BRGM
L'ENTREPRISE AU SERVICE DE LA TERRE
Illustration by Satellite Imagery of turbidity in
French shore waters of the Dover Strait
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Preliminary interpretation
Illustration par l'imagerie satellitaire de la
turbidité sur le littoral du Détroit du Pas-de-Calais
interprétation préliminaire
Ch. Vinchon
with the collaboration of:
J.P. Dupont^
R. Lafite 1
A. Matthews^
R
December 1992
34702 GEO SGN - 92
^Laboratoire de Sédimcntologie, Université de Rouen,
^Southampton University Department of Oceanography, UK.
BRGM
SERVICE GÉOLOGIQUE NATIONAL
Département Géologie
B.P. 6009 - 45060 Orléans Cedex 2 - France - Tel
:
(33) 38 64 34 34
Illustration by satellite imagery of turbidity in French shore waters of ihe Dover Strait
Abstract
Behaviour of the coastal waters turbidity plume offshore Cap Gris Nez, on the French side of
the Dover Strait is illustrated through archived SPOT and LANDSAT TM satellites imagery.
Absence of in-situ calibration of air and water reflectance and diffusion hinders quantification
and qualification of the turbidity. Therefore our approach is focused on the dynamic behaviour
of the plume, in relation with seasons and tides. Seasonal influence is essentially observed on
the density and the width of the plume, where tidal effect influences the shape, with a lag in the
time of response to the main tidal current. Shore morphology, man-built or natural, appears to
contribute to this lag. Wind seems to have little effect on the shape of the cloud.
RÉSUMÉ
Le comportement du panache de turbidité dans les eaux côtières est illustré à partir d'images
satellites SPOT et LANDSAT TM, en archives. En l'absence de mesures in situ permettant de
calibrer les phénomènes de réflectance et de diffusion de la lumière dans l'eau et dans l'air, on
ne peut envisager de quantifier ou qualifier la turbidité, et on s'attachera à décrire le
comportement dynamique du panache de turbidité, en fonction des saisons et des marées.
L'effet saisonnier se marque essentiellement sur la densité et la largeur du panache, tandis que
l'influence de la marée joue sur la forme de celui-ci, en marquant un temps de latence pour
répondre aux changements de sens des courants de marée. La morphologie de la côte, naturelle
ou anthropique, provoque un| effet de frottement qui s'ajoute à cette inertie du panache turbide.
Le vent a peu d'influence sur le panache turbide.
Rapport BRGM R 34702 GEO SGN - 92
Illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
Table of contents
1.
INTRODUCTION
5
2.
HYDRODYNAMIC DATA IN THE WESTERN DOVER STRAIT
6
2.1 - Bathymetry and morphology
2.2 - Tides and currents
2.3 - Salinity and temperature
2.4 - Turijidity
3. SPOT
4.
6
6
6
7
AND LANDSAT PARAMETERS
8
DESCRIPTION AND PRELIMINARY INTERPRETATION OF SPOT AND
LANDSAT TM IMAGERY
4.1 - Picture of the 5-7-1987
4.2 - Picture of the 7-9-1988
4.3 - Picture of the 21-12-1989
4.4 - Picture of the 11-1-1988
4.5 - Piaure of the 17-3-1990
4.6 - Picture of the 26-5-1986
9
11
12
12
13
13
5. SYNTHESIS OF THE DESCRIPTION AND CONCLUSION
5.1 - Interpretation
15
of the pictures in relation to seasonal and tidal variations
5.1.1 - Seasonal variations as observed on pictures
5.1.2 - Tidal influence on the dynamism of the turbidity plume
5.2 - Other parameters
5.3 - Complementarity
of different satellites imagery
16
18
List of tables
- Parameters
15
15
16
BIBLIOGRAPHY
1
15
16
5.4 - Further studies
Table
9
of the pictures acquisition
Table 2 - Tidal and meteorological conditions during acquisition
Rapport BRGfJI R 34702 GEO SGN - 92
of the pictures
Illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
Appendix
list of figures
Fig
1
Interpretation of the SPOT 5-7-87 picture in terms of turbidity.
-
Fig 2 -
Inteipretation of the 7-9-88 LANDS AT TM picture in terms of turbidity.
Fig 3 -
Interpretation of the SPOT picture of the 21-12-89 in terms of turbidity.
Fig 4 -
Interpretation of the SPOT picture of the 21-12-89 in terms of turbidity. Detail of the
eddy, north of Cap Blanc Gris Nez.
Fig 5-
Inteipretation of the LANDSAT T.M. picture of the 11-1-88.
Fig
Interpretation of the SPOT picture of the 17-3-90 in terms of turbidity.
6-
Fig 7 -
Interpretation of a detail cf the 17-3-90 SPOT picnire.
Fig
Interpretation of the 26-5-86 SPOT image.
8
-
Interpretation of the 26-5-86 SPOT picture.
Fig 9 -
Rg
10 - Comparison of the different images
within the tidal cycle.
LIST OF PLATES
Plate
1
-
SPOT image of 5-7-87 - Scenery 38247, view angle 18.1° - Pas de Calais.
Plate 2 -
LANDSAT TM. picture of 7-9-88.
Plate 3 -
Detail of the LANDSAT T.M. picture of the 7-9-88 - Infra-red Channel (thermal).
Plate 4 -
SPOT picture of 21-12-1989. Scenery 37247, view angle 7.6° - Pas de Calais.
Plate 5 -
Detail of SPOTpicmre of the 21-12-1989. Scenery 37247, North of Cap Gris Nez.
