Showing papers in "Journal of Waterway Port Coastal and Ocean Engineering-asce in 2004"

Near Field Characteristics of Landslide Generated Impulse Waves

Abstract: Landslide generated impulse waves were investigated in a two-dimensional physical laboratory model based on the generalized Froude similarity. The recorded wave profiles were extremely unsteady and nonlinear. Four wave types were determined: weakly nonlinear oscillatory wave, non-linear transition wave, solitary-like wave and dissipative transient bore. Most of the generated impulse waves were located in the intermediate water depth wave regime. Nevertheless the propagation velocity of the leading wave crest closely followed the theoretical approximations for a solitary wave. Between 4 and 50% of the kinetic slide impact energy propagated outward in the impulse wave train. The applicability ranges of the classical nonlinear wave theories to landslide generated impulse waves were determined. The main wave characteristics were related to the landslide parameters driving the entire wave generation process. The slide Froude number was identified as the dominant parameter. The physical model results were compared to the giant rockslide generated impulse wave which struck the shores of the Lituya Bay, Alaska, in 1958.

Using a Three-Dimensional Smoothed Particle Hydrodynamics Method for Wave Impact on a Tall Structure

Abstract: The impact of a single wave generated by a dam break with a tall structure is modeled with a three-dimensional version of the smoothed particle hydrodynamics method. The method is used to analyze the propagation of a long wave and the force it exerts on a tall structure located in a region with vertical boundaries. Velocities and forces obtained from a numerical model are shown to be in good agreement with laboratory measurements. The effect of having a dry bed in front of the dam prior to the dam break versus a wet bed in the experiment is discussed.

Generating Power from Tidal Currents

Abstract: Power generation by tidal currents requires the establishment of a pressure difference across a turbine. This limits the power output to a fraction of the undisturbed energy flux through the same cross-sectional area, though the maximum possible value of this fraction is not well established. It is further shown that an array of turbines in the entrance to a bay is most effective if it is uniformly distributed across the entrance. The maximum power available for a quadratic drag law occurs when the tidal range inside the bay is reduced to 0.74 of the original amplitude, suggesting that power generation is compatible with the maintenance of good flushing and with use of the bay for other purposes. Moreover, this exploitation of tidal power through continuous operation of current turbines in the entrance is not much less productive than more conventional schemes that rely on trapping the water at high tide and releasing it during a small part of the tidal cycle.

Modified Hunt’s Equation Incorporating Wave Setup

Abstract: Hunt’s equation provides a simple means of estimating the runup of breaking waves. However, breaking waves also induce setup on a slope. A modification to Hunt’s equation is advocated which accounts for the presence of wave setup. Support for the modification is provided by data from model tests involving random waves approaching smooth, impermeable slopes of 1:5, 1:10, 1:20, and 1:30. Coefficients to be used in the modified equation are given which allow the significant runup and other characteristic values to be estimated.

Finite-Element Model for Modified Boussinesq Equations. I: Model Development

Abstract: This paper and its companion paper describe the development of a finite-element model based on modified Boussinesq equations and the applications of the model to harbor resonance problems. This first of the two papers reports the model development and validations. A Galerkin finite-element method with linear elements is employed in the model. Auxiliary variables are introduced to remove the spatial third derivative terms in the governing equations, and an implicit predictor-corrector iterative scheme is used in the time integration. The treatments of various boundary conditions, including the perfect reflecting boundary with an irregular geometry, the absorbing (sponge layer) boundary, and the incident wave boundary, are described. Numerical results are obtained for several examples, and their accuracy is checked by comparing numerical results with either experimental data or analytical solutions.

Generation and propagation of water waves in a two-dimensional numerical viscous wave flume

Abstract: This study investigated the generation and propagation of water waves in a numerical viscous wave flume. The numerical scheme developed by Huang and collaborators for solving the unsteady two-dimensional Navier–Stokes equations for wavemaking problems was employed to generate different incident waves, including small- and finite-amplitude waves and solitary waves. The accuracy of the numerical results for the wave and velocity profiles was verified by comparison with the analytical solutions. The wave propagation in a numerical wave flume was also investigated. For periodic gravity waves on finite water depth, the results showed that waves with larger Ursell numbers are more stable than those with smaller Ursell numbers. The propagation of solitary waves in the channel is stable. For stable waves, the wave height attenuation caused by the energy dissipation in the wave motion was shown to be consistent with the theoretical results.

