Showing papers on "Wave height published in 2007"

Unified View of Sediment Transport by Currents and Waves. II: Suspended Transport

Abstract: The problem of suspended sediment transport in river and coastal flows is addressed. High-quality field data of river and coastal flows have been selected and clustered into four particle size classes 60-100, 100-200, 200-400, and 400-600 m. The suspended sand transport is found to be strongly dependent on particle size and on current velocity. The suspended sand transport in the coastal zone is found to be strongly dependent on the relative wave height Hs/h, particularly for current velocities in the range 0.2-0.5 m/s. The time-averaged over the wave period advection-diffusion equation is applied to compute the time-averaged sand concentration profile for combined current and wave conditions. Flocculation, hindered settling, and stratification effects are included by fairly simple expressions. The bed-shear stress is based on a new bed roughness predictor. The reference concentration function has been recalibrated using laboratory and field data for combined steady and oscillatory flow. The computed transport rates show reasonably good agreement within a factor of 2 with measured values for velocities in the range of 0.6-1.8 m/s and sediments in the range of 60-600 m. The proposed method underpredicts in the low-velocity range 0.6 m/s. A new simplified transport formula is presented, which can be used to obtain a quick estimate of suspended transport. The modeling of wash load transport in river flow based on the energy concept of Bagnold shows that an extremely large amount of very fine sediment clay and very fine silt can be transported by the flow.

Wave climate changes in the North Atlantic and North Sea

Abstract: This article presents a review of all available data describing long-term trends in the wave climate of the North Atlantic and North Sea, using both visual estimates and instrumental measurements of wave height. Long series of measurements from Seven Stones Light Vessel (1962–1986), sited off the south-west tip of England, and Ocean Weather Station Lima (1975–1988), at 57°N,20°W, are both examined for evidence of trends in mean wave height; the former data set is also examined for evidence of trends in annual maximum wave height and extreme (50-year return value) wave height. All available data show an increase in mean wave height over the whole of the North Atlantic in recent years, possibly since 1950, of about 2 per cent per year. There are insufficient data to say with confidence whether maxima or extremes have also risen.

Observation of gravity-capillary wave turbulence.

Abstract: We report the observation of the crossover between gravity and capillary wave turbulence on the surface of mercury. The probability density functions of the turbulent wave height are found to be asymmetric and thus non-Gaussian. The surface wave height displays power-law spectra in both regimes. In the capillary region, the exponent is in fair agreement with weak turbulence theory. In the gravity region, it depends on the forcing parameters. This can be related to the finite size of the container. In addition, the scaling of those spectra with the mean energy flux is found in disagreement with weak turbulence theory for both regimes.

Error Estimation of Buoy, Satellite, and Model Wave Height Data

Abstract: Triple collocation is a powerful method to estimate the rms error in each of three collocated datasets, provided the errors are not correlated. Wave height analyses from the operational European Centre for Medium-Range Weather Forecasts (ECMWF) wave forecasting system over a 4-yr period are compared with independent buoy data and dependent European Remote Sensing Satellite-2 (ERS-2) altimeter wave height data, which have been used in the wave analysis. To apply the triple-collocation method, a fourth, independent dataset is obtained from a wave model hindcast without assimilation of altimeter wave observations. The seasonal dependence of the respective errors is discussed and, while in agreement with the properties of the analysis scheme, the wave height analysis is found to have the smallest error. In this comparison the altimeter wave height data have been obtained from an average over N individual observations. By comparing model wave height with the altimeter superobservations for different v...

A nearshore model to investigate the effects of seagrass bed geometry on wave attenuation and suspended sediment transport

