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Stephen M. Henderson R.T. Guza Steve Elgar Britt Raubenheimer W.C. O’Rielly T.H.C. Herbers

Comparison of observed and modeled alongshore-variable surfzone currents. Stephen M. Henderson R.T. Guza Steve Elgar Britt Raubenheimer W.C. O’Rielly T.H.C. Herbers. Observations. Depth (m). Distance north (m). Distance east (m).

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Stephen M. Henderson R.T. Guza Steve Elgar Britt Raubenheimer W.C. O’Rielly T.H.C. Herbers

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  1. Comparison of observed and modeled alongshore-variable surfzone currents Stephen M. Henderson R.T. Guza Steve Elgar Britt Raubenheimer W.C. O’Rielly T.H.C. Herbers

  2. Observations Depth (m) Distance north (m) Distance east (m) • Observations collected during the Nearshore Canyon EXperiment (NCEX). • Two-Hertz times series of pressure and horizontal velocity measured at more than 34 locations.

  3. Wave refraction over canyon • At north end of beach, wave propagation direction within 20 deg of due east. Wave propagation direction (deg counterclockwise from East) 1 Nov. 7 Oct. time • Near canyon, wave propagation direction 0-40 deg N of due east. Wave propagation direction (deg counterclockwise from East) 1 Nov. 7 Oct. time Depth (m) y (m) x (m)

  4. Alongshore-variable currents • Alongshore variations in wave field lead to alongshore variations in currents. northward mean current (m/s) Depth (m) y (m) 7 Oct. 1 Nov. time northward mean current (m/s) 1 Nov. 7 Oct. x (m) time

  5. Can standard models for wave-generated currents predict the alongshore-variable flows observed during NCEX?

  6. Previous work • Long and Ozkan-Haller (2005) compared predicted NCEX currents with video observations. • Long and Ozkan-Haller are working to compare predicted NCEX currents with measured currents.

  7. Wave Models • Wave field observed 12km offshore in 550m depth was shoaled across inner shelf using Bill O’Rielly’s linear spectral ray-tracing model. • Wave dissipation and refraction shoreward of 6m simulated using a small-angle, shallow-water model with a Thornton-Guza breaking parameterization. After Magne et al. (2007)

  8. Mean-flow model , where Guassian velocity distribution assumed, angle brackets denote an average, and CD=f(breaker dissipation rate) with 0.001<CD<0.003. • Currents simulated using depth-averaged rigid-lid shallow water equations. • Waves assumed to have small angles and slow (but order-one) alongshore variations.

  9. Mean-flow model • Currents simulated using depth-averaged rigid-lid shallow water equations. • Waves assumed to have small angles and slow (but order-one) alongshore variations. Hyperdiffusivity D chosen to damp grid-scale motions within a few time steps

  10. Model application Distance north (m) Depth (m) Distance east (m) • Terrain-following grid. • Periodic alongshore boundary conditions >1km from region of interest. • Impermeable, hyperslippery walls at shore (20cm depth) and offshore. • Low resolution: 33117 grid, about 25 s timestep. • High resolution: 300700 grid, about 2 s timestep.

  11. High resolution, 23 October, 7am Mean current Eddy stress (hujuk) 2500 Particle rotation: Red=counterclockwise, Blue=clockwise (the most intense eddies take several minutes to complete a revolution) 2000 Predicted breaker dissipation (m2s-3) y (m) 1500 1000 -200 0 200 -200 0 200 x (m) x (m) • Alongshore currents separate from the shore. • Separation generates strong 0.001-0.01 Hz eddies.

  12. High resolution, 23 October, 7am

  13. Low resolution, 10th - 31st October • Model run with continuously varying forcing and depth. Data Model northward mean current (m/s) Depth (m) y (m) 7 Oct. 1 Nov. time Data Model northward mean current (m/s) 1 Nov. 7 Oct. 1 Nov. x (m) time

  14. Low resolution, 10th - 31st October • Model run with continuously varying forcing and depth. Data Model northward mean current (m/s) Depth (m) y (m) 7 Oct. 1 Nov. time Data Model northward mean current (m/s) 1 Nov. 7 Oct. 1 Nov. x (m) time

  15. Low resolution, 10th - 31st October • Every 1/2 hour, observed alongshore velocity regressed against predictions. 16 Oct., 9:00am-9:30am r2=0.75 Observed northward velocity (m/s) Predicted northward velocity (m/s)

  16. Low resolution, 10th - 31st October • Every 1/2 hour, observed alongshore velocity regressed against predictions. r2 31 Oct. 7 Oct. time rms northward mean current (m/s) 31 Oct. 7 Oct. time

  17. Conclusions • NCEX alongshore currents simulated using combined wave and current models. • Model has significant skill in predicting temporal and alongshore variability of surfzone currents . • Model has no significant skill in predicting currents outside surfzone in 6m depth.

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