Surf Zone Width Research Articles

Statistical Description of Longshore Transport Environment

The longshore transport environment at a coastal site is represented in terms of six parameters. The total population of longshore transport-rate observations at a site is separated into two populations, positive transport and negative transport, each of which canbe described by a log-normal probability distribution. The six parameters describing the environment are the two means and standard deviations that define the two log-normal distributions and the fractions of time the transport is positive and negative. The log-normal distribution appears to fit data from a wide variety of sites and for a wide range of types of wave data. Longshore transport rates calculated from longshore current observations depend on surf-zone width; thus the coefficient relating longshore transport rates to surf-zone conditions varies from site to site. Annual net longshore transport rates are normally distributed. Synthetic net transport-rate data at Indian River Inlet, Delaware, was generated having the same statistical parameters as the observed transport rates.

TOWARD AN IMPROVED EMPIRICAL FORMULA FOR LONGSHORE SAND TRANSPORT

This paper presents results of two field experiments performed using portable traps to obtain point measurements of the longshore sand transport rate in the surf zone. The magnitude of the transport rate per unit width of surf zone is found to depend on the product of the local wave height and mean longshore current speed, but correlation is much improved by including two correction terms, one accounting for local wave energy dissipation and the other for the fluctuation in the longshore current. The field transport rates are also found to be compatible with laboratory rates obtained under combined unidirectional and oscillatory flow. Total transport rates previously reported for this experiment program are revised with recently determined sand trapping efficiencies.

LABORATORY EXPERIMENTS ON THE INFLUENCE OF WIND ON NEARSHORE WAVE BREAKING

The influence of wind on nearshore breaking waves was investigated in a laboratory wave tank. Breaker location, geometry, and type depended upon the wind acting on the wave as it broke. Onshore winds tended to cause waves to break earlier, in deeper water, and to spill: offshore winds tended to cause waves to break later, in shallower water, and to plunge. A change in wind direction from offshore to onshore increased the surf zone width by up to 100%. Wind's effect was greatest for waves which were near the transition between breaker types in the absence of wind. For onshore winds, it was observed that microscale breaking can initiate spilling breaking by providing a perturbation on the crest of the underlying wave as it shoals.

Offshore Breakwaters in Laboratory and Field

Small scale model tests for single and multiple offshore breakwaters were performed in a spiral wave basin to examine the effects of their geometric parameters on the morphological change in their vicinity. The model results are compared with physical model studies reported by others and with offshore breakwaters in the field. All the horizontal lengths are nondimensionalized with respect to the offshore distance of the breakwater from the original shoreline, X\dB. Three dimensionless variables, X\u*\db(=X\db/X\dB), L\u*\dB(=L\dB/X\dB), and G\u*\dB(=G\dB/X\dB), are found to be important, in which X\db, L\dB, and G\dB are the surf zone width, the length of breakwater, and the gap spacing between adjacent breakwaters, respectively.

Distribution patterns of rip frequency and intensity in Algoa Bay, South Africa

A new technique of rip-current recording has been employed to provide quantitative data on rip frequency and intensity during twelve aerial surveys along the shores of Algoa Bay, South Africa. Distinct alongshore gradients in surfzone width and beach morphodynamic state were recorded. These gradients, themselves related to the geography of the bay, wind direction and the angle of deep-sea swells, result in a gradient of rip frequency and intensity in the bay. Rip currents always appear to be present along the Sundays River Beach (central and northern shores), but their presence along the southwest shore was restricted to easterly wind-wave conditions. Both rip frequency and intensity were positively and significantly correlated with surfzone width and beach state over the range of sea conditions encountered. Beach state accounted for 62% of the alongshore variability in rip frequency and 35% of the variability in intensity.

