Orientation of Outer Deltas ALW - 05/04/2002 Why are outer deltas formed? What determines the orientation (of the channels) of the outer delta + their dynamics? Why are outer deltas formed? What determines the orientation (of the channels) of the outer delta + their dynamics?

Conceptual model:Updrift: Downdrift: wave-dominated Tide-dominated Interaction shore parallel current and current in the strait Preferred sediment deposition on downdrift side Orientation of the Outer Delta :

Proposition: outer deltas are downdrift oriented w.r.t. the ‘large scale’ sediment flux Sediment flux Asymmetry due to ‘external’ asymmetry only. Hence the asymmetry in back-barrier area + alignment of islands not considered. Hypothesis:

From process-based models + observations M2 amplitudeM2 phase

M4 amplitudeM4 phase M0 amplitude

Model Setup: Parameters varied:M2 amplitude (default = 1 m) + phase M4 amplitude (default = 0 m) + phase M0 amplitude (default = 0 m) initial bed profile (default = flat)

M2 Amplitude Z2=1 Z2=-0.4Z2=1Z2=-1Z2=1

M2 Phase  deg Z2=1

Influence of M0 Default: Z2_l = 0.9 Z2_r = 1.0  = 40 deg Z0=0Z0=-0.1Z0=0 Z0=0.1

Influence of M4 phase (1) FD ED FD ED  deg Z4=0.2  Z4=0.2 

Influence of M4 phase (2)  deg Z4=0.2  Z4=0.2  FD ED FD ED

Initial bed not flat Z0=0Z0=-0.1Z0=0 Z0=0.1

Comparison with data M0 amplitudeRelative phase FD ED : updrift (UD) : downdrift (DD) FD: downdrift (DD) ED: updrift (UD) UD DD UD UD DD+UD However: M0 seems to be more ‘efficient’

Conclusion Orientation of outer deltas is downdrift with respect to sediment flux Correlates well with the Wadden Sea embayments Initial topography changes details, not overall picture

Future work Add waves Influence of shape of embayment and width of throat