HR: 15:20h
AN: OS23E-07    [Abstracts]
TI: Modeling Tidal and Wind-Induced Alongshore Currents in the Nearshore
AU: * Ruessink, B
EM: g.ruessink@geog.uu.nl
AF: Dept. of Physical Geography Faculty of Geosciences Utrecht University, P.O. Box 80.115, Utrecht, 3508 TC Netherlands
AB: Under storm conditions, obliquely incident breaking waves drive strong (1--2~m/s) alongshore currents in the nearshore. When wave breaking is either absent or confined to a very narrow region at the beachface, nearshore alongshore currents may still be significant (up to $\approx$ 1~m/s), driven largely by tidally induced alongshore surface slopes, wind, or buoyancy effects. There are, however, few detailed observational and modeling studies of alongshore nearshore currents in the absence of breaking waves. Here, predictions of a single-point model in the vertical driven by tidally induced 10--100 km scale alongshore surface slopes, wind stress, and Earth's rotation [Houwman, 2000] are compared to alongshore currents measured under nonbreaking conditions at two heights (0.3 and 1.2~m above the bed) at four positions (water depths 3--10~m) in the nearshore of Terschelling, Netherlands. The observations span several thousands of hours and include maximum near-bed alongshore currents of 0.8~m/s. All calculations are performed using a time-dependent eddy viscosity derived from a two-equation $k-\epsilon$ model and a quadratic partial slip bottom boundary condition. Because the overall performance of the model is satisfactory (e.g., skill $r^2$ exceeds 0.9 at all positions and heights), the model output for a single tide-wind situation is examined in more detail. In the selected situation, a shore-parallel wind (wind stress up to 0.5 N/m$^2$) in the direction of the (positive) flood current is predicted to yield positive near-bed currents even during the ebbing phase of the tide in water depths less than 6~m, whereas modeled alongshore currents in deeper water do change sign, in good quantitative agreement with the observations. Also, model simulations suggest that the wind strongly alters the vertical velocity profile of the alongshore current during a tidal cycle, related to a marked change in the temporal evolution of the eddy viscosity compared to a tide-only situation. Houwman, K.T., 2000. Tide, wind- and wave-driven flow processes in the nearshore zone. Ph.D. thesis, Utrecht University, 235 pp.
DE: 4255 Numerical modeling
DE: 4512 Currents
DE: 4546 Nearshore processes
DE: 4568 Turbulence, diffusion, and mixing processes
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting