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 amplitude M2 phase

M4 amplitude M4 phase

M0 amplitudeM0 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 AmplitudeZ2=1 Z2=1 Z2=-0.4 Z2=1 Z2=-1 Z2=1

M2 Phase

degZ2=1 Z2=1

Influence of M0 Default: Z2_l = 0.9 Z2_r = 1.0 = 40 deg

Z0=0 Z0=-0.1 Z0=0 Z0=0.1

Influence of M4 phase (1)

FD

ED

FD

FD

ED

degZ4=0.2

Z4=0.2

Influence of M4 phase (2)

degZ4=0.2

Z4=0.2 FD

ED

FD

ED

Initial bed not flat

Z0=0 Z0=-0.1 Z0=0 Z0=0.1

Comparison with data

M0 amplitude Relative phase

FD

ED

: updrift (UD): downdrift (DD)

FD: downdrift (DD)ED: updrift (UD)

UD DD UD UD DD+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