Floating Breakwater Response to Waves Action Using a Boussinesq Model Coupled with a 2DV Elliptic Solver

Abstract: The hydrodynamic behavior of fixed and heave motion floating breakwaters is studied in the present paper, using a finite-difference, mathematical model based on the Boussinesq type equations. The flow under the floating breakwater is treated separately as confined flow. The pressure field beneath the floating structure is determined by solving implicitly the Laplace equation for the potential Φ of the confined flow using the appropriate boundary conditions. The dynamic equation of heave motion is solved with the consequent adjustments of the continuity equation in the case of a heave motion floating breakwater. Numerical results, concerning the efficiency of fixed and heave motion floating breakwaters, are compared to experimental results satisfactorily. The ability of the numerical model to predict the pressure field beneath the floating structure and the vertical force acting on it is thoroughly examined by making comparisons of the numerical results with large-scale experimental data. The experiments were conducted in the CIEM flume of the Catalonia Univ. of Technology, Barcelona, Spain. The final goal is to study floating breakwaters efficiency in shallow and intermediate waters.

Predictions of Missing Wave Data by Recurrent Neuronets

Abstract: Real time wave measurements in Turkey are often interrupted because of operational difficulties encountered. Therefore, the lacking significant wave height, period and directions were simultaneously estimated from the dynamic Elman type recurrent neural networks. Their predictions were compared with the commonly applied static feed-forward multilayer neural networks and with the stochastic Auto Regressive (AR) and Exogenous Input Auto Regressive (ARX) models. Two distinct learning algorithms, the steepest descent with momentum and the conjugate gradient methods were employed to train the neural networks. It was concluded that, the recurrent neural network generally showed better performance than the feed-forward neural network in the concurrent forecasting of multiple wave parameters. Both artificial intelligence techniques demonstrated a good performance when compared to the predictions of AR and ARX models. Prediction methods are also compared using continuous artificial data generated with known properties by measuring their performance in predicting the removed segments of various lengths. The multivariate ENN model successfully predicted the removed segments of artificially generated wave data. Hence, the learning ability of artificial intelligence techniques was verified signifying the robustness and fault-failure tolerance of neural networks.

Finite-Element Model for Modified Boussinesq Equations. II: Applications to Nonlinear Harbor Oscillations

Abstract: The finite-element model for modified Boussinesq equations developed in Part I of this paper is used to study harbor resonance problems. The internal wavemaker technique is implemented on an unstructured finite-element mesh, and a sponge layer is used as an open boundary condition. Both linear and nonlinear harbor oscillations in rectangular and circular basins are examined, and experimental measurements and analytical solutions are used to validate the model. Finally, the wavelet transform is employed to analyze transient features of the nonlinear resonance problem.

Modeling Flow In and Above a Porous Beach

Abstract: The development of a numerical model for the simulation of water flow in and above a porous beach is described. The model, OTTP-1D, is based on a previous hydrodynamic model of the swash zone, OTT-1D, and builds on the numerical methods developed in the earlier model by the addition of a permeable layer representing a gravel beach. The porewater flow equations used in the present model are described along with the boundary layer approach used to determine the bed shear stress when the effects of exchange between the free-flow and porewater flow regions of the model are taken into consideration. These equations are transformed so as to make them amenable to solution by an upwind finite volume method in the same way as the earlier model. The model is verified against a simple analytical case of Darcy flow and an estimate of lower limit of applicable grain size made thereby. Validation against observed water depths and free-flow velocities recorded on a gravel beach at Slapton, U.K. in 2001 is also undertaken. The model accurately predicts both measured instantaneous water depths and velocities. DOI: 10.1061/~ASCE!0733-950X~2004!130:5~223! CE Database subject headings: Porous media; Porous flow; Numerical models; Validation; Beaches.

