ICES CM 2004/P:42, POSTER
A Digital Atlas for the Physical Environment of Eastern English Channel (CHARM Project)
C. S. Martin, A. Carpentier, F. Coppin, P. D. Eastwood, G. J. Meaden, M., Walkey, S.
Harrop, Z. Kemp and S. Vaz
ABSTRACT
The Dover Strait, which connects the English Channel to the North Sea, constitutes a
significant economic resource for a wide range of human activities including the extraction of
biotic (e.g. fisheries) and abiotic (e.g. aggregate) resources, plus ports and shipping.
Exploitation of these natural and human resources currently lacks integration and applies
pressure on the marine ecosystem, including fish stocks. There is a crucial need for
harmonizing environmental information in this area. CHARM (Eastern Channel Habitat Atlas
for Marine Resource Management) is a Franco-British Interreg IIIA project that intends to
meet this requirement through the development of a digital atlas.
Using Geographical Information Systems (GIS) technology, physical parameters such
as bathymetry, seabed sediments, shear seabed stress, estuaries and embayment, tidal fronts,
water temperature and salinity will be compiled into an atlas. Seasonal and inter-annual
variability of features such as water temperature and salinity will be investigated and
included in the atlas.
The data will be used to create a Marine Landscape map for the Dover Strait and
Eastern English Channel, with broad scale units based on geophysical, physiographic and
hydrodynamic characteristics of the seabed and the water column. Since biotic resources,
such as commercial fish species interact closely with their physical environment. this Marine
Landscape map is expected to aid ecosystem management in areas of the Dover Strait and
Eastern English Channel where fish abundance data is lacking.
KEY-WORDS:
Eastern English Channel, CHARM, Physical Environment, Digital Atlas, GIS
ADDRESSES:
C. S. Martin & G. J. Meaden: Fisheries GIS Unit, Department of Geographical and
Life Sciences, Christ Church University College, Canterbury CT1 1QU, U. K. [tel. (+44)
1227 767700 (ext. 2324), e-mail: [email protected]]. A. Carpentier, S. Vaz & F.
Coppin : Ifremer, Laboratoire Ressources Halieutiques, 150 quai Gambetta, BP699, 62321,
Boulogne/mer, France [tel : (+33) 3 21 99 56 00, fax : (+33) 3 21 99 56 01, e-mail
:[email protected]]. P. D. Eastwood, CEFAS, Lowestoft Laboratory, Lowestoft
NR33 0HT, U.K. [tel. (+44) 1502 562244, e-mail : [email protected]]. M. Walkey &
S. Harrop : Durrell Institute of Conservation and Ecology, Department of Anthropology,
University of Kent at Canterbury, Canterbury, Kent CT2 7NS, UK [tel : +44 (0) 1227 764000,
fax : +44 (0) 1227 827289, e-mail : [email protected]]. Z. Kemp : Department of
Computer Science, University of Kent at Canterbury, Canterbury, Kent CT2 7NS, UK [tel :
+44 (0) 1227 827698, fax : +44 (0) 1227 762811, e-mail :[email protected]].
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INTRODUCTION
The Dover Strait (FIGURE 1a & 1b, showing the study area), which connects the
English Channel to the North Sea, constitutes a significant economic resource for a wide
range of human activities including the extraction of biotic (e.g. fisheries) and abiotic (e.g.
aggregate) resources, plus ports and shipping. Exploitation of these natural and human
resources currently lacks integration and applies pressure on the marine ecosystem, including
fish stocks. There is a crucial need for harmonizing environmental information in this area.
CHARM (Eastern Channel Habitat Atlas for Marine Resource Management) is a FrancoBritish Interreg IIIA project that intends to meet this requirement through the development of
a digital atlas. The atlas is expected to be useful for a variety of purposes: potential users
include fisheries managers, aggregate extraction companies, researchers, but also the general
public since one of the project’s objectives is to increase the public’s awareness of the
environment of the Eastern English Channel.
STUDY SITE
The Dover Strait represents a unique transitional zone between two contrasting
environments: to the west lie the warmer, more saline waters of the Eastern English Channel,
whilst to the north are the colder, less saline waters of the North Sea (Corten & van de Kamp,
1996). The transitional hydrographic character of the region is reflected in the faunal
composition, with both the Dover Strait and southern North Sea containing species of
“boreal” (northerly) and “Lusitanean” (southerly) origins (Roger & Millner, 1996). The area’s
fauna is abundant and diverse: its richness is linked to abundant benthic populations (Nival,
1991), and makes it an important fishing ground.
