Joan Oltman-Shay

Observations of infragravity waves

Infragravity-wave (periods of one-half to a few minutes) energy levels observed for about 1 year in 8-m water depth in the Pacific and in 8- and 13-m depths in the Atlantic are highly correlated with energy in the swell-frequency band (7- to 20-s periods), suggesting the infragravity waves were generated locally by the swell. The amplification of infragravity-wave energy between 13- and 8-m depth (separated by 1 km in the cross shore) is about 2, indicating that the observed infragravity motions are dominated by free waves, not by group-forced bound waves, which in theory are amplified by an order of magnitude in energy between the two locations. However, bound waves are more important for the relatively few cases with very energetic swell, when the observed amplification between 13- and 8-m depth of infragravity-wave energy was sometimes 3 times greater than expected for free waves. Bispectra are consistent with increased coupling between infragravity waves and groups of swell and sea for high-energy incident waves.

A generalized equilibrium model for predicting daily to interannual shoreline response

Coastal zone management requires the ability to predict coastline response to storms and longer-term seasonal to interannual variability in regional wave climate. Shoreline models typically rely on extensive historical observations to derive site-specific calibration. To circumvent the challenge that suitable data sets are rarely available, this contribution utilizes twelve 5+ year shoreline data sets from around the world to develop a generalized model for shoreline response. The shared dependency of model coefficients on local wave and sediment characteristics is investigated, enabling the model to be recast in terms of these more readily measurable quantities. Study sites range from microtidal to macrotidal coastlines, spanning moderate- to high-energy beaches. The equilibrium model adopted here includes time varying terms describing both the magnitude and direction of shoreline response as a result of onshore/offshore sediment transport between the surf zone and the beach face. The model contains two coefficients linked to wave-driven processes: (1) the response factor (φ) that describes the “memory” of a beach to antecedent conditions and (2) the rate parameter (c) that describes the efficiency with which sand is transported between the beach face and surf zone. Across all study sites these coefficients are shown to depend in a predictable manner on the dimensionless fall velocity (Ω), that in turn is a simple function of local wave conditions and sediment grain size. When tested on an unseen data set, the new equilibrium model with generalized forms of φ and c exhibited high skill (Brier Skills Score, BSS = 0.85).

Effect of alongshore nonuniformities on longshore current predictions

Longshore currents have been primarily modeled by assuming alongshore uniformity. In this paper we revisit the problem of nonuniform longshore currents to refocus attention on the importance of accounting for alongshore nonuniformity in longshore current predictions and to provide an analytical estimate of the importance of this nonuniformity. We derive a semianalytical solution for the longshore currents that allows for weak alongshore variations in the bottom topography. The solution shows that the alongshore pressure gradient induced by alongshore variations in the bottom topography could contribute substantially to the forcing for the longshore currents. An example calculation shows that the longshore current could deviate by up to ±30% from the mean for a ±10% deviation of the bottom topography. Thus we suggest that in many practical cases it is important to include the alongshore pressure gradient to accurately model longshore currents.

Edge waves on nonplanar bathymetry and alongshore currents: A model and data comparison

The numerical solutions for the dispersion of linear, shallow-water edge waves are compared with observed alongshore wavenumber-frequency spectra of edge wave variance to examine their prediction of the effects of nonplanar bathymetry and moderate mean alongshore currents. Field observations were made at the two Southern California Nearshore Sediment Transport Study field sites, Torrey Pines (1978) and Leadbetter (1980) beaches. On Torrey Pines beach, where the bathymetry is very planar except for a concave (steepening landward) beach face, observed departures from the analytical plane beach dispersion solution, attributed to the concave face, are predicted by numerical dispersion solutions using the measured cross-shore depth profile. On both beaches, asymmetry in observed dispersion curves of upstream and downstream progressing edge waves are well predicted by numerical dispersion solutions using the cross-shore profile of measured mean alongshore current. Spectra of the alongshore velocity component from both beaches show changes in edge wave mode dominance with increased frequency. Oltman-Shay and Guza (1987) demonstrated that mode dominance transition with frequency, as observed from a fixed offshore location, is associated with the seaward decay of edge waves and that the approximate frequency of mode transition is predicted by the profile solutions for edge wave variance and the assumption of equal shoreline elevation variance amongst modes. A submerged concave beach face at Torrey Pines beach is shown to alter the predicted cross-shore scaling of the edge wave profiles from that predicted when mean water is below the concave beach face, and therefore to alter the mode transition frequency predictions. Different mode transition frequencies are observed in spectra from data runs with mean water levels both above and below the concave beach face, and the differences are shown to be reasonably predicted using edge wave profile solutions for the measured beach profile. For these data, observed and predicted mode transition frequencies did not change appreciably in the presence of mean alongshore current, implying that large changes in cross-shore scaling of the edge wave profiles did not occur. However, as has been previously shown by Howd et al. (1992), there are also local edge wave profile shape changes because of the cross-shore shear of the mean alongshore current. Verification of these predicted local shape changes can not be made with these data. Confidence in cross-shore profile solutions is of practical importance for estimates of shoreline variances from offshore measurements. On these two beaches, estimates using both numerical solutions for the measured depth and current profiles, and analytical solutions for plane beach approximations typically differ by only 10 to 40%.

