Capacitance Wave Gauges Research Articles

Experimental Investigation of Infragravity Wave Propagation on a Porous Reef

A laboratory experiment is carried out to study the generation mechanism and the evolution process of an infragravity wave (IG wave) on a typical permeable fringing reef. Fifteen capacitance wave gauges are installed along the reef model to record the water surface elevation under irregular wave action. Based on the experimental data, both IG wave generation and strong IG wave interaction are systematically analyzed. The experimental results indicate that the IG waves are generated by the time-varying breakpoint mechanism at the reef edge, and the tidal modulations on the IG wave height are mainly influenced by wave breaking and wave dissipation. Around the reef edge, the tidal modulation is controlled by wave breaking and the IG wave height monotonously decreases with the increase of the tide level. At the shoreline, the tidal modulation is controlled by wave dissipation at the low tide level and wave breaking at the high tide level. Therefore, the IG wave height first increases and then decreases with the increase of the tide level, and the maximum IG wave height occurs at the middle tide level.

Void fraction estimation using a simple combined wave gauge system under breaking waves

In this study, a unique experimental technique that is capable of quantifying void fraction under two-phase air-water flows based on simple combined resistance and capacitance wave gauge system is described and the subsequent applications are discussed. The technique is validated under various surf-zone breaking waves of different wave periods and wave heights in the 2-dimensional wave flume. The difference in the measured surface elevations by capacitance and resistance wave gauges is positively linked to the depth of aeration and depth-averaged void fraction, that is validated by the corresponding images. The measured depth-averaged void fraction at the passage of the air cavity formed by the overturning jet after breaking showed a distinct pattern in regular and cnoidal waves which is attributed to different amount of droplets generated during the jet-curling process. Based on wavelet analysis, the peak frequency-wave continuously spreads its energy to its super-harmonic components due to shoaling while instantaneous spreads of wave energy to the higher harmonics are superposed due to breaking and subsequent jet-overturning. Considering void fraction, a sharper decay of wave energy within the intensely aerated region immediately after breaking is observed and the dissipated energy is reduced as the wave steepness increases following an exponential decay.

EXPERIMENTAL STUDY ON GEOTEXTILE TUBE APPLICATIONS AS SUBMERGED BREAKWATERS FOR BEACH PROTECTION IN YUCATAN, MEXICO

The Yucatan coast, with a wide continental shelf and low sediment sources, is subject to a microtidal regime and low energy, short waves (Hs < 1 m, T=4-6s) with two main incident wave directions (from the E and NE) that generate a dominant westward longshore sediment transport. Approximately 60 of the 245 km extent of the Yucatan coastline have seen significant urban, industrial and touristic developments that have led to coastal erosion of 1 m/year over the last 10 years. In order to mitigate this trend, many structures (most of them emerged groins) have been constructed, but the majority of these efforts have been unsuccessful. From 2005 to 2007 geotextile tubes were empirically used as submerged breakwaters (GT-SBW) at several sites on the Yucatan coast, many of them placed between 10 and 30 m seaward from the shoreline. It was found that, in some cases, the beach was stabilized after placing the structure; but in other places the results were far from those expected. (Fig. 1). Considering the lack of knowledge in the design criteria and in the hydrodynamic performance of GT-SBW, this study analyses the results obtained through experimental tests using different model scales and geometries. The motivation for this study is to better understand the wave-structure interaction in terms of the effectiveness of the structure in minimizing the wave transmission coefficient (Kt). The main goal of this paper is to define optimal design criteria for the GT-SBW. The experimental tests were conducted in a wave flume (37x0.8x1.20 m) at the Coastal Engineering Laboratory of the Engineering Institute at UNAM, using two scales (1:5 and 1:10). The wave generation system is of a piston-type vertical paddle from HR Wallingford. Throughout statistical analysis of hindcast data, the mean wave regime was obtained in order to use representative data of the Yucatan coast. A total of 12 capacitance wave gauges were placed along the flume (before and after the structure) to record water surface elevations and to compute the transmission (Kt) and reflection (Kr) coefficients; Mansard and Funke’s (1980) separation method was used. During the experimental tests it was observed that the hydrodynamic behaviour of these structures is quite similar to that of low-crested breakwaters with very low permeability and that when the elevation of the breakwater crown is slightly below the mean water level, the waves break violently, thus dissipating wave energy. On the contrary, if the waves fail to break they are only steepened and the transmission coefficient can be even greater than one. From the data analysis it was found that the optimal freeboard should not exceed twice the design wave height. The experimental results and an empirical formula are shown in (Fig. 2).

