L.D. Wright

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.

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

Abstract 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 4T ( ⋍ 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 4T attenuates as Type 4 develops and is not present with Type 5 topography but the first subharmonic (2T) 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.

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.

Processes of marine dispersal and deposition of suspended silts off the modern mouth of the Huanghe (Yellow River)

The processes responsible for the transport and deposition of concentrated suspended silts over the delta front of the Huanghe were observed during three cruises and have been modeled numerically. Suspended sediment concentrations in the lower Huanghe average about 25 kg m−3 and exceed 200 kg m−3 during flood stage. Cruises were conducted during normal discharge conditions in spring 1985 and summer 1986, and during low-discharge storm-dominated conditions in autumn 1987. During the first two cruises, the shallow delta-front top (depth≤ 5m) was covered by a turbid water mass with suspended sediment concentrations of 1–10 kg m−3. Strong (∼1m s−1) parabathic tidal currents resuspended newly deposited muds and advected them alongshore. Near a break in slope, the turbid layers plunged beneath the ambient water and descended the delta-front slope as gravity-driven hyperpycnal underflows. In 1987 the hyperpycnal underflows occurred only during an intense strom that resuspended delta-front sediments to produce underflows with concentrations on the order of 100 kg m−3. We infer that gravity-driven underflows constitute the most important mode of suspended sediment transport across isobaths. Concentrated and channelized “point source” underflows, apparently associated with flood conditions, were not observed but were inferred from morphological evidence and were modeled numerically. Modeling results show that the Coriolis force and ambient momentum should cause appreciable curvature to the paths of underflows, while entrainment of ambient mass contributes to underflow decay. Early extinction of all underflow types is suggested by field and modeling results, and is considered to be responsible for extremely rapid delta-front deposition and for the fact that most of the sediments discharged by the Huanghe remain close to the mouth.

Modes of cross-shore sediment transport on the shoreface of the Middle Atlantic Bight

The mechanisms responsible for onshore and offshore sediment fluxes across the shoreface zone seaward of the surf zone were examined in a 3-year field study. The study was conducted in the southern part of the Middle Atlantic Bight in the depth region 7–17 m using instrumented tripods supporting electromagnetic current meters, pressure sensors, suspended sediment concentration sensors, and sonar altimeters. The observations embraced fairweather, moderate energy, swell-dominated, and storm conditions. Cross-shore mean flows ranged from near zero during fairweather to > 20 cm s−1 during the storm; oscillatory flows were on the order of 10 cm s−1 during fairweather and 100 cm s−1 during the storm. Suspended sediment concentrations at about 10 cm above the bed were 5 kg m−3 during the storm. Three methods were applied to evaluate the relative importance of incident waves, long-period oscillations, mean flows and gravity in effecting shoreward or seaward sediment flux: (1) an energetics transport model was applied to instantaneous near-bottom velocity data, (2) higher moments of near-bottom flows were estimated and compared, and (3) suspended sediment fluxes were estimated directly from the instantaneous products of cross-shore velocity and suspended sediment concentration. The results show that measurable contributions were made by all four of the processes. Most significantly, mean flows were seen to dominate and cause offshore fluxes during the storm and to contribute significantly to onshore and offshore flux during fairweather and moderate energy. Incident waves were, in all cases, the major source of bed shear stress but also caused shoreward as well as seaward net sediment advection. Low-frequency effects involving wave groups and long-period waves made secondary contributions to cross-shore sediment flux. Contrary to expectations, low-frequency fluxes were just as often shoreward as seaward. Whereas cross-correlations between suspended sediment concentration and the instantaneous near-bottom current speed were high and in phase under storm conditions, they were weak and out of phase during fairweather conditions. This suggests that simple energetics models are probably inadequate for predicting fairweather transport of suspended sediment.

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.