Dual Boundary Element Analysis of Normal Incident Wave Passing a Thin Submerged Breakwater with Rigid, Absorbing, and Permeable Boundaries

Abstract: In this paper, the dual integral formulation is derived for solving the scattering problem of normal incident wave passing a thin vertical and inclined barrier with rigid boundary condition which is descending from the water surface to a depth. Absorbing and porous boundary conditions are both considered. The breakwater thickness is assumed to be zero since it is negligible in comparison with the water depth and the wavelength of the incident wave. Although the multidomain boundary element method (BEM) can solve boundary value problems with degenerate boundaries by dividing the interesting domain into two subdomains, the hypersingular formulation provides the key to solve the problem more efficiently in a single domain. To demonstrate the effects of the breakwater with rigid, absorbing, and porous boundary conditions for the energy dissipation by the barrier, the transmission and reflection coefficients of the scattering problem are determined by the developed dual BEM program. In addition, the results are obtained for the cases of wave scattering by the barrier with zero thickness in constant water depth and are compared with the analytical solutions, the multidomain BEM solution, and the experimental data.

On the Use of Directional Wave Criteria

Abstract: Directional wave statistics can be used to optimize the design of offshore structures which are not symmetrical. Unfortunately there does not appear to be any consensus on how directional design criteria should be developed or used. Using simple examples, we show that commonly used methods are unconservative. In order to preserve the reliability given by designing to omnidirectional criteria, the product of the probabilities of nonexceedance from all of the direction bands must equal the omnidirectional probability. We suggest a simple method for assuring consistency between omnidirectional and directional criteria.

Optimal Engineering Design Method that Combines Safety Factors and Failure Probabilities: Application to Rubble-Mound Breakwaters

Abstract: This paper presents a new method for engineering design that allows controlling safety factors and failure probabilities with respect to different modes of failure. Since failure probabilities are very sensitive to tail assumptions, and safety factors can be insufficient, a double check for the safety of the engineering structure is done. The dual method uses an iterative process that consists of repeating a sequence of three steps: (1) an optimal (in the sense of optimizing an objective function) classical design, based on given safety factors, is done, (2) failure probabilities or bounds of all failure modes are calculated, and (3) safety factor bounds are adjusted. The three steps are repeated until convergence, i.e., until the safety factor lower bounds and the mode failure probability upper bounds are satisfied. In addition, a sensitivity analysis of the cost and reliability indices to the data parameters is done. The proposed method is illustrated by its application to the design of a rubble-mound breakwater.

Impacts of Dredging on Shoreline Change

Abstract: Nearshore dredging for construction aggregate or beach nourishment can result in a perturbation of natural littoral processes, changes in wave transformation patterns, and a net loss of sand from the littoral system. A method is described for estimating both direct and indirect effects of dredging on shoreline change. The direct effect results from infilling of the dredged pit via cross-shore sediment transport and is addressed statistically, assuming that the beach profile is in some arbitrary equilibrium shape. The indirect effect arises from project-induced wave transformation, which alters longshore sediment transport patterns, and is described using both spectral and monochromatic, numerical wave transformation models to provide input to a one-line model for shoreline change. Infilling of the pit is neglected when estimating the indirect effect, providing a worst-case estimate of the indirect effect. The methodology is applied to a site on the Turkish Black Sea coast, using hindcast wave data. The influences of pit location and geometry are investigated systematically, and recommendations regarding optimum pit dimensions and locations are made.

Annual Variations of Tide Level and Mudflat Profile

Abstract: Mudflat profiles were measured monthly along two cross-shore lines exceeding 1,000 m in the vicinity of a river mouth for 2 years All the measured profiles are shown to be approximated within average errors of 3 cm by a quadratic profile with three parameters that vary with time The temporal variations of the quadratic profile represented by the quadratic convexity, mean slope, and vertical displacement are correlated with available data on the tide level and range, significant wave height, wind speed, and river discharge in the vicinity of the field site The vertical displacement varied in phase with the annual tide level variation of 40 cm, but the mudflat displacement was only 20% of the tide level displacement The mudflat slope became gentler or steeper as the tide level rose or fell annually The proposed quadratic profile approach may be useful in examining intertidal mudflat morphology at other sites

Numerical Study of Tidal Effects on Seawater Intrusion in Confined and Unconfined Aquifers by Time-Independent Finite-Difference Method

