Waves and Mean Currents

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Along open ocean coasts, waves are nearly ubiquitous and contribute to shaping the morphology of the shallow seabed.Wind-generated ocean surface waves are the major driving force for nearshore circulation and sediment transport in the surf zone and inner continental shelf (Wright et al., 1991).As waves shoal in coastal waters, wave energy spectra evolve owing to refraction, nonlinear energy transfers to higher and lower frequencies (Elgar et al., 1990), and energy dissipation caused by wave breaking and bottom friction (Thornton and Guza, 1983).Less obvious, but equally important, are the effects of mean currents. Surf zone and inner shelf mean currents may be forced by a variety of mechanisms including waves, wind, tides, and regional pressure gradients, but the wave-driven surf zone component has been the most intensively studied (Hubertz, 1986; Thornton and Guza, 1986; Haines and Sallenger, 1994). Both longshore currents generated by oblique wave approach to the shoreline and strong near-bed offshore flows (undertow) are clearly wave-forced since current velocities drop to near zero outside the surf zone (Stive and Wind, 1986; Thornton and Guza, 1986; Haines and Sallenger, 1994). Of all the approaches explaining the generation of nearshore currents, those based on radiation stress, the excess flux of momentum due to the presence of waves, have the strongest theoretical basis (Longuet-Higgins and Stewart, 1964). However, predictions of nearshore currents using only wave breaking and bottom conditions (topography and roughness) may be in error in magnitude and direction if other forces such as wind, tide, or regional pressure gradients are significant (Whitford and Thornton, 1993).

At intermediate depths over the shoreface, tidal- and wind-forced currents are frequently stronger in the near-bed region than wave orbital velocities (Wright et al., 1991). In the Middle Atlantic Bight, wind-driven, jet-like, southerly currents produced by northeaster storms have been observed on the inner shelf and can produce secondary, but strong, downwelling. These upwelling and downwelling flows related to wind stress are among the more powerful mesoscale motions which operate seaward of the wave-dominated surf zone (Wright et al., 1986).

Previous studies have recorded near-bottom and interior fluid flows during fair weather and storm conditions (Hubertz, 1986; Wright et al., 1986; Wright et al., 1991; Cacchione et al., 1994; Wright et al., 1994a; Wright et al., 1994b) and concluded that inner shelf processes are dominated by storm-generated flows.These storm-generated cross-shore mean flows have been proposed as dominant mechanisms in both onshore and offshore sediment movement (Roelvink and Stive, 1989; Trowbridge and Young, 1989; Wright et al., 1991).Wave and current bottom stresses also cause sediment mobilization on the surf zone and inner shelf and determine the amount of sediment available for transport (Lyne et al., 1990; Cacchione et al., 1994; Vincent and Downing, 1994; Maa et al., 1995). On the continental shelf, bed stresses due to waves will dominate the resuspension of the bed materials, but the combined stresses due to the waves and currents are important for the net transport of sediment in either the longshore or cross-shore direction (Vincent and Downing, 1994).

In turbulent boundary layers, the bed shear stress, to, is related to the shear velocity, u*, by

                                                u* = (to/r)1/2                                                                 (1)

where r is water density (Wiberg and Harris, 1994). Because the local shear stress remains constant with elevation within the logarithmic flow layer, the elevation dependent mean current velocity, uc(z), can be used to calculate u*c, the shear velocity related to the mean current, and the hydraulic roughness length, zo ,

                                                uc(z) = (u*c ln (z/zo)) / K                                                                 (2)

where zo is given by the vertical intercept (where uc(z) = 0) in the extrapolated logarithmic velocity profile. K, von Karmon's constant, is 0.4, and z is distance above the seabed (Wright, 1995). A minimum of three velocity measurements within 1.5 m of the bed can be used to obtain a bed shear stress value, to, (Drake and Cacchione, 1992).

On the inner continental shelf, interaction of waves and mean flows determine the magnitude of bed shear stress which suspends sediments, while the oscillatory and mean flows may transport the sediments independently.Wiberg and Smith (1983) indicate that it is necessary to account for the presence of waves and wave-current interactions on the continental shelf when estimating bottom stresses, either from field data or theoretically.Waves and currents over sandy shorefaces experience an effective bottom roughness approximately consistent with existing semi-empirical representations of the roughness characteristics of wave formed sand ripples Trowbridge and Agrawal, 1995), thus knowledge of all three, waves, currents and bedforms, are needed for accurate prediction of bed shear stresses and resulting sediment transport. Estimates of wave energy dissipation due to bottom friction are derived from empirical parameterizations typically without the benefit of field measurements of bottom roughness or sediment type. This lack of quantitative data obtained in either the laboratory or the field has left a major deficiency in our understanding of the dissipative processes.

