Comprehensive Field Study of Swash-Zone Processes. II: Sheet Flow Sediment Concentrations during Quasi-Steady Backwash

Abstract

Sheet flow sediment concentration profiles were measured in natural conditions for the first time as part of a comprehensive field study on swash-zone hydrodynamics and sediment transport. Three conductivity concentration profilers (CCPs) measured the sediment concentration profile in the sheet flow layer with a 1 mm resolution in the swash zone of a dissipative beach. This paper focuses on sheet flow during quasi-steady backwash events generated by infragravity motion when the effects of phase lags, surface-generated turbulence and accelerations are small. The sheet flow sediment concentration profile has a linear shape in the lower section of the profile and a power-law shape in the upper section, with the transition occurring at sediment volume fractions of 0.20-0.30. The shape of the concentration profile is self-similar for measured sheet flow layer thicknesses ranging from 6 mm to 18 mm. Because of the self-similarity, a single concentration profile curve can be used to describe the normalized profile for the entire range of sheet thicknesses, leading to improved estimates of the sheet flow layer thickness in a simple analytical model. The sheet flow layer thickness and sheet load, the sediment mass mobilized in the sheet flow layer, are wellcorrelated with the hydrodynamic forcing represented by the mobility number (r 2 = 0.60 for sheet thickness and r 2 = 0.53 for sheet load).

A semianalytical model for sheet flow layer thickness with application to the swash zone

“…Lanckriet et al [2014] found good agreement of (6) with field measurements in the swash zone using a 5 1.73. Integration of (5) then yields:…”

“…Wilson [1987] derived (1) analytically from a force balance between the shear stress and the granular friction (based on a dynamic Coulomb yield criterion that was originally proposed by Bagnold [1956Bagnold [ , 1966) and estimated K % 10. Good agreement between (1) and measurements of (quasi-)stationary sheet flow has been observed [Sumer et al, 1996;Lanckriet et al, 2014]. Equation (1) has also been used to predict the quasi-steady and maximum sheet flow layer thickness under oscillatory flow Malarkey et al, 2003;Dong et al, 2013].…”

“…A. Puleo et al, Sediment transport partitioning in the swash zone of a large-scale laboratory beach, submitted to Coastal Engineering, 2014]. The importance of near-bed sediment transport in the swash zone is in part due to the strong hydrodynamic forcing, which generates sheet flow throughout a large portion of the swash cycle [Lanckriet et al, 2014], and in part because the small water depths constrain the capacity for suspended sediment transport, despite high observed suspended sediment concentrations [Butt and Russell, 1999;Puleo et al, 2000;Masselink et al, 2005].…”

On the role of infiltration and exfiltration in swash zone boundary layer dynamics

“…Shortly afterward, the flow evolves from subcritical to supercritical as the favorable pressure gradient becomes increasingly stronger during the backwash phase [Baldock and Hughes, 2006]. Sediment transport is then dominated by bed-generated turbulence [Longo et al, 2002;Cowen et al, 2003;Zhang and Liu, 2008;Lanckriet and Puleo, 2013] that may also mobilize sediment as sheet flow [Hughes et al, 2007;Lanckriet et al, 2014]. The relative dominance of the different flow phases (i.e., uprush versus backwash asymmetry) during a swash cycle largely depends on the beach face slope and incident wave conditions [Masselink and Puleo, 2006] and is known to have a large influence controlling the overall morphological response of the beach [Osborne and Rooker, 1999;Blenkinsopp et al, 2011;Puleo et al, 2014a;Masselink et al, 2009].…”

“…Previous studies have investigated the mean and turbulent flow field during the different phases of an individual swash cycle under natural [Puleo et al, 2000;Butt et al, 2004;Aagaard and Hughes, 2006;Raubenheimer et al, 2004;Blenkinsopp et al, 2011;Puleo et al, 2014b;Lanckriet et al, 2014] and laboratory [Cowen et al, 2003;Barnes et al, 2009;O'Donoghue et al, 2010;Sou et al, 2010;Sou and Yeh, 2011;Kikkert et al, 2012Kikkert et al, , 2013 conditions. Moreover, recent numerical studies [e.g., Brocchini and Baldock, 2008;Bakhtyar et al, 2009;Zhu and Dodd, 2013;Torres-Freyermuth et al, 2013] have provided additional information on the dynamics in this region, as they allow improved understanding of temporal and spatial variability of flow properties under different boundary/forcing conditions.…”