High resolution coastal DEM of the Dover Strait:
managing dynamic bedforms
Aurélie Maspataud, Thierry Schmitt, Laurie Biscara and Ronan Créach
SHOM, 13 rue du Chatellier, CS92803, 29228 Brest CEDEX 2, France
Garcia-Moreno, D., Verbeeck, K., Camelbeeck, T., De Batist, M., Oggioni, F., Zurita Hurtado
O., Versteeg, W., Jomard, H., Collier, J.S., Gupta, S., Trentesaux, A., Vanneste, K. 2015.
Fault activity in the epicentral area of the Dover Strait (Pas-de-Calais) earthquake
(northwestern Europe). Geophysical Journal International, 201, 528-542.
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Garlan, T., Le Faou, Y., Guyomard, P., Gabelotaud, I. 2008. French marine sand dune
project. Marine and River Dune Dynamics, 1-3 April 2008, Leeds, United Kingdom, 133-
Hebert, H. & the TANDEM project team. 2014. Project TANDEM (Tsunamis in the Atlantic
and the English ChaNnel: Definition of the Effects through numerical Modeling) (2014-
2018): a French initiative to draw lessons from the Tohoku-oki tsunami on French coastal
nuclear facilities, EGU 2014, Vienna, 28 April-2 May 2014, Vol. 16, Poster.
Le Bot, S., Idier, D., Garlan, T., Trentesaux, A., Astruc, D. 2000. Dune dynamics: from field
measurements to numerical modelling. Application to bathymetric survey frequency in
the Calais-Dover Strait. In Marine Sandwave Dynamics, International Workshop,
Université de Lille 1, 23-24 mars 2000, 101-108.
Maspataud, A., Biscara, L., Hébert, H., Schmitt, T., Créach, R. 2015. Coastal Digital
Elevation Models (DEMs) for tsunami hazard assessment on the French coasts. EGU
2015, 13-17 avril 2015, Vienna, Autriche. Poster.
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Geoscience, Vol. 7, August 2014, p. 550.
Roger, J. & Gunnell, Y. 2012. Vulnerability of the Dover Strait to coseismic tsunami hazards:
insights from numerical modeling. Geophysical Journal International, 188, 2, 680-686.
The present work is undertaken within the framework of the TANDEM project
(Tsunamis in the Atlantic and the English ChaNnel: Definition of the Effects through
numerical Modeling ; 2014-2017). This project is dedicated to the appraisal of
tsunami waves on coastal areas along the French coastlines (Owen and Maslin, 2014),
with a special focus on the Atlantic and The Channel coasts where French civil nuclear
facilities have been operating since about 30 years (Hébert et al., 2014).
Scope of work For further information see TANDEM
project website: www-tandem.cea.fr
Seamless integrated high resolution (10 - 20m) topo-bathymetric coastal DEMs are
generated for specific coast configurations (embayment, estuary, bay, harbor
approaches…) (Maspataud et al., 2015) and will be used to simulate expected wave
heights for a set of defined tsunami generation scenarios, such as earthquake (Roger
et Gunnel, 2012 ; Garcia-Moreno et al., 2015) or landslide triggering.
Characteristics of the study area
High resolution Dover Strait
topo-bathymetric DEM (20 m)
Fig 1. Location of the studied area and the coastal
topo-bathymetric DEM on the Dover Strait.
• A strong alternative character of tidal currents enhanced by the strait
• A dynamic seabed with a diversity of sedimentary features ;
• A local complexity of the area composed of superimposed bedforms: dynamic
active sand dunes as part of giant dune fields, or as associative bedforms usually
well developed on the flanks of sandbanks.
In the French side, known regular surveys of subaqueous sand dunes and
sandbanks were performed by SHOM for the last decades, to ensure the safety of
navigation (Garlan et al., 2007, 2008).
with variable density and
from many different types
and from multiple sources
services, state and
and private engineering
topographic data (LIDAR)
Topographic LIDAR DEM (RGE ALTI 1m® IGN)
Echo sounding surveys (SHOM)
Multibeam surveys (SHOM)
Multibeam surveys (french harbour authorities)
Echo sounding surveys (french harbour authorities)
Multibeam surveys (UKHO)
Composite DEM LIDAR 2 m (Environment Agency)
SRTM 90 m V.4 DEM (NASA-NGA)
Echo sounding (Vlaamse Hydrografie)
Single beam echosounder
Paper and Electronic
to be digitized
Low resolution composite
1- Inventory of
2- Harmonization of the different datasets
Fig 2. Source, type and coverage of selected datasets, available for each country, on the extend of DEM.
Red dash and solid lines respectively represent the extracted data area and the final DEM coverage.
Fig 4. Example of (A) DEM interpolation without and with
specific selection between available bathymetric surveys, and
(B) difference between 2003 and 2012 SHOM’s bathymetric
surveys in the North Sea.
• Ensure the vertical and horizontal precision of the data by getting access to
corresponding metadata or reports and processing (punctual editing, tide
correction…) them if needed, to minimise potential artefacts.
• In order to converge to a similar vertical reference (ellipsoid) prior to the
interpolation, SHOM takes advantage of (national and/or local) datum conversion
grids based on the BathyElli® product in French waters (e.g. CD/ell.) and the VORF
project in British waters (LAT/ell.), and the conversion grid RAF09 from IGN for
French topographic data.
