocean under the wave sensors and their signals were recorded digitally. The data analysis focused on comparisons of the crest height measurements from the various sensors and comparisons of the crest height distributions derived from the sensors and from theories. Some of the sensors had greater than expected energy at high frequencies. Once the measurements were filtered at 0.64 Hz, the Baylor, EMI and Vlissingen crest height distributions matched quite closely, while those from the other sensors were a few percent higher. The Baylor and EMI crest distributions agreed very well with the statistics from second order simulations, while previous parameterizations of the crest height distribution were generally too high. We conclude that crest height distributions derived from second order simulations can be used with confidence in engineering calculations. The data were also used in investigations of crest and trough shapes and the joint height/period distribution.

Intercomparison of Sea-State and Zero-Crossing Parameters From the WACSIS Field Experiment and Interpretation Using Video Evidence

During the WACSIS field experiment, wave elevation time series data were collected over the period December 1997 to May 1998 on and near the Meetpost Nordwijk platform off the coast of the Netherlands from an EMI laser, a Saab radar, a Baylor Wave Staff, a Vlissingen step gauge, a Marex radar and a Directional Waverider. This paper reports and interprets, with the help of simultaneous dual video recordings of the ocean surface, an intercomparison of both single wave and sea state wave parameters.

The noise spectral parameters and the energy of breaking waves experimental study

The process of wave breaking and whitecap creation is one of the most important and least understood phenomena associated with the evolution of the surface gravity waves in the open sea. This process is the main way of energy and momentum transfer between ocean and atmosphere. However, it is very difficult to estimate, under real sea conditions, the frequency of breaking wave events or the fraction of sea surface covered by whitecaps and the amount of dissipated energy produced by wave breaking. A controlled experiment was carried out in the Ocean Basin Laboratory at MARINTEK, Trondheim (Norway). The simulation of random waves of the prescribed spectra provided a very realistic pattern of the sea surface. The number of breaking waves was estimated using photography method and wave staff recording. Acoustic measurements during the experiments were conducted in order to examine the relationship between the noise spectral parameters and both the whitecap coverage and dissipation energy of breaking waves for different types of waves. A comparison of simultaneous video observations, wave staff records of the surface wave above the hydrophones, with the spectral parameters of acoustical signals made it possible to find physical links between processes.

Low-energy Morphodynamics of a Ridge and Runnel System

A field experiment was carried out over a period of five weeks on an intertidal ridge and runnel system on the Northumberland Strait coast of Nova Scotia, Canada. The area is microtidal with a spring tidal range of just under 2 m. The main purpose of the research was to examine the effects of changing water depth and bar emergence on the morphodynamics of the system and to determine the controls on the stability of the ridges during non-storm conditions. The system is developed on a gently sloping platform over 300 m wide at spring low tide with an average gradient of 0.004. It is characterised by the presence of 5 or 6 ridges in the intertidal zone and 2 bars in the subaqueous zone. The ridges are 0.35–0.50 m in height, 50–60 m in wave length, and the continuity of the ridges alongshore is broken by drainage channels. The system at Linden Beach is similar to others that have developed in a number of areas along this coast on platforms resulting from recession of relatively weak sandstone cliffs. Topographic surveys were carried out along 10 profiles spaced 25 m apart using a total station and the position of the ridge crests and troughs was also mapped using a GPS system. Measurements of wave transformation, water motion and suspended sediment concentration over individual tidal cycles were carried out along a profile across the second ridge and associated troughs using electromagnetic current meters, resistance wave staffs and OBS nephelometers. There were no major storms during the monitoring period but there were a number of days with significant wave heights >0.4 m and the measurements spanned the full range from neap to spring tides. During the five week period the ridge crests exhibited a high degree of stability with maximum movement <5 m. The effects of tidal currents were isolated through measurements made during calm conditions with light winds. Measurements indicate that wave shoaling and breaking across the ridge crest at mid to high tide have the potential to transport large quantities of sediment landward and thus to induce landward migration under non-storm conditions. The stability of the bars appears to be controlled by a combination of offshore flows across the bar crests due to undertow and tidal currents near high tide, and through the transport of sediment alongshore in the troughs and offshore in the drainage channels on the ebb tide. The dynamics of the system more closely resembles that of sub-tidal multiple parallel bars than that of intertidal swash bars.

Wave Crest Distributions: Observations and Second-Order Theory

Many empirical and heuristic distribution functions for wave crest heights have been proposed, but their predictions differ considerably. Part of the lack of agreement is due to the difficulty of making measurements that accurately record the true height of the wave crests. Surface following buoys effectively cancel out the second-order nonlinearity by making a Lagrangian measurement. Pressure transducers filter the nonlinear components of the signal in complicated ways. Wave staffs have varying degrees of sensitivity to spray. The location of the instruments also plays an important role. There is clear evidence from measurements in the North Sea that spurious crests due to spray are a problem downwind even from mounting supports that appear transparent. Much of the theoretical nonlinearity can be captured by calculations correct to second order. Explicit calculation of the interactions of each pair of components in a directional spectrum is straightforward although computationally intensive. This technique has the advantage that the effects of wave steepness, water depth, and directional spreading are included with no approximation other than the truncation of the expansion at second order. Comparisons with measurements that are believed to be of the best quality show good agreement with these second-order calculations. Simulations for a set of JONSWAP spectra then lead to parametric crest distributions, which can be used easily in applications.