Rip-current type, spacing and persistence, Narrabeen Beach, Australia

Long-term daily rip observations on Narrabeen Beach, Australia, were made together with daily measurements of wave height, period, direction, surf-zone width and beach state. A total of 3513 rips were observed on 270 days over a 19-month period. Results show that rips accompany moderate to high waves and are associated with intermediate beach types. Rip spacing follows wave conditions increasing in spacing, size and intensity as waves rise, and conversely as they fall. Rips are a function of both the prevailing and antecedent wave conditions and the rate and direction of change in wave conditions. Rips are classified into three types: erosion, mega and accretion. Erosion rips are initiated by rising seas in the transverse bar and rip and higher beach states. They accompany general beach erosion. They are widely spaced (Narrabeen y s = 300–500+ m, σ = 100–200 m) increasing in size and intensity with the waves until the fully dissipative (ripless) state is reached. They are both temporally and spatially highly variable persisting in one location for only hours to a day or so, except where topographically controlled. They are a major mechanism for the seaward transport of sediment and under extreme conditions may extend over a kilometer seaward of the breaker zone. Mega rips are large-scale (>1 km) topographically controlled erosion rips that persist when nearshore and/or embayment topography prevents the development of the fully dissipative state by inducing wave refraction and persist longshore gradients in surf-zone dynamics that in turn drive the rip circulation. Accretion rips prevail during stable or falling wave conditions, usually following erosion rip formation. They are associated with general beach accretion, are more closely spaced (Narrabeen y s = 170–250 m, σ = 100 m), less intense and become increasingly topographically arrested by crescentic bars and rhythmic bar-beach morphology of the longshore bar-trough, rhythmic bar and beach and transverse bar and rip beach states. They are relatively stable both spatially and temporally and may persist in one location for days to weeks if wave conditions remain favorable. They are fair weather rips which may disappear by infilling (in the low-tide terrace beach state) or be destroyed by rising seas. Rips generally are absent from the reflective, low-tide terrace and fully dissipative beach states.

Storm-induced response of a nearshore-bar system

A nearshore-bar system was surveyed periodically through a storm and the following recovery period. The data showed a very rapid response of morphology to changing wave conditions and allowed various models on bar formation to be tested. Under low-energy conditions prior to the storm a small bar was surveyed 13 m offshore. Both the high reflectivity of the beach and the cross-shore distance to the bar are consistent with a model of sediment convergence at the node or antinode of a standing wave of incident period. Such a small-scale bar may be a common feature on beaches with steep foreshores and more gentle offshore slopes. With the increase in wave height during the storm, the bar became better developed and migrated offshore at rates up to 2.2 m h −1. The bar maintained its form in that the ratio of trough depth to crest depth ( h t h c ) remained roughly constant. The bar was in no way related to processes which would cause the convergence of sediment in the breaker zone; through most of the storm the bar-crest distance offshore was typically only 10% of the surf-zone width. Analysis of the bar distance offshore in terms of a standing wave motion showed that the causative wave period must have been much longer than that of incident waves, probably on the order of a minute. Surf-zone wave data showed significant energy in the infragravity band at these periods although no definite link has been made. After the height of the storm, the bar had a crescentic morphology. The development of this morphology occurred very rapidly with parts of the bar migrating onshore at rates up to 1.2 m h −1. In contrast to the storm, during the recovery period h t h c varied by nearly a factor of three. Analysis of the offshore and longshore length scales showed the bar to be similar to one which would be generated by a standing mode 1 edge wave of period on the order of one minute.

Beach and nearshore facies: Southeast Australia

The morphology, texture and facies sequence on seven sand beaches, located in low, moderate and high wave energy, microtidal environments in southern Australia were investigated using box coring and Scuba observations. Systematic variation in facies occur both within and between the beaches. Low-energy reflective beaches are limited in lateral and vertical extent and in facies to beach laminations separated by coarse step deposits from finer nearshore cross-lamination facies. Moderate-energy intermediate beaches characterised by rip circulation possess increasingly wider surfzones with ridge and runnel and bar-trough facies separating the beach and step facies from the more extensive nearshore sequence. High-energy dissipative beaches may have 500 m wide surfzones containing multiple bar-trough topography. Fine beach laminations with backwash structures grade into 4–5 m thick bar-trough sequences then the extensive nearshore facies. As wave energy increases from low ( H b < 1 m) to high ( H b > 2.5 m) the vertical extent of the beach to nearshore sequence increases from <10 m to approximately 30 m, and the width from 100 m to several kilometres. Consequently one would expect higher-energy paleo-beach sequences to be represented more by diagonal than vertical facies sequences.

Effects of nonlinear inertial forces on nearshore currents

A perturbation method is developed to investigate the effects of nonlinear inertial forces on breaking-wave-induced nearshore currents. Obliquely incident waves are considered and the bottom topography varies periodically in the shoreline direction. For simplicity, lateral turbulent mixing is ignored. Numerical examples are given for a specified bottom topography with various angles of incidence and surf zone width. The nearshore currents of interest mainly belong to the meandering type. When the nonlinear terms are included, there appears an advective shift as well as an amplitude reduction in the current pattern.