Plate 6 -
LANDSAT TM picture of the 1 1-1-1988. Cap Gris Nez.
Plate 7 -
SPOT picture of the 17-3-1990. Scenery 37247, view angle 3.1° - Pas de Calais.
Plate 8 -
Detail of the 17-3-1990 SPOT picture,
Plate 9 -
SPOT picture of the 26-5-1986. Scenery 36247, view angle 0°, Pas de Calais.
Plate 10 - Detail
of the 26-5-1986 SPOT picture. Scenery 36247, view angle 0°.
Rapport BRGM R 34702 GEO SGN - 92
Illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
1.
Introduction
Within the framework of a European project on hydrodynamics and sediment input from the
Eastern English Channel towards the North Sea, (FLUXMANCHE, MAST CEE), study of
satellite imagery (SPOT and LANDSAT-TM) has been carried out. The aim was to illustrate the
behaviour of turbidity clouds in relation to the coast morphology, linked to seasonal and tidal
variations.
This report gives a first account of our observations ; at this step of the study, SPOT images in
possession of the BRGM were used, from different seasons and tides, showing the shore
between Boulogne and Dunkerque (4 pictures). A LANDSAT T.M picture was purchased, for
comparison of data given by different satellites.
Other satellite imagery (LANDSAT T.M.) has been collected in the frame of the MAST project,
by the Remote Sensing Laboratory of the Uiuversity of Southampton (U.K), and is compared
with our archives.
Within the frame of MAST I, many data on Dover Strait's hydrodynamic behaviour have been
collected; this report will be completed, in a ftirther step, by the Fluxmanche and Task Force
research groups.
Existing data on the Dover Strait will be summarised in a first part, followed by
iiiformation on SPOT and LANDSAT image interpretation parameters.
a
brief
The second part of this report will give the description and preliminary interpretation of
observed images, through seasonal and tidal parameters, and entail the demonstration of the
interest and limits in using SPOT and T.M. satellite imagery to obtain a 2D picture of
suspended particles matter (SPM) distribution in shore water masses.
Rapport BRGM R 34702 GEO SGN - 92
Illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
2. Hydrodynamic data in the western dover strait
2.1 - BATHYMETRY AND MORPHOLOGY
Bathymetry and morphology of the Dover Strait has been compiled by C. AUGRIS et aL, (1990
a, b, c):
The depth in the Dover Strait neariy never exceeds 50 m, slowly increasing towards the sea,
with a maximum slope of 2 to 3°. The morphology of the sea floor is smooth, organised on the
eastern side of the Charmel in elongated sand banks, belonging to a wider structure called
"Flemish banks". Those banks are 5 to 50 km long, 5 km wide, 25 m high, oriented NNW south
of Cap Gris Nez, and NW north of Cap Gris Nez, Their shape is, most of the time, slightly
asymmetric, with the gentle slope facing seaward. The sandy structures are overlying a gravel
bed, itself lying on the bedrock (Cretaceous and Jurassic sediments).
The surfaces of the sand bodies are moulded in ripples of different wavelength and shapes
The coastal front, from South to North, is made of wide sandy beaches limited landwards by
dunes (Bay d'Authie to Cap d'Alprech), then narrow gravely beaches, limited landwards by
bedrock cliffs, also exposed on the beach (from Cap d'Alprech to Cap Gris-nez), and wide
sandy beaches with outcrops of bedrock (from Cap Gris-Nez to the Belgian boarder).
The beaches are organised in sandy bars and runnels ("ba ches") parallel to the shore.
2.2 - TIDES AND CURRENTS
The tidal range in the eastern part of the Dover Strait is important, up to 7.7 m in spring tides, in
Boulogne. It is decreasing towards Dunkerque. The tidal waves shows a delay of about 1 hour
between Boulogne and Dunkerque. Flood tide lasts 5 h to 5h30, ebb tide lasts approximately
7 hours.
Tidal currents are dominant in the Dover Strait, altematively southwards during flood tide,
reversing one hour before high tide, and northwards during ebb tide, reversing lh30 before low
tide. Flood current is stronger. Surface currents can reach a speed of 3.4 knots in spring tides
at Cap Gris Nez, decreasing South and North of this cape. Bottom currents decrease of 20 to
30% ofl'shore, and still more near the coast , where swell currents are also interfering.
2.3 - SALINITY AND TEMPERATURE
Measurement of temperature and salinity has been carried out, offshore Equihen, south of the
Cap Gris Nez (J.M. BRYL1NSKI,Y. LAGADEUC, 1990; J.M. BRILINSKI et al., 1991) and
underiines the existence of a front between coastal and offshore waters, at 2 to 5 miles from the
coast line. It is mainly a salinity and density front, (34.5 g/1 salinity front, with lower salinity
and density in coastal waters), the temperature gradient being insignificant The coastal water
mass is also identified by its high content in phyioplancton.
Rapport BRGM R 34702 GEO SGN 92
Illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
The shape of this front is strongly tidal-influenced : it is vertical during spring tides, oscillating
landwards and seawards respectively at flood and ebb. It is oblique, with ascending slope
seawards during neap tides; this shape can be exaggerated by soft water river input.
New data on a transect through the Strait have been collected by FLUXMANCHE; results are
being processed,
2.4 -TURBIDITY
Turbidity profiles (J,P, DUPONT et ai, 1991) are very irregular. There is a high turbidity level
near the shore, linked to the coastal turbidity plume, and another offshore, where the tidal
current is the strongest. Other turbidity peaks are linked to the above-described front, as if it
worked as an hydrodynamic barrier for particles concentration and exchange.