Wave attenuation experiments over living shorelines over time: a wave tank study to assess recreational boating pressures

With sea level rise, erosion, and human disturbances affecting coastal areas, strategies to protect and stabilize existing shorelines are needed. One popular solution to stabilize while conserving intertidal habitat is the use of “living shoreline” techniques which are designed to mimic natural shoreline communities by using native plants and animals. However, little information is available on the success of living shoreline stabilization. This project evaluated the wave energy attenuation associated with living shorelines that contained Crassostrea virginica (eastern oyster) and/or Spartina alterniflora (smooth cordgrass) in a wave tank. Four living shoreline techniques were assessed, including a control (sediment only), oysters alone, cordgrass alone, and a combination of oysters plus cordgrass. Time since deployment (newly deployed, one-year after deployment) was also assessed to see how wave energy attenuation changed with natural oyster recruitment and plant growth. Wave energy was calculated for each newly deployed and one-year old shoreline stabilization treatment using capacitance wave gauges and generated waves that were representative of boat wakes in Mosquito Lagoon, a shallow-water estuary in Florida. All one-year old treatments attenuated significantly more energy than newly-deployed treatments. The combination of one-year old S. alterniflora plus live C. virginica was the most effective as this treatment reduced 67 % of the wave energy created by a single recreational boat wake, compared to bare sediment. Natural resource managers and landowners facing shoreline erosion issues can use this information to create effective stabilization protocols that preserve shorelines while conserving native intertidal habitats.

Experimental study of wave dynamics in coastal wetlands

This paper presents laboratory experiments of wave-driven hydrodynamics in a three-dimensional laboratory model of constructed coastal wetlands. The simulated wetland plants were placed on the tops of conically-shaped mounds, such that the laboratory model was dynamically similar to marsh mounds constructed in Dalehite Cove in Galveston Bay, Texas. Three marsh mounds were placed in the three-dimensional wave basin of the Haynes Coastal Engineering Laboratory at Texas A&M University, with the center of the central wetland mound located in the center of the tank along a plane of symmetry in the alongshore direction. The experiments included two water depths, corresponding to emergent and submerged vegetation, and four wave conditions, typical of wind-driven waves and ocean swell. The wave conditions were designed so that the waves would break on the offshore slope of the wetland mounds, sending a strong swash current through the vegetated patches. Three different spacings between the wetland mounds were tested. To understand the effects of vegetation, all experiments were repeated with and without simulated plants. Measurements were made throughout the nearshore region surrounding the wetland mounds using a dense array of acoustic Doppler velocimeters and capacitance wave gauges. These data were analyzed to quantify the significant wave height, phase average wave field and phase lags, wave energy dissipation over the vegetated patches, mean surface water levels, and the near-bottom current field. The significant wave height and energy dissipation results demonstrated that the bathymetry is the dominant mechanism for wave attenuation for this design. The presence of plants primarily increases the rate of wave attenuation through the vegetation and causes a blockage effect on flow through the vegetation. The nearshore circulation is most evident in the water level and velocity data. In the narrowest mound spacing, flow is obstructed in the channel between mounds by the mound slope and forced over the wetlands. The close mound spacing also retains water in the nearshore, resulting in a large setup and lower flows through the channel. As the spacing increases, flow is less obstructed in the channel. This allows for more refraction of waves off the mounds and deflection of flow around the plant patches, yielding higher recirculating flow through the channel between mounds. An optimal balance of unobstructed flow in the channel, wave dissipation over the mounds, and modest setup in the nearshore results when the edge-to-edge plant spacing is equal to the mound base diameter.

Application of CCD cameras as a versatile measurement tool for flume tank

Recently, computer vision technique becomes a popular tool in flume experiment. The accuracy of computer vision method relies on the exactness of camera calibration, quality of the image and effectiveness of the image processing technique. Camera calibration is a delicate and difficult task to carry out in field work. However, the features on flume window are planar. This research adopted an idea called modified plane-based camera calibration to develop a method suitable for flume experiments. The calibration is simplified as map projection, and the associated time and efforts are reduced. Therefore, cameras can be installed at any location to meet the observation needs. The performance of the calibration is studied by comparing the measurements with the theoretical values of a known curve which mimics a beach slope. The robustness was examined by comparing the results acquired under two conditions: The camera was mounted on a secure fixture and on a mobile tripod. Overall measurement results are studied by comparing seabed profiles and surface waveforms obtained by our method and capacitance wave gauges. With inexpensive web cameras as the imaging devices, the RMS error was smaller than one millimeter when the target feature was measured 70cm away.

