Short wave modulation due to infragravity waves in the nearshore zone, with applications

Abstract

The omni-present low frequency wave motion (30-300 sec) contains a substantial fraction of the total wave energy inside the surf zone. A more complete description of nearshore wave processes considers incident short period wind waves superposed on, and interacting with, long standing waves. The wind waves are modulated in amplitude, wavenumber and direction due to relatively slowly varying depth changes caused by the long waves. The energy in the wind wave band is enhanced by side band growth at the sum and difference frequencies of short and long waves (order 15% at the shoreline). The modulation is identified in the analysis of field data as a positive correlation between the long waves and the wind wave envelope near the shoreline. Considering oblique incident waves, a steady longshore current showing a non-vanishing current at the shoreline is found as a result of the non-linear interaction between monochromatic incident and infragravity waves. An analytical solution describing the unsteadiness of the longshore current is developed. Applying the derived longshore current solution, longshore sediment transport is reformulated to include the infragravity waves, giving improved comparisons with field measurements.