Chlorophyll profile gives the following distribution: the highest concentration is encountered
offshore, the lowest in the transitional zone of the front. It increases landwards in the coastal
water zone, but decreases abruptly in the turbid coastal fringe.
Floristic associations proved to be an useful tool for the qualification of sources, and in a
further step, for the characterisation of water masses: coastal input is identified by fossil
cocoliths, and benthic-intertidal diatoms, demonstrating reworking of coastal and intertidal
sediment, river soft waters and brines input by a specific association of benthic and planctonic
diatoms and terrigeneous fine silts. Offshore waters are characterised by a high production of
phyioplancton, and local reworking of bottom sediments and water by strong tidal currents in
the central zone
of the Strait.
Exchange is favoured by a stratified front (neap tide, river flood) up to a factor of 10^ to 10^.
FLUXMANCHE data have been collected on granulometry and floristic association in the
different water masses during this last year, and are being processed.
Rapport BRGM R 34702 GEO SGN - 92
Illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
3. Spot AND landsat parameters
This work aims to illustrate the dynamic behaviour, in 2D, of water masses above described or
measured, by using reflectance properties of the water. Two types of images have been
collected: airborne remote sensing imagery (CASSI), and satellite imagery (SPOT and
LANDSAT), This report deals with the satellite imagery.
Pure water has got a low intensity in reflectance, mainly within the green wavelengths, which
appears in this report with false colour treatment (in blue or stretched red and yellow).
The intensity of reflectance increases with suspended-particles matter, enhanced by small size
granulometry and mineral content, by scattering the light. Chlorophyll content, on the contrary,
inhibits the reflectance, by absortjing the light, (RJ, CURREN and E,M, NOVO, 1988),
However, by the fact that the described imagery is archived imagery, this relation between
SPM concentration and water reflectance, in relation to chlorophyll and mineral content cannot
be calibrated by reliable in-situ measurements. For the same reasons, atmospheric effect on
water colour has not been considered, due to the lack of surface measurement of reflectance.
Therefore, the purpose of this report will be to describe the dynamic behaviour of the turbidity
plume, in regards to tide an season induced parameters. Empirical approximations are made,
when data exist on similar tidal or seasonal situation, but have to be considered as such. In
further studies however, we hope to be able to calibrate turbidity composition and SPM content
and atmospheric effects by in-situ measurements in real time with the picture (C, MARTEL,
1990; R, SANTER etal., 1991; M, VIOLLIER, 1980).
SPOT satellite is a multichannel satellite (equipped with two sensors for visible light, and one
in the close infrared), with a periodicity of 26 days. Images are taken at variable angles between
23°W to 23°E, at approximately 9h30 U,T; this decreases the periodicity to 3 to 4 days, giving a
similar scenery at slightly different angles, which explain a slight deformation between the
different herein described sceneries,
A scenery is 60 km wide. The
size
of a pixel is 20 m. The maximum depth resolution is
15
m,
depending on the turbidity.
Traditional colour composition has been made on the SPOT image so that green appears blue,
red appears green, and infrared appears red (Plates 1, 4, 5, 7, 8), Specific stretching was
sometimes used to underline some features (Plates 9 and 10),
multichannel satellite , with a periodicity of 18 days; the considered
scenery is taken at nadir, at approximately 9h40 U,T, A scenery is 120 km wide, with pixel
sizes depending on channel and wavelength, from 30 m in visible light to 120 m for far I.R,
Resolution depth also varies with wavelength and channel, between a few nannometers to
LANDSAT satellite is
a
15 meters.
The herein LANDSAT described pictures are a composition of the pictures obtained by the
different channels (Plates 2 and 6), The detail picture (Plate 3) is a selection on the thermal
channel (infra-red).
Rapport BRGM R 34702 GEO SGN - 92
Illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
4. Description and preliminary interpretation of
spot and landsat xm. imagery
spot images are archived pictures of BRGM purchased for another project, on land and water
use. Sceneries are framed on the Eastern part of Dover Strait, from Dunkerque to South of
Boulogne, The LANDSAT picture is also an archived picture, purchased for this project,
(table 1).
Satellite
Date
Sensor
Scenery
View
angle
5,7.87
21.12,89
17,3,90
17,3.90
26.5.86
7,9,88
11,1.88
Table
38247
37247
37247
37248
36247
SPOT
SPOT
SPOT
SPOT
SPOT
LANDSAT
LANDSAT
1
- Parameters
TM
TM
18.1°
7.6°
3.1»
3.10
QO
QO
0»
of the pictures acquisition.
The pictures were taken at different seasons and tides. Conditions are given in table 2,
Description of the pictures allows to draw the front of the turbidity plume and possible internal
features. In a first step, we will adopt a seasonal chronology, independent of the real
chronology, and start the description in summer, when the turbidity is low. The second approach
will be the comparison of pictures in relation to tide situation and associated currents.
4.1 - PICTURE OF THE 5-7-1987
This picture (SPOT scenery 38247/248), (Plate 1, fig. 1) was taken in summer, during ebb-tide
(HT -I- 4 h in Boulogne, HT + 3 h in Dunkerque), during a neap tide period (coefficient 50). The
tidal current is trending northwards and weak, not long before reverse.
Meteorological conditions were a high pressure of 1014 hpa, little nebulosity, average visibility,
quiet sea. The wind is blowing from Southwest, stable, with a speed of 6 m/s.
The water is clear, and the information obtained on this picture is mainly linked to the
bathymetry and coastal morphology. Within a distance of 2 km from the shoreline, the "blue"
colour of the picture is clearer, correlated with shallow water, or emerging banks (Banc à la
Ligne-Wissant). Those irregularities of the bottom relief are surrounded by a short distance
plume, also underiining the direction of tidal currents (low current, northwards, just before
reversal). Bedrock exposures offshore the Cap Gris Nez also provoke a diffuse turbidity cloud.
