Bedload and suspended load contributions to breaker bar morphodynamics

Abstract

This study presents measurements of sheet flow processes, grain sorting, and bedload plus suspended load transport rates around a medium-sand breaker bar in a large-scale wave flume. The results offer insights in effects of wave breaking on bedload and grain sorting processes and in the quantitative contributions by bedload and suspended transport to breaker bar morphodynamics. Sheet flow layer dynamics are highly similar to observations under non-breaking waves, revealing clear effects by velocity asymmetry but no evident effects by breaking-generated turbulence, bed slope, or the cross-shore non-uniform flow. The sheet flow layer thickness can be predicted using existing empirical formulations based on local hydrodynamic forcing. At locations covering the shoaling region up to the bar crest the cross-shore variation in bedload transport rates is explained by variations in wave shape (i.e. velocity skewness and asymmetry). At locations between bar crest and bar trough, bedload transport rate magnitudes correlate positively with bed slope and turbulent kinetic energy. Bedload and suspended load transport rates are of similar magnitude but of opposite sign. Bedload transport is onshore-directed and dominates in the shoaling zone, but after wave breaking, the offshore-directed suspended sediment transport increases in magnitude and exceeds bedload transport rates in the breaking and inner surf zones. Bedload and suspended load transport contribute notably differently to bed profile evolution: bedload transfers sand grains from the offshore slope to the bar crest and additionally leads to erosion of the shoreward bar slope and deposition at the bar trough, while suspended load transport induces an opposite pattern of erosion at the bar trough and accretion at the bar crest. Grain size analysis of suspended sediment samples reveals size-selective entrainment and vertical size segregation in the inner surf zone, but suggest size-indifferent entrainment and vertical mixing by energetic vortices in the breaking region. Size-selective transport by bedload and suspended load leads to a cross-shore coarsening of the bed from shoaling to inner surf zone, with local additional sorting mechanisms around the breaker bar due to bed slope effects.

Near‐Bed Turbulent Kinetic Energy Budget Under a Large‐Scale Plunging Breaking Wave Over a Fixed Bar

“…The results of the present experiment can be compared with observations of sand transport processes during an accompanying mobile-bed experiment involving similar wave conditions and bed profile (van der Zanden et al, , 2017a(van der Zanden et al, , 2017b. The overall distribution of near-bed TKE seen in the present study is qualitatively and quantitatively similar to observations from the mobile-bed experiment .…”

Relative Contributions of Bed Load and Suspended Load to Sediment Transport Under Skewed‐Asymmetric Waves on a Sandbar Crest

“…(2004b), the vertical gaps were filled by fitting a third-order spline to wave-averaged flux profiles such that the vertically integrated flux profiles matched measured transport rates from sediment traps. In other cases, where gaps in observations make it impossible to directly compute bed load, net bed load transport rates are estimated by subtracting the measured suspended sediment transport rate from the total net transport rate obtained through evaluating the Exner equation (van der Zanden, van der A, Hurther, Cáceres, O'Donoghue, Hulscher, et al, 2017). The validation of net transport rates presented in section 4.3 using the Exner equation was not possible with the BARSED setup, since cross-shore gradients in net sediment transport rate were not collected.…”

“…While mobile bed layer thickness is very thin relative to the water depth (δ s ≪ h), average sediment concentrations in the mobile bed layer are 1-3 orders of magnitude greater than those in suspension (ϕ sf ≫ ϕ ss ). Consequently, it is possible that the contribution of intrawave sheet flow transport to the total transport rate is similar in order of magnitude to the contribution of the intrawave suspended load (Mieras et al, 2017b;O'Donoghue & Wright, 2004b; van der Zanden, van der A, Hurther, Cáceres, O'Donoghue, Hulscher, et al, 2017).…”

“…Thus, there is a severe lack of detailed simultaneous observations of suspended sediment and sheet flow processes under realistic wave forcing. Since 2009 when the SANTOSS database was compiled, several more small-scale (Berni et al, 2013(Berni et al, , 2017 and large-scale (Anderson et al, 2017;Brinkkemper et al, 2017;Mieras et al, 2017a;Schretlen, 2012;van der Zanden, van der A, Hurther, Cáceres, O'Donoghue, Hulscher, et al, 2017;Yoon & Cox, 2012) wave flume studies have contributed to fill the gap in full-scale surface wave experiments in the sheet flow regime. Recent development of a new sensor, the conductivity concentration profiler (CCP; Lanckriet et al, 2013), has enabled instantaneous sediment concentration profiles in the sheet flow layer to be measured with high resolution in the swash zone (field and large wave flume; Lanckriet et al, 2014;Puleo et al, 2014Puleo et al, , 2016 and the shoaling/surf zone (large wave flume; Anderson et al, 2017;Mieras et al, 2017a).…”

On Bedload and Suspended Load Measurement Performances in Sheet Flows Using Acoustic and Conductivity Profilers

“…Novel techniques were developed later, aimed at measuring local concentration and streamwise velocity measurements involving the use of pointwise capacitance probes (Horikawa et al, 1982;Ribberink & Al-Salem, 1994;Sumer et al, 1996), borescopic techniques (Cowen et al, 2010), two-component particle velocity and mean concentration profiles through video imaging (Armanini et al, 2005;Spinewine et al, 2011). and pointwise intrusive conductivity meters with automated bed level tracking capability (van der Zanden et al, 2015;van der Zanden et al, 2017). Only recently have advanced acoustic and conductivity techniques been developed Mieras et al, 2017) and used Mieras et al, 2017;Naqshband, Ribberink, Hurther, Barraud, et al, 2014;Puleo et al, 2016Puleo et al, , 2014Revil-Baudard et al, 2015) to profile such challenging media at sufficiently high temporal (O(0.1 s)) and spatial (O(0.001 m)) resolutions to actually measure the multiscale benthic boundary layer sediment transport processes.…”