contents

problem description

observed phenomena

degradation history

key mechanisms

conclusions from analysis

countermeasures

Elbe

Ems

Weser

Loire

WesternScheldt

problematic estuaries in NW Europe

common denominators

access to harbour / shipyard activities

loss of intertidal area

access channel artificially deepened / enlarged

access channel dominates other channels

net import of fine sediment > increasing turbidity

environmental degradation

primary responses

tide:

(1) increase of tidal range (resonance)

(2) increase of ebb-flood asymmetry (velocity)

(3) increase of HW/LW asymmetry (water level)1

sediment:net import

> more pronounced turbidity maximum

> more extensive turbidity maximum

oxygen:

- reduction

1) different for Western Scheldt

positive feedbacks1

tide:

friction reduced by high sed. conc.

enhanced tidal amplification

sediment:overall turbidity

interaction with flow > fluid mud

floc size & location

estuarine circulationenhanced by high sediment conc.

1) not observed in Western Scheldt

positive

feedbacks

enhancing

sediment

import

degradation history

stage 0: ‘regular’ estuary

- well-defined turbidity maximum

at the head of the estuarine circulation

- balance between effects of

(1) river flow (export),

(2) tidal asymmetry (import), and

(3) estuarine circulation (import)

degradation history

stage 1: first effects of deepening

- well-defined turbidity maximum

with elevated concentrations

- enhanced estuarine circulation

due to higher sediment content

- less tidal asymmetry (realtively),

but flood-dominance remains

- tidal amplitude increases

- effect of river flow decreases

NET EFFECT:

ENHANCED SEDIMENT IMPORT

degradation history

stage 2: continued effects of deepening

- broad turbidity maximum,

local fluid mud formation

- effect estuarine circulation

relatively unimportant

- during flood: well-mixed water column

during ebb: highly stratified water column

- sediment load determined by capacity

conditions during ebb flow

NET EFFECT:

FURTHER ENHANCED SEDIMENT IMPORT

degradation history

stage 3: continued effects of deepening

- high SPM concentrations throughout

the estuary

- hyperconcentrated regime

- ubiquitous fluid mud formation in

certain phases of the tide

- sediment-induced drag reduction

enhanced tidal amplification

- internal asymmetry enhanced by floc

effects

- peak velocity asymmetry dominant

NET EFFECT:

STILL FURTHER ENHANCED SEDIMENT IMPORT

flood flow ebb flow

high peak velocity

turbulent mixing

dominates

density effects

largest flocs high

in water column

low peak velocity

density effects

dominate

turbulent mixing

largest flocs low

in water column

flocculation asymmetry

tidal pumping

z

ct1

t2

z

t3 = t1

t4 = t0

time

ve

loc

ity

entrainment phase

settling phase

z

c

t1

t0

z

ct1

t2

z

ct1

t2

z

t3 = t1

t4 = t0

z

t3 = t1

t4 = t0

time

ve

loc

ity

entrainment phase

settling phase

z

c

t1

t0

z

c

t1

t0

accelerating flow:

- fluid mud layer turbulent

- water entrainment at top

- sediment entrainment

at bottom

rising lutocline

decelerating flow:

- SPM-concentration above

capacity

- turbulence collapses

- high concentrations near

the bottom

falling lutocline

source: Schrottke & Bartholomä, 2008

… confirmed by data

salinity and turbidity uncoupled

salinity

sed. conc.

at flood

salinity

sed. conc.

at ebb

transport regimes

0

0 0.2 0.4 0.6 0.8 1

volumetric concentration f

flu

x R

ich

ard

so

n n

um

be

r R

i f

super-saturated conditions (1)

low/high-conc. sub-sat. suspension (2a/b)

hyper-conc. sub-sat. suspension (2c)

Ricr

U1

U2; U2 < U1

low-conc. susp.

high-conc.

susp.

1a

1b3

2negative

buoyancydominates

turbulentmixing

dominates

regime 1a:

- sub-saturated

- low concentration

- no sediment-fluid interaction

- no density stratification

- transport below capacity

regime 1b:

- sub-saturated

- high concentration

- sediment-fluid interaction

- density statification

- transport near capacity

increase of destabilises

transport regimes

0

0 0.2 0.4 0.6 0.8 1

volumetric concentration f

flu

x R

ich

ard

so

n n

um

be

r R

i f

super-saturated conditions (1)

low/high-conc. sub-sat. suspension (2a/b)

hyper-conc. sub-sat. suspension (2c)

Ricr

U1

U2; U2 < U1

low-conc. susp.

high-conc.

susp.

1a

1b3

2negative

buoyancydominates

turbulentmixing

dominates

regime 3:

- hyper-concentrated

- hindered settling

- sub-saturated

- strong sediment-fluid int.

- fluid mud formation

regime 2:

- super-saturated

- turbulence collapses

- sediment settles

- fluid mud formation

observations suggest: Lower Ems in regime 3

increase of stabilises

conclusions of analysis

longitudinal profiles of near-surface SPM

cross-stream distribution of SPMthe Lower Ems is probably in stage 3 now

it has undergone a severe regime shift

undoing this will take drastic measures

reducing tidal asymmetry will not be enough

hyper-concentrated regime needs to be breached

proposed measure: ground sill

reduces tidal asymmetry

(velocity & water level)

yet, flood dominance remains

but sed. input not down to zero

low water slack period longer

more efficient sediment trap (?)

internal asymmetry enhanced

NET EFFECT?

first get out of

hyperconcentrated regime,

then this may help

proposed measure: tidal retention

reduces tidal asymmetry

reduces tidal amplification

maintains larger channel

requires large storage area

sediment trapping efficiency?

undesired long-term effects in the

outer estuary?

sufficient to get out of the

hyperconcentrated regime?

proposed measure: tidal control

low water level set-up the

most efficient control mode?

velocity control as an

alternative?

sediment flushing

additional structural

measures needed

(e.g. bed protection)

additional measures needed

to get rid of the mud

(exit hyperconc. regime)

median conditions

Loire: natural flushing

N.B. Temporary effect!

puzzle: Western Scheldt response

deviations from other estuaries:

- water level asymmetry (HW vs LW)

- SPM import turbidityHOW COME?

Western Scheldt: multiple-loop system

conclusion

the Lower Ems has undergone a regime change

restoring a good state requires intrusive measures

no ready-to-use solution

many questions remaining

we might learn from other estuaries