Echoes of coastal relief or manmade structures can initiate deviation of the direction of the
cloud, drawing a feathered structure north of Cap Gris Nez, a well formed eddy within the port
of Boulogne, and less structured eddy offshore the port of Calais.
Rapport BRGM R 34702 GEO SGN - 92
33
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Date
co
5.7.87
21.12.89
17.3.90
17.3.90
26.5.86
7.9.88
11.1.88
Ticial conditions
in Boulogne
HT + 4h
HT+3h30
HT-5h
HT-5h
HT-4h
HT+ Ih
HT + 6h
Tidal
coefficient
50
43
67
67
93
45
57
Table 2
:
Tidal current
trend
NNE
NNE
SSW
SSW
SSW
NNE
SSW
Tidal current
speed
0.4Kn
1.0 kn
2.5 kn
2.5Kkn
3.3 kn
2.0 kn
l.lkn
Wind direction
rose of 36
6
18
14
14
24
16
26
Wind
speed(m/s)
6
14
6
6
4
3
11
Tidal and meteorological conditions during acquisition of pictures
Atmospheric
prassure(hpa)
1014
989
1019
1019
1012
1018
1012
Sea
-5
roughness
quiet
rough
g
3"
quiet
C^
:t
tn
3O
î
CD
o
cr
CD
CO
Illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
Stretch colour effect in green underlined, offshore the 2 km wide band structure, the
morphology of sea bottom, and the different bottom structures due to distribution of sand and
gravel, described by C. AUGRIS etal. (1990), can be recognised.
The narrowness of the turbidity plume fits with in-situ measurement made by J,P, DUPONT et
ai, during MAST 1 1990 sea campaigns, giving a low content of SPM at station FX6 in July
and August Oower than 10 mgA SPM),
4.2 - PICTURE OF THE 7-9-1988
This picture (LANDSAT T.M) was taken at 9h40 U,T. The scenery covers the whole Dover
Strait, and the French coast down to Pays de Caux (120 x 120 km). This picture was taken a
few days before the September 1990 cniise of FLUXMANCHE.
The tidal conditions are the beginning of ebb tide (HT+1) in Boulogne, and high tide in
Dunkerque. The period is neap tide (coefficient 45). The tidal current is strong (about 1.3
Knot), trending northwards.
The wind is strong, from Southeast , with a speed of 13 m/s.
On the general view (all channels), the coastal band also observed on SPOT imagery can be
clearly seen, and appears more patchy, south of Cap Gris Nez. The Somme estuary generates a
large plume, which diffuses North and South; it covers the Authie plume, northwards. The cape
d'Ailly, south of Dieppe, generates a small plume, oriented Northwest.
North of Cap Gris Nez, the plume is more continuous, decreasing in intensity seawards, up to
sharp limit, in the centre of the strait
a
The western side of the Strait shows a high density turbidity cloud, continuous along the coast
and gradually decreasing seawards, except for the centre of the Strait limit.
The distribution of the plumes south of Cap Gris Nez are cleariy linked to water input from the
Somme and Authie rivers, and also shows evidence of coastal erosion at Cap d'Ailly, south of
Dieppe. North of Cap Gris Nez, the plume structure is rather linked to the current direction.
The same view (plate 3) on channel 6 (red, thermal) shows a wide stain offshore Boulogne,
limited seawards by this above-quoted north to south limit. This thermal patch is likely linked
to the higher concentration of chlorophyll along the French coast as measured by fluorimetry
(SHIMWELL et al., 1991) This patch is enhanced on this picture by infrared channel, and
absorbed by blue and green colour on the composed picture (plate 2),
This North-South limit draws a front between two water masses, an eastern mass warmer with
higher content of chlorophyll, but little turbid, a western mass, cooler, more turbid, with a
turbidity gradient decreasing seaward.
Rapport BRGM R 34702 GEO SGN - 92
11
Illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
4.3 -PICTURE OF THE 21-12-1989
This picture (SPOT Scenery 37247), (plates 4 and 5, fig, 3 and 4) was taken in winter, during
ebb tide (HT + 3h30 in Boulogne, HT + 2h 30 in Dunkerque) at a neap tide period (coefficient
43), The tidal current is weak, trending towards North-Northeast,
Meteorological conditions are shown by small cumulus on the Northwest of the picture, and on
land. The zoom pictures show foam on waves edges, oriented N to NE, There is a low pressure
(990 hpa), high nebulosity average visibility, agitated sea: the wind is blowing from North to
North East, with a speed of 14 m/s.
The light "blue" band following the coastal line is, here, not dependent on bottom morphology;
its width vary between two (offshore Boulogne) to five kilometres (offshore Wissant) and
appears as a superposition of turbidity plumes of decreasing colour density seawards and
northwards, within one unit, and between different units. This superposition can be observed
offshore Boulogne, North of Cap Gris Nez, offshore Blanc Nez, and Calais,
The outer edges of turbidity plumes are more or less diffuse, or feather-like; the outer cloud,
North of Cap Gris Nez (plate 3) specially shows a sharper limit with dark "blue" water
offshore; inside limits of individual plumes are diffuse. Coastal manmade structures induce
small eddies, within the wider band.
