Andrew D. Short

Morphodynamic variability of surf zones and beaches: A synthesis☆

Abstract A synthesis of some results obtained over the period 1979–1982 from a study of beach and surf zone dynamics is presented. The paper deals with the different natural beach states, the process signatures associated with these states, environmental controls on modal beach state, and the temporal variability of beach state and beach profiles. Hydrodynamic processes and the relative contributions of different mechanisms to sediment transport and morphologic change differ dramatically as functions of beach state, that is depending on whether the surf zone and beach are reflective, dissipative or in one of several intermediate states. Depending on beach state, near bottom currents show variations in the relative dominance of motions due to: incident waves, subharmonic oscillations, infragravity oscillations, and mean longshore and rip currents. On reflective beaches, incident waves and subharmonic edge waves are dominant. In highly dissipative surf zones, shoreward decay of incident waves is accompanied by shoreward growth of infragravity energy; in the inner surf zone, currents associated with infragravity standing waves dominate. On intermediate states with pronounced bar-trough (straight or crescentic) topographies, incident wave orbital velocities are generally dominant but significant roles are also played by subharmonic and infragravity standing waves, longshore currents, and rips. The strongest rips and associated feeder currents occur in association with intermediate transverse bar and rip topographies. Long-term consecutive surveys of different beaches with contrasting local environmental conditions provide the data sets for empirical—statistical assessment of beach mobility, direction of change and response to environmental conditions. Conditions of persistently high wave energy combined with abundant and/or fine grained sediment results in maintaining highly dissipative states which exhibit very low mobility. Relatively low mobility is also associated with persistently low-steepness waves acting on coarsegrained beach sediments. In such cases, the modal beach state is reflective. The greatest degree of mobility is associated with intermediate but highly changeable wave conditions, medium grained sediment and a modest or meager sediment supply. Under such conditions, the beach and surf zone tend to alternate among the intermediate states and to exhibit well-developed bar trough and rhythmic topographies. A good association is found between beach state and the environmental parameter Ω = H b ( w s T ) where Hb is breaker height, w s is mean sediment fall velocity and T is wave period. Temporal variability of beach state reflects, in part, the temporal variability and rate of change of Ω, which, in turn depends on deep-water wave climate and nearshore wave modifications.

Wave, beach and dune interactions in southeastern Australia

Abstract 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.

Short-term changes in the morphodynamic states of beaches and surf zones: An empirical predictive model☆

A time series of 6.5 years of daily observations of beach states and wave characteristics from southeastern Australia was analyzed with the aim of determining the degree to which time-varying beach state can be explained and predicted. The six commonly occurring beach states (dissipative, longshore bar trough, rhythmic bar and beach, transverse bar and rip, low-tide terrace, and reflective), discussed earlier, were related, by means of discrete discriminant analyses, to the parameter Ω = Hb(WsT) (where Hb is breaker height, Ws is the fall velocity of the beach sand, and T is peak breaker period) using a total of 1545 cases. Two Ω values were used in the analyses: the immediate value occurring on the day the particular state was observed and a weighted mean value expressing recently antecedent conditions. The immediate value made a negligible contribution to explaining day-to-day beach state observations; however, the antecedent conditions showed a strong relationship and provide a successful means of prediction. By examining cases where the time derivatives of both state and Ω were near zero, it was possible to define the equilibrium conditions associated with each state. Directions of change (erosional or accretionary) are predicted in terms of departures of Ω from the equilibrium value appropriate to the beach state prevailing at the time change begins.

The Southern Oscillation Index, wave climate, and beach rotation

Short embayed beaches bounded by headlands are a common feature along the southern and central coastline of New South Wales, Australia. Many of these embayed beaches have experienced severe erosion at their southern end over the last decade. Previous studies have suggested that this erosion may be the result of an oscillatory medium-term phenomenon known as beach rotation. The present study was undertaken with the objectives of: (1) establishing definitive links between the Southern Oscillation Index (SOI), wave climate, and beach rotation, and (2) determining the physical processes governing beach rotation. Data from two similar New South Wales beaches were analysed using time series analysis techniques and image processing techniques (using ARGUS video images) in this study. The results indicate that the northern end of this type of beach accretes during El Nino phases, while the southern end erodes, resulting in a net clockwise rotation of the beach. The opposite occurs during La Nina phases resulting in a net anti-clockwise rotation of the beach. The beach response at the northern and southern ends lags SOI trend shifts by 3 and 17 months, respectively. Waves are predominantly incident from the southeast during both El Nino and La Nina phases. Offshore wave height is positively correlated with the SOI while offshore wave direction is negatively correlated with the SOI. On average, the number of storms per year doubles from El Nino to La Nina. Based on these links between beach width fluctuations, SOI, and incident wave conditions, a conceptual model of beach rotation is presented. The model describes the combinations of cross-shore and longshore sediment transport and hydrodynamic processes that are expected to result in the observed clockwise and anti-clockwise beach rotation during El Nino and La Nina phases, respectively.

Australian Beach Systems—Nature and Distribution

Abstract The Australian coast contains 10,685 beach systems, which occupy half the coast and can be classified into 15 beach types. These include six wave-dominated, three tide-modified, and four tide-dominated types which are a product of wave-tide and sediment conditions and two types which are influenced by intertidal rocks and fringing reefs. Wave-dominated beaches occupy the higher energy, microtidal southern coast exposed to persistent Southern Ocean swell. Tide-modified and tide-dominated beaches are most prevalent around the more tropical northern coast, which experiences meso-, macro-, and mega-tides and receives lower seas, as well as some sheltered and mesotidal southern locations. This article assesses the roles of waves, sediment, and tide range in contributing to beach type, particularly through the dimensionless fall velocity and relative tide range. It also describes their regional distribution, together with the occurrence of rip currents, multibar beach systems, and the influence of geological inheritance and marine biota.

