Research papers Sediment resuspension and nepheloid layers induced by long internal solitary waves shoaling orthogonally on uniform slopes D. Bourgault a,n , M. Morsilli b , C. Richards c , U. Neumeier a , D.E. Kelley d a Institut des sciences de la mer de Rimouski, Université du Québec à Rimouski, 310 Allée des Ursulines, Rimouski, QC, Canada G5L 3A1 b Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, Via G. Saragat 1, 44100 Ferrara, Italy c Woods Hole Oceanographic Institution, 266 Woods Hole Rd, MS# 21, Woods Hole, MA 02543-1050, USA d Dalhousie University, Department of Oceanography,1355 Oxford Street PO BOX 15000, Halifax, NS, Canada B3H 4R2 article info Article history: Received 20 February 2013 Received in revised form 25 October 2013 Accepted 29 October 2013 Available online 7 November 2013 Keywords: Internal solitary wave Sediment Nepheloid layer Numerical modelling Sedimentary structures abstract Two-dimensional, nonlinear and nonhydrostatic field-scale numerical simulations are used to examine the resuspension, dispersal and transport of mud-like sediment caused by the shoaling and breaking of long internal solitary waves on uniform slopes. The patterns of erosion and transport are both examined, in a series of test cases with varying conditions. Shoreward sediment movement is mainly within boluses, while seaward movement is within intermediate nepheloid layers. Several relationships between properties of the suspended sediment and control parameters are determined such as the horizontal extent of the nehpeloid layers, the total mass of resuspended sediment and the point of maximum bed erosion. The numerical results provide a plausible explanation for acoustic backscatter patterns observed during and after the shoaling of internal solitary wavetrains in a natural coastal environment. The results may be useful in the interpretation of some sedimentary structures, and suggest an effective mechanism for offshore dispersal of muddy sediments. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Long internal solitary waves (ISWs or simply waves hereafter) typically found in coastal environments are a special class of mode-1 internal waves characterized by large amplitude relative to the surface mixed layer thickness and high frequency relative to the buoyancy frequency at the pycnocline (Helfrich and Melville, 2006). To give a sense of scale, in estuaries and fjords such waves may be characterized with  10 m amplitude,  100 m wave- length and  100 s period (e.g. Farmer and Armi, 1999; Bourgault et al., 2007) while in coastal seas, such as the South China Sea, these wave characteristics may be an order of magnitude greater with, for example, amplitude that can reach 100 m (e.g. Helfrich and Melville, 2006). Internal solitary waves have emerged in recent studies as poten- tially effective agents for resuspending sediments in comparatively shallow environments such as coastal seas, shelves and estuaries (Bogucki et al., 1997, 2005; Bourgault et al., 2008; Carter et al., 2005; Hosegood et al., 2004; Hosegood and van Haren, 2004; Johnson et al., 2001; Klymak and Moum, 2003; Quaresma et al., 2007; Richards et al., 2013). 1 Although episodic, wave-induced resuspension is hypothesized to be important enough to shape the topography and to impact coastal marine ecosystems by disturbing diagenesis processes and trans- porting particulate organic matter and other biologically sensitive compounds such as iron, nutrients, or contaminants (Sandstrom and Elliott, 1984; Hosegood and van Haren, 2004; Bogucki and Redekopp, 2008; Pan et al., 2012; Pomar et al., 2012). Furthermore, recent hypotheses suggest that sediment mobilization and transport caused by internal waves in general, and internal solitary waves in particular, may be at the origin of some sedimentary structures found in the sedimentary rock record, termed internalites (Bádenas et al., 2012; Pomar et al., 2012), and also the hummocky-cross stratification (Morsilli and Pomar, 2012). While the bottom shear stress associated with stably propagating waves may be sufficient to mobilize sediment within the wave- induced bottom boundary layer, wave destabilization and subsequent turbulence are required to efficiently propel sediment out of the bottom boundary layer and further up into the water column. To date, essentially three destabilizing mechanisms relevant to wave-induced sediment resuspension have been studied. One mechanism relates to the interaction of waves propagating over flat, hydrodynamically smooth bottoms and laminar boundary layers with vertically sheared background currents. Under some Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/csr Continental Shelf Research 0278-4343/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.csr.2013.10.019 n Corresponding author. Tel.: þ1 418 723 1986x1763. E-mail address: daniel_bourgault@uqar.ca (D. Bourgault). 1 Note that for the benefit of conciseness we have excluded any discussion about internal tide and sediment resuspension. The subject is vast and the dynamic and scales of internal tide is too different from that of internal solitary waves to be (footnote continued) treated together in a comprehensive manner in a single specialized study. This study therefore focuses only on large-amplitude and high-frequency, horizontally propagating internal solitary waves. Continental Shelf Research 72 (2014) 21–33