EFFECT OF RIP CURRENT BARRIER ON HARBOR SHOALING

Prototype experiments on rip currents and sediment transport around structures were conducted at two fishery harbors on microtidal high energy beaches facing the Pacific Ocean. The purpose of the experiments was to examine the performance and mechanism of rip current barrier structures on harbor shoaling. Based on the results of five experiments, the wave breaker heights during which varied from 1.1 m to 3.0 m, it is concluded that shore-parallel rip current barriers are effective if their length is greater than the surf zone width and if they are located outside the surf zone. When the above conditions are satisfied, the rip current barrier is a cost-effective measure against shoaling of small craft harbors.

Wave and tidal-current sorting of sand on a wide surf-zone beach

Experiments with fluorescent tracers have shown that on a wide surf-zone beach, selective transport of different grain-size material may play an important role in the sorting of beach sediments by waves and tidal currents. In the alongshore direction the results have shown that at the mid-tide level small particles in general move more rapidly than coarse material, although there is evidence to suggest that with increasing particle size the net rate of movement passes through a minimum. Such behaviour would be consistent with Komar's (1977) observations and suggestion of a minimum corresponding to the transition from bedload to suspended load transport of sediment. Across the beach, fluorescent tracer samples have shown onshore—offshore sorting about the mean position of the breaker line with coarse material moving offshore and fine material moving onshore.

Wave, beach and dune interactions in southeastern Australia

A morphodynamic classification of surfzones, beaches and dunes of the microtidal, low- to high-energy southeast Australian coast is presented. The first section (A: Waves and beaches) briefly deals with the transformation of deep-water wave energy as it crosses the shelf, nearshore and surfzone. Depending on the deep-water wave characteristics and shelf and inshore morphology, resultant breaker wave energy may be high, moderate or low, and the accompanying beach surfzone morphodynamic state in fine to medium sand beaches either dissipative, intermediate or reflective. Dissipative beaches have wide surfzones with shore parallel bar(s) and channel(s) and predominantly shore-normal circulation; intermediate beaches are characterised by rip circulation, crescentic-transverse bars and megacusps; and reflective beaches by a barless surfzone and steep, cusped or bermed beach. Each beach form has a characteristic level of beach stability, zone of sediment storage and mode of beach and dune erosion. Landward aeolian sediment transport of swash-deposited sand (Section B) is dependent on the subaerial beach topography and the aerodynamic flow regime across that topography. Characteristic profile shapes are ascribed to each beach type (dissipative, intermediate and reflective). Aeolian sand transport rates are potentially highest on dissipative beaches, moderate on intermediate beaches and lowest on reflective beaches. These rates determine the potential size of foredunes which are correspondingly largest on dissipative beaches and smallest on reflective beaches. The combination of mode and frequency of beach/dune erosion, rates of aeolian sand transport, and foredune volume and morphology provide an explanation of the nature and morphology of landward-occurring, large-scale dune systems. Dissipative beaches are frequently characterised by large-scale transgressive dune sheets; intermediate, by a trend from large-scale parabolic dune systems (high-wave energy) to small-scale blowouts (low-wave energy); and reflective beaches by minimal dune development.

Dynamics of a high-energy dissipative surf zone

Pressure and horizontal current ( u, v) time series were measured at different positions across the inner 150 m of a wide (≈500 m) surf zone of a microtidal high wave-energy beach. Incident waves had average heights of 3–4 m with maxima of 5 m and periods of 12 to 15 sec. Bores of broken waves diminished in height at a nearly constant rate as they progressed across the surf zone. The ratio, γ, of bore height H to local water depth h was everywhere less than 1 for even the highest bores and was on the order of 0.40 for the significant bores at incident wave frequencies. Rip circulation was weak or absent but a moderate longshore current was present. Shore-normal flows were vertically segregated with strong net onshore flows prevailing just below the surface accompanied by weaker net seaward flows near the bed. Spectra of water surface oscillations, η as determined from pressure, u, and v reveal that most of the energy in the inner surf zone was at infragravity frequencies (periods greater than 30 sec). Shoreward decay of wave energy at incident wave frequencies was accompanied by shoreward growth of infragravity energy. Near the beach the infragravity oscillations had heights on the order of 1 m. Cross-spectra show that the infragravity oscillations were standing in the shore-normal direction. From the relative magnitudes of infragravity versus incident wave currents, it is inferred that the surf beat may be an order of magnitude more important than incident waves to the transport of sediment in the inner surf zone.

