: The BASS Rake is an acoustic travel time current meter designed to make spatially and temporally dense velocity profile measurements in the continental shelf wave bottom boundary layer. The thinness of the layer is responsible for high levels of bottom shear stress which are important contributors to the sediment entrainment process and which enhance turbulent dissipation of flow energy. The BASS Rake is a modification of BASS, the Benthic Acoustic Stress Sensor, using a new geometry to image flow in the WBBL. A laboratory prototype has previously demonstrated the features and near bed capabilities of the new design. The field prototype described here was constructed with standard BASS components to evaluate the measurement technique and the performance of the support frame in the near shore zone. The field prototype measures the horizontal velocity vector at ten heights from the bottom up to 30 cm above the bottom. The results of tow tank calibration and cosine response measurements are presented. After calibration the field prototype was deployed in 3 m of water immediately outside the surf zone of a local beach. Profiles were recorded continuously at 1 Hz for approximately 3.5 weeks in December of 1996. This period includes both calm and storm conditions. Selected data from this deployment are presented.
Two Depth Inversion Algorithms (DIAs) were developed and validated using on results of computations for the shoaling of periodic waves over mild slopes, in a two-dimensional numerical wave tank, based on fully nonlinear potential flow theory. The first algorithm, DIA1, uses sets of values of wave celerity c, height H, and spatial wavelengths L-c and L-t, simultaneously measured at a number of locations x(i) (i = 1,..., N) in the direction of wave propagation (e.g., using remote sensing techniques), to predict the depth variation h(x(i)). The second algorithm, DIA2 uses spatial wave asymmetry s(2)/s(1) calculated from wave phase, instead of H. Results indicate that state-of-the-art depth inversion methods based on the linear dispersion relation may lead to large errors (50-70%) for the depth prediction in very shallow water, whereas the present methods are 3-10 times more accurate.
The paper describes comparison between DELILAH field data and model predictions using the quasi-3D SHORECIRC circulation model with forcing provided by the REF/DIF short wave model. The results are in much better agreement than earlier modelling efforts but still show principal errors. A discussion is provided on the possible sources of those inaccuracies.
The experimental studies of the breaking effects on wave statistics for deep-water random waves are presented. It is especially focused on the behavior of kurtosis of surface elevations due to wave breaking. Wave breaking suppresses the maximum limit of kurtosis of the surface elevation, although skewness depends on characteristic wave steepness. The mean instantaneous wave steepness of breaking waves defined using the zero-down-crossing method was much lower than expected from the Stokes waves.
The extreme wave climate of a Pacific Northwest littoral cell has been characterized by examining four long-term databases of measured wave parameters. Winter waves in the region average 3.0 m in significant wave height and 12 s in period, while summer waves average 1.5 m and 8 s, respectively. There is a distinct seasonality in monthly mean wave direction, with winter storm waves arriving from the southwest and the milder summer waves arriving from the northwest. Projections of the 50-year design wave height vary greatly, depending on the data source, ranging from 8.5 m to 12.2 m. A probabilistic model, combining these long-term measurements of waves with measurements of tides and local beach morphology, has been applied to a sub-region of the littoral cell. The model quantifies the susceptibility of coastal properties to erosion by predicting the frequency with which wave runup impacts either sea cliffs or sand dunes. The coastal erosion model is based on wave runup measurements obtained on high-energy dissipative beaches typical of the Pacific Northwest. Statistics of wave runup maxima are well correlated with simple relationships between commonly measured wave and beach parameters. Measurements of extreme tides have been shown to be greater than predicted due to a variety of factors, particularly the occurrence of El Nino events. The 1997/98 El Nino has produced some of the highest water levels on record, increasing the potential for erosion within the region.
The common problem of wave reflection in hydraulic laboratory wave studies is often solved by a spectral analysis technique presented by Gods and Suzuki in Coastal Engineering 1976. This method is capable of estimating the incident and reflected wave trains from a multi-wave-reflection system such as a wave tank By applying this method, it is possible to test reflective structures in continuous runs without worrying about the multi-reflection set up in the tank due to the structure, the wave paddle, and the tank itself The Goda and Suzuki method implements a Fourier analysis of wave data which requires a simultaneous record of wave data for two gages Recent wave height transmission tests of submerged breakwaters at Florida Tech required a wave gage array containing four capacitance type wave gages.Signal interference between wave gage probes of separate wave gages was found analyzed, and solved by redesigning the wave gage array circuitry. Each wave gage was found to function properly individually, however the electrical signals became erratic when more than one gage was placed in the same body of water. The signals of each gage were analyzed simultaneously with the use of a dual-trace oscilloscope and a digital voltmeter. The voltage output signal was observed to slowly incase to a certain point, then rapidly decrease to a certain point, and continue the cycle when multiple gages were placed in stillwater. A minor solution of grounding the wave tank water helped to stabilize the signal The cyclical voltage output change of multiple gages was salved by synchronizing the reference oscillators on each gage's circuit board. The wave gage circuitry was therefore redesigned so that each gage collecting date simultaneously was connected to only one reference oscillator.
