Hans Burchard1, Henk M. Schuttelaars2, and Rockwell W. Geyer3

1. Leibniz Institute for Baltic Sea Research Warnemünde, Germany

2. TU Delft, The Netherlands3. Woods Hole Oceanographic Institute, MA, USA

*) J. Phys. Oceanogr., under review.

Residual sediment fluxes in weakly-to-periodically stratified

estuaries and tidal inlets*

Question:

What are the processes driving residual sediment fluxes into the Wadden Sea?

Velocity and sediment profile data from Spiekeroog 2011

MacCready & Geyer (2010) after Jay & Musiak (1994)

Longitudinal density gradients & tidal oscillations lead to:

Tidal straining

Residual velocity

Flood sediment Ebb sediment

Observations of tidal pumping

Scully & Friedrichs (2007)

Simpson number

Key non-dimensional numbers

Rouse number

Horizontal buoyancy gradientWater depthBottom friction velocity scale

Settling velocity

Estuaries and tidal inlets

Analytical solution of most simple setting:Stationary exhange flow with parabolic eddy mixing

Analytical solution of most simple setting:Sediment fluxes in the Si – Ro parameter space

seaward

landwardHow does this compare to asymmetric tidal forcing?

Approach = GOTM

unlimited bottom pool limited bottom pool no bottom pool

Define time scale for bottom sediment pool:

Te = Time needed to empty bottom pool at mean bed stress

Effect of Si on bed stress

Decomposition of sediment flux

fluctuation flux

transport flux

total flux

Tidally averaged profiles

unlimited bottom pool limited bottom pool no bottom pool

total flux fluctuation flux transport flux

unlim

ited

botto

m p

ool

no b

otto

m p

ool

Sediment flux in parameter space

Adding an M4 tidal forcing component

Long flood-to-ebb transition

Short flood-to-ebb transition

Stronger flood

Stronger ebb

Adding an M4 tidal forcing component

Long flood-to-ebb transition Stronger flood

Short flood-to-ebb transition Stronger ebb

Long

floo

d-to

-ebb

tran

sitio

n

Stro

nger

floo

d

Shor

t floo

d-to

-ebb

tran

sitio

n

Stro

nger

ebb

Stro

nger

floo

d

Long

floo

d-to

-ebb

tran

sitio

nSh

ort fl

ood-

to-e

bb tr

ansit

ion

Stro

nger

ebb

Campaign in Lister Deep (April 2008)

Becherer et al. (GRL 2011)

shoals

Becherer et al. (GRL 2011)

Campaign in Lister Deep (April 2008)

d

Campaign in Lister Deep (April 2008)

Conclusions for PACE

• Classical picture of estuarine sediment transport: transport flux dominates.

• Observations of Scully and Friedrichs indicate important role of tidal pumping (=fluctuation flux).

• The present study supports this.

• When high ammounts of sediment are available, then fluctution flux is dominant.

• The M4 phasing (and probably other higher harmonics) determines whether net sediment flux is landward or seaward.

• Since sediment flux depends on so many parameters, it may actually be unpredictable?