Changes In Wave Height Research Articles

North Atlantic Ocean Wave Climate Change Scenarios for the Twenty-First Century

Using the observed relationships between sea level pressure (SLP) and significant wave height (SWH) as represented by regression models, climate change scenarios of SWH in the North Atlantic were constructed by means of redundancy analysis (for seasonal means and 90th percentiles of SWH) and nonstationary generalized extreme value analysis (for seasonal extreme SWH). SWH scenarios are constructed using output from a coupled climate model under three different forcing scenarios. Scenarios of future anomaly seasonal statistics of SWH are constructed using climate model projections of anomaly seasonal mean SLP while projections of seasonal extreme SWH are made using projections of seasonal mean SLP and seasonal SLP gradient index. The projected changes in SWH are assessed by means of a trend analysis. The northeast Atlantic is projected to have increases in both winter and fall seasonal means and extremes of SWH in the twenty-first century under all three forcing scenarios. These changes are generally accompanied by decreases in the midlatitudes of the North Atlantic and increases in the southwest North Atlantic. The rate and sign of the projected SWH change is not constant throughout the twenty-first century. In the Norwegian and North Seas, the projected SWH changes are characterized either by faster increases in the late decades than in the early decades, or by decreases in the early decades followed by increases, depending on the forcing scenario and the specific location. Using lower or higher rates of increase in greenhouse gases forcing generally leads to reduced or increased rates of change, respectively, in ocean wave heights. The sign and rate of future wave height changes in the North Sea in particular appear to be quite dependent on the forcing conditions. In general, global warming is associated with more frequent occurrence of the positive phase of the North Atlantic Oscillation (NAO) and strong cyclones, which leads to increases of wave heights in the northeast Atlantic.

Long wave forcing on a barred beach

We present new laboratory data on long wave forcing over a barred beach profile under random wave breaking conditions. The data include incident and radiated wave amplitudes, wave set-up, and detailed measurements of the cross-shore variation in long wave amplitude, including shoreline (swash) amplitudes. The total surf zone width was varied via changes in both wave height and the water level over the bar crest. The data obtained from the barred beach are also compared with previous data obtained from a plane beach under essentially identical short wave forcing conditions. The presence of the bar induces a frequency downshift in the spectral peak of the radiated long waves, a consequence of the increased surf zone width on the barred beach and a clear signature of long wave forcing by a time-varying breakpoint. Further comparisons of the two data sets suggest that the bar leads to resonant trapping and amplification (or suppression) of the shoreline motion at discrete long wave frequencies. Well-defined standing long wave motion occurs at discrete frequencies inside the bar and the resonant response is consistent with a simple seiche between the bar crest and shoreline, in agreement with previous numerical model studies. The long wave structure offshore of the breakpoint depends on the relative positions of the bar, shoreline and breakpoint, and is inconsistent with a numerical solution for a free standing long wave over the barred beach profile.

WAVE INTERACTION WITH PARABOLIC CORRUGATED AND PERFORATED WAVE ABSORBERS

ABSTRACT The wave reflection characteristics of Parabolic (P), Parabolic Corrugated (PC) and Parabolic Corrugated and Perforated (PCP) wave absorbers were investigated using physical models. The profile of the parabolic absorber is Y2 = 0.12 X. The investigations were carried out both with regular and random wave fields for a wide range of wave periods and heights. It is found that the coefficient of reflection, Kr varies from 0.30 to 0.90 for regular waves, when the relative water depth, d/L is decreased from 0.33 to 0.07. For the same range of relative water depths, for the case of random waves, Kr value varies from 0.23 to 0.45 only. Empirical equations are proposed to predict Kr by using Surf Similarity parameter, ξ and relative water depth, d/L as an independent parameter. The relative water depth, d/L is found to be better when compared to ‘ξ’ in predicting the wave reflection from parabolic absorbers. PCP type wave absorber is better by about 5% to 10% when compared to PC and P type wave absorber. All the three wave absorbers are about 5 limes sensitive for change of wave period than change of wave height.

Effects of Sand Mining on Physical Processes and Biological Communities Offshore New Jersey, U.S.A.