SCAWVEX is an EU MAST project providing data sets measuring the spatial and temporal variability of ocean waves and currents. Wave measurements obtained during three experiments are discussed in this paper. These include measurements using bottom-mounted pressure sensors, HF and satellite radar and directional wavebuoys. The project has demonstrated the suitability of HF radar for simultaneous leave and current monitoring in coastal waters. The availability of satellite data in coastal waters is limited by the requirements of global coverage. The experiments have revealed interesting insights into wave-current interaction processes.
Transformation characteristics of double peak spectral waves are investigated by doing laboratory experiments and numerical calculations using a hybrid theoretical model which consists of a modified spectral KdV equation model and a probabilistic bore breaking model. Since energy densities around higher frequency peak decrease faster than those around lower frequency peak in shallow water zone, waves become single peak wave train; which causes that the occurrence probability of double peak spectral waves is higher in offshore region than that in coastal area. The hybrid model can be used to predict the changes of energy spectra, significant wave heights, and significant wave periods.
A new directional wave deformation model due to random wave breaking is developed. The model combines Kweon-Goda's breaking model (1996) for uni-directional wave and Karlsson's energy balance equation (1969) with its numerical scheme for directional waves. The spreading in a 3-D system is dealt with the modification of the wave decay factor after breaking which was introduced in Kweon-Goda's model. The modification process is calibrated by comparison with two existing models for uni-directional wave breaking. Verifications were made with two sets of experimental data involving 3-D wave breaking phenomena which were investigated by two laboratories. The model is found to be capable of predicting reasonably well the variations of significant wave heights over shoals measured at two laboratories.
Two irregular wave tests were conducted on quasi-equilibrium terraced and barred beaches consisting of fine sand to investigate the cross-shore variations of the probability distributions and statistics of the free surface elevations and horizontal velocities in the shoaling, surf and swash zones. The exponential gamma distribution with the measured mean, standard deviation and skewness is shown to be capable of describing the measured probability distributions in a unified manner. The probability distribution of the free surface elevation in the swash zone becomes approximately exponential with the skewness s similar or equal to 2 and the root-mean-square wave height H-rms similar or equal to root 8f (h) over bar with = mean water depth in the region of (h) over bar greater than or similar to 1.0 cm. The local use of linear long-wave theory is shown to be effective in relating to the mean and standard deviation of the horizontal velocity with those of the free surface elevation. The measured horizontal velocity statistics are found to be fairly constant over depth in the surf zone.
Numerical wave models are used to estimate wave transformation over irregular bathymetry for a variety of coastal engineering problems. In using these tools, coastal engineers require knowledge of the performance, limits of application, and expected errors of such models. A 1:100 scale physical model of Ponce De Leon inlet, Florida was utilized to obtain a wave transformation data set for model evaluation using both monochromatic and spectral wave conditions covering a range of wave periods and directions. Wave height data were collected along two linear arrays consisting of twelve gauges each, spaced at a prototype interval of 100 meters. The numerical wave models RCPWAVE, REF/DIF 1, and STWAVE are commonly used by the US Army Corps of Engineers for coastal design problems and were selected for this evaluation on that basis. Each model is evaluated against the measured data and characteristics and trends of each model are discussed. STWAVE estimates of wave height are found to most closely represent the measured data, and RCPWAVE and REF/DIF are found to overestimate wave heights, especially at the nearshore array.
Sixteen years of hourly wave data measured from four National Data Buoy Center (NDBC) buoy stations in the northeastern Pacific Ocean were used to study design wave heights. It is shown that the data selection is crucial for determining the design wave height, and the annual maximum method is not suitable for measured data. The selected data fit the 3-parameter Weibull distribution better than the FT-1 distribution. Effects and stability of return wave heights computed from various years of data and various 10-year periods were examined. The results show that, if enough moderate extreme values are in the selected data set, the return wave heights are stable and reliable and the effect from a few very extreme values is insignificant.
Nearshore berms are mechanically placed as stable or active profile features. Active nearshore berms are designed to provide material to the beach profile. Within the active profile, increasing placement depth diminishes the percentage of placed material moving landward to enhance the profile (McLellan, 1990). The optimum construction depth for ensuring that the berm will be active varies with sediment type and incident wave climate. Nearshore berms designed and constructed as stable, quasi-permanent features can serve as long-term protection to beaches. A stable berm's crest elevation must be high enough to selectively filter erosive waves as they propagate landward. By allowing accretionary waves to pass unhindered while inducing breaking of erosive waves, such features can provide beach stabilization. Research indicates that the height of the nearshore berm crest (h(c)), above the seafloor should be about half the local water depth (d(s)) or less to remain stable. Increasing crest width has shown to improve wave breaking ability and reduce wave heights in the lee of nearshore berms. Stability and wave-filtering capabilities are ultimately a function of placement depth, crest elevation, crest width, grain size, placement method, and incident wave climate. The work presented in this paper focuses on wave transformation over stable nearshore berms.
