Farquharson Gordon

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Farquharson
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  • Article
    Oblique internal hydraulic jumps at a stratified estuary mouth
    (American Meteorological Society, 2017-01-04) Honegger, David A. ; Haller, Merrick C. ; Geyer, W. Rockwell ; Farquharson, Gordon
    Observations and analyses of two tidally recurring, oblique, internal hydraulic jumps at a stratified estuary mouth (Columbia River, Oregon/Washington) are presented. These hydraulic features have not previously been studied due to the challenges of both horizontally resolving the sharp gradients and temporally resolving their evolution in numerical models and traditional observation platforms. The jumps, both of which recurred during ebb, formed adjacent to two engineered lateral channel constrictions and were identified in marine radar image time series. Jump occurrence was corroborated by (i) a collocated sharp gradient in the surface currents measured via airborne along-track interferometric synthetic aperture radar and (ii) the transition from supercritical to subcritical flow in the cross-jump direction via shipborne velocity and density measurements. Using a two-layer approximation, observed jump angles at both lateral constrictions are shown to lie within the theoretical bounds given by the critical internal long-wave (Froude) angle and the arrested maximum-amplitude internal bore angle, respectively. Also, intratidal and intertidal variability of the jump angles are shown to be consistent with that expected from the two-layer model, applied to varying stratification and current speed over a range of tidal and river discharge conditions. Intratidal variability of the upchannel jump angle is similar under all observed conditions, whereas the downchannel jump angle shows an additional association with stratification and ebb velocity during the low discharge periods. The observations additionally indicate that the upchannel jump achieves a stable position that is collocated with a similarly oblique bathymetric slope.
  • Article
    Magnetic and gravity surface geometry inverse modeling of the TAG active mound
    (American Geophysical Union, 2021-09-29) Galley, Christopher ; Lelièvre, Peter G. ; Haroon, Amir ; Graber, Sebastian ; Jamieson, John W. ; Szitkar, Florent ; Yeo, Isobel A. ; Farquharson, Colin ; Petersen, Sven ; Evans, Rob L.
    Seafloor massive sulfide deposits form in remote environments, and the assessment of deposit size and composition through drilling is technically challenging and expensive. To aid the evaluation of the resource potential of seafloor massive sulfide deposits, three-dimensional inverse modeling of geophysical potential field data (magnetic and gravity) collected near the seafloor can be carried out to further enhance geologic models interpolated from sparse drilling. Here, we present inverse modeling results of magnetic and gravity data collected from the active mound at the Trans-Atlantic Geotraverse hydrothermal vent field, located at 26°08′N on the Mid-Atlantic Ridge, using autonomous underwater vehicle and submersible surveying. Both minimum-structure and surface geometry inverse modeling methods were utilized. Through deposit-scale magnetic modeling, the outer extent of a chloritized alteration zone within the basalt host rock below the mound was resolved, providing an indication of the angle of the rising hydrothermal fluid and the depth and volume of seawater/hydrothermal mixing zone. The thickness of the massive sulfide mound was determined by modeling the gravity data, enabling the tonnage of the mound to be estimated at 2.17 ± 0.44 Mt through this geophysics-based, noninvasive approach.
  • Article
    Microwave radar cross sections and Doppler velocities measured in the surf zone
    (American Geophysical Union, 2005-12-23) Farquharson, Gordon ; Frasier, Stephen J. ; Raubenheimer, Britt ; Elgar, Steve
    The relationship between microwave imaging radar measurements of fluid velocities in the surf zone and shoaling, breaking, and broken waves is studied with field observations. Normalized radar cross section (NRCS) and Doppler velocity are estimated from microwave measurements at near-grazing angles, and in situ fluid velocities are measured with acoustic Doppler velocimeters (ADVs). Joint histograms of radar cross section and Doppler velocity cluster into identifiable distributions. The NRCS values from pixels with large NRCS and high Doppler velocities (>2 m/s) decrease with decreasing bore height to the shoreline, similar to scattering from a cylinder with decreasing radius. The Doppler velocities associated with these regions in the histograms agree well with theoretical wave phase velocities. Radar and ADV measurements of fluid velocities between bore crests have similarly shaped energy density spectra for frequencies above about 0.1 Hz, but energy levels from the radar are an order of magnitude higher than those of the ADV data. Instantaneous interbore Doppler velocities are correlated with ADV measured fluid velocities but are offset by 0.8 m/s. This offset may be due to Bragg wave phase velocities, wind drift, range and azimuth sidelobes, the finite spatial resolution of the radar, and differences between mean flows measured at the surface with radar and flows measured below the surface with ADVs. Shoaling and breaking waves measured through radar grating lobes significantly affect both the Doppler velocities near the edges of the images and the scattering from the rear faces of waves, causing large Doppler velocities to be observed in these regions.
  • Article
    Turbulence from breaking surface waves at a river mouth
    (American Meteorological Society, 2018-02-22) Zippel, Seth F. ; Thomson, James M. ; Farquharson, Gordon
    Observations of surface waves, currents, and turbulence at the Columbia River mouth are used to investigate the source and vertical structure of turbulence in the surface boundary layer. Turbulent velocity data collected on board freely drifting Surface Wave Instrument Float with Tracking (SWIFT) buoys are corrected for platform motions to estimate turbulent kinetic energy (TKE) and TKE dissipation rates. Both of these quantities are correlated with wave steepness, which has previously been shown to determine wave breaking within the same dataset. Estimates of the turbulent length scale increase linearly with distance from the free surface, and roughness lengths estimated from velocity statistics scale with significant wave height. The vertical decay of turbulence is consistent with a balance between vertical diffusion and dissipation. Below a critical depth, a power-law scaling commonly applied in the literature works well to fit the data. Above this depth, an exponential scaling fits the data well. These results, which are in a surface-following reference frame, are reconciled with results from the literature in a fixed reference frame. A mapping between free-surface and mean-surface reference coordinates suggests 30% of the TKE is dissipated above the mean sea surface.