Abstract Physical processes and biological data were collected and analyzed for eight sand resource areas on the New Jersey Outer Continental Shelf to address environmental concerns raised by the potential for mining sand for beach replenishment. Nearshore wave and sediment transport patterns were modeled for existing and post-dredging conditions, with borrow site sand volumes ranging from 2.1 to 8.8 × 106 m3. Wave transformation modeling indicated that minor changes will occur to wave fields under dominant directional conditions and selected sand extraction scenarios. Localized seafloor changes at borrow sites are expected to result in negligible impacts to the prevailing wave climate at the coast. At potential impact areas along the New Jersey coast, wave height changes averaged approximately ±3 to 15% when compared with wave heights for existing conditions. For all selected sand borrow sites offshore New Jersey, average variation in annual littoral transport was approximately 10% of existing values. Be...

Scattering from a single bubble near a roughened air-water interface: laboratory measurements and modeling.

The problem of scattering from a single bubble located close to a slightly roughened, air-water interface is studied both theoretically and experimentally. Two well-controlled laboratory experiments were performed to investigate the effects of surface roughness on the scattering response of the bubble. In the first experiment, a bubble of radius 1200 microm was placed on a fine thread at a variable distance, d, from the mean-still-water level of the surface, which was roughened using a wind source. In the second experiment, a bubble of radius 800 microm was utilized, while the water surface was roughened using a plunger wave-making source. The waveheights and important characteristic length scales associated with each experiment were quantified using digital photography. The wind source produced waveheights that were represented by a Gaussian distribution, while the plunger source produced waveheights that were represented by a bimodal distribution. To model the acoustic measurements, an expression describing the four scattering paths, from source to bubble to receiver, was used. A random phase shift due to the surface roughness was added to the paths that interacted with the surface, and expectations of this phase shift were computed based on the analytical representations for the waveheight distribution. The data show good agreement with the simulations and the sensitivity of scattering from a subsurface bubble to small changes in waveheight is illustrated. The experiments highlight important parametric dependencies, which are summarized here, and the relation between monostatic and bistatic scattering is also discussed.

The typhoon swell Doppler effect

The swell of a moving typhoon has the same Doppler effect as a moving sound source. Both originate from the variations in an energy flux. In a sound source this is called a frequency shift. In a typhoon it is a swell wave height change. A distant typhoon may generate very high waves through the Doppler effect as the typhoon approaches. A modification λ is proposed for engineering practice.

The accumulation and decay of near-bed suspended sand concentration due to waves and wave groups

High resolution acoustic measurements were made of suspended sand and bedform dimensions caused by prototype-scale waves, both regular and in groups, over a mobile sand bed, in a very large wave channel. The changes in wave height at the beginning of the regular waves and within wave groups provides an opportunity to examine the time lag in the response of the sediment. For regular waves suspended sand concentrations lagged the forcing waves with the lag increasing with distance from the seabed. Typically, near-bed (1–2 cm) concentrations reached an equilibrium one to two wave-periods after the waves themselves had reached their steady height while at elevations of 10–15 cm the lag was longer. This lag was interpreted as due to the continual injection of turbulence into the water column from vortex processes associated with the oscillatory wave boundary layer over bedforms. A similar pattern was seen for wave groups, with the sand concentration near the bed lagging by the waves by one to two wave-periods and increasing with distance from the bed. Despite the controlled nature of these prototype-scale suspension experiments, with detailed measurements of bedforms and attempts to achieve ‘equilibrium’ bedforms, considerable variability (±30%) in the suspended sand concentration occurred between ‘similar’ forcing conditions, both at a wave-to-wave level and on the scale of groups and longer. The results suggest that considerable variability (a factor of two or more) should be expected in the suspension due to turbulence produced from wave boundary layers in natural environments, where bedforms are frequently continually evolving as the waves change their height, period and direction. A simple wave-average suspended-load model is used to describe the major temporal features of the suspension and to quantify the lag of the suspended sediment in relation to the waves and wave groups. Quantification of the lag is essential for assessing the transport of sand at infra-gravity frequencies. A decay rate of 0.06 s −1, applied to antecedent waves, decreased the mean average error (MAE) by a factor of 3 when predicting the suspended load of the repeating wave groups. When tested against five further data sets (including random waves and wave records from the SANDYDUCK field experiment) including the decay rate of 0.06 s −1, resulted in a decrease in the MAE of a factor of 1.5–2 (compared to the same model with no lag). The entrainment (pickup) constant in the same model was variable and no consistent pattern was found, although there were suggestions of a link to the location of the measured profile relative to the bedforms.