The shape of turbidity plumes suggests a general movement towards the North or Northeast,
and inside dislocation of the plume by a slight reverse movement. The main currents arc, at this
time of the ebb tide, oriented northwards. The width of the plume appears associated with the
current speed, its movement to the current trend. The diffuse internal limits of individual
plumes can be explained by a residual current linked to a shear effect of the shore morphology
and/or to inertia of the SPM plume compared to water movement.
Offshore Boulogne, the above-described light "blue" plume front is close (2 nautic miles from
the shore) to the 20 mg/l SPM concentration isovalue measured in-situ in December 1990
(SHIMWELL et al., 1991), The station FX6 is inside the plume. The width of the plume
increases north of Cap Gris Nez up to Dunkerque (3 to 4 nautic miles) and showing more
irregular edge seawards, as an echo of coastal morphology. The decreasing intensity of light
"blue" seawards may be associated to a stratified shape of the front, oblique and ascending
seawards, at ebb tide, neap tide, still enhanced by higher river discharge in winter
(J.R DUPONT eifl/,, 1991),
4.4 - PICTURE OF THE 11-1-1988
This picture (LANDSAT T,M,) (plate 6, fig 5) belongs to Southampton University Department
of Oceanography,
It was taken in winter, at low tide in Boulogne, during a neap tide period (coefficient 57),The
tidal current trend is southwards, increasing to 1 knot.
Wind is blowing from West-Southwest with a speed of
11
m/s. The pressure condition are high
(1012 hPa)
Rapport BRGM R 34702 GEO SGN 92
12
illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
The picture that we dispose of for this report is a composed colour picture. It shows the coastal
front offshore Cap Gris Nez; the shape of the plume is complex, made of composite plumes, the
external plume reaches more that 3 nautical miles, with a lobate shape south of Cap Gris Nez,
FX6 is on the southern edge of this lobate shape. An internal plume is observed, denser than the
lobate plume, 1,5 nautical mile wide South of Cap Gris Nez, more than two nautical miles north
of Cap Gris Nez, with a diffuse limit towards north. This internal shape is similar to the above
described plume of the 21-12-1989 picture,
4.5 - PICTURE OF THE 17-3-1990
This picture (SPOT scenery 37247), (Plates 7 and 8, fig, 6 and 7) was taken, in late winter, one
week after a strong storm, at the beginning of flood tide, (neap tide) (HT-5 at Boulogne -LT),
Tidal current trend were southwards, strong (2,0 knots).
Meteorological conditions were a south-eastern wind, with a speed of 14 m/s. The pressure was
high (1019 hPa), This picture was taken one week after a strong stonn.
The light "blue" structure appears complex (plate 7): it is wide (up to 4 miles offshore
Dunkerque), and subdivided in elongated shapes (one mile large, for about 5 miles long,
subparallel to the coast line. Those last shapes have a better definition North of Cap Gris Nez,
Within the plume (plate 8), eddies are formed nearshore manmade structures (Port of Calais and
Dunkerque),
Zoomed images on the external plume North of Boulogne suggest a slight movement
southwards; on the contrary, two diffuse structures on the outer edge of the internal plume,
offshore Cap Blanc Nez and between Calais and Dunkerque, suggest a slight northward trend
South of Cap Gris Nez, the band limit is narrower but even more diffuse; no movement is
suggested. The southwards movement is associated to the tidal current situation, although the
internal plume seems to oppose a certain inertia to the current strength.
Seasonal parameters (high river and coastal discharge after the storm, mainly terrigeneous,
stratified water) are likely to interfere on the width of the light "blue" band, and its intensity,
enhancing the colour dispersion. It is likely to be close to the definition
of the coastal water
mass.
4.6 - PICTURE OF THE 26-5-1986
This picture (SPOT scenery 36247), (Plates 9 and 10, fig, 8 and 9) was taken in late spring,
during flood tide, (HT-4 h at Boulogne, HT-5 h at Dunkerque), during spring tide period
(coefficient 93), Tidal currents are oriented southwards, and strong.
Meteorological conditions at 9 h U,T are good: a high pressure of 1012 hP, low nebulosity,
visibility of 2500, quiet sea, wind from Northeast, speed 4 m/s.
Rapport BRGM R 34702 GEO SGN 92
13
illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
The water appears clear and the main plume is narrow, less than one mile offshore. Stretching
on colours (plates 9 and 10) underiines bottom topography offshore Boulogne (C, AUGRIS et
ai, 1990). This internal plume is very similar to the 5-7-1987 picture, despite different tidal
parameter. Small turbidity plumes are essentially linked to bottom irregularities and manmade
structures, inducing small eddies. A secondary plume, less dense, is observe between offshore
Boulogne, between 4 (North of Gris Nez) and ten miles South, where it draws two lobated
structures (plate 10) appearing like large "stains" at the surface of the water.
Rapport BRGM R 34702 GEO SGN - 92
14
illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
5. Synthesis of the description and conclusion
Preliminary exploitation of archived satellite imagery from different tidal and seasonal
situations leads to a dynamic notion of the turbidity plume behaviour. Complementary studies
on pictures calibrated in relation to reflectance and diffusion parameters of water and
atmosphere should allow further interpretation, on concentration and composition of SPM.
The weight of seasonal, tidal and other parameters on the turbidity plume behaviour is
underlined in the following paragraphs. Appreciation of the data given by different satellite
imagery is given.
5.1 - INTERPRETATION OF THE PICTURES IN RELATION TO SEASONAL
AND TIDAL VARIATION
There is not enough imagery to obtain a complete cycle either seasonal or tidal. Nevertheless,
effect of seasonal parameters and/or tidal parameter can be recognised. Pictures are a
combination of those parameters.
