2
Book of Abstracts
NCK – Days
March 18th to 20th, 2015
Strandpaviljoen STRUIN, Camperduin
32
Bed-level motions and sheet-flow processes in the swash zone
Joep van der Zanden1, José M. Alsina2, Iván Cáceres3 and Jan S. Ribberink1
1 University of Twente, j.vanderzanden@utwente.nl; j.s.ribberink@utwente.nl 2 Imperial College, j.alsina@imperial.ac.uk
3 Universitat Politècnica de Catalunya (UPC), i.caceres@upc.edu
1. Introduction
The swash-zone is a highly dynamic area with large sediment fluxes within a shallow, transient water layer. Connecting the surf zone to the beach, the swash is a relevant area for coastal morphology. Obtaining high-quality measurements in the swash has been proven challenging. Therefore, the complex physical processes driving sediment transport in the swash are poorly understood. We present results of recent large-scale wave-flume experiments, focusing on sheet-flow sediment transport in the swash zone.
2.Methodology:
Experiments were done in the CIEM wave flume at UPC, Barcelona. The campaign focused on the role of grouping period on near-shore morphology and sediment transport processes. Here we present results for one erosive condition, involving a bichromatic group (period Tgr=15 s) propagating
from a water depth of 2.48 m towards a constantly sloping (1:15) beach built with medium sand (D50=0.25 mm).
Figure 1: Overview of instrumental set-up. Figure 1 shows the instrumental set-up close to the shoreline. Along the surf and swash zone, the evolution of the wave groups (water levels) is measured through a series of Pore Pressure Transdu-cers (PPTs) and Acoustic Wave Gauges (AWGs). Water velocities (ADVs) and sediment concentrations (OBSs) measure sediment fluxes. Of particular notice are two recently developed CCM+
tanks, that can be used to measure bed-levels and sediment concentrations in the sheet-flow layer.
3. Results
Figure 2 shows the ensemble-mean results for two characteristic wave groups in the lower swash. The intra-wave bed-level varies, with net erosion during uprush and net accretion during the backwash. Despite
hydrodynamics being largely similar, the groups show substantial differences in terms of magnitudes of the bed-level changes. Sheet-flow transport can be identified during uprush and backwash. The time-variant bed-level and absence of upper sheet-flow layer mark clear differences from oscillatory sheet-flow studies.
Figure 2: Ensemble-mean results for two characteristic wave groups (left & right). (a,b)
Water surface levels η; (c,d) horizontal water velocities u; (e,f) time-varying concentrations in
the sheet-flow layer at heights z’ w.r.t. group-averaged bed-level.
4. Conclusions
We present for the first time simultaneous bed-level and sheet-flow measurements for a continuous swash cycle. Sheet-flow dynamics in the swash differ from oscillatory flow conditions, with (i) a time-variant intra-group bed-level and (ii) absence of typical upper sheet-flow behaviour. Both observations are likely due to the strong flow non-uniformity in the swash, as a result of which horizontal advective processes dominate local sheet-flow layer dynamics.
Acknowledgments
The experiments were funded by the EU through the HydraLab IV access program. The CCM+
instrumentation was developed within the SINBAD project, funded by STW and EPSRC.
-10 -8 -6 -4 -2 0 2 4 6 8 10 -1 -0.5 0 0.5 1 x (m) h ( m ) TANK1 TANK2 AWG OBS ADV PPT CCM+ tank 1 CCM+ tank 2
