1. 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

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

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

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

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

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 Terrain-following grid. Periodic alongshore boundary conditions >1km from region of interest. Distance east (m) Distance north (m) Depth (m) 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 Predicted breaker dissipation (m 2 s -3 ) Alongshore currents separate from the shore. Separation generates strong 0.001-0.01 Hz eddies. 1000 1500 2000 2500 y (m) -2000200-2000200 x (m) Mean currentEddy stress (hu j u k ) Particle rotation: Red=counterclockwise, Blue=clockwise (the most intense eddies take several minutes to complete a revolution)

12. High resolution, 23 October, 7am

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

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

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

16. Low resolution, 10 th - 31 st October time Every 1/2 hour, observed alongshore velocity regressed against predictions. time r2r2 rms northward mean current (m/s) 7 Oct. 31 Oct. 7 Oct. 31 Oct.

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.