5.1.1 - Seasonal variations as observed on pictures
The description of those ¡mages, both SPOT and LANDSAT, underiines the high variability of
the structuration of the turbid front. The seasonal situation seems to affect very little the shape
of the turbidity cloud. The main eñ"ect is on its density and width.
This is observed on late spring and summer images, where most of the information is obtained
by bottom reflectance in shallow water, with localised plumes likely to be of terrigeneous
content, suggesting coastal input and bottom resuspcnsion of limited extension. The main
plume is narrow and close to the shore
In autumn, winter, and early spring, the reflectance is higher, likely to be linked to a higher
quantity of SPM, of terrigeneous content, and a stratified structure of the front, due to some
tidal conditions (neap tide), and fresh water input. Some meteorological conditions can also
enhance the picture of the front
Further interpretation towards characterisation and evaluation of the SPM content needs further
calibration of the atmospheric effect, of the surface reflectance, and diffusion effect of the water
mass.
5.1.2 - Tidal influence on the dynamism of the turbidity plume
Variations due to tidal situations are essentially on the shape of the plume (fig, 10), The trend of
the tidal current induces a general movement of the plume in the same trend, enhanced by a cap
effect, south of Gris Nez, But, in detail, some of the pictures show a residual plume (7/9/1988
Rapport BRGM R 34702 GEO SGN - 92
15
illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
LANDSAT) or a reverse trend to the dominant cun-cnt (21/12/1989 SPOT), Those features are
attributed to the inertia of the turbidity plume, inducing a time lag of the response to the change,
compared to the water mass and current measured at the surface of water. It can also be due to a
differential vertical current
5.2 - OTHER PARAMETERS
Tidal and seasonal effects are the major parameters on shape, width and density of the turbidity
plume as seen by satellite imagery and are intimately combined. Nevertheless, other parameters
interfere on detail features, such as coastal morphology (capes) and manmade structures (ports)
which generate nearshore eddies, inducing a friction, and adding to the time lag due to inertia of
the plume. This can be observed on pictures from 17-3-1990 and 21-12-1989 (SPOT images).
Wind effect appears to have little influence either on shape or width of the turbidity plume
(fig. 10),
5.3 - COMPLEMENTARITY OF DIFFERENT SATELLITE IMAGERY
The study of satellite imagery underiines the interest in getting a 2D picture to be compared
with punctual or linear measurement at sea,
SPOT high resolution gives a rather precise image
interpretation on its behaviour.
of the distribution of turbidity,
and allows
Information given by the LANDSAT TM picture applies to a wider scenery but compiles
different parameters. Resolution of the plume imagery is not so precise than SPOT imagery, but
supeiposition of thermal, turbidity, and swell information cleariy individualises, on the 11-91988 picture, two main water masses in the Strait The same image shows along the French
coast different types of input (river input coastal erosion).
This information is likely to be also obtained by NOAA surface themiicity imagery. This last
imagery has a very wide scenery (English Channel, and North Sea), and little resolution, but
can be taken daily, and compiles different channel. The infonnation is limited to the surface of
water.
Data on atmospheric and water reflectance and diffusion, and calibration with in-situ
measurement (chlorophyll-terrigencous content) is nevertheless necessary to assess the degree
of accuracy of the picture in relation to the coastal water mass ("fleuve côtier"),
5.4 - FURTHER STUDIES
This preliminary work on archived satellite imagery emphasises the usefulness of this approach
to gain a two-dimension picture of the turbidity plume. Analysis of its behaviour, in relation to
in-situ-measuremem, underiines the influence of seasonal output on density and width of the
plume, and the effect of tide on its shape, with a inertia lag. This reinforces the knowledge of
Rapport BRGM R 34702 GEO SGN - 92
16
Illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
in-situ measurements made within of the FLUXMANCHE I program (J,M, BRYLINSKI etal.,
1991; J,R DUPONT etal., 1991), on a transect from Doverto Boulogne,
Calibration of pictures by in-situ measurements in real time with the acquisition of the picture
should allows to go further in our approach, by adding a quantification and qualification
(Chlorophyll versus mineral content) to this first study.
Being done within the framework of FLUXMANCHE I, (L. CABIOC'H et al., 1991), this
preliminary study was focused on the Dover Strait output of the English Channel into the
North Sea, It is wished to focus, in the future, on input zones, such as Baie de Seine, or Baie de
Somme; preliminary observations on archived pictures of this zone have been made and let
think that tidal influence is the main parameter on the behaviour of the plume. Further studies
are necessary to assess this hypothesis.
Rapport BRGM R 34702 GEO SGN -92
17
Illustration by satellite imagery of turbidity in French shore waters of the Dover Strait
Bibliography
AUGRIS C CLABAUT R, VICAIRE O. (1990) - Le domaine marin du Nord - Pas-deCalais ; Nature, morphologie et mobilité des fonds. Publication IFREMER - Région Nord - Pasde-Calais.
BRYLINSKI J.M., LAGADEUC Y. (1990) - L'interface eaux côtières /eaux du large dans le
Pas-de-Calais (côte française) : une zone frontale. C. R. Acad. Sci. Paris, 311, 2, p. 535-540,
BRYLINSKI J,M LAGADEUC Y GENTIHOMME V DUPONT J,R, LAFITE R
DUPEUBLE RA HUAULT M,F, AUGER Y, PUSKARIC Y WARTEL M., CABIOC'H L,
(1991) - Le "fleuve côtier" : un phénomène hydrologique important en Manche orientale.
Exemple du Pas-de-Calais, Oceanologica acta, Actes du colloque international sur
l'environnement des mers epicontinentales. Special issue. H,CHAMLEY (ed,), n° 11,
December 1991,
CABIOC'H L. (1991) - FLUXMANCHE, First annual progress report. June 1990-May 1991.