Abstract: This study presents a two-dimensional time-independent finite difference model to simulate tidal effects on the intrusion of seawater in either a confined or phreatic aquifer. The model considers a sloped beach face. The irregular beach face and varying ground water surface are mapped to a square domain and the time dependence of the moving boundary is removed by an appropriate coordinate transformation. The finite-difference expression of the governing equations is, therefore, written in a time-independent mesh system, and a time-independent finite difference scheme is established. The numerical technique is confirmed by the reported results of the traditional Henry's and modified Henry's problems. Idealized saltwater intrusion, known as Henry's problem, was extended to simulate seawater intrusion on sloped beach faces accounting for tidal effects. Tidal effects enhance the migration of seawater. The dominant feature of a confined aquifer is that seawater intrudes inland during the flood tide. The saltwater continues to intrude further inland and reaches its furthest upstream position at ebb tide. In an unconfined aquifer, the tidal fluctuation causes the variation of the intrusion profile in the vertical direction only. The fluctuation of the water table oscillates according with the fluctuating tide with a time lag, but the influence of the tide is damped out with horizontal distance from the beach. The variation of the distance through which the seawater intrudes also oscillates with the tide, with a constant time lag of 0.25T, where T is the tidal period. The shape of the aquifer also affects the intrusion of saltwater and the velocity of intrusion. The rate of seawater intrusion and the distance through which the seawater intrudes increases with the slope of the bank.

High-frequency water level fluctuations in Lake Michigan

Abstract: During certain meteorological events, water level oscillations up to 1.5 m with periods of less tha n 2 h have been observed in the Great Lakes. The squall line events of 7-11 March 1998, 29 May-2 June 1998, and 8 -12 November 1998 in Lake Michigan, are analyzed by spectral and analytical methods. Dominant periods of less than 2 h are identified in the spectra of water level fluctuations and coherencies among the spectral peaks of water levels of different station pairs are calculated to determine whether the oscillations are localized or basin wide. Explicit numerical calculations of normal mode periods and structures using a Lanczos procedure show that the dominant periods in the observed data are consistent with the structures and periods of some of the calculated modes. The March 1998 and the November 1998 episodes show higher surges with a gradual rise of water level, while the episode of May 1998 shows an abrupt rise in the water level at Calumet Harbor and about ten times higher spectral signature than the former two. Many of the high-frequency modes have large amplitude at or near Calumet Harbor and the periods are close to the periods of edge waves that would be generated by a squall line similar to the May 1998 squall line. The trapping of energy and localized higher modes in a water body can work together to excite edge waves and localized seiches causing abrupt water level fluctuations.

Settling Distance and Incipient Motion of Sandbags in Open Channel Flows

Abstract: This study is concerned with two hydrodynamic parameters that are essential for the construction of submerged dikes in rivers with sand-filled geosynthetic bags. First, laboratory study and field observation are conducted to investigate the horizontal settling distance of sandbags in open channel flows. Sandbag models used for laboratory tests are prepared according to similarity requirements. An analytical formula for evaluating the settling distance is then derived based on the experimental data. The predictions are then compared to the field measurements. The observed settling distance varies significantly and its probability density distribution is positively skewed. This study also presents a formula for computing the critical flow velocity at incipient sandbag motion for two extreme conditions.

Incorporation of Wind Effects into Boussinesq Wave Models

Abstract: Recent advances in the Boussinesq modeling of nearshore hydrodynamics offer a platform for the study of wind effects on wave transformation and breaking-generated nearshore circulation. The paper documents: (1) the new parameterization of the momentum flux transferred from the wind to surface gravity waves in the coastal region on the basis of the field observations; (2) the implementation of the parameterized wind stress into phase-resolving Boussinesq wave models; (3) the development of empirical breaking criteria with the wind effect based on the existing laboratory data; and (4) the tests of the extended Boussinesq model against field observations and empirical results with respect to wind drag coefficients over shoaling waves and wave growth on a shallow lake. Fairly good agreement between the model results and measurements is observed. The methodology for the parameterization of the wind stress as a function of wave steepness and wind speed as well as the extended Boussinesq model incorporating the wind forcing can be used as a tool to improve our understanding of wind effects on nearshore wave propagation and horizontal circulation.

