Modeling wave-mud interaction on the central chenier-plain coast, western Louisiana Shelf, USA I. Safak a,⇑ , C. Sahin b , J.M. Kaihatu c , A. Sheremet b a Department of Environmental Sciences, University of Virginia, 291 McCormick Road, Clark Hall, Charlottesville, VA 22904, USA b Department of Civil and Coastal Engineering, University of Florida, 365 Weil Hall, P.O. Box 116590, Gainesville, FL 32611, USA c Zachry Department of Civil Engineering, Texas A&M University, 3136 TAMU, College Station, TX 77843, USA article info Article history: Available online 25 November 2012 Keywords: Wave modeling Surface waves Nonlinear waves Wave dissipation Muddy seafloor Mud Viscosity Bottom boundary layer Louisiana Shelf abstract The strong coupling between hydrodynamics and seafloors on shallow muddy shelves, and resulting bed reworking, have been extensively documented. On these shelves, spectral wave transformation is driven by a complex combination of forcing mechanisms that include nonlinear wave interactions and wave energy dissipation induced by fluid-mud at a range of frequencies. Wave-mud interaction is investigated herein by using a previously validated nonlinear spectral wave model and observations of waves and near-bed conditions on a mildly-sloping seafloor off the muddy central chenier-plain coast, western Lou- isiana Shelf, United States. Measurements were made along a cross-shelf transect spanning 1 km between 4 and 3 m water depths. The high-resolution observations of waves and near-bed conditions suggest presence of a fluid mud layer with thickness sometimes exceeding 10 cm under strong long wave action (1 meter wave height with 7 s peak period at 4 meter depth). Spectral wave transformation is modeled using the stochastic formulation of the nonlinear Mild Slope Equation, modified to account for wave- breaking and mud-induced dissipation. The model is used in an inverse manner in order to estimate the viscosity of the fluid mud layer, which is a key parameter controlling mud-induced wave dissipation but complicated to measure in the field during major wave events. Estimated kinematic viscosities vary between 10 4 -10 3 m 2 /s. Combining these results of the wave model simulations with in-depth analysis of near-bed conditions and boundary layer modeling allows for a detailed investigation of the interaction of nonlinear wave propagation and mud characteristics. The results indicate that mud-induced dissipa- tion is most efficient when the wave-induced resuspensions of concentrations > 10 g/L settle due to rel- atively small bottom stresses to form a fluid mud layer that is not as thin and viscous as a consolidated seafloor in absence of wave action but also not as thick and soft as a near-bed high concentration layer that forms during strong wave action. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction In nearshore muddy environments, energetic surface waves have been observed to soften the initially consolidated seafloor and cause resuspension of sediment which finally settles to form fluid mud layers (e.g., Jaramillo et al., 2009; Sahin et al., 2012), the thickness of which depends on site-specific mud properties and hydrodynamic conditions. Interaction of waves with these high concentration mud layers causes significant wave energy dis- sipation (e.g., Sheremet et al., 2005). The dissipation rate was re- ported to be more significant during the phase of hindered settling of the resuspended material when a fluid mud layer forms (Sheremet et al., 2011a), and dramatically greater than that observed over sandy shelves (e.g., Ardhuin et al., 2003). Mud-in- duced wave energy dissipation is observed at both low frequencies and at the short wave band of the spectrum. This short wave band is not kinematically coupled to the seafloor; energy loss in this band was hypothesized to be due to nonlinear energy transfers across the spectrum, i.e., triad interactions (Sheremet and Stone, 2003). An early study of wave propagation on muddy seafloors (Gade, 1958) considered a two-layer system consisting of water overlaying a viscous fluid representing the muddy seafloor. The resulting mod- el of Gade (1958) is valid for long waves; with it, wave heights were seen to exponentially decay as waves propagate. This model was later improved with the inclusion of viscous effects in both layers, and an extension to dispersive waves (Dalrymple and Liu, 1978). An analytical limit to the model of Dalrymple and Liu (1978) was derived by Ng (2000). This simplification is valid when the thickness of the mud layer (h o ) is comparable to the Stokes’ 1463-5003/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ocemod.2012.11.006 ⇑ Corresponding author. Tel.: +1 508 457 23 08; fax: +1 508 457 23 10. E-mail addresses: ilgar@virginia.edu (I. Safak), cisahin@ufl.edu (C. Sahin), jkaihatu@civil.tamu.edu (J.M. Kaihatu), alex@coastal.ufl.edu (A. Sheremet). Ocean Modelling 70 (2013) 75–84 Contents lists available at SciVerse ScienceDirect Ocean Modelling journal homepage: www.elsevier.com/locate/ocemod