Wind stress, bed roughness and sediment suspension on the inner shelf during an extreme storm event

Abstract Instrumented bottom boundary layer tripods were deployed on the inner shelf at depths of 13 and 8 m off the U.S. Army Corps of Engineers Field Research Facility at Duck, NC, U.S.A., over a 2-week period that included the severe and prolonged “Halloween Storm” of late October 1991. The storm persisted for 5 days and generated waves with heights and periods of up to 6 m and 22 s. Although the instrumentation was destroyed, current profile and suspended sediment concentration profile data were recovered from the 13 m site. Mean currents attained speeds of nearly 0.5 m s −1 at 0.29 m above the bed and were directed about 10° offshore from shore-parallel. These strong currents are shown to be wind driven and result in predictions of a wind-drag coefficient, C a = 4.7 × 10 −3 . The currents were recorded simultaneously with root-mean-square (rms) wave orbital velocity amplitudes in the 0.6–1.0 m s −1 range. During the peak of the storm suspended sediment concentrations exceeded 1 kg m −3 throughout the lower 1 m of the water column. Analysis of current profiles, accounting for the presence of waves, is performed to obtain an equivalent bottom roughness, k n , of approximately 15 times the median sediment diameter, i.e. k n ⋍ 15 d 50 . Analysis of the suspended sediment concentration profiles, using the experimentally obtained hydrodynamic characteristics, results in a value of 4 × 10 −4 for the resuspension parameter, γ 0 , with the reference concentration taken 7 d 50 above the bed. From the severity of the storm condition it is inferred that our estimates of k n and γ 0 correspond to sheet flow conditions.

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.

Across-shelf benthic transports on the inner shelf of the Middle Atlantic Bight during the Halloween storm of 1991

Instrumented bottom boundary layer tripods were deployed on the inner shelf at depths of 13 m and 8 m off the U.S. Army Corps of Engineers Field Research Facility at Duck, North Carolina, USA over a two week period that included the severe and prolonged “Halloween storm” of late October 1991. The storm persisted for five days and generated waves with heights and periods of up to 6 m and 22 s. Although the instrumentation was destroyed, current profile and suspended sediment concentration profile data were recovered from the 13 m site. Wind-driven mean along-shelf currents at 1.24 m above the bed attained speeds of nearly 0.5 m s−1; across-shelf flows, primarily seaward-directed, had speeds varying from 0.05 to 0.15 m s−1. These seaward flows intensified in association with groups of high waves. Total, mean current, and skin-friction bed shear stresses were increased, relative to moderate energy values, by more than an order of magnitude. Notably, the highest shear stresses occurred in association with high, long period swell during the late phase of the storm after winds had turned offshore creating shoreward mean flows near the bed. Suspended sediment concentrations exceeded 1 g 1−1 throughout the lower meter of the water column and at elevations well above the top of the wave boundary layer. Net across-shelf suspended sediment fluxes were strongly dominated by mean flows. Cospectral analyses show that important, but secondary, roles were also played by infragravity oscillations and by wave orbital velocities.

VIMS Sea Carousel: a field instrument for studying sediment transport

Preliminary results from field experiments using a newly developed annular sea-bed flume, vims Sea Carousel, are presented to illustrate a method of in situ determination of critical threshold shear stress necessary for sediment entrainment. This flume has been deployed in the lower Chesapeake Bay and on the inner shelf of the Middle Atlantic Bight. We applied a series of controlled bed shear stresses by changing the stress sequentially, to examine the responses of nearly undisturbed natural sediment beds. An optical backscatter sensor mounted on the flume indicated the occurrences of resuspension of bed material. We identified clear threshold values of bed shear stress for resuspension. Generally, the sandy inner shelf sediment had a higher critical bed shear stress (0.22 Pa) for resuspension than that for silty sediments (0.1-0.19 Pa). Data from partially cohesive sediments in the lower Chesapeake Bay indicate significant seasonal differences in critical bed shear stress, 0.19 Pa at the beginning of summer and 0.11 Pa at the end of summer. These differences may be attributed to bioturbation, the seasonal difference in the microflora living on the grains, or spatial heterogeneity of bed.