One-Dimensional Wave Bottom Boundary Layer Model Comparison: Specific Eddy Viscosity and Turbulence Closure Models

Abstract: Six one-dimensional-vertical wave bottom boundary layer models are analyzed based on different methods for estimating the turbulent eddy viscosity: Laminar, linear, parabolic, \Ik\N—one equation turbulence closure, \Ik\N–e—two equation turbulence closure, and \Ik\N-ω—tw equation turbulence closure. Resultant velocity profiles, bed shear stresses, and turbulent kinetic energy are compared to laboratory data of oscillatory flow over smooth and rough beds. Bed shear stress estimates for the smooth bed case were most closely predicted by the \Ik\N–ω model. Normalized errors between model predictions and measurements of velocity profiles over the entire computational domain collected at 15° intervals for one-half a wave cycle show that overall the linear model was most accurate. The least accurate were the laminar and \Ik\N–e models. Normalized errors between model predictions and turbulence kinetic energy profiles showed that the \Ik\N–ω model was most accurate. Based on these findings, when the smallest overall velocity profile prediction error is required, the processing requirements and error analysis suggest that the linear eddy viscosity model is adequate. However, if accurate estimates of bed shear stress and TKE are required then, of the models tested, the \Ik\N–ω model should be used.

Nearshore Impacts of Dredging for Beach Nourishment

Abstract: A methodology is presented for predicting the impacts of nearshore dredging on shoreline change, using numerical models for waves, currents, and shoreline change. The dredge pit, outside of the surf zone, was assumed to be steady state, but alters waves reaching the surf zone. The methodology accounts for the transformation of directional wave energy spectra due to spatial gradients in both mean currents and bathymetry, wind inputs, and energy dissipation via several mechanisms. The wave transformation model SWAN (Simulating Waves Nearshore) was used to simulate waves at Folly Island, S.C., which served as the test case. Wave model results were validated using short-term field measurements at three locations. Longer-term (1 year) simulations were driven using hindcast wave and wind data. Velocity fields and tidal stages were simulated using the hydrodynamic model Environmental Fluid Dynamics Code in two-dimensional mode, after calibrating with field measurements. Wave model results were used to estimate longshore sediment transport and shoreline change due to longshore gradients of the longshore sediment transport rate. Empirical sediment transport coefficients were chosen to match model results to measured beach volume changes derived from beach profile data. The calibrated model was then used to predict impacts of dredging on long-term shoreline change. Results reproduced observed trends of erosion and accretion along approximately 90% of the shoreline.

Soft Cliff Recession under Oblique Waves: Physical Model Tests

Abstract: Laboratory experiments have been performed to investigate the recession of soft cliffs due to oblique wave actions. The experiments were conducted in a wave tank with irregular waves. A test cliff ...

Longshore sediment transport by nonlinear waves and currents

Abstract: A time-dependent model for obliquely incident nonlinear waves is developed and applied to predict longshore current and sediment transport. The wave model is based on the Boussinesq equations for breaking and nonbreaking waves. Wave breaking is introduced by adopting the surface roller concept. Longshore current velocity is calculated using the time-averaged alongshore momentum equation, including the effects of the cross-shore circulation on the dispersion of momentum. The wave module provides the longshore current and the sediment transport modules with all required hydrodynamic information such as radiation stress, bottom velocity, undertow velocity, and eddy viscosity coefficient. The Dibajnia and Watanabe formula is adopted to predict sheet-flow transport; whereas, for the suspended load, an energetics approach is used. Model results are compared with experimental data as well as with the Kamphuis and Costal Engineering Research Center formulas for the total alongshore sediment transport rate.

Transformation of progressive waves propagating obliquely on gentle slope

Abstract: This paper reports the results of a new mathematical derivation for the transformation of a progressive wave propagating obliquely on a gentle slope. On the basis of the conservation principle of wave motion and in a wave-ray coordinate system, an explicit expression for the velocity potential of the wave field is derived as a function of the bottom slope a perturbed to a second order in an Eulerian coordinate system. Wave profile is then obtained in the Lagrangian system. Together, these enable the description of the features of wave shoaling and refraction in the direction of wave propagation from deep to shallow water, particularly, the process of successive deformation of a wave profile.