CURRAN P.J., NOVO E.M. (1988) - The relationship between suspended sediment
concentration and remotely sensed spectral radiance : a review. Journal of Coastal Research 4,
3, p. 351-368.
DUPONT J.P,, LAFITE R HUAULT M,F DUPEUBLE RA BRYLINSKI J,M
GUEGUENIAT R, LAMBOY M., CABIOC'HL. (1991) - Apports des mesures physico¬
chimiques et caractérisation des matières en suspension dans l'étude de la dynamique des
masses d'eaux en Manche Orientale, Oceanologica acta, Actes du colloque international sur
l'environnement des mers epicontinentales, H.CHAMLEY (éd.). Special issue, December 1991,
n° 11, p. 177-186,
MARTEL C, (1990) - Analyse géométrique et physique d'images d'observation de la
terrcEléments d'hydrodynamique côtière. Thèse de doctorat, Univ, Paul Sabatier, Toulouse,
SANTER R., HERMAN M TANRE D LENOBLE J, (1988) - Characterization of the
stratospheric aerosol,/. G. R 93, Dl, p, 14209-14221,
SHIMWELL S,J LAFITE R., DUPONT J.R, HUAULT M.E, LAMBOY M., COLLINS M.B,
(1991) - Suspended material fluxes through the strait of Dover, Subtask SI, in: Hydrodynamics
ans Biogeochemical fluxes in the Eastern Channel; fluxes into the North Sea, FLUXMANCHE,
First annual progress report. June 1990 - May 1991, Coordinator: L. CABIOC'H,
VIOLLIER M, (1980) - Télédétection des concentrations de seston et pigments chlorophylliens
contenus dans l'océan. Thèse de doctorat d'état - Sciences physiques. Univ. Sci. et Tech., Lille.
Rapport BRGM R 34 702 GEO SGN -92
18
Appendix
ILLUSTRATION BY SATELLITE IMAGERY
OF TURBIDITY IN FRENCH SHORE WATERS
OF THE DOVER STRAIT. Preliminary interpretation.
ILLUSTRATION PAR L'IMAGERIE SATELLITAIRE
DE LA TURBIDITE SUR LE LITTORAL
DU DETROIT DU PAS DE CALAIS. Interprétation préliminaire.
SATELLITE IMAGERY and INTERPRETATIONS
Illustration by satellite imagery of turbidity in french sfiore waters of the Dover Strait
^
^-^ '
^.'"
1
'"-"' ^^
/ / fi
j
5/7/87 - SPOT
High tide 4 4h
neap tide
/
^^
{¡Cap Gris
*
,
Nez
(
,.\'
;\\\
!
'
U
1
\
\
FX6
1
Í
U
M
\
:i\
;
/
i/^/U
//
'
;/
tl
t*
."
'
,
.
It
»
m'
»
J
*
".'V-->
fl
1
r-'l \
'.''
o/î>
if
1/
BOULOGNE
\
1,2l<m
1
Limit of the turbidity plume
Internal structures of the plume
^
"^^
Figure
External structures, underlining bathymorphology and associated
suspended matter
Tidal current trend
Wind direction
1 : Interpretation of tlie SPOT 5/7/87 picture in terms of turbidity.
(to be compared with plate 1)
Rapport BRGM R 34 702 GEO SGN 92
A/2
1
Illustration by satellite imagery of turbidity infrench shore waters of the Dover Strait
Plate 1 - SPOT image of 5-7-87-Scenery 38247, view angle 18.1° - Pas de Calais,
(processed by B R G M )
Rapport BRGM R 34 702 GEO SGN 92
A/3
Illustration by satellite imagery of turbidity in french shore waters of the Dover Strait
DOVER
^^^
-/ N 'f^^
~'^ /'^^'\ >:^^:0',/^^^^^^
'^^
Cap
7/9/88 - LANDSAT
High tide * 1h
neap tide
d'A/tl/
,8l<m
I
:.:;: ^
High to low turbidity
Limit of the turbidity plume and internal structures
-^
Tidal current trend
Wind direction
Figure 2 : Interpretation of the 7/9/88 LANDSAT TM picture in terms of turbidity
(to be compared with plate 2)
Rapport BRGM R 34 702 GEO SGN 92
A/4
1
Illustration by satellite imagery of turbidity ¡nfreneh shore waters of the Dover Strait
Plate 2 : L A N D S A T T M picture of 7/9/88
(Processed by B R G M )
Rapport BRGM R 34 702 GEO SGN 92
A/5
Illustration by satellite imagery of turbidity infrench shore waters of the Dover Strait
Plate 3 : Detail of the L A N D S A T T . M picture of the 7.9. !. Infra-red
channeUthermic)
(processed by B R G M )
Rapport BRGM R 34 702 GEO SGN 92
A/6
Illustration by satellite imagery of turbidity in french shore waters oftfte Dover Strait
DUNKERQUE
21/12/89 - SPOT
High tide
3h30
neap tide
1
3.7kin
I
limit of the turbidity plume
Internal structures of the plume
Tidal current trend
Wind direction
Interpretation of the SPOT picture of the 21/12/89 in terms of turbidity
(to be compared to plate 4)
Figure 3
:
Rapport BRGM R 34 702 GEO SGN 92
A/7
1
Illustration by satellite imagery of turbidity infrench shore waters of the Dover Strait
Plate 4 : S P O T picture of the 21/12/89 - Scenery 37247- view angle 7.6°.Pas de
Calais
(processed by B R G M )
Rapport BRGM R 34 702 CEO SCN 92
A/8
Illustration by satellite imagery of turbidity in french shore waters of the Dover Strait
/
."''