Depth and seabed sediments
The physical characteristics of the seabed in the Dover Strait are heavily influenced by
the strong tidal dynamics encountered in the region. Much of the inshore waters are relatively
shallow and generally do not exceed 10 m in depth, whereas offshore depths range up to 6070 m. The slope of the seabed is relatively gentle throughout most of the region, but is more
steeply-shelving in the immediate vicinity of the Dover Strait. A narrow deep (40 m) water
channel runs through the centre of the Strait.
The seabed sediments in the region consist predominantly of sands and gravels. Sands
are found along much of the French, Belgian and UK coasts, except in the immediate vicinity
of the Dover Strait where gravely sediments dominate. A few localised areas of muddy
sediments are found in the outer Thames estuary, along the southern UK coast, and along the
Belgian coast to the east of the Dover Strait. The most prominent seabed feature within the
region is the large number of sandbanks aligned roughly parallel to the coast in both nearshore
and offshore waters.
Oceanography
Physical dynamics: The currents in the Dover Strait are dominated by tidal flows.
Superimposed onto the tidal regime are wind-driven currents, and to a lesser extent, currents
caused by density gradients from the mixing of fresh and saline coastal waters. The general
direction of the tidal streams are from west to east with oceanic water entering the English
Channel from the north-east Atlantic and then flowing through the Dover Strait into the
southern North Sea. The speed of the tidal flow increases in the narrowing channel between
the Cherbourg peninsula (Cotentin) and the Isle of Wight, and also as it travels through the
relatively restricted passage of the Dover Strait. Maximum current speeds in the Strait are
typically 1.5 ms-1, and the tidal range is high towards the eastern end of the English Channel,
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reaching 8-9 m along the French coast. The water column in the Dover Strait and surrounding
sea area remains well-mixed throughout the year, as turbulent mixing in the relatively shallow
waters prevents the development of a thermocline during the summer months. Density
gradients, however, can be found along the coastal margins near to areas of freshwater input.
Temperature: A coastal-offshore temperature gradient exists throughout much of the
year, particularly near to regions of freshwater influence: coastal waters tend to be warmer
than the offshore waters in the summer, and colder in winter. Coastal waters near to sources
of freshwater input experience a greater range of temperatures, such as along the French coast
to the south of the Strait. The flow of warmer water that runs through the English Channel and
extends into the southern North Sea ensures that offshore temperatures rarely fall below 5 C
in the winter, although colder waters can be found near to the coast.
Salinity: The salinity of the water throughout the region is typical of oceanic water.
The waters along the south-east coast of England are an exception, as they receive minimal
freshwater inputs and so a higher salinity is maintained throughout the year. In contrast, the
French coast of the Eastern English Channel is characterised by a region of freshwater known
as the “fleuve côtier”, or coastal flow (Brylinsky et al., 1991). The flow, which can extend to
6 miles from the coast, is maintained by freshwater inputs from the numerous rivers
discharging along the French coast. The less saline coastal waters are separated from the
offshore waters of north Atlantic origin by a tidal front, which varies in intensity depending
on the tidal state.
METHODS
Datasets
Work so far has concentrated on short-listing the physical parameters which would
best describe the physical environment of the Eastern English Channel. Then, work focused
on finding suitable data providers and obtaining datasets. It was usually necessary to
standardise and validate data obtained from different sources in order to subsequently allow
displaying them at meaningful temporal and spatial scales within the atlas. For example, all
data had to be provided in WGS (World Geodetic System) 1984 geographic coordinate
system. It was important to achieve consistency among environmental data layers, especially
because most of them are to be used for the final stage of fish habitat suitability modelling
(i.e. for the mapping of habitat suitability indices), another aspect of the CHARM project.
From survey data points to regular grids of interpolated data points
Some datasets (e.g. water temperature) were provided as data points (e.g. data
obtained from oceanographic surveys), from which continuous raster maps were to be created.
Geostatistics is a methodology for estimating the values of a parameter of interest in areas
where the parameter has not been sampled directly. Kriging is the general term for
geostatistical estimation, and is different from other interpolation techniques (e.g. inverse
distance weighted) because it uses a model of the spatial variation within the dataset – the
variogram. The method used in this study is best described in Webster & Oliver (2001).
Geostatistical methods and kriging were used to interpolate survey data points into regular
grids of interpolated data points, which were then used as basis for the creating of continuous
raster maps (see next paragraph). The grid’s spatial resolutions were chosen according to the
spatial resolutions of the original survey dataset (i.e. a survey with spatially closer data points
could be kriged in grids of higher resolution, and conversely).
