On the Intersite Variability in Inter-Annual Nearshore Sandbar Cycles

Walstra, D.J.R.; Wesselman, Daan; van der Deyl, Eveline; Ruessink, BG

doi:10.3390/jmse4010015

On the Intersite Variability in Inter-Annual Nearshore Sandbar Cycles

Title

On the Intersite Variability in Inter-Annual Nearshore Sandbar Cycles

Author

Walstra, D.J.R. (TU Delft Coastal Engineering; Deltares)
Wesselman, Daan (Universiteit Utrecht)
van der Deyl, Eveline (Universiteit Utrecht)
Ruessink, BG (Universiteit Utrecht)

Date

2016-02-25

Abstract

Inter-annual bar dynamics may vary considerably across sites with very similar environmental settings. In particular, the variability of the bar cycle return period (Tr) may differ by a factor of 3 to 4. To date, data studies are only partially successful in explaining differences in Tr, establishing at best weak correlations to local environmental characteristics. Here, we use a process-based forward model to investigate the non-linear interactions between the hydrodynamic forcing and the morphodynamic profile response for two sites along the Dutch coast (Noordwijk and Egmond) that despite strong similarity in environmental conditions exhibit distinctly different Tr values. Our exploratory modeling enables a consistent investigation of the role of specific parameters at a level of detail that cannot be achieved from observations alone, and provides insights into the mechanisms that govern Tr. The results reveal that the bed slope in the barred zone is the most important parameter governing Tr. As a bar migrates further offshore, a steeper slope results in a stronger relative increase in the water depth above the bar crest which reduces wave breaking and in turn reduces the offshore migration rate. The deceleration of the offshore migration rate as the bar moves to deeper water—the morphodynamic feedback loop—contrasts with the initial enhanced offshore migration behavior of the bar. The initial behavior is determined by the intense wave breaking associated with the steeper profile slope. This explains the counter-intuitive observations at Egmond where Tr is significantly longer than at Noordwijk despite Egmond having the more energetic wave climate which typically reduces Tr.

Subject

morphodynamic feedback loop
Egmond
Noordwijk
inter-annual bar dynamics
process based modeling
Unibest-TC
sandbars
bar switch
morphodynamic modeling
cyclic bar behavior
Jarkus