Assessment of Wood Pile Deterioration due to Marine Organisms

Abstract: In this paper a description of the major groups of marine organisms causing significant wood pile damage is presented. These organisms are divided into two groups: (1) fungi and (2) marine borers. The basic physical and biological characteristics of these organisms are presented, as well as the type of damage that they cause in marine wood piles. The objective of the study presented in this paper is to characterize deterioration of wood piles due to marine organisms and to assess damage in the wood pile zones of a typical waterfront installation. Marine borer activity in Maine coastal waters is assessed through a survey directed to harbor masters; the results of the survey are correlated with historic data. In order to illustrate the type and extent of wood pile deterioration, two case studies in Maine harbors are presented.

Spread parameter of extreme wave height distribution for performance-based design of maritime structures

Abstract: A new parameter is proposed to characterize the tail spreading performance of extremal distribution functions as defined by the ratio of the 50-year return wave height to the 10-year height, and it is called the spread parameter. Extreme data sets of annual maximum wind speed and peaks-over-threshold significant wave heights have been analyzed for best-fitting distribution functions, for which the spread parameter is evaluated. The observed values of the spread parameter are in good agreement with the predicted values based on the coefficient of variation of an extreme data sample. Formulas are given to derive the scale and location parameters from a given value of the spread parameter. Some typical values of the spread parameter are presented for promotion of performance-based design of maritime structures in the world.

Estimation of Contraction Scour in Riverbed Using SERF

Abstract: Contraction scour in a firm-clay estuarine riverbed is estimated at an oil-unloading terminal at the Port of Haldia in India, where a scour hole attained a maximum depth greater than 5 m relative to the original bottom. A linear equation for the erosion flux as a function of the excess bed shear stress was semicalibrated in a rotating-cylinder device called SERF ~Simulator of Erosion Rate Function! and coupled to a hydrodynamic code to simulate the hole as a clear-water scour process. SERF, whose essential design is based on previous such devices, additionally included a load cell for in situ and rapid measurement of the eroded sediment mass. Based on SERF's performance and the degree of comparison between measured and simulated hole geometry, it appears that this device holds promise as a simple tool for prediction of scour in firm-clay beds.

Visualization analysis of runup and mixing of internal waves on an upper slope

Abstract: Laboratory experiments were conducted in a 2D wave tank, where a Plexiglas plate was installed in order to understand the runup and breaking of internal waves over a sloping boundary. During the experiments, a fluid consisting of fresh water and salt water was prepared to imitate the density-stratified ocean. An image processing technique was used to reveal the profile of internal waves and the mixture of the upper-layer and lower-layer water. The measured wave profiles were compared with the calculated results by the method of characteristics. The major feature of the runup processes was well reproduced by the theoretical solution, except for the upper slope region. Calculation using the k-e model was carried out, and the predicted density variation due to mixing was compared with a set of luminance data analyzed by the image processing technique.

Stokes Correction for Nonlinearity of Wave Crests in Heavy Seas

Abstract: The application of Stokes fifth-order wave theory to random deep-water waves is considered in connection with nonlinearity in crest amplitudes The theory is shown to apply, on average, to crest amplitudes and wave heights, although differences exist for individual waves as a result of variation in wave frequency Corrections to the Rayleigh law and associated predictions of fractions of waves exceeding crest threshold levels are shown to be in close agreement with measurements for heavy seas when the significant wave height and average wave period are specified accurately

Behavior of a Contractile Floating Reservoir for the Confinement and Recovery of Oil Slicks

Abstract: A new system for the confinement and recovery of oil slicks or other floating materials is tested in a physical model to evaluate its feasibility and effectiveness. It consists of encircling a polluted area by a specially designed boom using light-weight material, in such a way as to trap entirely the oil slick inside a circular reservoir, making its dispersion less widespread. The system allows the oil slick to be slowly towed to safer zones, where it can be pumped and recycled after separating the reservoir into several compartments in order to increase the efficiency of pumping and treatment. The experimental investigation was carried out in a small-scale model and demonstrated the effectiveness of the system in practical situations. In terms of oil loss, the limits for the critical waves and towing velocities were determined. Carefully selected polymer granulate layers were used to reproduce the characteristics of oil slicks.