Measure Wave Attenuation Research Articles

Performance Assessment of Three Living Shorelines in Cedar Key, Florida, USA

A community-driven effort in Cedar Key, Florida, USA, resulted in the construction of three living shoreline retrofits intended to bolster failing coastal infrastructure and restore habitat functions in Daughtry Bayou. A multi-year monitoring program tracked changes in elevation and vegetation communities across the entire shoreline profile from lower-intertidal to upland/transitional zones and measured wave attenuation during typical and extreme (hurricane) conditions. Overall, these living shoreline retrofits served to soften more than 30% of the bayou’s shoreline, dramatically reducing the extent of armored shoreline in direct contact with tidal influence. The extent of vegetated habitat area has increased at all three sites, despite sediment export from higher elevation zones driven largely by repeated impacts from hurricanes and tropical storms. These living shorelines reduced wave energy by 33 to 79% in typical conditions and by up to 28% in hurricane conditions, consistently outperforming armored shorelines, even during an extreme event (Hurricane Idalia). Our monitoring efforts were sufficient to capture project trajectories and assess performance relative to project goals, but our program had limitations that could have been overcome with additional resources and increased focus on capturing spillover effects. The living shoreline retrofit projects assessed here have persisted through and shown signs of recovery after multiple tropical storms and hurricanes, while providing important energy reduction services. Thus, living shoreline retrofits continue to be a cost-effective shoreline management strategy in the short term for this area. However, our analyses suggest that persistence of these shorelines could be threatened by the combination of sea-level rise (by 2040), upland armoring, and an increasing risk of more intense tropical systems. Therefore, future interventions should more carefully consider these threats in conjunction with habitat enhancement goals.

Prototype-Scale Physical Model of Wave Attenuation Through a Mangrove Forest of Moderate Cross-Shore Thickness: LiDAR-Based Characterization and Reynolds Scaling for Engineering With Nature

This study investigates the potential of a Rhizophora mangrove forest of moderate cross-shore thickness to attenuate wave heights using an idealized prototype-scale physical model constructed in a 104 m long wave flume. An 18 m long cross-shore transect of an idealized red mangrove forest based on the trunk-prop root system was constructed in the flume. Two cases with forest densities of 0.75 and 0.375 stems/m2 and a third baseline case with no mangroves were considered. LiDAR was used to quantify the projected area per unit height and to estimate the effective diameter of the system. The methodology was accurate to within 2% of the known stem diameters and 10% of the known prop root diameters. Random and regular wave conditions seaward, throughout, and inland of the forest were measured to determine wave height decay rates and drag coefficients for relative water depths ranging 0.36 to 1.44. Wave height decay rates ranged 0.008–0.021 m–1 for the high-density cases and 0.004–0.010 m–1 for the low-density cases and were found to be a function of water depth. Doubling the forest density increased the decay rate by a factor two, consistent with previous studies for other types of emergent vegetation. Drag coefficients ranged 0.4–3.8, and were found to be dependent on the Reynolds number. Uncertainty in the estimates of the drag coefficient due to the measured projected area and measured wave attenuation was quantified and found to have average combined standard deviations of 0.58 and 0.56 for random and regular waves, respectively. Two previous reduced-scale studies of wave attenuation by mangroves compared well with the present study when their Reynolds numbers were re-scaled by λ3/2 where λ is the prototype-to-model geometric scale ratio. Using the combined data sets, an equation is proposed to estimate the drag coefficient for a Rhizophora mangrove forest: CD = 0.6 + 3e04/ReDBH with an uncertainty of 0.69 over the range 5e03 < ReDBH < 1.9e05, where ReDBH is based on the tree diameter at breast height. These results may improve engineering guidance for the use of mangroves and other emergent vegetation in coastal wave attenuation.

Experimental study on vegetation flexibility as control parameter for wave damping and velocity structure

Vegetation can contribute to coastal defence by damping incoming waves. However, prior studies have shown that attenuation varies greatly among plant species. Plant flexibility is a mechanical property that is commonly omitted, but varies considerably between shrubs and grasses on salt marshes. Therefore, we present an experimental study in a laboratory wave flume with artificial vegetation that differs in flexibility only. We measured wave attenuation and water particle velocities around rigid and flexible salt marsh vegetation. Waves were measured using a series of gauges and Particle Image Velocimetry (PIV) was used to measure spatio-temporal variations of water particle velocities in the x-z plane around the vegetation. Our results show that flexible vegetation attenuates waves up to 70% less than rigid vegetation due to swaying of flexible plants. Furthermore, we find that rigid vegetation modifies the velocity structure, whereas flexible vegetation does not. Specifically, a mean current in the direction of wave propagation develops around the canopy and the horizontal particle velocities are amplified directly above the canopy. These results indicate that plant flexibility is a key parameter in the wave-vegetation interaction that controls wave damping and velocity structure.