'
t
f
I
^^
1
-"./
WISSANT
21/12/89 - SPOT
High tide
3h30
neap tide
0.6km
I
Limit of the turbidity plume
internal structures
tidal current trend
reverse currents
wind direction
Figure 4 : Interpretation of the SPOT picture of the 21/12/89 in terms of turbidity.
Detail of the eddy north of Cap Blanc Nez
(to be compared with plate 5)
Rapport BRGM R 34 702 GEO SGN 92
A/9
1
Illustration by satellite imagery of turbidity infrench shore waters of the Dover Strait
Plate 5 : Detail of S P O T picture of the 21-12-89-scenery 37247. North of Cap Gris
Nez
(processed by B R G M )
Rapport BRGM R 34 702 GEO SGN 92
A/10
Illustration by satellite imagery of turbidity in french shore waters of the Dover Strait
11/01/87 - LANDSAT
Low tide
neap tide
\
\
V
0,5km
Limit of the internal plume
Limit of the distal plume
^
Figure
Tidal current trend
Wind direction
Interpretation of the LANDSAT TM picture of the 1 1/1/88
(to be compared with plate 6)
5 :
Rapport BRGM R 34 702 GEO SGN 92
A/Il
Illustration by satellite imagery of turbidity infrench shore waters of the Dover Strait
Plate 6 : L A N D S A T T M picture of the 11 -1.88 - Cap gris nez
(processed by Southampton University Department of Oceanography)
Rapport BRGM R 34 702 CEO SGN 92
A/12
Illustration by satellite imagery of turbidity in french shore waters of the Dover Strait
DUNKERQUE
CALAIS
17/03/90 - SPOT
High tide - 5h
spring tide
BOULOGNE
/
3.6km
I
Limit of the turbidity plume
Internal structures : manmade constructions induced eddies
Tidal current trend
Wind direction
Figure 6 : Interpretation of the SPOT picture of the 17/3/90 in terms of turbidity
(to be compared with plate 7)
Rapport BRGM R 34 702 GEO SGN 92
A/13
1
Illustration by satellite imagery of turbidity infrench shore waters of the Dover Strait
Plate 7 : S P O T picture of the 17/3/90 - Scenery 37247. view angle 3.1O-Pas de
Calais.
(processed by B R G M )
Rapport BRGM R 34 702 GEO SGN 92
A/14
Illustration by satellite imagery of turbidity in french shore waters of the Dover Strait
19/03/90 - SPOT
High tide - 5h
spring tide
0,6km
I
eddies structure inside the turbidity plume, induced by man made
construction
Tidal current trend
Wind direction
Figure 7: Interpretion of a detail of the 17/3/90 SPOT picture
(to be compared with plate 8)
Rapport BRGM R 34 702 GEO SGN 92
A/15
1
Illustration by satellite imagery of turbidity infrench shore waters of the Dover Strait
Plate 8 : Detail of 17/3/90 S P O T picture
(processed by B R G M )
LQË&ÎL fît 17/3/90 S P O T picture (copyright B R G M )
Rapport BRGM R 34 702 CEO SGN 92
A/16
Illustration by satellite imagery of turbidity in french shore waters of the Dover Strait
26/05/86 - SPOT
High tide -4h
spring tide
3.9km
1
limit of the distal turbidity plume
limit of the proximal turbidity plume
:==-
internal structures in the distal turbidity plume
Tidal current trend
Wind direction
Figure 8 : Interpretation of the 26/5/86 SPOT image
(to be compared with plate 9)
Rapport BRGM R 34 702 GEO SGN 92
A/17
1
Illustration by satellite imagery of turbidity infrench shore waters of the Dover Strait
Plate 9 : S P O T picture of the 26/5/86 . Scenery 36247. view angle 0°. Pas de Calais
(processed by B R G M )
Rapport BRGM R 34 702 GEO SGN 92
A/18
Illustration by satellite
imagery of turbidity
in french shore waters
of the Dover Strait
Figure 9
:
Cap
Gr/s f/ez
.
Interpretation
of 26/5/86 SPOT picture
Detail of figure 8. to be
compared with plate 10
Limit of the distal
and internal turbi¬
dity plumes
Internal structures
-^
Tidal current trend
Wind direction
/
/.
BOUL
/
I
W
/
\
y
V
y
/
/
/
\
>
I
I
'
I
I
I
1.2km
I
Rapport BRGM R 34 702 GEO SGN 92
A/19
1
Illustration by satellite
imagery of turbidity
infrench shore waters
of the Dover Strait
Plate 10 : Detail of
the 26/5/86 S P O T
picture
Scenery 36247.
view angle 0°
(Processed by B R G M )
Rapport BRGM R 34 702 GEO SGN 92
A/20
Illustration by satellite imagery of turbidity infirench shore waters of the Dover Strait
11/03/90 - SPOT
HT -5h
spring tide
3.6kni
26/05/86 - SPOT
HT -4h
spring tide
3.6km \
07/09/88 - LANDSAT
HT Ih
neap tide
21/12/89 - SPOT
HT 3h30
neap tide
TM
3.6km
3.6km
/
05/01/91 - SPOT
HT *kh
neap tide
11/01/87 - LANDSAT
LT
neap tide
FX6
3.6km
3.6km
Tidal current
Wind
Figure 10
:
Comparison
of the different images within
RAPPORT BRGM R 34 702 GEO SGN 92
the tidal cycle.
A/21