Bedload and suspended load contributions to breaker bar morphodynamics

Joep Van Der Zanden, Dominic Alexander Van der A, David Hurther, Ivan Caceres, Thomas O'Donoghue, Suzanne J.M.H. Hulscher, Jan S. Ribberink

Research output: Contribution to journalArticle

7 Citations (Scopus)
4 Downloads (Pure)

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.
Original languageEnglish
Pages (from-to)74-92
Number of pages18
JournalCoastal Engineering
Volume129
Early online date20 Sep 2017
DOIs
Publication statusPublished - Nov 2017

Fingerprint

Sorting
Suspended sediments
Erosion
Sand
Sediment transport
Coarsening
Kinetic energy
Vortex flow
Turbulence
Hydrodynamics

Keywords

  • Breaking wave
  • sheet flow
  • morphodynamics
  • grain size sorting
  • Bedload transport
  • breaker bar

Cite this

Bedload and suspended load contributions to breaker bar morphodynamics. / Van Der Zanden, Joep; Van der A, Dominic Alexander; Hurther, David; Caceres, Ivan ; O'Donoghue, Thomas; Hulscher, Suzanne J.M.H.; Ribberink, Jan S.

In: Coastal Engineering, Vol. 129, 11.2017, p. 74-92.

Research output: Contribution to journalArticle

Van Der Zanden, Joep ; Van der A, Dominic Alexander ; Hurther, David ; Caceres, Ivan ; O'Donoghue, Thomas ; Hulscher, Suzanne J.M.H. ; Ribberink, Jan S. / Bedload and suspended load contributions to breaker bar morphodynamics. In: Coastal Engineering. 2017 ; Vol. 129. pp. 74-92.
@article{6d725891372f4a1ba108de659d1bbbe0,
title = "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.",
keywords = "Breaking wave, sheet flow, morphodynamics, grain size sorting, Bedload transport, breaker bar",
author = "{Van Der Zanden}, Joep and {Van der A}, {Dominic Alexander} and David Hurther and Ivan Caceres and Thomas O'Donoghue and Hulscher, {Suzanne J.M.H.} and Ribberink, {Jan S.}",
note = "The authors wish to thank the staff of CIEMLAB, in particular Joaquim Sospedra, Oscar Galego and Ricardo Torres, for their hospitality and hard work during the experimental campaign. We thank Sjoerd van Til for his contributions to the grain size analysis. The authors are also grateful to fellow SINBAD researchers for their feedback on preliminary results and to two anonymous reviewers for their constructive feedback on the draft manuscript. The research presented in this paper is part of the SINBAD project, funded by STW (12058) and EPSRC (EP/J00507X/1, EP/J005541/1). We further acknowledge the European Community’s FP7 project Hydralab IV (contract no. 261520) for funding the accompanying SandT-Pro experiments.",
year = "2017",
month = "11",
doi = "10.1016/j.coastaleng.2017.09.005",
language = "English",
volume = "129",
pages = "74--92",
journal = "Coastal Engineering",
issn = "0378-3839",
publisher = "Elsevier",

}

TY - JOUR

T1 - Bedload and suspended load contributions to breaker bar morphodynamics

AU - Van Der Zanden, Joep

AU - Van der A, Dominic Alexander

AU - Hurther, David

AU - Caceres, Ivan

AU - O'Donoghue, Thomas

AU - Hulscher, Suzanne J.M.H.

AU - Ribberink, Jan S.

N1 - The authors wish to thank the staff of CIEMLAB, in particular Joaquim Sospedra, Oscar Galego and Ricardo Torres, for their hospitality and hard work during the experimental campaign. We thank Sjoerd van Til for his contributions to the grain size analysis. The authors are also grateful to fellow SINBAD researchers for their feedback on preliminary results and to two anonymous reviewers for their constructive feedback on the draft manuscript. The research presented in this paper is part of the SINBAD project, funded by STW (12058) and EPSRC (EP/J00507X/1, EP/J005541/1). We further acknowledge the European Community’s FP7 project Hydralab IV (contract no. 261520) for funding the accompanying SandT-Pro experiments.

PY - 2017/11

Y1 - 2017/11

N2 - 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.

AB - 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.

KW - Breaking wave

KW - sheet flow

KW - morphodynamics

KW - grain size sorting

KW - Bedload transport

KW - breaker bar

U2 - 10.1016/j.coastaleng.2017.09.005

DO - 10.1016/j.coastaleng.2017.09.005

M3 - Article

VL - 129

SP - 74

EP - 92

JO - Coastal Engineering

JF - Coastal Engineering

SN - 0378-3839

ER -