PhD Defence Joep van der Zanden

sand transport processes in the surf and swash zones

Coastal regions are in many ways relevant to society, but are widely threatened by erosion. Long-term predictions of beach morphology using numerical models can help coastal managers to develop cost-effective coastal protection strategies. Such models operate at a large spatial and temporal domain and rely on semi-empirical parameterizations to account for underlying small-scale processes. Parameterizations for sand transport have mainly been developed on the basis of measurements under non-breaking waves. When these sand transport models are applied to the wave breaking region and the swash zone – where waves run up and down on the beach – they fail at properly predicting sand transport rates. This is attributed to a lack of understanding of how sand transport is affected by additional breaking-related processes.

The aim of this thesis is therefore to improve understanding of sand transport physics in the breaking and swash zone. This objective is pursued through controlled experiments in a large-scale wave flume with a mobile medium-sand bed during two campaigns: one focusing on the breaking region, the other on the swash zone. During both campaigns, the use of novel instruments enables measurements of sediment transport processes near the bed with much higher resolution than during previous studies. Effects of wave breaking and flow non-uniformity on sediment transport dynamics are identified  by comparing results with existing knowledge of sand transport dynamics for non-breaking waves. The results are then used to suggest improvements for sand transport models in the breaking and swash zone.


Chapters 2 to 4 present measurements of hydrodynamics and sand transport dynamics under a monochromatic plunging breaking wave around an evolving breaker bar. Chapter 2 focuses on the hydrodynamics in the breaking region, with particular interest on flow and turbulence over the near-bed region which includes the wave bottom boundary layer. The measurements in this region, obtained with a prototype acoustic concentration and velocity profiler (ACVP), are believed to be the first measurements of the complete wave bottom boundary layer under large-scale breaking waves. Wave breaking leads to large turbulence production and an increase in turbulent kinetic energy (TKE) in the complete water column. Breaking-generated turbulence also invades the boundary layer. Near the plunge point, this invasion occurs during two instances of the wave cycle: a first occurrence rapidly after wave plunging, and a second occurrence during the wave trough phase when undertow and periodic velocities transport TKE towards the breaker bar. The invasion results in an increase in TKE inside the wave bottom boundary layer with a factor 3 from shoaling to breaking region. Breaking-generated turbulence travels back and forth between breaking and shoaling region due to advection by orbital and undertow velocities. Consequently, the near-bed region affected by wave breaking extends horizontally to about 3 m offshore from the plunge point. Time-averaged velocities in the wave bottom boundary layer are offshore-directed and are generally dominated by the undertow. The non-dimensional wave bottom boundary layer thickness increases with a factor 2-3 in the breaking region, which is caused by flow divergence induced by the bar geometry and/or by effects of breaking-generated turbulence.

Measurements during the same experiment are used in Chapter 3 to assess wave breaking effects on suspended sediment transport processes and to examine the spatial distribution of sediment fluxes. Sediment concentrations are measured at outer-flow elevations using a transverse suction system and near the bed using the ACVP. The ACVP also measures velocities and allows the examination of collocated TKE and suspended sediment concentrations and fluxes in the near-bed region with much higher resolution than previous studies. Results show that suspended sediment concentrations increase by up to one order of magnitude from shoaling to breaking region. This is due to effects of breaking-generated turbulence, which does not only enhance vertical mixing but is in the present study also identified as the main driver for sediment pick-up. Outer-flow wave-averaged sediment fluxes are offshore-directed over most of the water column, but significant onshore contributions are found at elevations between wave trough and wave crest level in the breaking region and at elevations inside the wave bottom boundary layer at locations offshore from the plunge point. The latter is due to a significant onshore-directed wave-related suspended sediment transport contribution that is generally confined to the wave bottom boundary layer. The measurements are used to relate the spatiotemporal variation in suspended sediment concentrations to horizontal advection and to vertical exchange between the bedload and suspension layer. This analysis reveals that the entrainment of sediment in the bar trough occurs primarily during the wave trough phase, when both near-bed velocity magnitude and near-bed (breaking-generated) TKE are highest. The entrained particles are almost instantly advected offshore during the wave trough phase, and are deposited near the bar crest during the wave crest phase when velocity magnitudes reduce. The suspended particles further follow an intra-wave onshore-offshore excursion between shoaling and breaking region. This excursion is consistent with spatiotemporal patterns in TKE, which suggests that sediment particles are trapped in breaking-generated vortices that are advected back and forth following the orbital motion.

Chapter 4 focuses on effects of wave breaking on bedload and grain-size sorting processes, and on bedload and suspended transport contributions to the breaker bar morphodynamics. Two novel conductivity-based concentration measurement (CCM+) tanks measured concentrations and particle velocities in the bedload (sheet flow) layer at the breaker bar crest. Sheet flow layer concentrations and thicknesses do not reveal a significant effect of breaking-generated turbulence, but are affected by cross-shore advection of sediment. This shows that bedload dynamics are not fully controlled by local hydrodynamic forcing. Net bedload transport rates show strong cross-shore variation which relates firstly to variations in acceleration and velocity skewness (for locations from shoaling zone up to bar crest), and secondly to variations in local bed slope and near-bed TKE (in breaking region along shoreward-facing bar slope). During the experiment the bar slowly migrates onshore whilst its crest grows and its trough deepens. This occurs under the influence of (i) onshore-directed bedload transport, which erodes the offshore bar slope and accretes the bar crest, and (ii) offshore-directed suspended load, which induces net pick-up at the bar trough and net deposition at the bar crest. Both transport components are of similar magnitude, but bedload dominates in the shoaling zone while suspended load is larger in the breaking and inner surf zone. Grain-size distributions of suspended sediment samples show that sediment pick-up and vertical mixing is size-selective (i.e. the fraction of fine sediment is relatively large) in the inner surf zone in presence of vortex ripples, but size-indifferent (i.e. also coarsest grains are entrained) in the breaking region in presence of large and energetic breaking-induced turbulent vortices. Selective transport by both bedload and suspended sediment transport leads to a cross-shore coarsening of sediment in the bed from shoaling to inner surf zone.

Sediment transport in the swash zone is studied through another experimental campaign (Chapter 5). The novel CCM+ is deployed for the first time, and is used to study the response of the sheet flow layer and the intra-swash bed level to energetic swash events with strong wave-swash interactions. The highly non-uniform flow conditions under these swash events lead to intra-swash bed level changes of up to 1 cm, with rapid erosion during the early uprush phase and gradual accretion during the backwash. The bed level changes are explained by cross-shore advection of sediment between the lower and mid/upper swash zone at an intra-swash time scale. This advection also affects sheet flow concentrations and thicknesses, leading to much larger sheet flow thicknesses than expected based purely on the horizontal oscillatory velocities. Moreover, the sheet flow thickness increases temporally under events of strong wave-backwash interactions that enhance local turbulence. Both factors (sediment advection and bore turbulence) may affect sheet flow transport rates in the swash zone.

The general discussion (Chapter 6) reflects on the implications of the present study’s results for the future development of engineering-type sand transport and morphodynamic models.

Starting-time 14.30h Building Waaier - Prof.dr.G. Berkhoff-zaal