Faculty of Geosciences

(1)

Faculty of Geosciences

Research group River and delta morphodynamics

-10 0 10 20

-7.5 -5 -2.5

0 2.5

5 7.5

elevation relative to MSL [m]

-15 -10 -5 0

Ba rr ie r

Inlet River inflow

& coh. sed.

supply

Sea mudflats

0 2 4

BI

active

Sand Marine Fluvial Both

0 1 2 3 4

W

estuary

[km ]

W_inactive

W_active

25 50 75

Area [% ]

^Aintertidal

A_mud

0 5 10 15 20 distance [km]

-1.5 -1 -0.5 0 0.5

U

x

[m/s]

U_Flood

U_ebb

1 2 3 4

BI

active

Sand Marine Fluvial Both

1.6 1.8 2

2.2 2.4 2.6 2.8

W

estuary

[km ]

0 25 50

Area [% ]

^A^intertidal

A_mud

0 0.02 0.04 0.06 0.08 0.1 Mud supply [kg/m

³

]

300 400 500

Surface area [km

3

]

0 20 40 60 80 100

cumalitive area [%]

-10 -8 -6 -4 -2 0 2

elevation [m]

mud=0 mud=5e-4 mud=1e-3 mud=5e-3 mud=1e-2 Dovey

high-low water

0

Turning the tide: Effects of mud supply on large-scale estuarine morphology

Lisanne Braat, Maarten Kleinhans, Thijs van Kessel, Laura Bergsma

l.braat@uu.nl, m.g.kleinhans@uu.nl, thijs.vankessel@deltares.nl, l.s.m.bergsma@students.uu.nl

Project summary

Q = 0-150 [m3/s]

fluvial mud supply

= 0-100 mg/L tidal amplitude

= 0-2 m

marine mud supply

= 0-100 mg/L

Parth-Kr EH

Delft3D numerical modelling (depth-averaged)

Method

Modelling results

Potentially, sandy river estuaries have great economic and ecologic values, but a better under- standing is required about the effect of mud on large-scale morphodynamics to optimise maintenance strategies. Very few studies actually include the effects of mud on morphody- namics on decadal and centennial timescales due to model limitations and lack of spatially and temporally dense data of mud in the bed. Here we aim to isolate the effects of cohesive sediment supply on equilibrium estuary shape, bar-channel patterns and dynamics.

Only sand

Marine mud supply

Fluvial mud supply

Fluvial and marine mud supply

• Marine mud confines the estuary mouth and extends the coast

• Fluvial mud is spread through the whole estuary and has a stronger confining effect

• Channel width decreases with fluvial mud, but not with marine mud supply

• Estuary length decreases with fluvial mud supply

• Less meandering upstream with fluvial mud supply

Morphology

• Widening with sand, narrowing with mud

• Mud leads to smaller channels

• Mudflats are quickly formed

• Less migration of channel with mud supply

Dynamics

Mud cover starts increasing at low water level

Channel depth

Mudflats Increase total mud cover

Higher elevation with mud supply

Future experiments

• Mud supply confines estuaries

• More area is coverd with mud for higher supply concentrations.

• Marine mud supply narrows the estuary mouth and has limited effect upstream.

• Fluvial mud is distributed over the whole estuary and had therefore a stronger effect by confining the whole estuary.

• Less estuarine dynamics are observed with an increase in mud supply.

Conclusions

Fig. 1. Final bathymetries of model runs with and without

mud supply from river or sea. Fig. 2. Final mud deposits of model runs with mud supply

from river or sea. Fig. 3. Bathymetry timestack of cross section near the mouth

of model runs with and without mud supply from river or sea. Fig. 4. Mud deposit timestack of cross section near the mouth of model runs with mud supply from river or sea.

Fig. 5. Braiding index, estuary width, area and velocity along estuary

for runs with no mud, marine supply, fluvial supply and both. Fig. 6. Mud supply plot against braiding index, estuary width

and surface area. All model runs represented as one point. Fig. 7. Hypsometric curves for a range of fluvial mud supply. Dotted lines are the cummulative percentages of surface area covered with mud of the total area.