NEARSHORE SUSPENDED SEDIMENT LOAD DURING STORM AND POSXTSTORM CONDITIONS

As part of the DUCK-X experiment at the CERC field research facility at Duck, North Carolina in September, 1978, suspended sediment measurements were made along the CERC pier. In situ bulk water samples were collected during a moderate northeast storm and two days later during post-storm wave conditions. Concentrations varied from approximately 0.01 g/1 to over 10.0 g/1. Vertical arrays of suspended sediment samples indicated that concentration decreases rapidly up to two meters above the bed, then remains relatively constant, reflecting the nature of the suspension; intermittent suspension of sand near the bed, and continuous washload higher in the water column. Concentrations were at a maximum during storm conditions when measured values were 3 to 5 times higher than during non-storm conditions. The total load of sediment in a pier cross section during sampling periods in storm and post^storm conditions was calculated from arrays of 49 samples each. With H1/3 exceeding 2.3 HI and the surf zone width over 300 m during the storm, the total load of sediment in suspension was approximately 10 times higher than during poststorm conditions (Hi 73 - 1.2 m and surf zone width approximately 100 m) . Estimates of the longshore flux of suspended sediment indicate that as much as 60 times more sediment was transported during storm than during post-storm conditions. Longshore transport of sediment measured from 5 cm above the bed to the surface reached the equivalent of 22,330 m^/day. This value corresponds very closely to longshore transport predicted from wave energy flux. During post-storm conditions, on the other hand, transport of suspended sediment accounts for less than one-third of the transport predicted from wave energy flux.

BEACH RESPONSE TO VARIATIONS IN BREAKER HEIGHT

The breaker wave height required to produce a particular beach-surfzone morphology is examined for micro-tidal, medium to fine sand beaches. Waves &gt;2.5m produce dissipative beach systems with wide surfzones and shore parallel bar/s and channel/s. Breakers between 1 and 2.5m result in rip circulation and associated rhythmic morphology (crescentic bars, rip channels, megacusps, etc). Low waves (

Edge waves on complex beach profiles

In order to interpret field data in terms of edge wave modes, investigators usually assume that the beach profile is linear, allowing the use of simple analytical solutions for edge wave structure and wavelength. The validity of this assumption is checked by using a numerical model to find the edge wave modes on a typical concave beach. Results show that estimates of edge wave wavelength using the plane beach assumption can be wrong by ±100% at a fixed mean sea level with a further error of ±50% introduced by the tides. Greatly improved estimates can be made if the value of beach slope is chosen to reflect suitably the offshore extent of the edge wave. A rule of thumb for calculating the effective beach slope βe is , where h is the water depth at distance x′ from shore, L is the edge wave wavelength, and n is the edge wave mode number. The motion of the offshore profile of the edge wave due to a change of tidal elevation is dramatically different on a concave beach than on a plane beach. The tendency for the profile to move offshore for a drop in tide is compensated by the decrease in mean beach slope and hence reduction in wavelength and offshore extent of the wave. For a fixed instrument a drop in tide may lead to a profile shift onshore or offshore or no shift at all. Edge wave damping on a concave beach is shown to change as a function of tide. At low tide the surf zone width is increased while edge wave wavelength is decreased, both factors increasing damping. Thus edge wave spectra taken at low tide may be less energetic than those at high tide, particularly at high frequencies.

Surf-beach dynamics in time and space — An Australian case study, and elements of a predictive model

Inshore morphology and circulation on a medium- to high-energy surf beach — Durras, N.S.W., Australia — is analysed from a semi-continuous run of data maps covering a 2.25-km length of beach monitored at 50-m intervals. Twenty surveys were made over about five weeks, the only significant gap being an interval of twelve days between the thirteenth and fourteenth records. Nearshore waves and winds varied considerably during the observation period; inshore morphology was highly changeable and longshore and rip circulation rarely was well fitted to morphology. From statistical analysis of morphologic and circulation patterns we identify a set of seven inshore states, related not only to immediate wave climate but also to antecedent morphology and wave changes. We address the problem of adjustment to energy changes, for the two-dimensional inshore—nearshore profile, using a model in which wave-work on the bed is uniformly distributed. Model profiles fit closely to our bathymetric data, and appear to predict reasonably the surf-zone width variations resulting from energy changes.

Morphodynamics of reflective and dissipative beach and inshore systems: Southeastern Australia