WAVE GROUP FORCING OF LOW FREQUENCY SURF ZONE MOTION

The nearshore potential vorticity balance of Bowen and Holman (1989) is expanded to include the forcing from wave group induced radiation stresses. Model results suggest that the forcing from these radiation stresses can drive oscillations in the longshore current that have a spatial structure similar to linear shear instabilities of the longshore current. In addition, the forced response is nearly resonant when the forcing has scales (k,σ) similar to the linearly most unstable mode. Thus, we suggest that wave groups may provide an initial perturbation necessary for the generation of shear instabilities of longshore currents and also act as a source of vortical motions on beaches where linear instabilities are completely damped. Data from the SUPERDUCK (1986) field experiment were analyzed for the presence of spatially coherent wave groups. The analysis confirms that wave groups with periods and longshore spatial structures comparable to the observed shear wave motions were sometimes present on this open coast. This indicates that wave groups with the required spatial and temporal structure to initiate the low frequency oscillations in the longshore current can exist.

OBSERVATIONS OF THE SWASH EXPRESSION OF FAR INFRAGRAVITY WAVE MOTIONS

Tanah Lot Temple is situated in Tabanan Regency - Bali, on the coast of the Indonesian Ocean. Due to continuous wave attack, wind force, and weathering of the rock bank where the Temple stands, abrasion has occured which is more and more threatening the existence of the Temple. Considering that Tanah Lot Temple is a sacred place for the Hindu Balinese people and a place of high cultural value, and also an important tourism, steps to save the Temple are imperative. The Central as well as the Regional Authorities, and also the Bali nese community are very much interested in the effort to keep the Temple intact. Measures have been undertaken to protect both the seaside and land-side banks of the Temple rock bank. This paper only discusses counter measures of the sea —side bank of the Temple.

ON THE VISCOUS DESTABILIZATION OF LONGSHORE CURRENTS

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.

LOW FREQUENCY SURF ZONE RESPONSE TO WAVE GROUPS

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.

EVOLUTION OF INFRAGRAVITY VARIANCE DURING A STORM

Tanah Lot Temple is situated in Tabanan Regency - Bali, on the coast of the Indonesian Ocean. Due to continuous wave attack, wind force, and weathering of the rock bank where the Temple stands, abrasion has occured which is more and more threatening the existence of the Temple. Considering that Tanah Lot Temple is a sacred place for the Hindu Balinese people and a place of high cultural value, and also an important tourism, steps to save the Temple are imperative. The Central as well as the Regional Authorities, and also the Bali nese community are very much interested in the effort to keep the Temple intact. Measures have been undertaken to protect both the seaside and land-side banks of the Temple rock bank. This paper only discusses counter measures of the sea —side bank of the Temple.

Influence functions for edge wave propagation over a nonplanar bottom bathymetry

The problem of edge waves propagating over a nonplanar bottom bathymetry is examined. Assuming that the variation from the planar case is small, we examine the problem using a perturbation expansion. This assumption allows an analytical estimate of how nonplanar features change the frequency and spatial structure of an edge wave with a given wavenumber. We find that these changes can be conveniently expressed in terms of “influence functions.” For example, the change in frequency, σ1,n, of a given edge-wave mode, n, can be expressed as σ1,n/σ0,n=∫0∞h1Iσdx where x is the cross-shore coordinate, h1(x) is the deviation from the planar topography, σ0,n is the frequency of the edge wave on a plane beach, and Iσ is the influence function. Similar results are also derived for the spatial structure of the edge wave. The results show that 1) in general, the spatial structure of the edge wave is more sensitive to bottom perturbations than the frequency, and 2) at a given wavenumber, the higher modes are more sensit...