Cartography using GIS (Geographical Information Systems)
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Grids of interpolated data points resulting from kriging were imported into ArcGIS
(ESRI), plotted and projected to a common reference system. A custom Transverse Mercator
projection was chosen because it was applicable for the study area and preserved distance and
area as best possible. The grids’ data points were then interpolated into raster maps using the
default kriging parameters available in ArcGIS’s Spatial Analyst tool (ESRI, ArcGIS version
8.2). Care was taken to limit the spatial extent of the interpolated maps so as to avoid keeping
data resulting from extrapolation (i.e. in areas where no sampling had taken place). All maps
created are to be later compiled into a digital atlas.
RESULTS TO DATE & DISCUSSION
Work so far has concentrated on selecting the physical parameters which would best
describe the physical environment of the Eastern English Channel: bathymetry, seabed
sediments, seabed stress due to the M2 tide, sea surface temperature and salinity.
Bathymetry
As mentioned above, most of the data layers are to be used for the final stage of fish
habitat suitability modelling (another aspect of the CHARM project). Hence, it was decided
that bathymetry corrected for mean sea level would be biologically relevant for estimating
fish habitat suitability indices. Classical bathymetric maps (i.e. corresponding to the lowest
astronomical tide) were thought to be less informative with regards to fish habitat. Sea level
above zero at mid-tide (tide coefficient of 70) was obtained from a hydrodynamic model of
IFREMER labs in the form of a regular grid of data points (resolution 4 km2), which was then
combined to a classical bathymetric raster map. This resulted in a 4 km2 “bathymetry with
mean sea level” raster map (FIGURE 2). This relatively low resolution is thought not to
impact on model outputs since its resolution is comparable to (and better than) most of the
other atlas’ data layers (particularly the fish abundance distribution maps).
Seabed sediments
Among the GIS datasets available for seabed sediment in the study area, the vector
map created by Larsonneur (1979) was found to provide the best coverage. Although the
classification covers > 20 types of sediments, a simpler classification (FIGURE 3) was
adopted: (1) fine sand, (2) coarse sand, (3) gravel & pebbles, (4) mud, since it was found to be
sufficient for the modelling part of the project.
Seabed stress due to the M2 tide (Lunar Semidiurnal Tide)
This dataset, which was obtained as a grid from POL (Proudman Oceanographic Labs,
U. K.), is an Output from a two-dimensional hydrodynamic model of the north-west European
shelf and is used to predict the depth-mean M2 tidal current (in m. s-1) at a spatial resolution
approximately 8 km2. Bed stresses (FIGURE 4, in Newtons) due to the M2 tide were then
calculated using a quadratic expression, with stress dependent on the predicted maximum
ellipse current and an appropriate bed friction coefficient (in this case with an assumed value
of 0.0025).
Sea Surface Temperature
In situ measurements: Sea Surface Temperature (SST) data measured in situ were
provided by IFREMER (Channel Ground Fish Survey, CGFS, October months) and CEFAS
(Beam Trawl Survey, BTS, August months). Temporal and spatial coverages were uneven,
but relatively good datasets were available for 1997-2003 (CGFS) and for the years 1989,
1990, 1993, 1994, 1998-2002 (BTS). Annual continuous raster maps for the months of August
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(BTS) and October (CGFS) and mean continuous raster maps (covering all available years)
for these months were created (e.g. FIGURE 5 and 6a, total: 18 maps).
Satellite imagery: Sea Surface Temperature (SST) data were also freely available for
download at the U.S. NASA/PO.DAAC web site. For the period 1988 to 2002, SST ASCII
grid data derived from AVHRR sensor were downloaded for each month (January –
December). For the year 2003, ASCII grid data were derived from MODIS/AQUA sensor
(AVHRR ended 2002). AVHRR-derived SST data (9 km resolution) presented a coarser
resolution compared to MODIS/AQUA-derived SST data (4 km resolution).
In order to compare SSTs derived from satellite imagery with SSTs measured in situ
(during oceanographic surveys), continuous SST raster maps were created using SST ASCII
grid data derived from AVHRR and MODIS (e.g. FIGURE 6b). These raster maps are soon to
be compared to those created using in situ SST measurements (August for BTS survey and
October for CGFS). If the maps are comparable, it might then be possible to utilize data
derived from satellite imagery for those years when survey data points were lacking. This will
be particularly pertinent for the fish habitat modelling part of the project since models have so
far been designed using in situ data (survey data points).