Field experiments involving direct measurement of surf and inshore current spectra, inshore circulation patterns, and depositional morphology have been replicated under different energy conditions and in several environmentally contrasting beach localities on the high-energy coast of New South Wales, Australia. The region exhibits compartmentalized beach systems and is dominated by a highly variable wind-wave climate superimposed on persistent high-energy swell ( T = 10–14 sec). Two general types of beach system occur: (1) predominantly reflective systems in which much of the incidentwave energy is reflected from the beach face; and (2) dissipative systems with wide surf zones and high turbulent energy dissipation. Reflective systems are characterized by steep, linear beach faces, well-developed berms and beach cusps, and surging breakers with high runup and minimum setup; rip cells and associated three-dimensional inshore topography are absent. Wave height and current spectra from reflective beaches consistently have their dominant peaks at incident wave and subharmonic frequencies, and cross-spectra indicate the existence of low-mode edge waves at those frequencies. Infra-gravity peaks are negligible. Under low-energy conditions subharmonic peaks are low relative to incident-wave peaks; however, increasing breaker height tends to be accompanied by increasing subharmonic dominance. Analyses of shore-normal currents near the bed show that under all conditions the strongest shoreward motions are induced by the incident waves; however, seaward motion near the beach face is subharmonic-dominated. Dissipative systems characterize the exposed open coast and are fronted by concaveupward nearshore (seaward of break) profiles and wide flat surf zones. Waves break 75–300 m seaward of the beach and dissipate much of their energy before reaching the beach, creating significant radiation-stress gradients and setup. Topography is much more complex and varied than in the case of reflective beaches; one or more bars, three-dimensional inshore topography and different scales of rip cells are normally present. The commonly occurring time-and-environment-dependent morphologic states can be classified into six general types. The greatest total dissipation is associated with Type 1 which prevails in the regions of most abundant inshore sediments or during and immediately after severe storms. Setup is highest and runup is lowest (relative to incident-wave height) with this type and the dominant energy near the beach is in the surf-beat part (80–120 sec) of the spectrum. As the bar migrates shoreward (Types 2 and 3) and beach face steepens and a deep trough develops within which the partially dissipated waves reform. Although the outer surf zone remains dissipative, synchronous and subharmonic resonance occurs near the beach face and, as with reflective beaches, low-mode edge waves form beach cusps. A conspicuous feature of Types 2 and 3 is the occurrence of pronounced resonant spectral peaks at 4 T ( ⋍ 40–50 sec ) within the trough and on the bar. Edge waves at this frequency may be responsible for the development of the crescentic bar forms (Type 3). Lower frequency surf beat peaks also remain present but are secondary. The peak at 4 T attenuates as Type 4 develops and is not present with Type 5 topography but the first subharmonic (2 T) becomes more pronounced, particularly with high tide. Type 6 morphology represents the fully accreted beach state and occurs only after prolonged periods of low swell. This type is a reflective beach with a steep linear beach face and a very high berm which remains continuous for long distances alongshore; rips are always absent. Wave and current spectra are also similar to those described for reflective beaches.

Finite Element Modeling of Nearshore Currents

A finite element model is developed to compute the nearshore currents induced by breaking waves in the surf zone. The normal incident wave system is employed so as to study the effects of beach topography on the current circulation patterns. The beach topography considered here is of linear plane beach shape with minor undulations in the longshore direction. Ignoring the lateral turbulent diffusion, the finite element representation of the governing equations of mean currents is obtained by the method of weighted residuals. It is shown that, due to the flexible grid discretization, this model can be used to study problems containing more complex beach topography within a large area of interest. Two types of alongshore beach undulations are investigated: rhythmic topography and localized irregular topography. The locations of rip currents depend on the surf zone width and the on-offshore variation of beach profile.

MORPHODYNAMIC VARtABILITY OF HIGH-ENERGY BEACHES

Field observations of beach and inshore morphology and of surf and inshore current spectra using an array of pressure transducers and low-inertia bidirectional flow meters interfaced with an in-field mini-computer/logging system have been replicated on several beaches in southeastern Australia under a range of energy conditions. Two broad extremes of beach conditions are distinguished spatially and temporally: (1) reflective systems In which much of the incident wave energy Is reflected from the beach face; and (2) dissipative systems with wide surf zones and high turbulent viscosity. Reflectivity increases as the ratio of wave steepness to beach (or bed) steepness decreases. Compared to steep, unbarred reflective beaches which are common in deeply indented or partially protected compart-, ments, the topography of exposed dissipative systems is more complex and varied: six time-and-environment-dependent morphologic types with different bar patterns and bar-beach relationships are recognized. The greatest total dissipation prevails In regions of most abundant inshore sediments or during and immediately after severe storms. Between the reflective and dissipative extremes there is a hierarchy of observed resonant frequencies with the highest frequency resonance occurring in the most reflective cases. Results indicate that near the beach face, motions associated with resonance at periods greater than incident wave periods exhibit strong net seaward resultants and are probably important sources of beach erosion. Reflective beaches are sensitive to resonant excitations over a wider range of frequencies and under lower energy conditions than are dissipative beaches. Hence, although reflective beaches represent the accretive end point of a "beach cycle" they are also more delicately poised with a higher potential for erosion.