Showing papers on "Wave flume published in 2019"

The role of beach and sand dune vegetation in mediating wave run up erosion

Abstract: Coastal dunes are often the first and only line of physical defense for communities subjected to damaging storms and waves. Planting vegetation on them has been proposed as one way to increase their protective capacity, but it is unknown how dune plant architecture reduces erosion. We conducted wave flume and field experiments to address this question and found that dune plants primarily reduced erosion by attenuating wave swash and run up bores with their stems and leaves, while their roots initially enhanced erosion through uprooting. After excavation, the roots also attenuated waves and reduced erosion. We then sampled the biophysical attributes of a broad distribution of plants, and found that herbaceous non-Graminoid (non-grass) species that inhabited the lowest latitudes and most seaward zones had the most efficient structures for erosion reduction. Our results suggest that there is a fundamental tradeoff in the ability of dune plant species to respond to hydrodynamic versus Aeolian processes, based on the relative allocation of aboveground versus belowground biomass. Through the combination of flume experiments, field survey, and meta-analysis, our findings show that vegetation provides on average ∼1.6 factor of safety over bare sand across a wide range of latitudes in the northern hemisphere - translating into a reduction of wave run up erosion by approximately 40% for dunes.

Experimental Evidence of a Hydrodynamic Soliton Gas.

Abstract: We report on an experimental realization of a bidirectional soliton gas in a 34-m-long wave flume in a shallow water regime. We take advantage of the fission of a sinusoidal wave to continuously inject solitons that propagate along the tank, back and forth. Despite the unavoidable damping, solitons retain their profile adiabatically, while decaying. The outcome is the formation of a stationary state characterized by a dense soliton gas whose statistical properties are well described by a pure integrable dynamics. The basic ingredient in the gas, i.e., the two-soliton interaction, is studied in detail and compared favorably with the analytical solutions of the Kaup-Boussinesq integrable equation. High resolution space-time measurements of the surface elevation in the wave flume provide a unique tool for studying experimentally the whole spectrum of excitations.

Numerical solutions of waves-current interactions by generalized finite difference method

Abstract: In this paper, a meshless numerical wave flume, based on the generalized finite difference method (GFDM), is adopted to accurately and efficiently simulate the interactions of water waves and current. The GFDM, a newly-developed meshless method, is truly free from mesh generation and numerical quadrature. The proposed meshless numerical wave flume is the combination of the GFDM, the second-order Runge–Kutta method, the semi-Lagrangian approach, the sponge layer and the ramping function. The problems of wave-current interactions in flumes with horizontal and inclined bottoms are accurately and stably investigated by the proposed meshless scheme, respectively. The changes of waveform can be obviously found, while the cases of coplanar, opposing and no currents are stably simulated. Besides, the distribution of steady current in the flume with inclined bottom, which is governed by an inverse Cauchy problem, is acquired by the GFDM in a stable manner. Numerical results of wave-current interactions are compared with other solutions to verify the accuracy of the proposed meshless scheme. Additionally, different parameters of the proposed meshless numerical scheme are examined to validate the consistency and stability of the proposed numerical wave flume for solutions of wave-current interactions.

Development of PARISPHERE as the particle-based numerical wave flume for coastal engineering problems

Abstract: The paper presents a new numerical wave flume packaged as PARISPHERE code based on the Incompressible Smoothed Particle Hydrodynamics (ISPH) method with an improved framework for enhancement of the...

Computational Model for Wave Attenuation by Flexible Vegetation

Abstract: Coastal vegetation has a well-known effect of attenuating waves; however, quantifiable measures of attenuation for general wave and vegetation scenarios are not well known, so field and laboratory studies must be performed for individual setups. The standard practice of performing these studies for such scenarios is extremely expensive, and it is difficult to change parameters and setups. We presented and validated a computational model for a wave flume that can be used for studies of wave attenuation over flexible vegetation based on the previously developed immersed-structure method for fluid–vegetation interaction, thereby augmenting field and laboratory studies with a more-flexible and less-expensive alternative. The main advantage of this computational framework is that almost all terms are derived from first principles without requiring a large number of empirically determined parameters. A series of computational experiments were performed, and an analysis of the wave attenuation with respect to wave heights, spectra, and energy was conducted. Results were compared to results from experiments that the computational wave flume was designed to replicate.

Numerical Investigation on Hydrodynamic Performance of an OWC Wave Energy Device in the Stepped Bottom

Abstract: The efficiency response of an oscillating water column (OWC) wave energy converter is analysed by adopting a numerical approach and using various stepped bottom configurations for the seabed. A two-dimensional, fully non-linear higher-order boundary element method (HOBEM) model is developed to simulate the hydrodynamic characteristics and fluid structure interaction for a fixed OWC located in a numerical wave flume. A number of model tests are conducted for various regular incident wave conditions by modifying the wave amplitudes and wavelengths. The measured free surface elevation at the chamber centre and the oscillatory air pressure generated by the fluctuating free surface within the chamber are recorded. The hydrodynamic efficiency of the OWC is determined using these parameters. The simulation results are validated by comparing against previously published experimental and numerical data. Good agreement is observed in both cases. The geometric dimensions of the step are modified by altering the step height and step length. Furthermore, by altering the step length, the location of the front face of the step relative to the front wall of the OWC is adjusted. Therefore, the positional significance of the step can also be analysed in terms of its relative location to the OWC chamber and to the flow field development. It is shown that the geometry of the step and the position of the vertical face of the step relative to the OWC influences the hydrodynamic efficiency. The research demonstrates that by optimising the step geometry and position for a given wave condition a higher operational efficiency can be achieved.