Water salinity
Surface water salinity data measured in situ were provided by IFREMER (CGFS,
October months) and CEFAS (BTS, August months). Temporal and spatial coverage were
uneven, but relatively good data were available for 1997-2003 (CGFS) and for the years 1989,
1990, 1993, 1994, 1998-2002 (BTS). Annual raster maps for the months of August (BTS) and
October (CGFS) and mean raster maps (covering all available years) for these months were
created (FIGURE 7, total: 18 maps).
CONCLUSION
Most of the data layers of the digital atlas have been standardised and are being
compiled. Potential sources for dataset describing estuaries and embayments are being
investigated and will be added to the atlas. An important part of future work will involve
validating the use of sea surface temperature data obtained by satellite telemetry for those
years when survey surface temperature were not collected. Seasonal and inter-annual
variability of features such as water temperature and salinity will also need to be investigated
and included in the atlas.
The digital atlas is intended to be used to create a Marine Landscape map (Golding et
al., 2004) for the Eastern English Channel, with broad scale units based on geophysical,
physiographic and hydrodynamic characteristics of the seabed and the water column. Since
biotic resources, such as commercial fish species interact closely with their physical
environment. this Marine Landscape map is expected to aid ecosystem management in areas
of the Eastern English Channel where fish abundance data is lacking.
ACKNOWLEDGMENTS
The CHARM (Eastern Channel Habitat Atlas for Marine Resource Management) project is
funded at 50% by the EU under the INTERREG IIIA scheme (with help from the Region
Haute Normandie). The authors would like to show their gratitude to NASA/PO.DAAC (U. S.
A.) for the use of AVHRR and AQUA/MODIS satellite imagery, and to Proudman
Oceanographic Laboratories (POL, U. K.) for kindly providing seabed stress data. Many
thanks to Frank Dumas (IFREMER Brest) for providing data on mean sea level for the
English Channel.
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REFERENCES
Corten, C. L. & van de Kamp, G., 1996. Variation in the abundance of southern fish species
in the southern North Sea in relation to hydrography and wind. ICES Journal of
Marine Science, 53, 1113-1119.
Brylinsky, J. M., Lagadeuc, Y., Gentilhomme, V., Dupont, J. P., Lafite, R., Dupeuble, P. A.,
Huault, M. F., Auger, Y., Puskaric, E., Wartel, M., Cabioch, L., 1991. The “coastal
flow”: an important hydrological phenomenon in the Eastern English Channel.
Example of the Dover Strait. Oceanological Acta, 11 special issue, 197-203.
Golding, N., Vincent, M. A., Connor, D. W., 2004. Irish Sea Pilot - Report on the
development of a Marine Landscape classification for the Irish Sea. JNCC. and online
at www.jncc.gov.uk/irishseapilot.
Larsonneur, C., Vaslet, D., Auffret, J. –P., 1979. Les Sediments Superficiels de la Manche,
Carte Geologique de la Marge Continentale Francaise. Bureau des Recherches
Geologiques et Minieres, Ministere de l’Industrie, Service Geologique National,
Orleans, France
Nival, P., 1991. Manche et mer du Nord : riches et fragiles. Science et Vie n° 176. Sept. 91.
Hors-série. La vie des océans : pp. 106-113.
Roger, S. I. & Millner, R. S., 1996. Factors affecting the annual abundance and regional
distribution of English inshore demersal fish populations: 1973 to 1995. ICES Journal
of Marine Science, 53, 1094-1112.
Webster, R. & Oliver, M. A., 2001. Geostatistics for Environmental Scientists. Wiley,
Chichester.
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FIGURE 1a. The English Channel in relation to surrounding seas
FIGURE 1b. The Dover Strait. The study area of the CHARM project is shown in blue.
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FIGURE 2. Bathymetry with mean sea level (corresponding to a tide coefficient of 70) (m).
FIGURE 3. Seabed sediments according to Larsonneur’s map (1979).
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FIGURE 4. Seabed stress (N).
FIGURE 5. Mean Sea Surface Temperature (SST) over nine years of BTS surveys in August
(years 1989, 1990, 1993, 1994, 1998-2002). The sampling stations upon which geostatistical
analysis and kriging were based are shown.
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FIGURE 6a. Sea Surface Temperature (SST) for the month of August 1994 (created from data
collected during the BTS survey). The sampling stations upon which geostatistical analysis
and kriging were based are shown.
FIGURE 6b. Sea surface Temperature (SST) for the month of August 1994 (created from data
collected by the AVHRR sensor).
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FIGURE 7. Mean surface salinity (SSALN) over seven years of CGFS surveys in October
(years 1997 to 2003). The sampling stations upon which geostatistical analysis and kriging
were based are shown.
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