Human Footprint on the Seafloor
Brussel - RBINS 2/9/2011
Suspended particulate matter
dynamics in coastal turbidity
maximum areas
Michael Fettweis
Royal Belgian Institute of Natural Sciences
Management Unit of the North Sea Mathematical Models
Gulledelle 100, B-1200 Brussels, Belgium
Human Footprint on the Seafloor
Brussel - RBINS 2/9/2011
Coastal turbidity maximum?
Turbidity is a measure of water clarity (optical property
of the water) and contains colored dissolved organic
matter and Suspended Particulate Matter.
Turbidity maximum occurs in coastal waterways (river
plume, estuaries) and is caused by tidal forcings
resulting in trapping and flocculation of SPM at the
salt-fresh water wedge (salinity is 2 ppt)
Human Footprint on the Seafloor
Brussel - RBINS 2/9/2011
Coastal turbidity maximum !
Sediment supply (clays, cabonates,..)
from English Channel and local erosion.
Trapping due to hydrodynamics & meteo
effects: Residual transport is reduced
resulting in a congestions of the SPM in
the coastal zone
Strong tidal currents and slack waters
stimulate flocculation and thus settling &
deposition of SPM
Human Footprint on the Seafloor
Brussel - RBINS 2/9/2011
Aim of Q4D SPM research:
SPM dynamics and human impact
1.
Natural variability (see mainly next presentation)
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ƒ
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2.
Instrumentation
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3.
Multiparametric sensors to investigate SPM dynamics: optical and acoustic
backscattering, laser diffraction
Statistical properties and sampling methods
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4.
Near bed processes: high concentrated mud suspensions and fluid mud
Extreme events (storms)
Meteorological and climatological effects
Evaluation of in situ and remote sensing (MODIS) techniques
SPM concentration as indicator to detect changes in the environment
Case study of human impact
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Disposal of dredged material and SPM concentration
Long term changes in SPM concentration and mud deposits
Human Footprint on the Seafloor
Brussel - RBINS 2/9/2011
Cohesive sediment dynamics
Highly Interdependent Processes : Deposition, Erosion, Sedimentation, Resuspension, Aggregation, Breakup, and so on. (Maggi, 2005)
Human Footprint on the Seafloor
Brussel - RBINS 2/9/2011
SPM concentration as indicator of anthropogenic impact:
long-term data set
Human Footprint on the Seafloor
Brussel - RBINS 2/9/2011
Wind Speed
Wind Direction
360
270
180
90
0
°
28
21
14
7
0
11
Water Depth
9
m
m s−1
Time series
7
Alongshore Subtidal Flow
Cross−shore Subtidal Flow
0
m
−0.5
3
240
Hs
d50
2
1
0
1.8
m
Seabed Altimetry
2.2
OBS 2.0mab
OBS 0.2mab
ADP 2.0mab
3
10
mg l−1
80
0
2
2
10
1
10
160
28
30
32
34
36
38
40
J Day
Variation in SPM concentration is related to tides, storms and also
seasonal changes and human impacts.
µm
m s−1
5
0.5
Human Footprint on the Seafloor
Brussel - RBINS 2/9/2011
SPM concentration: log-normal distribution
SPM concentration can be defined as a statistical population.
SPM concentration time series are sub-samples of the whole population
Human Footprint on the Seafloor
Brussel - RBINS 2/9/2011
Monitoring the effect of disposal of dredged material
Differentiating between natural and anthropogenic induced
signal using statistical techniques
tripod
disposal site
experiment conducted by
Maritime access division
Human Footprint on the Seafloor
Brussel - RBINS 2/9/2011
Time series of SPM concentration at
0.2 m above bed (SPM1) and 2 mab (SPM2)
Nov – Dec 2006
May – Jun 2008
during
after
May – Jun 2009
Human Footprint on the Seafloor
Brussel - RBINS 2/9/2011
Cumulative probability distribution of SPM concentration
measured at 2 mab and 0.2 mab
1
Probability
0.8
6a: during dredging experiment
6a (May 2009)
1-5, 6b
Stand. dev.
0.6
0.4
Significant increase in near bed
bottom layer Æ formation of HCMS
layers
0.2
0
0
500
1000
1500
Median at 0.2 mab is very high during
disposal exp. (612 mg/l) this is about
240 mg/l higher than during winter and
330 mg/l higher than during spring
2008)
SPM concentration 2 mab (mg/l)
1
Probability
0.8
0.6
6a (May 2009)
1-5, 6b
Stand. dev.
0.4
0.2
0
0
1000
2000
SPM concentration 0.2 mab (mg/l)
3000
Human Footprint on the Seafloor
Brussel - RBINS 2/9/2011
HCMS layers
Acoustic detection layer via altimetry of ADV & ADP: the signal is
reflected at high SPM concentration Æ decrease in sea level
erosion of HCMS
0
1.8
0
m
Seabed Altimetry
2
2.2
OBS 2.0mab
OBS 0.2mab
ADP 2.0mab
3
mg l−1
10
2
10
1
10
28
30
32
34
36
J Day
38
40
Human Footprint on the Seafloor
Brussel - RBINS 2/9/2011
HCMS layers
13
Human Footprint on the Seafloor
Brussel - RBINS 2/9/2011
Take Home Messages
SPM concentration distribution can be used as indicator for
environmental changes
High concentrated mud suspension have been identified thanks to
combination of different instrumentation
SPM dynamics is governed by near-bed processes: New sensors,
measuring techniques are needed (see this afternoon)