Nonlinear wave dynamics in the presence of mud-induced dissipation on Atchafalaya Shelf, Louisiana, USA

The interplay between wave nonlinearity and mud-induced dissipation is studied here using wave and sediment transport measurements collected in winter 2008, on the Atchafalaya Shelf, Louisiana, between the 8-m and the 4-m isobaths. This study focuses on the relatively energetic storm that occurred on March 4th (2-m wave height in 8-m water depth), which caused significant bed reworking and left in its wake a 20-cm layer of hindered-settling fluid mud. While the net wave dissipation rate was maximal during the hindered-settling phase after the storm, consistent with previous observations, significant dissipation was observed throughout the storm duration, with a secondary maximum associated with the peak of the storm and the maximum bed-reworking effects. The effects of mud-induced dissipation on the nonlinear shoaling process are investigated here using TRIADS, a newly-developed spectral model for nonlinear shoaling of waves. With mud-parameter values estimated using a crude inverse modeling approach, the model was used to separate the effects of mud-induced dissipation from nonlinear transfers, during both the erosion and deposition phases of bed reworking. Numerical simulations show that nonlinear transfers are always active, even under strong dissipation regimes. Remarkably, they also suggest that heavy mud-induced dissipation causes the nonlinear energy cascade to revert direction, with bulk transfers occurring from high-frequency to low-frequency spectral bands. This supports the hypothesis that the interplay between mud-induced dissipation, which is much more significant than the bottom dissipation over sandy beds, and wave nonlinearity can drain energy from the entire spectrum, and not just from the frequency bands that interact directly with the seafloor. The decay of high-frequency variance induced by the reversal of spectral flux in effect reduces the nonlinearity of the wave field.