DYNAMICS OF A HIGH-ENERGY DISSIPATIVE SURF ZONE

~Vright, L.D., Guza, R.T. and Short, A.D., 1982. Dynamics of a high-energy dissipative surf zone. Mar. Geol., 45: 41--62. 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, % 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, ~ 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.

Decadal Scale Patterns in Beach Oscillation and Rotation Narrabeen Beach, Australia—Time Series, PCA and Wavelet Analysis

Abstract Twenty-six years of monthly beach profiles located along 3.6 km Narrabeen beach were analyzed using time series, principle components (PCA) and wavelet analysis. The time series reveal both beach oscillation (erosion-accretion) and rotation between the boundary headlands. The rotation phenomenon is confirmed by the 2nd PCA component, explaining 58% of the remaining data variance. The scale of beach oscillation is on the order of 70 m, with 30 m of this oscillation attributed to beach rotation. Continuous wavelet transform analysis identified decadal scale patterns in beach response. This analysis indicates that variability in ensemble beach width and beach rotation exhibit long-term non-stationary variability. Longer-term cycles of behavior in overall beach erosion/accretion and beach rotation appear to coincide with each other, while short term fluctuations suggest a variety of physical processes are responsible for beach width variation and the interannual rotation phenomenon.

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

Abstract 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 ys = 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 ys = 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.

Morphodynamics of a macrotidal beach

Abstract An intensive field investigation of hydrodynamic processes, processes of sediment entrainment and suspension, and morphologic change was carried out on an unprotected macrotidal beach near Broome in northwestern Australia. The spring-tide range was 9.5 m; waves had heights of 0.5–1.2 m and periods of 9–13 s. The beach had an overall concave-upward profile with low-gradient and dissipative subtidal and low-tidal zones, and steeper more reflective mid-tidal and high-tidal zones. Direct measurement of energy-flux dissipation over the intertidal profile showed dissipation rates on the order of 1–2 W m−2 of bed and indicated an approximate balance between shoaling and dissipation of unbroken waves so as to maintain a constant wave height. Time-averaged predictive estimates of wave work over the lunar half cycle for different points on the intertidal profile show similar dissipation rates and reveal a relatively uniform distribution of work over most of the profile but with maxima in the middle of the low-tidal zone and over the lower part of the high-tidal zone. Most of the work over the low- and mid-tidal zones was performed by unbroken shoaling waves rather than by surf-zone processes; surf-zone processes only dominate over the high-tidal zone. The nature of the surf-zone processes varied across the profile as local gradient and degree of reflectivity changed with changing tide level. The growth of standing waves and infragravity (“surf-beat”) oscillations, as identified from spectra and cross spectra of surface elevation, η, and currents, u and v, was inhibited over most of the profile. However, well-developed secondary standing wave energy, particularly at infragravity frequencies, was observed over the high-tidal zone at spring high tide and over the mid-tidal zone at neap high tide. Over the low-tidal and subtidal zones, strong shore-parallel tidal currents were subordinate only to the orbital velocities of unbroken incident waves. Over the subtidal zone asymmetrical tidal currents, skewed toward the north, attained maximum speeds of 0.5 m s−1 just after high water. Field measurements of suspended-sediment concentration profiles under broken and unbroken waves showed very good fit to a diffusion model for wave-induced sediment suspension and suggested that sediment suspension was probably attributable largely to waves. Northerly advection of wave-suspended sediment by asymmetrical tidal currents over the subtidal and low-tidal zones accounted for a net northerly longshore transport. The greatest morphologic mobility of the intertidal profile occurred over the lower high-tidal and upper mid-tidal zones corresponding to the position of the coarsest material and secondary maximum of time averaged wave work and to a beach state intermediate between the reflective and dissipative extremes. Temporally, the greatest mobility of the profile as a whole was observed on the short, within-tidal-cycle time scale. Net changes over the longest time scale of a lunar half cycle were negligible.

Beach systems of the central Netherlands coast : processes, morphology and structural impacts in a storm driven multi-bar system

Abstract The 124 km long central Netherlands coast consists of a sand barrier system fronted by beach and surf zone containing 2 to 3 bars. The beach and surf zone are the product of wind waves generated in the North Sea, interacting with the medium to fine shoreface sands, in a micro tidal environment. The inner bar is usually attached to the beach as a ridge and runnel cut by drains and rips, the second and third bars are highly rhythmic and are characterised respectively by transverse bars and rips and rhythmic to longshore bar and trough bar types. Structural impacts influence 48 km of shore with harbour moles inducing lower wave energy and shoreline progradation, a 4.5 km long dyke replaces part of the beach but apparently has had little impact on mesoscale bar dynamics, while 158 groynes in three fields have produced more intermediate, rip driven, surf zones. Natural beach processes however dominate the entire coast with the hierarchical bar morphology related to cross-shore wave breaking. The equilibrium beach concept of Wright et al. (1987) was used to empirically predict beach/bar type. The results explain both the hierarchy of bar type and the temporal variation in beach-bar type. Finally a multi-bar “beach model” for the coast is presented which builds upon the single bar model of Wright and Short (1984).