Rip Currents: Coupling and Feedback between Waves, Flows, and Morphology

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

Rip currents transport biota, pollutants, and sediment from the shoreline to the continental shelf, and are a hazard to swimmers with over 100 fatalities per year. These narrow, transient, offshore-directed jets are difficult to observe, and thus there are few field tests of hypotheses for their generation, evolution, three-dimensional structure, and feedback with evolving seafloor morphology. Rip currents can be driven by alongshore variations in wave breaking, often associated with gaps in sandbars or depressions in the surf zone seafloor. Existing field observations near channels dredged across the surf zone and a numerical model tested with the field observations will be used to investigate the coupling and feedback between waves, currents, and morphology in and near rip channels. Although the main focus of the project is on the fundamental physics of hydrodynamics and morphodynamics near large perturbations to the surfzone seafloor, the project investigators will be collaborating with colleagues from federal agencies with more applied interests in nearshore processes. In particular, the post-doctoral fellow will work closely with the USGS colleagues who developed the model and other federal agency scientists to perform model simulations designed to provide results in line with their missions. The model will be a useful tool for these agencies to predict waves, flows, and beach changes, including responses to storms and engineering projects. In addition, the project investigators will collaborate with National Weather Service and National Ocean Service scientists to improve forecasts and warnings of rip currents and other hazardous beach conditions. In 2012 large shore-perpendicular channels were dredged in the surf zone with the underlying hypothesis that alongshore changes in wave breaking over the channels would drive strong rip current circulation cells (shore-parallel feeder currents converging and turning seaward at an offshore-directed rip jet). The three-dimensional circulation and evolving morphology was measured for a range of wave conditions and channel sizes, and included transitions from rip currents flowing offshore through the channels to alongshore currents flowing across the channels, and channels that migrated and filled. The field observations of waves, currents, and bathymetry in the presence of dredged shore-perpendicular channels will be used to ground truth and calibrate the numerical model. The numerical model will be used to investigate the hydrodynamic and morphologic response for a range of wave forcing and bathymetric perturbations. Although this study will focus on the rip channels we excavated, the results will be applicable to a wide range of nearshore systems, with or without complex bathymetric features. Specifically, the following questions will be addressed: (i) How does the three-dimensional nearshore circulation respond to different wave conditions and bathymetries, and what forces control transitions between alongshore flows and rip currents? (ii) What are the couplings and feedbacks between waves, currents, sediment transport, and morphological evolution that lead to channel migration, growth, or decay?