On the determination of directional wave spectra for practical applications

The performance of various directional instruments for practical oceanographic and coastal engineering applications is examined. The emphasis is put on the application of conventional current meters equipped with high resolution pressure sensors and three element arrays. Two simulation techniques have been used to produce input data with known frequency spectrum and known directional spreading. The directional spreading is determined by the maximum likelihood method and the resulting spreading is compared with the input spreading. The performance of a conventional current meter equipped with a high resolution pressure sensor depends on the width of the directional spreading of surface waves and on the frequencies under consideration. Even for very narrow directional spreading, the current meter response is acceptable for practical applications and for shallow water deployment. In general, the current meter directional response does not depend on the direction of the incident waves. The spatial array of three wave staffs deployed in shallow water shows a similar performance to that of the current meters when the dimension of the spatial array is of the order of 1 m. This performance also does not depend on the direction of the incident waves.

Variability of directional spectra of wind-generated waves, studied by means of wave staff arrays

Parameterization of the directional distribution of the wave energy obtained by means of arrays of wave probes is suggested. The parameterization encompasses a wide frequency band both above and below the peak frequency as well as a broad range of wave ages. The parameterization is developed for the integral width of the directional spectrum, which does not depend on a specific form of the approximating function. The width of the directional distribution depends on the stage of wave development and relative frequency. The narrowest directional spectra are observable, on average, 5% below the peak frequency. Above that frequency, the width of the directional spectrum increases as the frequency rises, and the rate of widening does not depend on the wave age. On the forward face of the spectrum, the rate of widening of the directional distributions towards low frequencies depends strongly on the stage of wave development and, at early stages, the directional spectra in this range are unchangeable. In general, the directional distributions of wave energy become narrower as the waves develop.

Error‐reducing strategies for estimates of high‐resolution directional spectra using closely spaced sensors

When an array of closely spaced sensors is applied, even if the number of sensors is very large, the error magnification phenomenon will render some cross spectra of wave records useless to the estimates of directional spectra because these cross spectra contain about the same information as others. Consequently, only few cospectra or quadrature (quad) spectra can provide truly useful information about the directional distribution when the ordinary spectral analysis procedures are applied. In this paper the sources of certain magnified errors are identified and several error‐reducing strategies are developed so that three more cospectra or quad spectra become useful to the estimates of directional spectra when a compact array with three closely spaced biaxial current meters plus a wave staff is utilized. While most of the strategies are directly relevant to reduction of the random errors, the influences of other data acquisition and processing errors are also fully taken into account. These error‐reducing strategies can improve the directional resolution of all estimators. Their effectiveness is illustrated through a series of model tests with the truncated Fourier series method (TFSM) and the Long and Hasselmann (1979) and Herbers and Guza (1990) minimum nastiness methods. In the TFSM analysis the additional three cospectra or quad spectra allow determination of the third and fourth angular harmonics of directional spectra, after which even the fifth and sixth harmonics can be determined approximately by a simple iterative scheme in most cases. For the modern Long and Hasselmann and Herbers and Guza techniques, the improvements by the present strategies are a bit marginal when the nonnegativity constraint of these techniques can very effectively optimize the estimates, but this may depend on the array geometry, and in some cases there is a clear improvement.

Directional Wave Spectra from a Swath Ship at Sea

Abstract During the Surface Wave Dynamics Experiment (SWADE), the swath ship Frederick G. Creed was equipped with an array of wave staffs for the estimation of wave directional spectra. This paper reports on the first such estimates taken from a ship at sea. An algorithm for removing the effects of the ship motion, including those resulting from the Doppler shifting of observed frequencies, is presented along with some results from the SWADE experiment. A comparison with directional wave spectra taken from a nearby buoy shows the fidelity of the method.

Wave Grouping in Locally Generated Seas on Short Fetches

Wave staff data from the U.S. Army Corps of Engineers Floating Breakwater Prototype Test Program in Puget Sound, Wash., are analyzed for wave grouping. Most previous studies have focussed on ocean conditions, where fetches, propagation distances, and wave periods are much longer than in Puget Sound. Probabilities for groups of various length closely match previous numerical simulations. Thus grouping in short‐fetch, locally generated seas at this site is well predicted by linear combination of spectral components. Sequences of waves within groups show no strong, physically justifiable trends. There is no evidence that very high waves tend to occur together in the same groups. Data are included on wave groups above the significant height and several other thresholds.

Flux measurements from a SWATH ship in SWADE

The Surface Wave Dynamics Experiment (SWADE) took place east of the U.S. Coast in the winter of 1990–1991. A major objective of the research program is to refine our understanding of the relationship between fluxes to the sea surface and the sea state as determined from directional wave spectra. Simultaneous measurements of turbulent fluxes of mass, momentum and energy between sea and air, with the directional wave spectra, were required to meet this objective. In this short article we describe the process of obtaining turbulent flux measurements from a small water-plane-area twin hull (SWATH) ship. We measured turbulent fluxes of momentum, heat and water vapor from a tall mast at the bow of the SWATH ship Frederick G. Creed by the eddy correlation method, while the ship was moving into the wind. Directional wave spectra were obtained from a wave staff array ahead of the bow of the ship. The motion of the ship was recorded and a coordinate rotation was performed for each data sample. After all instrument response and motion corrections have been accounted for, we compare our calculated turbulent fluxes with values computed from another standard method, viz. the inertial dissipation method. This approach is not susceptible to platform motion but relies on assumptions that are not always valid. However, the two methods agree on average within 12%, 20% and 31% for momentum, water vapor and heat fluxes, respectively.