An experimental study on gravity waves through a floating viscoelastic cover

We report the results of a laboratory study conducted to investigate the dispersion relation and attenuation of surface waves under viscoelastic floating covers. The study uses oil-doped polydimethylsiloxane (PDMS) with different viscoelastic properties quantified in-situ using a rheometer. An additional test is made with a floating PVC film to estimate possible attenuation in the boundary layer under the cover. Within the frequency range tested, the wavelength deviates from open water case negligibly under the PVC film. But for the PDMS covers, depending on the material property, cover thickness and incident wave period, both wave lengthening and shortening are observed. With thin PDMS covers, the measured wavelength matches well with three theoretical predictions based on different viscoelastic rheological models, while the thick cover shows some discrepancy especially for shorter wave periods. In terms of wave attenuation, however, large discrepancies are observed between the measured data and theoretically predicted results for all PDMS covers. Various mechanisms that could contribute to the discrepancies are considered. The most important mechanism is the boundary layer effect, as demonstrated by the measured wave attenuation from the PVC film. The overall results indicate that the attenuation due to the oscillatory boundary layer underneath the floating cover, which has not been included in the viscoelastic theories, contributes significantly to the discrepancies. Direct measurement of the fluid flow under a periodically flexing cover and the related attenuation rate is a challenging but highly important future study.

The evaluation of distributed damage in concrete based on sinusoidal modeling of the ultrasonic response

Ultrasonic wave attenuation is an effective descriptor of distributed damage in inhomogeneous materials. Methods developed to measure wave attenuation have the potential to provide an in-site evaluation of existing concrete structures insofar as they are accurate and time-efficient. In this study, material classification and distributed damage evaluation were investigated based on the sinusoidal modeling of the response from the through-transmission ultrasonic tests on polymer concrete specimens. The response signal was modeled as single or the sum of damping sinusoids. Due to the inhomogeneous nature of concrete materials, model parameters may vary from one specimen to another. Therefore, these parameters are not known in advance and should be estimated while the response signal is being received. The modeling procedure used in this study involves a data-adaptive algorithm to estimate the parameters online. Data-adaptive algorithms are used due to a lack of knowledge of the model parameters. The damping factor was estimated as a descriptor of the distributed damage. The results were compared in two different cases as follows: (1) constant excitation frequency with varying concrete mixtures and (2) constant mixture with varying excitation frequencies. The specimens were also loaded up to their ultimate compressive strength to investigate the effect of distributed damage in the response signal. The results of the estimation indicated that the damping was highly sensitive to the change in material inhomogeneity, even in comparable mixtures. In addition to the proposed method, three methods were employed to compare the results based on their accuracy in the classification of materials and the evaluation of the distributed damage. It is shown that the estimated damping factor is not only sensitive to damage in the final stages of loading, but it is also applicable in evaluating micro damages in the earlier stages providing a reliable descriptor of damage. In addition, the modified amplitude ratio method is introduced as an improvement of the classical method. The proposed methods were validated to be effective descriptors of distributed damage. The presented models were also in good agreement with the experimental data.

A Method to Estimate Shear Quality Factor of Hard Rocks

Attenuation has a large influence on ground motion intensity. Quality factors are used to measure wave attenuation in a medium and they are often difficult to estimate due to many factors such as the complex geology and underground mining environment. This study investigates the effect of attenuation on seismic wave propagation and ground motion using an advanced numerical tool—SPECFEM2D. A method, which uses numerical modeling and site-specific scaling laws, is proposed to estimate the shear quality factor of hard rocks in underground mines. In the numerical modeling, the seismic source is represented by a moment tensor model and the considered medium is isotropic and homogeneous. Peak particle velocities along the strongest wave motion direction are compared with that from a design scaling law. Based on the field data that were used to derive a semi-empirical design scaling law, it is demonstrated that a shear quality factor of 60 seems to be a representative for the hard rocks in deep mines to consider the attenuation effect of seismic wave propagation. Using the proposed method, reasonable shear quality factors of hard rocks can be obtained and this, in turn, will assist accurate ground motion determination for mine design.

Pulse-echo method cannot measure wave attenuation accurately

A number of techniques with different degrees of accuracies have been devised for the measurement of acoustic wave attenuation in solids and liquids. Still, a wide variation is observed in the attenuation values in different materials reported in the literature. Present numerical study based on a ‘propagating wave’ model analysis clearly shows that the attenuation constant obtained from exponential fitting of the echo heights in pulse-echo method differs from the exact value of intrinsic attenuation in the medium, even in the ideal situation of plane wave propagation without diffraction, dispersion or scattering.