Abstract: The effects of seagrass bed geometry on wave attenuation and suspended sediment transport were investigated using a modified Nearshore Community Model (NearCoM). The model was enhanced to account for cohesive sediment erosion and deposition, sediment transport, combined wave and current shear stresses, and seagrass effects on drag. Expressions for seagrass drag as a function of seagrass shoot density and canopy height were derived from published flume studies of model vegetation. The predicted reduction of volume flux for steady flow through a bed agreed reasonably well with a separate flume study. Predicted wave attenuation qualitatively captured seasonal patterns observed in the field: wave attenuation peaked during the flowering season and decreased as shoot density and canopy height decreased. Model scenarios with idealized bathymetries demonstrated that, when wave orbital velocities and the seagrass canopy interact, increasing seagrass bed width in the direction of wave propagation results in higher wave attenuation, and increasing incoming wave height results in higher relative wave attenuation. The model also predicted lower skin friction, reduced erosion rates, and higher bottom sediment accumulation within and behind the bed. Reduced erosion rates within seagrass beds have been reported, but reductions in stress behind the bed require further studies for verification. Model results suggest that the mechanism of sediment trapping by seagrass beds is more complex than reduced erosion rates alone; it also requires suspended sediment sources outside of the bed and horizontal transport into the bed.

The Role of Wave Erosion on Sloping and Horizontal Shore Platforms in Macro- and Mesotidal Environments

Abstract: Waves were measured over tidal cycles on a sloping, basaltic shore platform at Scots Bay in the macrotidal Bay of Fundy (large tidal range 13.5 m), and on a horizontal, argillite platform at Mont Louis in Gaspe, Quebec (large tidal range 3 m). Video cameras were used to record the height and period of the waves against a series of graduated metal poles anchored along surveyed, shore-normal profiles. Field measurement and theoretical considerations suggest that wave height increases with elevation at Scots Bay, reflecting the occurrence of a gently sloping tidal flat below the midtidal level and higher gradients on the upper than on the lower parts of the platform. Calculated pressures generated within the rock along joints and other discontinuities suggest that wave conditions are suitable for mechanical-wave erosion at Scots Bay. Waves generally break on the low tide cliff at the seaward edge of the platform at Mont Louis, preventing any wave action on the shallow, flooded platform surface behin...

Wave climate and long-term changes for the Southern North Sea obtained from a high-resolution hindcast 1958–2002

Abstract: An analysis of the extreme wave conditions in 1958–2002 in the North Sea as obtained from a regional model hindcast is presented. The model was driven by hourly wind fields obtained from a regional atmosphere model forced with reanalysis data from the National Center for Environmental Prediction (NCEP/NCAR). Furthermore, observed sea ice conditions from the Norwegian Meteorological Institute have been accounted for in the simulation. It is shown that the model is capable of reproducing extreme wave height statistics at a reasonable degree of approximation. The analysis of severe wave height events reveals that for much of the Southern North Sea, their number has increased since the beginning of the simulation period (1958), although the increase has attenuated later and leveled off around about 1985. On the other hand, the intensity and duration of severe wave height events decreased within the last few years of the simulation so that annual 99%-ile wave heights have also reduced since about 1990–1995. For the UK North Sea coast, a different behavior was found characterized by a reduction in severe wave conditions over much of the hindcast period.

On the extreme statistics of long‐crested deep water waves: Theory and experiments

Abstract: [1] Quasi-resonant four-wave interactions may influence the statistical properties of deep water long-crested surface gravity waves. As a consequence, the wave height exceedance probability can substantially deviate from the expected distribution obtained by assuming that waves are linear. Here the occurrence probability of extreme events recently derived by N. Mori and P. Janssen (2006) is compared with wave tank data, where strong departures from Gaussian behavior are observed. Experimental wave height, maximum wave height distribution, and probability of occurrence of freak waves are compared with theoretical expectations. The theory well predicts extreme waves in nonlinear wavefields.

Investigation into wave—current interactions in marine current turbines

Abstract: The paper looks at the use of blade element-momentum theory for predicting the torque and thrust on a marine current turbine and the results of wave tank tests using a 400 mm-diameter rotor model. To include the effects of waves, linear wave theory particle velocities and accelerations were integrated into the mathematical model. Comparison with test data shows a good agreement which implies that the theory can be effective in analysing the wave-current interactions in marine current turbines. The paper also carried out parametric studies into related parameters, which include wave height, wave frequency, and tip-speed ratio. The interaction of waves with the current may increase or decrease the torque and hence power output of the turbine. The paper also explains the selection and manufacture of the rotor and the experimental setup.