CULTIVATION CAPACITY OF A CULTIVATION FACILITY-INTEGRATED STRUCTURE BASED ON DISSOLVED OXYGEN BALANCE

In Hokkaido, cultivation (underwater farming) for shipment control of seafood or intermediate breeding of fries is conducted in some waters behind breakwaters with the seawater exchange function.This study discusses the cultivation capacity of a cultivation facility-integrated structure with the seawater exchange function, using the optimum dissolved oxygen content for the sea urchin, which is the species being cultivated, as the basis. As a result, the following matters were clarified(1) Temporal changes in the dissolved oxygen content in the preserve, in which 4, 000 sea urchins were continuously cultivated for a month, were observed using the wave data of January 1991. Changes in wave height and dissolved oxygen corresponded mostly with each other, and the dissolved oxygencontent in the preserve, which was 5.4mg/l 120 hours later, recovered to 8.0mg/l, the same level as the open sea water, by the effect of waves approximately 40cm in wave height. The lowest value of dissolvedoxygen in one month was 5.4mg/l, and the lower limit of optimum dissolved oxygen (2.7mg/l) was satisfied.(2) The dissolved oxygen content in the preserve decreased almost linearly in relation to the number of sea urchins cultured.(3) When sea urchins were cultivated for one month in a preserve using the wave data of 36 months from 1989 to 1991, the cultivation capacity in winter (February and March) was 5, 000 sea urchins, greater than the capacity of 4, 000 in summer

Hydrodynamic variability on megatidal beaches, Normandy, France

Several experiments aimed at characterising the hydrodynamics of megatidal beaches outside the surf zone were carried out between 1990 and 1994 on the Cotentin coast of the Cherbourg Peninsula in Normandy. The database was established from the records of several electromagnetic current meters and pressure sensors and from field surveys. The mean spring tidal range on these beaches varies between 9.3 and 11.4 m. The results show the prevalence of strong longshore currents, with velocities up to 0.5 m s −1, on the low- and mid-tidal beach zones. Mostly oriented northward, these currents reflect both a progressive tidal wave and a strong longshore gradient in water level between the Channel Islands embayment and the English Channel. While varying largely during a typical tidal cycle, these longshore velocities are maximum at high tide, reflecting the progressive nature of the tides. This high-tide maximum velocity increases by a factor of 1.5 between the mean tide and mean spring tide, and between the mid- and low-tidal zones due to bed friction effects. Cross-shore velocities are generally weak (<0.1 m s −1), but sometimes stronger in smaller water depths. In the low-tidal zone, they are commonly oriented onshore at the beginning of the rising tide and offshore during the falling tide. This circulation results from a west–east cross-shore gradient in water level that is particularly important around the mean water level. Towards high tides, weak offshore steady flows were observed in the presence of waves. Site-specific relationships were defined in order to characterise the modulation of significant wave height by sea level fluctuations both on the shoreface and in the intertidal zone. The water depth variability during the tidal cycle induces fluctuations in the dissipation by bottom friction, resulting in wave height changes. The influence of tidal currents on the wave height proved to be very small in this context. The tidal fluctuations also influence the instantaneous near-bed currents induced by simultaneous action of non-breaking waves and the tides. During stormy conditions, wave-induced gravity orbital motions dominate the steady flows in the mid-tidal zone, outside the surf zone. At this location, the shallow water friction effect results in weak steady longshore currents, and low water depths explain strong orbital motions. The opposite conditions prevail in the low-tidal zone, where the steady tidal currents are stronger than gravity orbital velocities during a few hours around high tide. Outside this period, with the decrease in water depth and in steady current intensity due to friction effects, the tidal and gravity wave-induced currents have comparable intensities. In both the low- and mid-tidal zones, infragravity motions are weak outside the surf zone. The foregoing results show that outside the surf zone, these megatidal beaches are characterised by wave-dominated mid-tidal zones and tide-dominated low-tidal zones during spring tides. We suggest the term “mixed wave-tide-dominated” for these beaches with very large tidal ranges.