Directional wave data measured from two PUV gages and two ocean buoys in the intermediate water about 10 miles offshore of St. Marys Entrance, Georgia, were intercompared. These PUV gages and ocean buoys were closely located with each other, with the longest distance between two individual gage units being about 20 km. Since the PUV gages and ocean buoys were located in the intermediate water, where shallow water effects were still insignificant, the measured PUV and buoy data are ideal for comparison of consistency and accuracy of the data. Intercomparison of wave data was made for basic wave parameters including significant wave height, dominant wave period, and the corresponding mean wave direction. These comparisons showed good agreement for wave height and wave period measurements between the PUV gages and ocean buoys. However, less agreement was seen for the dominant wave directions between the PUV gages and ocean buoys. The larger difference in the measured directions is attributed to the difference in the PUV and buoy measuring systems as well as the stochastic theory and spectral analysis technique applied to the data.
Wave data from altimeter measurements on the TOPEX/POSEIDON satellite are described. Data from 1994-5 in the Gulf of Mexico are paired in time and spatial location with U.S. Army Corps of Engineers wave hindcasts. Satellite data and hindcast significant wave height estimates are compared. Satellite data is a promising source of information for improving hindcasts, especially in constricted locations where hindcasting is more difficult.
As part of a monitoring study of project improvements at Barnegat Inlet, New Jersey, a directional wave gauge was placed oceanward and a pressure gauge was placed shoreward of the ebb shoal. The gauges were operated simultaneously for a 6-month period. In addition, an Acoustic Doppler Current Profiler was installed in the inlet for one month to measure tidal currents. From these data and numerical model simulations, the effects of the ebb shoal and tidal currents on wave transformation at an inlet entrance were investigated. For larger waves (>1.5 m), bathymetry and water level have the strongest influence on wave transformation. The influence of tidal currents is most prevalent for smaller (< 1 m) waves. Smaller waves increase in wave height 10% more for ebb currents than flood currents. Frequency analysis shows energy dissipation over the ebb shoal is usually greatest at the peak frequency with some energy gains at the higher harmonics.
A physical model study sponsored by the US Army Corps of Engineers investigated effects on runup on coastal structures of the strong onshore winds usually associated with design storm conditions. Strong winds were shown to increase runup, particularly on steeply-sloped structures. However, it was found that the wind in the flume study changed the spectral shape of the incident wave train and may affect wave groupiness. This paper examines spectral shape in the frequency domain in terms of spectral width and peakedness parameters, and examines wave grouping in the time domain in terms of run length and number of runs with a run defined as number of consecutive wave heights greater than H-s. NO significant effects on runup are found over the range of spectral width parameter peakedness parameter, run length, or number of runs encountered in the test series.
The Harvest Project consists of the Harvest Experiment (HarvEx), a four-month, co-located deployment of four different types of directional wave gages in a high energy environment, and the Harvest Intercomparison (HarvIn), a detailed statistical analysis that quantifies the uncertainty in analyzed wave data products. This paper describes HarvEx - the experimental design, the gages, the sampling and analysis plan, and the data products. A world-wide-web version of this report provides, in addition, all of the measured data in a retrievable medium. Measured time series, analyzed spectra, and reduced parameters can be viewed in tabular or graphical form, or downloaded for use by the engineering and research community.
In the framework of the NEPTUNE project studies have been carried out to develop a methodology to derive the combined extremes of wave conditions and water levels for the purposes of coastal engineering design. An important part of this methodology is the transformation of offshore wave conditions to nearshore wave conditions using numerical modelling techniques. Therefore a large number of wave model runs has been performed for the Friesche Zeegat, a tidal inlet between two Wadden islands in the north of the Netherlands. The results of these computations have been parameterized and integrated into an efficient method to transform the offshore wave conditions to nearshore. This method has been verified for a number of conditions and it has proved to be an efficient tool for the transformation of joint extremes to the coastal zone.
The paper deals with the development of an Online Absorption Control System for laboratory wave generation in a two-dimensional wave channel near to prototype scale. Specially for long-term tests the reflections of the test structure re-reflected by the wave paddle has to be absorbed. This must be done online superimposed with the wave generation process to get well defined wave conditions over the total test run.The accurate consideration of all phase shifts and the combination of two independent control loops show best results in efficiency and stability of the absorption control system. Giving the wave train signal directly to the wave paddle allows variable amplifications to the absorption process without affecting the current wave generation. The system works with all kinds of regular and irregular wave trains and will not be influenced by the tested coastal structures. Long time tests with a constant energy level are practicable.