The Boundary Value Problem for the Nonlinear Shallow Water Equations

Abstract: We propose an approximate analytical solution of the boundary value problem (BVP) for the nonlinear shallow waters equations. Our work, based on the Carrier and Greenspan [1] hodograph transformation, focuses on the propagation of nonlinear nonbreaking waves over a uniformly plane beach. Available results are briefly discussed with specific emphasis on the comparison between the Initial Value Problem and the BVP; the latter more completely representing the physical phenomenon of wave propagation on a beach. The solution of the BVP is achieved through a perturbation approach solely using the assumption of small waves incoming at the seaward boundary of the domain. The most significant results, i.e., the shoreline position estimation, the actual wave height and velocity at the seaward boundary, the reflected wave height and velocity at the seaward boundary are given for three specific input waves and compared with available solutions.

Onshore Migration of a Swash Bar During a Storm

Abstract: The morphodynamics of a swash bar was monitored over two tidal cycles during a storm event at Skallingen, Denmark. Higher water levels during the first tidal cycle forced waves to break either landward of, or at, the bar crest. The bar was unable to significantly alter the pattern of wave breaking over the foreshore slope, creating a potential for bar erosion through an offshore transport of sediment. In response, the bar became more symmetric in cross-section, and its height decreased. Lower water levels during the second tidal cycle forced waves to break on the seaward slope, causing the bar to rebuild and its crest to migrate onshore. The onshore migration of the swash bar occurred despite little change in the offshore wave height from one tidal cycle to the next and in conditions that are typically associated with offshore transport and bar migration. The results further illustrate a strong tidal dependency in which water depth relative to bar height is an important control on the morphodynam...

Effect of a Shallow Water Obstruction on Long Wave Runup and Overland Flow Velocity

Abstract: A study is presented to examine the one-horizontal dimension effect of a shallow shelf obstacle on nonlinear long wave runup. Due to the large horizontal-vertical aspect ratio of this problem, it is not well suited for experimental analysis, and therefore this study is purely numerical. Simulations are performed for various incident wave conditions, obstacle height and widths, and final beach slopes. Many of the setups involve breaking, either through approaching the obstacle as a large breaking bore, incipient breaking on top of the obstacle, or breaking during the beach uprush. The general conclusion of this study is that, for highly nonlinear waves wave height/shelf water depth 0.5, the obstacle will always act to reduce the runup and the maximum overland velocity. However, for very small obstacle lengths, particularly for extremely large waves, this reduction may be practically inconsequential. Interestingly, for weakly nonlinear waves 0.1, due to front steepening over the obstacle, greater overland velocities can result from increasing obstacle length. Consistent with previous studies, it is found that the final beach slope is of primary importance for determining the runup.

Short-term morphological change and sediment dynamics in the intertidal zone of a macrotidal beach

Abstract: This paper describes the morphological and sedimentological evolution of amacrotidal beach over a 20 day period under varying hydrodynamicconditions (significant breaker heights of 0AE3–2 m and tidal ranges of 2–5 m).During the field campaign, an intertidal bar developed around the mid-tidelevel, migrated onshore, welded to the upper beach and was then flattenedunder energetic wave conditions. The bar had a wave breakpoint origin and itsformation was triggered by a reduction in tidal range, causing more stationarywater-level conditions, rather than an increase in wave height. Most of theonshore bar migration took place while the bar was positioned in the inner tomid-surf zone position, such that the bar moved away from the breakpoint andexhibited ‘divergent’ behaviour. The depth of disturbance over individual tidalcycles was 10–20% of the breaker height. Such values are more typical of steepreflective beaches, than gently sloping, dissipative beaches, and are consideredto reflect the maximum height of wave-generated ripples. The grain sizedistribution of surficial sediments did not vary consistently across the beachprofile and temporal changes in the sedimentology were mostly unrelated tothe morphological response. The lack of clear links between beach morphologyand sedimentology may be in part due to shortcomings in the samplingmethodology, which ignored the vertical variability in the sediment sizecharacteristics across the active layer.Keywords Beach sediment, depth of disturbance, intertidal bar, macrotidalbeach, morphology.INTRODUCTIONBeaches respond rapidly to changing wave andtide conditions through the redistribution of theirsediments by oscillatory and quasi-steady cur-rents, resulting in spatial patterns of erosion(sediment divergence) and accretion (sedimentconvergence). Given sufficient time, these bed-level changes modify the beach shape by theformation, modification, destruction and/ormigration of secondary morphological features,such as berms, beach cusps, swash bars, ripchannels and nearshore bars. The dynamics ofthese secondary morphological features, and theirrole in affecting beach stability, has been a majorfocus of beach research over the last few decades(e.g. Komar, 1998; Short, 1999).Most short-term (days to weeks) field investi-gations addressing the morphological response ofsandy beaches to wave and tide forcing have beenconducted in microtidal or mesotidal settings.Beaches in macrotidal settings have, on the otherhand, only received limited attention, despitetheir common occurrence (Short, 1991). A mainreason for this imbalance is that macrotidalbeaches are often planar and lack pronouncedsecondary morphological features. Indeed, theonly type of macrotidal beach that does featureprominently in the coastal literature is the ‘ridgeand runnel’ beach, which is characterized bySedimentology (2007) 54, 39–53