An experimental study of two-dimensional surface water waves propagating on depth-varying currents. Part 1. Regular waves

This paper describes an experimental study of two-dimensional surface water waves propagating on a depth-varying current with a non-uniform vorticity distribution. The investigation is divided into two parts. The first concerns the ‘equilibrium’ conditions in which the oscillatory wave motion and the current co-exist. Measurements of the water-surface elevation, the water-particle kinematics, and the near-bed pressure fluctuations are compared to a number of wave and wave–current solutions including a nonlinear model capable of incorporating the vertical structure of the current profile. These comparisons confirm that the near-surface vorticity leads to an important modification of the dispersion equation, and thus affects the nature of the wave-induced orbital motion over the entire water depth. However, the inclusion of vorticity-dependent terms within the dispersion equation is not sufficient to define the combined wave–current flow. The present results suggest that vorticity may lead to a significant change in the water-surface profile. If a current is positively sheared, dU/dz &gt; 0, with negative vorticity at the water surface, as would be the case in a wind-driven current, a wave propagating in the same direction as the current will experience increased crest–trough asymmetry due to the vorticity distribution. With higher and sharper wave crests there is a corresponding increase in both the maximum water-particle accelerations and the maximum horizontal water-particle velocities. These results are consistent with previous theoretical calculations involving uniform vorticity distributions (Simmen &amp; Saffman 1985 and Teles da Silva &amp; Peregrine 1988).The second part of the study addresses the ‘gradually varying’ problem in which there are changes in the current, the wavelength and the wave height due to the initial interaction between the wave and the current. These data show that there is a large and non-uniform change in the current profile that is dependent upon both the steepness of the waves and the vorticity distribution. Furthermore, comparisons between the measured wave height change and a number of solutions based on the conservation of wave action, confirm that the vorticity distribution plays a dominant role. In the absence of a conservation equation for wave action appropriate for nonlinear waves on a depth-varying current, an alternative approach based on the conservation of total energy flux, first proposed by Longuet-Higgins &amp; Stewart (1960), is shown to be in good agreement with the measured data.

Daily Fish and Zooplankton Abundances in the Littoral Zone of Lake Texoma, Oklahoma-Texas, in Relation to Abiotic Variables

Many studies have shown the effects of yearly or monthly environmental conditions on the structure of fish and zooplankton communities. Environmental conditions can also vary greatly on much shorter time scales. We tested the effects of abiotic conditions on the daily abundance of fish and zooplankton in the littoral zone of Lake Texoma, Oklahoma-Texas. After date was removed statistically from the analysis, no environmental variables were particularly important in determining the daily abundance of zooplankton, whereas, numbers of fish in the littoral zone were related to changes in wind velocity and wave height. Regressions of daily fish abundance against wave height showed that the response differed among species and among life-history intervals within species. Numbers of juvenile Dorosoma petenense, Notropis atherinoides, and Morone chrysops were positively correlated with wave height, whereas juvenile Menidia beryllina were negatively correlated with wave height. We suggest that changes in the abundance of particular species may be associated with (1) avoidance of inshore areas to escape possible physical damage by wave induced turbulence, (2) attraction to inshore areas to feed on prey organisms suspended in the water column by wave induced turbulence, and (3) avoidance of inshore areas to escape high predator abundance and increased possibility of being eaten due to turbulence.

一軸圧縮試験での弾性波速度および波高の減衰と横歪の関係 一軸引張試験結果との関係に注目した検討

The study was started mainly to investigate horizontal velocity change and attenuation not only in pre-failure region but also in post-failure region. Beyond the strength failure point, ten rocks were tested in uniaxial compression and elastic-wave velocity and wave height in lateral direction were measured. Typically, the velocity and the wave height started decreasing at relatively low stress level in pre-failure region. The wave height decreased more rapidly than velocity; (Wave height change) / (Initial value of wave height) = 3 (Velocity change) / (Initial value of velocity). It was also found that the velocity and the wave height were correlated with the lateral strain which increased with crack number and length within a rock specimen.The experimental results in compression were compared with those in tension which had been carried out by the authors, and two interesting findings were obtained. 1) At the strength failure point of Inada granite and Akiyoshi marble, the lateral strain in uniaxial compression test was close to that in uniaxial tension test. 2) Energy loss in a loading-unloading cycle in uniaxial tension test was compared with the attenuation of elastic wave in uniaxial compression test, and a good correlation was found between the two.

