Alongshore momentum in the outer surf zone under hurricane wind and wave forcing

Abstract:

Wave breaking is the dominant physical process acting along beaches and coasts exposed to surface waves during storms, causing large gradients in wave momentum that generate strong currents in the nearshore zone. At the outer edge of the wave breaking region, strong horizontal current shear between the alongshore currents and alongshelf flows can develop and drive strong shear and mixing between these regions. Wave and current observations from a cross-shore array of six acoustic sensors in water depths of 2-11 m at Duck, NC, are used to elucidate the balance of momentum under storm wave conditions with wide surf zones, corresponding to offshore hurricanes. Offshore hurricanes that are distal (calm local wind) and proximal (local wind up to 25 m/s) generate large wave heights (3-5 m) and very strong alongshore currents (up to 2 m/s). We find that horizontal shear contributes strongly to momentum at the seaward limit of the surf zone and that radiation stress gradients, bottom stress, wind stress, mixing and advection play important roles at different times and locations in the cross-shore integrated momentum balance. The results provide insight into the cross-shore distribution of the alongshore current and the connection between alongshore flows inside the surf zone and outside it during major storms.