Wave attenuation and dispersion due to floating ice covers

Abstract: Experiments investigating the attenuation and dispersion of surface waves in a variety of ice covers are performed using a refrigerated wave flume. The ice conditions tested in the experiments cover naturally occurring combinations of continuous, fragmented, pancake and grease ice. Attenuation rates are shown to be a function of ice thickness, wave frequency, and the general rigidity of the ice cover. Dispersion changes were minor except for large wavelength increases when continuous covers were tested. Results are verified and compared with existing literature to show the extended range of investigation in terms of incident wave frequency and ice conditions.

Study of Floating Wind Turbine with Modified Tension Leg Platform Placed in Regular Waves

Abstract: In this study, the typical ocean environment was simulated with the aim to investigate the dynamic response under various environmental conditions of a Tension Leg Platform (TLP) type floating offshore wind turbine system. By applying Froude scaling, a scale model with a scale of 1:200 was designed and model experiments were carried out in a lab-scale wave flume that generated regular periodic waves by means of a piston-type wave generator while a wave absorber dissipated wave energy on the other side of the channel. The model was designed and manufactured based on the standard prototype of the National Renewable Energy Laboratory (NREL) 5 MW offshore wind turbine. In the first half of the study, the motion and structural responses for operational wave conditions of the North Sea near Scotland were considered to investigate the performance of a traditional TLP floating wind turbine compared with that of a newly designed TLP with added mooring lines. The new mooring lines were attached with the objective of increasing the horizontal stiffness of the system and thereby reducing the dominant motion of the TLP platform (i.e., the surge motion). The results of surge translational motions were obtained both in the frequency domain, using the response amplitude operator (RAO), and in the time domain, using the omega arithmetic method for the relative velocity. The results obtained show that our suggested concept improves the stability of the platform and reduces the overall motion of the system in all degrees-of-freedom. Moreover, the modified design was verified to enable operation in extreme wave conditions based on real data for a 100-year return period of the Northern Sea of California. The loads applied by the waves on the structure were also measured experimentally using modified Morison equation—the formula most frequently used to estimate wave-induced forces on offshore floating structures. The corresponding results obtained show that the wave loads applied on the new design TLP had less amplitude than the initial model and confirmed the significant contribution of the mooring lines in improving the performance of the system.

Statistical Analysis of the Stability of Rock Slopes

Abstract: Physical model tests were performed in a wave flume at Deltares with rock armoured slopes. A shallow foreshore was present. At deep water, the same wave conditions were used, but by applying different water levels, the wave loading on the rock armoured slopes increased considerably with increasing water levels. This allowed an assessment of the effects of sea level rise. Damage was measured by using digital stereo photography (DSP), which provides information on each individual stone that is displaced. Two test series were performed five times. This allowed for a statistical analysis of the damage to rock armoured slopes, which is uncommon due to the absence of statistical information based on a systematic repetition of test series. The statistical analysis demonstrates the need for taking the mean damage into account in the design of rock armoured slopes. This is important in addition to characterising the damage itself by erosion areas and erosion depths. The relation between damage parameters, such as the erosion area and erosion depth, was obtained from the tests. Besides tests with a straight slope, tests with a berm in the seaward slopes were also performed. A new method to take the so-called length effect into account is proposed to extrapolate results from physical model tests to real structures. This length effect is important, but is normally overlooked in the design of rubble mound structures. Standard deviations based on the presented model tests were used.

Laboratory Tests on the Reflection Coefficient of a Perforated Caisson

Abstract: This paper presents new tests on a perforated vertical wall caisson. The tests aim at evaluating the reduction of the reflection coefficient caused by different perforations. The reflection coefficient is estimated using the novel nonlinear methods proposed by Lykke Andersen et al. (2017) and Eldrup and Lykke Andersen (2019). The structure under investigation is, at prototype scale, 21.50 m high, 35.55 m long and 13.90 m wide, divided into 3 rows of 8 cells. The caisson has a recurved ‘nose’ and air vents on the parapet wall. An identical plain wall caisson without absorbing chambers is also tested for comparison. Two-dimensional laboratory tests have been carried out at Department of Engineering of Roma Tre University with both irregular and regular waves, reproducing mild wave conditions and design wave conditions. Two water levels have been used, reproducing the mean water level and a set-up condition of +0.5 m. Four structural layouts are tested in order to study various perforation types. Furthermore, the effect of the wave obliquity and of the short crestedness on the reflection coefficient is discussed using some 3D laboratory data obtained back in the 1995 but not published before.

Pore water pressure responses in silty sediment bed under random wave action

Abstract: We studied pore water pressure responses in silty seabed under random wave action through a series of experiments in a wide wave flume. Unlike previous experiments involving regular waves, we focus on random waves including wind-induced short waves and long waves so as to gain further insights into seabed responses and liquefaction risks posed by random waves. In particular, the study investigated how the secondary long waves that were induced by incident short wave groups affected the seabed responses. The test results revealed that these long waves could cause much larger seabed responses than the short waves (eight times larger in our flume tests). Although they had smaller wave heights than the short waves, the long waves were found to contribute much more significantly to the cumulative pore pressure than previously recognized. The likely reason is that the long waves are disproportionally effective in generating cumulative excess pore pressure, confirming qualitatively some of the earlier theoretical predictions. One of the implications from these research findings is that the existing design methods when applied to random waves could grossly underestimate liquefaction potential in silty sediment bed if either spectrum-based mean wave parameters or significant wave parameters were used.

Introduction to Physical Experiment Building of Kiost in Busan

Abstract: Upon the movement of Korea Institute of Ocean Science and Technology (KIOST) from Ansan to Busan, the Physical Experiment Building (PEB) was recently built in the new campus of KIOST, where several experimental facilities are being established. As of May 2019, PEB holds a wave basin, a wave flume, and a tilting flow channel. In addition, second wave flume will be commissioned by the end of this year. This paper describes an overview of those facilities with regard to design, capabilities, and foreseen applications.

Physical model study on geo-tube with gabion boxes for the application of coastal protection

Abstract: Shoreline erosion takes place due to the movement of sand by tides, wave actions, and wave-induced currents. The conventional techniques used for coastal protection such as artificial armor units and rubble mound are costly, and transportation is difficult in remote areas. Moreover, they may not be suitable for poor soil conditions. In this paper, the detailed physical experimental studies were carried out for a geo-tube saline embankment with ten geo-tubes in a four-layer configuration in a wave flume. This soft engineering solution may be particularly viable since the location is remote with poor soil strata. The studies include a scaled model (1:10) of geo-tube embankment with and without gabion boxes to check its possible utilization for coastal protection. The model is tested in the deep wave flume equipped with a piston type wave maker. The embankment is studied for two different water depths of 0.4 m and 0.5 m. The chosen 0.4-m water depth characterizes the high tide condition, while the 0.5-m water depth reproduces the combined high tide and storm surge conditions. The regular wave heights are varied in the range of 2 to 16 cm with a corresponding wave period of 1.5 to 2.2 s. Three probe methods are used to obtain the hydrodynamic parameters, based on which a geo-tube embankment protected with gabion box as armor is designed and constructed along the coast of Pentha (Odisha, India).

Experimental Study of Wave Attenuation in Trapezoidal Floating Breakwaters

Abstract: A comprehensive experimental study was carried out on the regular wave attenuation with a trapezoidal pontoontype floating breakwater (FB) in deep water. The functionalities of two simple FB geometries consist of a rectangle and a trapezoid with the slope of 60° were investigated under the wave attack. A two-dimensional wave flume was used in the experiment; the incident, transmitted waves, mooring line forces and motion responses of the floating breakwaters were measured. Also the influence of the sea state conditions (incident wave height and wave period) and structural parameters (draught of the structure) were investigated using the trapezoidal FB. Our experimental results indicated that the trapezoidal FB significantly reduced the wave transmission and mooring line force when compared with rectangular FBs. A new formula was developed in order to predict the value of the transmission coefficient in trapezoidal FBs with the slope of 60°. Experimental data showed to be consistent with the results of the formula.

Physical Model of Natural Coastal Protection System: Wave Transmission Over Mangrove Seedling Trees

Abstract: Yuanita, N.; Kurniawan, A.; Setiawan, H.; Hasan, F., and Khasanah, M., 2019. Physical model of natural coastal protection system: Wave transmission over mangrove seedling trees. In: Lee, J.L.; Yoon, J.-S.; Cho, W.C.; Muin, M., and Lee, J. (eds.), The 3rd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 91, pp. 176-180. Coconut Creek (Florida), ISSN 0749-0208.Erosion is one of main problem in coastal area. In order to solve erosion problem, currently natural coastal protection using vegetation such as mangrove is preferable in many places in the world. However, there are challenges in development of this natural coastal protection, e.g. mangrove-seedling-trees have been damaged by the waves or current, before they are growth strongly which required at least 2 years of plantation. To solve this problem, a natural coastal protection system consists of combination of main natural protection and temporary manmade structures is proposed. The study aimed to quantify the wave height reduction with various mangrove densities as well as the influence of mangrove seedling trees arrangements on wave reduction. The laboratory experiments were conducted in a narrow wave flume using model of mangrove as main natural protection and geotextile-geobag models as temporary manmade structure. Various wave conditions were generated during this laboratory test. This paper focus on the experiment results of wave transmission over mangrove seedling trees in order to determine the most effective configuration of mangrove trees plantation against wave. The results showed that the wave height reduction in area with mangroves was about two times larger compared to that in bare land. The wave reduction difference between tandem and staggered arrangements of trees was less than 20 %. It is also found that the temporary structure is significantly reduce wave height and protect mangrove seeds grow from waves attack.