Changes of wind waves in the North Atlantic over the last 30 years

In order to evaluate long-term climatic changes in wind wave height, visual wave estimates available from the Comprehensive Ocean–Atmosphere Data Set (GOADS) were updated for the period from 1964 to 1993. Analysis of the accuracy of visual estimates shows that observations from merchant ships can be used for the study of climate changes in storminess. Climate changes obtained in significant wave height, computed on the basis of the voluntary observed data, are quite consistent with those shown by the instrumental records at OWS L, Seven Stones Light Vessel and NDBC buoys. The linear trends in significant wave height, as well as in the wind sea and swell heights, were computed for the entire North Atlantic. Significant wave height increases of 10–30 cm/decade over the whole of the North Atlantic, except for the western and central subtropics were found. Changes in the swell height are very consistent with those seen in significant wave height. Nevertheless, wind sea indicates strong upward tendencies only in the central mid-latitudinal North Atlantic and does not show any significant trends in the Northeast Atlantic, where instrumental records of Bacon and Carter report secular changes of about 1% a year. Wind waves of smaller occurrences show significantly negative changes in the Northeast Atlantic; that is in agreement with the wind sea periods changes. Possible mechanisms driving the swell changes with no pronounced increase of the sea height and wind velocity are discussed. Changes in the intensities of intramonthly variability in different synoptic ranges are considered as the major agent of the increasing swell. The conclusion is made that the upward swell changes are driven by the intensification of high frequency synoptic processes. Copyright © 1999 Royal Meteorological Society

Fifteen years of global wave hindcasts using winds from the European Centre for Medium‐Range Weather Forecasts reanalysis: Validating the reanalyzed winds and assessing the wave climate

The ERA (European Centre for Medium‐Range Weather Forecasts Reanalysis) project resulted in a homogeneous data set describing the atmosphere over a time span of 15 years, from 1979 to 1993. To validate (part of) these data against independent observations we use the ERA surface winds to drive the WAM wave model. The modeled significant wave heights are then compared with observations. From this comparison the quality of the forcing winds is assessed. The patterns of computed wave heights agree well with observed patterns, and they are of the right magnitude. This confirms the realistic nature of the ERA winds. If one looks in detail, it appears that the significant wave heights resulting from the model are systematically lower than the observed ones in areas of high winds and waves and higher in areas of low winds and waves. It is argued that underestimation at high winds speeds is most likely a resolution effect, as wind and thus wave peaks are missed by finite resolution in space and time, while overestimation at low wind speeds most likely results from internal WAM errors. It is concluded that the monthly mean ERA winds are slightly (less than 5%) too low in areas of high winds, while from this study it is not possible to draw a decisive conclusion on the quality of ERA winds at low wind speeds. At the same time, the hindcast data form a 15‐year climatology of global waves. This climatology is analyzed in terms of annual cycle and trends. The largest trends in significant wave height occur in the North Atlantic with an increase of more than 12 cm/yr in January, and south of Africa where the increasing trend exceeds 7 cm/yr in July. These trends, however, are only marginally significant. Furthermore, they exhibit a large month‐to‐month variability, so that on a seasonal basis the trends are significant only in small parts of the ocean. In conclusion, we are unable to confirm a significant change in wave height during the ERA period.

Control of estuarine sediment dynamics by interactions between currents and waves at several scales

The effect of interactions between continuous (tidal currents) and intermittent (waves) processes on sediment dynamics and transport is addressed by presenting detailed field measurements of waves, boundary-layer currents and suspended sediment from an estuarine channel and an adjacent intertidal sandflat in Manukau Harbour, New Zealand. The aim is to determine in what ways it is necessary to couple waves and currents in numerical models, and thereby put limits on the fundamental structure of process-based estuarine sediment transport models. Waves were important on the intertidal flat: turbidity switched on and off with the appearance and disappearance of waves; wave groups dominated entrainment of bed sediment; a wave-current boundary-layer model explained measured bed shear stress and hydraulic roughness; and the measured near-bed time-averaged suspended-sediment concentration was mostly well predicted by a pure-wave model. Both the waves themselves and wave-related processes varied markedly over the tidal cycle. The variation in the former (principally changes in wave height) was related to changes in fetch caused by the harbour-wide emergence and submergence of intertidal regions. The variation in the latter was related to changes in water depth relative to the wavelength of the waves, which controlled the penetration to the bed of wave-orbital currents. In addition to the variation over the tidal cycle in the ‘intensity’ of wave processes, there was also a change in ‘kind’, which occurred with the arrival at the measurement site of the ‘turbid fringe’, which is the narrow, highly turbid edge of the estuarine water body. The relationship between suspended-sediment concentration and wave-orbital velocity in the turbid fringe was radically different to the relationship in the estuarine water body proper, which suggests a change in dynamics, perhaps related to breaking waves. A ‘hybrid’ modelling approach is required, i.e. one that treats discrete events but resolves tidal-cycle-scale variation within the event. There is a need to resolve the variation in the wave train over the tidal cycle and the penetration to the bed of wave-orbital motions, both of which could only be done adequately within an estuary tidal model. In contrast to the situation on the intertidal flat where waves intermittently entrained sediment, sediment transport in the channel was continuous, driven by tidal currents. To predict sediment flux in the channel we need to know the upstream sediment-transporting capacity of the flow (including that contributed by waves), the character of the bed sediment, and the sediment-settling time scale. These factors confounded even the simplest notion of flood and ebb dominance, which frequently has been applied to understand estuarine morphodynamics.