Impulse wave run-over: experimental benchmark study for numerical modelling

This research intends to provide a detailed data basis for numerical modelling of impulse waves. Three tests are described involving a rectangular wave channel, in which a trapezoidal ‘breakwater’ was inserted to study wave run-over. In addition, a reference test is also described, in which the breakwater was removed. Two-dimensional impulse waves were generated by means of subaerial granular slides accelerated by a pneumatic landslide generator into the water body. Wave propagation and run-over over the artificial breakwater are documented by a set of high-quality photographs. Water surface profiles were recorded using capacitance wave gages upstream and downstream of the breakwater, and velocity vector fields were determined for the run-over zone by means of Particle Image Velocimetry. The measurements are compared with predictive formulae for wave features and wave non-linearity. The present data set involves both simple channel topography and wave features to allow for numerical simulations under basic laboratory conditions.

Plane Impulse Waves in Reservoirs

The characteristics of impulse waves generated in reservoirs by the impact of variable density mass flows were assessed using two-dimensional model experimentation. Particle image velocimetry (PIV) was applied within the wave generation area at the slide impact location and the water wave profiles were measured by seven successive capacitance wave gages. The maximum relative wave amplitude and the normalized wave amplitude of the propagating wave train were correlated with the dimensionless slide quantities and the relative propagation distance, respectively. The impact Froude number was identified as the dominant parameter for slow impacting slides, whereas the water depth and the slide thickness governed the maximum possible wave amplitude for large impact Froude numbers. Four wave types were distinguished due to three different levels of wave nonlinearity associated with variable impact Froude numbers, relative slide densities, and characteristic slide geometries: nonlinear transient bore, transition wave, oscillatory wave, and nonbreaking solitary wave. Moreover, the density effect on the wave generation process was investigated with small impact Froude numbers using sequential PIV velocity vector fields.

Evaluation of Two Numerical Wave Models with Inlet Physical Model

This paper evaluates the performance of two numerical wave models, GHOST and STWAVE, with measurements made in an idealized inlet physical model. The emphasis of this paper is on the overall performance of these models in coastal inlets. Both wave models are similar in that they employ a finite-difference method to solve the wave action conservation equation for the steady-state wave spectral transformation. However, these models differ in the computation of diffraction, reflection, wave breaking, and representation of the directional spectrum transformation. The models’ performance is compared with a new set of physical model data for four different idealized inlet configurations. Wave height is measured in the physical model by a linear array of capacitance wave gauges, and wave direction is measured by a remote-sensing video-camera system. The comparison with data is presented as mean absolute relative errors of wave height and mean absolute difference of wave direction. Both wave models produced similar results, but neither could accurately describe waves observed in the physical model in inlets and near structures. The mean absolute relative error of wave height prediction from models was between 22 and 40% as compared with the measured data. The mean absolute error of wave direction estimates ranged from 5to12degrees. Overall, wave direction estimates from GHOST in inlets and near structures compared slightly better with measurements.

The flow in weak turbulent hydraulic jumps

Velocity and surface measurements in three turbulent hydraulic jumps are analysed in detail. The velocity measurements were obtained in a hydraulic ume using laser- Doppler velocimetry and the surface measurements were obtained using capacitance wave gauges. The purpose is to extract information about the flow field and the stresses in the jumps and the geometrical extension of the recirculating region called the roller. To achieve this we first ensure that the overall continuity and momentum conservation for the flow is satisfied. This includes comparison with the classical results for hydraulic jumps. It is found that deviations from uniform-depth Reynolds- averaged velocities and hydrostatic pressure give small but important corrections to the conservation equations. Careful evaluation of the velocity measurements in conjunction with the continuity equation makes it possible to determine the lower limit of the recirculating roller region. Measurements of the Reynolds stresses are analysed, in particular in the roller region, and a simple model established that illustrates the mechanisms of the flow in the roller and predicts well the stresses along the lower limits of the roller. In combination with the analysis of the vorticity distribution, this leads to a qualitative description of the processes for the generation of vorticity. It suggests both similarities and differences from the hypothesis of the existence of a shear layer inside the jump. Similarity profiles are developed for the variation of stresses, vorticity, and eddy viscosity along the lower limit of the roller. Finally, the total angular momentum for the jumps is examined and compared to the analysis of Hornung, Willert &amp; Turner (1995).

Laboratory experiments of tsunami runup on a circular island

Laboratory experiments of a 7.2-m-diameter conical island were conducted to study three-dimensional tsunami runup. The 62.5-cm tall island had 1 on 4 side slopes and was positioned in the center of a 30-m-wide by 25-m-long flat-bottom basin. Solitary waves with height-to-depth ratios ranging from 0.05 to 0.20 and “source” lengths ranging from 0.30 to 7.14 island diameters were tested in water depths of 32 and 42 cm. Twenty-seven capacitance wave gages were used to measure surface wave elevations at incident and four radial transects on the island slope. Maximum vertical runup measurements were made at 20 locations around the perimeter of the island using rod and transit. A new runup gage was located on the back or lee side of the island to record runup time series.