In most inner shelf environments, waves coexist with wind-driven and tidal currents, causing the thin oscillatory boundary layer of waves to be nested at the base of the thicker current boundary layer.Bottom friction is enhanced in combined wave and current boundary layers, and the total bed stress is greater than a linear addition of the solitary wave and current contributions. A notable effect of the waves is to increase the apparent roughness height, zo, estimated by extrapolation of the current log-layer profile (Wright, 1995). The boundary layer structure of the overlying fluid and the roughness elements of the bed comprise a morphodynamic feedback loop (Sherman and Greenwood, 1984).  Changes in the overlying water column directly impact the surface of the seabed that in turn will modify motions in the overlying fluids. The few local measurements of wave bottom boundary layer (WBBL) dynamics have also been concentrated on coasts with relatively smooth, gently sloping sandy bottoms.  On these shelves the vertical extent of the WBBL is typically small, on the order of a few centimeters, making it difficult to accurately measure small-scale velocity profiles given the resolving capabilities of existing technology (Foster et al., 1994). Subsequently, dissipation estimates have large uncertainty. Moreover, measurements in these regions are often complicated by the presence of nonstationary, migrating ripple fields of variable dimension, particularly when the bed elevation changes by more than the thickness of the WBBL. As a result, it has thus far been unrealistic to quantify the overall damping in a shoaling wave field over smooth, slowly varying topography from point measurements of dissipation rates.  Sediment transport and bedform migration are two processes that are driven by this fluid-sediment interaction.

The transition from measurements of wave and current activity to predictions of sediment transport and bedform activity during storm conditions is difficult at best.Even with these challenges, examining wave and current induced sediment suspension over time scales of fractions of seconds to hours with simultaneous time series of flow velocities and sediment concentration is one method of investigating sediment transport which has met with increasing success (Madsen et al., 1993; Beach and Sternburg, 1996; Amos et al., 1999).  Correlation of morphological changes with measured rates and directions of suspended sediments on the shoreface has been partly successful (Cacchione and Drake, 1982; Aagaard and Greenwood, 1994), and nearshore depth changes during autumn storms have been recorded where mean flows are the driving force behind sediment transport (Hay and Bowen, 1993; Thornton et al., 1996). Though many studies have concentrated on the mechanisms of transport and the forces which can initiate and sustain sediment transport, the actual amplitudes and nature of bed responses on the shoreface have usually been inferred indirectly, not measured, until a series of deployments were begun on the shoreface of the Middle Atlantic Bight.

Research on the shoreface of Duck, NC has documented a variety of fluid motions and associated bed elevation changes in fair and foul weather conditions through numerous deployments of tripods to support electromagnetic current meters, arrays of optical backscatter sensors (OBS), acoustic altimeters, and pressure sensors. Wright et al. (1994b) deployed two tripods in 8 and 13 m depths during the "Halloween storm" of 1991 when wave heights exceeded 6 m and periods reached 22 s.  The 8 m tripod was lost entirely and only the current and sediment concentration data were recovered from the 13 m site.  Despite the loss of the instrumentation, data analysis of the recovered records showed suspended sediment fluxes were dominated by the contribution from mean flows, but infragravity oscillations and wave orbital velocities were also important (Wright et al., 1994b). Wind-driven mean longshore currents at 1.24 m above the bed reached 50 cm/s.  Seaward directed cross-shore flows varied from 5-15 cm/s and intensified with groups of higher waves (Wright et al., 1994b).  A previous series of four tripod deployments in 7-17 m depth measured cross-shore flows from near zero in fair-weather conditions to greater than 20 cm/s offshore during storms (Wright et al., 1991).  From these deployments, they conclude mean flows dominate in storms and cause offshore fluxes of sediment.  Incident waves were the dominant source of bed shear stress and caused both shoreward and seaward transport.


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The content for this web page was taken from the Dissertation Storm Sedimentation on the Surf Zone and Inner Continental Shelfof Rebecca Lenel Beavers in the Department of Geology of Duke University

This web site was created by Doug Call (Contract Student, University of Virginia) on July 31, 2001.