Wave attenuation characteristics of a tethered float system

Wave attenuation characteristics of a tethered float system have been investigated for various wave heights, wave periods, water depths, depths of submergence of floats and float sizes. As the floats are similar in size and shape, only a single tethered spherical float is considered for the theoretical analysis. Float motion is determined through the dynamical equation of motion, developed for a single degree of freedom. From incident and transmitted wave powers, transmission coefficients are computed. The results show that transmission coefficient does not vary with changes in wave height or water depth. When depth of submergence of float increases, wave attenuation decreases, showing that the system performs well when it is just submerged. As float velocity decreases with increase in float size, transmission coefficient increases with increase in float size. The influence of wave period on wave attenuation is remarkable compared to other parameters. The effect of drag on wave attenuation is studied for varying drag coefficient values. Theoretical results are compared with experimental values and it is found that theory overestimates wave attenuation which may probably be due to various linearisations involved in the theoretical formulation.

The corrections for significant wave height and attitude effects in the TOPEX radar altimeter

The routine ground processing of data from the NASA radar altimeter of TOPEX/POSEIDON includes instrument corrections for the effects of significant wave height and attitude angle changes on the altimeter's estimates of range, backscattered power, and significant wave height. This paper describes how these instrument corrections were generated and how they are applied. Detailed waveform fitting to telemetered waveform samples is used to assess the effectiveness of the corrections. There are several altimeter hardware‐caused small waveform departures from the model waveforms and these departures, designated waveform “features,” are described in detail. A consequence of the waveform features, and their positioning relationship to range rate, is that range data for ground tracks moving toward the equator may differ systematically by about a centimeter compared to range data for ground tracks moving away from the equator. The results and discussion are limited to side A of the redundant altimeter, as only side A has been operated on orbit.

Estimation of wave height duration statistics

A theoretically founded parametric model for the estimation of duration statistics for significant wave height is established. Use of the model requires information both on the distribution and the average absolute rate of change of significant wave height. Results from analyses of data indicate that the model provides fairly accurate estimates of mean duration of exceedance and non-exceedance of a threshold level. The durations of exceedance and non-exceedance are found to vary considerably about the mean.

THE ECOLOGY OF THE HONG KONG LIMPETS CELLANA GRATA (GOULD, 1859) AND PATELLOIDA PYGMAEA(DUNKER, 1860): DISTRIBUTION AND POPULATION DYNAMICS

Cellana grata is a high-zoned, exposed-shore limpet with a lifespan of 3 years. Patclloida pygmaeais a low-zoned, sheltered-shore limpet with a lifespan of I year. C. grata migrates up-shore in winter, but occurs lower on the shore in summer. This migration is synchronized with variations in algal availability high on the shore; such variations are, however, themselves related to seasonal changes in wave height. P. pygmaea is restricted to the low shore, year round. The movements upwards in winter and downwards in summer are still evident. Algal species richness increased from September to reach a peak between December and March, and to abruptly diminish between May and June. The encrusting algae Brachytrichia maculans, Hildenbrandia prototypus, Lithothamnion sp. and Ralfsia vcrrucosa were present year round. The timing of juvenile recruitment of C. grata and P. pygmaea was from October to July and February to October, respectively. Temperature may be important in enhancing juvenile survival by affecting the seasonal pattern of algal occurrence on the shore. Seasonal fluctuations in the rate of growth were recorded for both species with the time of fastest growth of C. grata and P. pygmaea occurring between October and March and September and February, respectively. Mortality of both species occurred mainly in summer. Summer heat, predation and intra-specific competition for food appear to be the major causes of death.

Velocity Field around Large Cylindrical Structures

This paper aims to discuss the velocity field around large cylindrical structures under wave action, based on the laboratory experiments and numerical calculation. The wave-induced currents around large cylinders are revealed to direct offshoreward through the whole depth except near bottom and above the wave trough. When the diffraction parameter D/L (D is the diameter of a cylinder, L is the wavelength) is large, the wave height and velocity change complicatedly in space.