3- Filtering and processing to enhance morphological coherency
Key points in high resolution coastal DEM development
managing with complex morphology and bedforms mobility
Fig 3. Examples of morphological artefacts of sedimentary
bedforms encountered during bathymetric data assessment.
5- Validation and coherency 4- Interpolation
6- Vertical datum conversion
High resolution topo-bathymetric DEM of the Dover Strait
• This step consists in
interpolating the entire dataset
in order to fill gaps and get a
continuous and regular sea-
• The implementation of the
Multilevel B-Spline inter-
polation (from SAGA-GIS
software) is used for its
efficiency to honor variable
density data while providing a
smooth and precise continuous
• Locally, gaps between marine
and terrestrial data also
required the introduction of
new methods and tools to
This work is supported by the French national research agency (ANR) program TANDEM in the frame of "Investissements d’Avenir",
under the grant ANR-11-RSNR-00023-01. The authors thank H. Hébert (CEA), coordinator of the TANDEM Project team.
The authors are grateful to I. Laeremans (Vlaamse Hydrografie), J. Thomas (UKHO), F. Guyot (IGN), the Archive data team (Environment Agency), the BELGICA team, and French
harbor authorities (Boulogne-sur-Mer, Calais, Dunkerque) for providing complementary bathymetric and topographic data. The efficient technical help provided by S. Thépaut
and J. Genevier (SHOM) for data cleaning and digitization was greatly appreciated.
• This final step leads to use vertical datum conversion
grids (as cited in step 2) to obtain a series of DEMs
referenced to a variety of vertical datums, to meet the
needs of different users.
• Currently, final products are vertically provided at:
- the lowest astronomical tide (LAT) and
- the mean sea level (MSL).
• This task deals with the inspection of the resulting DEM, through its visual
analysis (2D, 3D) along with slope computation and comparison with the
initial and/or not included soundings.
• The lack of coherence or the presence of artifacts is treated by further
processing of the original dataset prior to new interpolation: new
deconfliction of appropriate datasets, and/or cutting of overlapping
Until now the deconfliction between numbers of
datasets (neighbors, redundant, overlapping ...) is
manually performed by the operator in charge of the
building of DEM, based on such previously acquired
knowledge on the working area.
On the overall of the gathered datasets, 67.4% were
selected among the approximately 651 available
surveys, i.e. 439 selected surveys including 38% carried
out from multibeam sensors.
Sediment dynamics can
locally induce artifacts on
the resulting DEM or
poor temporal accuracy
of the representation of
In some cases, it leads to
inappropriate shifting of
the crests on dune fields
(Fig. 3B ; Fig. 4A) or of
the overall shape of
sandbanks (Fig. 3A, C, D).
This complex sedimentary environment led us to carefully consider
the moving rates of sand dunes in this area and estimate whether
these values are significant with respect to the expected resolution of
the final DEM. Locally, differences with previous bathymetric surveys,
on sandbanks and dune fields, are sometimes larger than the grid
Previous studies that were based on an optimum recurrence of re-
surveying (annual, bi-annual or decadal) identified: 1) the highly
sensitive sectors, 2) the direction in which the dunes preferentially
move (either NE or SW in this area ; Fig. 5), and 3) the mean rates of
displacement, or those of particular structures (Le Bot et al., 2000 ;
Garlan et al., 2008).
In the case of the Dover Strait, considerations about the local
sedimentology have been taken into account from:
• decadal survey of specific dunes reveals that they can move from
about 50 to 100 m to the SW (or NE elsewhere ; Garlan et al., 2008) as
experienced in Fig. 4B ;
• dunes movements could reach several meters to several dozen of
meters per year, up to 17 to 23 m per year, according to their exposure
to main currents ;
• isolated dunes in southern North Sea could move 1.5 times faster
than dune grouped in dune fields (Garlan et al., 2008).
Fig 5. Extract of the "Distribution of sand dunes of the French continental shelf"
product by SHOM (V1.1, 2013 ; available on data.shom.fr). The Geographic
Information System of the Dunes (Dunes GIS) was developed by the cell
"Sedimentology" of SHOM during the Dunes I project (2006-2010). Its
development continues in the Dunes II project (2013-2017). This product includes
the representation of the dune crest of near 3000 subaqueous dunes digitized
from regular bathymetric surveys conducted by SHOM (Garlan, 2009).
The Dover Strait is representative for its high diversity of dynamic seabed
features. The navigation channel has been extensively surveyed for decades.
SHOM holds repeated coverage of some areas where the mobility of the bedforms
can be assessed.
The data composing the presented DEM are rich in information but required, in
our high resolution DEM production line, a large and attentive work in the
selection of the appropriate surveys (varied extent, age and type) in order to
provide a coherent and realistic modeling of the area. In order to update this DEM
frequently further methodological work is currently undertaken to improve the
selection of the most recent surveys.
This DEM will be available on data.shom.fr under an open data license (with a
citation and an associated DOI number). Indeed several regional and coastal DEMs
were already released in 2015 and 2016 and populated data.shom.fr. In this way
the portal will be gradually fed by future productions along the French
metropolitan and overseas coasts.
Consequently, the collected datasets were first assessed
internally for both quality and accuracy and then
externally with other to ensure consistency and gradual
topographic/bathymetric transitioning along limits of
Direction of dune migration:
Toward East – North-East
Toward West – South-West
Coastal topo-bathymetric DEM prototype of the Dover Strait (20 m)