Considerations on wave-induced fluid mud streaming at open coasts

Abstract

Wave-induced streaming of fluid mud at open coasts under damped, nonbreaking waves is examined as a likely mechanism to explain nearshore bottom evolution in such an environment. For a two-layered, water-fluid mud domain in which mud is considered to be a viscous continuum, an expression for the streaming velocity is obtained. Preliminary data on alongshore streaming of fluid mud in a wave basin show an order of magnitude agreement with theory. Previous observations on the rates of mud movement along the coasts of Surinam, Guyana and Louisiana are shown to yield alongshore velocities that are commensurate with those obtained by considering streaming to be the prevalent cause of mud transport. These velocities are considerably lower than those which would occur due to breaking waves. Off the southwestern coast of India, shoreward streaming of mud under monsoonal waves results in the formation of shore-fast mudbanks. It is argued that among transport mechanisms which govern the dynamics of microtidal muddy coasts, streaming may be an important means by which fluid mud influences the formation, migration and dissipation mudbanks in areas where wave breaking is significantly attenuated by viscous damping.

Review of wave-driven sediment resuspension and transport in estuaries

“…Fluid mud layers are typically clearly delineated at the top by a lutocline , but the lower interface with the bed is harder to define since that interface may not correspond with the zero‐velocity plane [ Ross and Mehta , ]. Fluid mud viscosity ranges from 2 to 4 orders of magnitude greater than that of water [ Rodriguez and Mehta , ], which has a profound effect on the rheology and therefore the dynamics of the mud itself and on the dynamics of waves traveling overhead [e.g., Mathew et al ., ; Calliari et al ., ; Sheremet et al ., ].…”

Decadal-scale morphological evolution of a muddy open coast.

“…The following hydrometeorological factors have been identified to explain such temporal and spatial variability of the shoreline ( Fig. 2): (i) trade-wind wave intensity and direction (Augustinus, 2004;Eisma et al, 1991;Gardel and Gratiot, 2005;Gratiot et al, 2007;Rodriguez and Mehta, 1998), (ii) solitary waves (Wells et al, 1978;Wells and Coleman, 1981a), (iii) the nodal tidal cycle (18.6yrs) Wells and Coleman, 1981b), (iv) tidal and coastal currents (Bourret et al, 2008;Chevalier et al, 2008;Gibbs, 1976;Pujos and Froidefond, 1995), (v) the Northern Atlantic Oscillation (Walcker et al, 2015) and (vi) other secondary factors for which correlations have not been clearly demonstrated, e.g., sea-level variation (NEDECO, 1968;Wong et al, 2009) and irregular ENSO variation Pujos et al, 1996).…”

“…2, the migration of mud banks is driven by several sources of forcing acting at different spatial and temporal scales. Among these forcings, wind-generated waves play a predominant role in longshore transport by liquefying the muddy bottom and by maintaining cohesive sediment in gel-like form (Allison et al, 2000;Augustinus, 2004;Eisma et al, 1991;Gratiot et al, 2007;NEDECO, 1968;Rodriguez andMehta, 1998, Wells et al, 1978;Wells andColeman, 1981a, 1981b). Variability in the rate of migration is influenced by shoreline orientation (Augustinus, 1978;Gardel and Gratiot, 2005), the effects of river flow on mud banks (Anthony et al, 2013;Jolivet et al, 2019), and local shoreline irregularities (Gratiot et al, 2007).…”

“…10). Gratiot et al (2007) extracted a wave ratio based on wave heights (H 0 ) and wave periods (T) following an earlier formulation by Rodriguez and Mehta (1998). Due to the partial coverage of the outer mud bank during 2008, 2010 and 2018 bathymetric surveys, the computation of yearly eroded and deposited volumes was performed between 2011 and 2017.…”

Hydrodynamics in a Mud Bank Regime during Nonmonsoon and Monsoon Seasons

“…For example, the critical parameters of viscosity and shear modulus of the mud, required for their viscoelastic bottom mud model, were obtained from laboratory tests, which may not be representative of real field conditions. More recently, RODRIGUEZ and MEHTA (1998) suggested that based on laboratory investigations the shoreward streaming of mud under monsoonal waves is a plausible mechanism for mud bank formation with the onset of monsoon. LI and PARCHURE (1998) examined the physical factors influencing the suspended sediment concentration profiles and presented a semiempirical model that accounts for the vertical fluxes of fine sediment in suspension due to waves and currents.…”