Nearshore Fluid Dynamics


Investigators

R. T. Guza, Steve Elgar

Objectives

The long-term research goal of this program is to understand the dynamics of surface gravity waves and wave-driven circulation in shelf and nearshore waters. The near-term scientific objectives are to develop an improved understanding of wave-driven circulation in the surf zone.

Approach

Because the fluid dynamics of the nearshore are very complex, many theoretical and conceptual advances have been substantially guided by field observations. The acquisition and interpretation of data from field experiments is the key element in this ongoing program.

Results

A new digital data acquisition system, and internal digitization electronics for electromagnetic current meters and pressure sensors, were designed, constructed and deployed in the Duck94 field experiment. Data were acquired for about 2 months from about 15 bottom-mounted flowmeters and pressure sensors, and from vertical arrays of electromagnetic current meters mounted on 6-m and 4-m tall structures, deployed on a single cross-shore transect extending 700 m from the shoreline to about 8 m depth.

Graduate student Falk Feddersen and collaborators are using data collected during Duck94, and the companion CooP project, to study quasi-steady (e.g. hourly averaged) alongshore currents in depths less than about 8m. Breaking sea and swell waves obliquely incident to a beach force alongshore currents. The total wave-induced, mean alongshore momentum flux available to drive steady currents, known as the radiation stress (Longuett-Higgins, 1970), was estimated from measurements with an array of pressure sensors in 8 m depth deployed by the Army Corps of Engineers. Alongshore flows on the inner continental shelf are often driven by the alongshore component of the wind stress (e.g. Winant,1980). Winds were measured at the end of the Duck pier and stresses estimated with standard algorithms (S. Lentz, personal communication).

Models for surf zone currents usually neglect the wind stress, and models for inner shelf circulation often neglect forcing by breaking waves. The poorly understood transition from wind-driven shelf to wave-driven surf zone circulation is being examined. The hourly averaged alongshore currents shown in figure 1 were observed during a time period when the wind and radiation stresses were of comparable moderate magnitude, but had opposite signs (the alongshore wind stress was towards the South). The observed cross-shore variation of wave height (not shown), indicates that the surf zone extended from the shoreline approximately to the crest of the sand bar (located at cross-shore position 250m). Near-bottom mean alongshore currents flowed Southward (e.g followed the wind) offshore of the sand bar but flowed Northward (e.g. followed the breaking waves) inshore of the sand bar (fig 1b). Alongshore currents are relatively uniform over the vertical and do not reverse sign (fig 1a). These observations show that wind-forced currents can contribute significantly to the flow field very near, and perhaps within the surf zone. In general, the alongshore wind and wave radiation stresses have the same sign (fig 2a), so the spatial regions of dominant wind or wave forcing are not as obvious as when these stresses are opposed (figure 1).

The relative importance of wind and wave forcing, integrated over the region from the shoreline to 8m depth, are shown for a 45 day period in figure 2a. When wind and/or wave stresses are large, the alongshore wind stress has is about half the magnitude of the radiation stress. The alongshore wind stress is negligible only during the energetic waves occurring around day 21 (figure 2a). The combined forcing by wind and waves (e.g. the sum of the stresses shown in figure 2a) can be balanced against bottom friction. The velocities observed at each flowmeter were used to calculate a local bottom stress assuming a quadratic dependence of stress on velocity, and the transect of instruments was used to estimate the total bottom stress between the shoreline and 8m depth. As shown in figure 2b, the total applied stress (wind plus waves) is approximately balanced by the total bottom stress. (The friction factor Cf = 0.0015 used to relate bottom stress to the squared near-bottom velocity is consistent with previous estimates.) The approximate closure of the alongshore momentum balance (figure 2b) suggests that the cross-shore structure of the alongshore flow was resolved adequately by the measurements, and that the model physics are approximately correct. Future analysis includes the testing of models which predict the spatial structure of the alongshore currents (e.g. figure 1) forced by both waves and wind.

References

Longuet-Higgins, M. S., Longshore currents generated by obliquely incident sea waves, J. Geophys. Res.,75,6778-6801, 1970

Winant, C.D., Coastal Circulation and Wind Induced Currents. Ann. Rev. Fluid Mech., 12, 271-301, 1980


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