Laboratory and numerical studies of wave damping by emergent and near-emergent wetland vegetation

Wetlands protect mainland areas from erosion and damage by damping waves. Yet, this critical role of wetland is not fully understood at present, and a means for reliably determining wave damping by vegetation in engineering practice is not yet available. Laboratory experiments were conducted to measure wave attenuation resulting from synthetic emergent and nearly emergent wetland vegetation under a range of wave conditions and plant stem densities. The laboratory data were analyzed using linear wave theory to quantify bulk drag coefficients and with a nonlinear Boussinesq model to determine numerical friction factors to better represent wetland vegetation in engineering analysis.

Microwave measurement of electron density and collision frequency of a pine fire

Pinus caribea (pine) litter flame is a weakly ionized medium. Electron–neutral collisions are a dominant form of particle interaction in the flame. Assuming flame electrons to be in thermal equilibrium with neutrals and average electron–neutral collision frequency to be much higher than the plasma frequency, the propagation of microwaves through the flame is predicted to suffer signal intensity loss. A controlled fire burner was constructed where various natural vegetation species could be used as fuel. The burner was equipped with thermocouples and used as a cavity for microwaves with a laboratory quality network analyser to measure wave attenuation. Electron density and collision frequency were then calculated from the measured attenuation. The parameters are important for numerical prediction of electromagnetic wave propagation in wildfire environments. A controlled pine litter fire with a maximum flame temperature of 1080 K was set in the burner and microwaves (8–10.5 GHz) were caused to propagate through the flame. A microwave signal loss of 1.6–5.8 dB was measured within the frequency range. Based on the measured attenuation, electron density and electron–neutral collision frequency in pine fire were calculated to range from 0.51–1.35 × 1016 m−3 and 3.43–5.97 × 1010 s−1, respectively.

Prediction and Measurement of Electron Density and Collision Frequency in a Weakly Ionised Pine Fire

Pine litter flame is a weakly ionised medium. Electron-neutral collisions are a dominant form of particle interaction in the flame. Assuming flame electrons to be in thermal equilibrium with neutrals and average electron-neutral collision frequency to be much higher than the plasma frequency, the propagation of microwaves through the flame is predicted to suffer signal intensity loss. A controlled fire burner was constructed where various natural vegetation species could be used as fuel. The burner was equipped with thermocouples and used as a cavity for microwaves with a laboratory quality network analyzer to measure wave attenuation. Electron density and collision frequency were then calculated from the measured attenuation. The parameters are important for numerical prediction of electromagnetic wave propagation in wildfire environments. A controlled pine litter fire with a maximum flame temperature of 1080 K was set in the burner and microwaves (8–10.5 GHz) were caused to propagate through the flame. A microwave signal loss of 1.6–5.8 dB was measured within the frequency range. Based on the measured attenuation, electron density and electron-neutral collision frequency in pine fire were calculated to range from 0.51–1.35 × 1016 m−3 and 3.43–5.97 × 1010 s−1 respectively.

Parameter inversion from measured data by the modified random trial-and-error method

To find unknown parameters with the ordinary method of inverting the measured wave propagation data is difficult, especially when multiple scattering occurs. One can remove this difficulty somewhat if one uses the least-squares estimator with the modified random trial-and-error method. The number density or sizes of randomly distributed particles embedded in a uniform medium can be obtained from the measured wave transmittance, reflectance or attenuation by this method. Two examples are given. They are the distribution of raindrops in the atmosphere by inverting the measured wave attenuation with single scattering considered, and the number density and average size of polystyrene particles in a water layer with multiple scattering considered. The results show the validity of this method.

Improved measurement of the electron temperature of a low density shock-heated argon plasma by means of microwaves

The determination of electron temperature of a low density shock-heated argon plasma (3 Torr ≤p1≤ 10 Torr, 5500 °K ≤ 9500 °K) by means of microwave diagnostics is improved with a test section which permits the simultaneous transmission of two microwave beams at different frequencies, and with two corrections applied to the measured attenuation of transmitted waves. The purpose of these corrections is to obtain the true attenuation due to wave power dissipation into the plasma, which is dependent on the electron temperature and density. The first correction takes into account the diffraction of waves by apertures of the test section and the wave defocusing by the plasma. The second correction considers the effect of wave beam bending due to the electron density gradient in the flow direction of the plasma. Both corrections diminish the measured wave attenuation. Consequently a lower ratio of attenuation to phase shift of transmitted waves is determined, which in turn yields lower values of electron collision frequency and electron temperature. This report describes the electron temperature evaluation technique in detail, and presents results. The electron temperature values obtained have a range of uncertainty of – 20 % and +10% only, with exceptions at the beginning of the test time. In most tests, the electron temperature is equal or lower by approximately 1000 °K in comparison with the heavy particle temperature.