Daily to seasonal cross-shore behaviour of quasi-persistent intertidal beach morphology

Abstract: In this study, an intertidal bar and trough system on the beach of Noordwijk, The Netherlands was monitored over a 15-month period in order to examine the daily to seasonal sequential cross-shore behaviour and to establish which conditions force or interrupt this cyclic bar behaviour. The beach morphology (bars and troughs) was classified from low-tide Argus video images based on surface composition. From the classified images, time series of the landward boundary of the bar and of the trough were extracted. The time series of the alongshore-averaged boundary positions described sawtooth motion with a period between 1 and 4 months, comprising gradual landward migration followed by abrupt seaward shifts. The abrupt seaward shift appeared to be a morphological reset induced by storm events, which lasted at least 30 h with a large average root-mean-square wave height (≥2 m) and offshore surge level (≥0·5 m), and a small trough (<20 m wide) in the pre-storm beach morphology. The time series of the boundary positions exhibited very little longer (seasonal) scale variability, but somewhat larger smaller (daily) scale variability. The bar boundary was found to be more dynamic than the trough boundary. Copyright © 2007 John Wiley & Sons, Ltd.

Estimates of wave climate in the northern Baltic Proper derived from visual wave observations at Vilsandi

Abstract: Wave conditions, their seasonal cycle and long-term variations in the northern part of the Baltic Sea Proper are studied, based on visual wave observations at the Vilsandi Island near the coast of Saaremaa in 1954-2005. Typical wave periods are from 2 to 4 s. The monthly mean wave height follows the seasonal variation in wind speed and varies from about 0.4 m in April-July to almost 0.8 m in January. The annual mean wave height shows a quasiperiodic behaviour. The wave activity varied insignificantly in the 1960s and 1970s, considerably increased in the 1980s, was the highest just before the turn of the century and is decreasing starting from about 1998.

Application of a POT model to estimate the extreme significant wave height levels around the Balearic Sea (Western Mediterranean)

Abstract: CANELLAS, B.; ORFILA, A.; MENDEZ, F.J.; MENENDEZ, M. and TINTORE, J., 2007. Application of a POT model to estimate the extreme significant wave height levels around the Balearic Sea (Western Mediterranean). Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), pg – pg. Gold Coast, Australia, ISBN Extreme value wave climate analysis at a particular site requires predicting long-term wave height levels from short duration records. In the present work we used the Peak Over Threshold (POT) model, assuming the frequency as a Poisson process and the intensity to be Pareto distributed, to characterize the spatial variability of the long-term extreme value wave climate along the Balearic Sea. Wave data used is part of the HIPOCAS database, a 44-years high resolution, spatial and temporal, wave hindcast, covering an area between 38oN-42oN and 1oW-6oE, of the western Mediterranean Sea. The use of data from a homogeneous grid, instead of a single location wave data record, allows describing the spatial variability of the long-term extreme wave height levels, over the whole Balearic Basin. Results show that extreme values for a 50-year return period level around 11 m are found in the north sector of the Balearic Islands while in the southern part much lower extreme values are found. This is due to the shadow effect of the islands over the severe north-eastern storms.