High resolution coastal DEM of the Dover Strait: managing dynamic bedforms

Abstract

The Dover Strait has a high diversity of dynamic seabed sedimentary features. In this paper we present recent work undertaken to compile a high resolution 20 m grid spacing bathymetric digital elevation model extending along British and French coastlines. At this resolution, it is important to take particular precautions on the precision of the datasets (both vertically and horizontally). More importantly, in a highly dynamic environment, we had to take care of the temporal aspect of the dataset and their potential overlap to ensure the best continuity of the bedforms. In order to get the best coverage, a variety of dataset from different data providers, from different surveying systems, referenced at different vertical and horizontal datums, had to be harmonized prior to be interpolated. This DEM is intended to be used by hydrodynamic modeler in the context of the assessment of tsunami wave propagation.

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Marine and River Dune Dynamics – MARID V – 4 & 5 April 2016 – North Wales, UK
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1. INTRODUCTION
Bathymetric DEMs are essential input elements for
numerous applications in geosciences, research
and civil engineering, hydrodynamic modeling,
planning and resource exploitation, mapping and
positioning, data validation, military operational
works... Specifically, building accurate and up-to-
date coastal DEMs is a prerequisite for accurate
modeling and forecasting of hydrodynamic
processes at local scale in the context of marine
flooding, originating from tsunamis, storm surges
or waves (Eakins and Taylor, 2010). They are
computed from a synthesis of bathymetric
information sampled in an appropriate manner that
conveniently represents underwater bedforms.
However, because of practical considerations, this
synthesis can only be composed of various datasets
collected through times by various sounders, with
various precision and density. A detailed work is
needed to select the appropriate sources in order to
sample the seafloor accurately, with respect to its
temporal component. This is especially true in
complex sedimentary environments, where seabed
experienced migration and geometry changes.
This study presents the main challenges
encountered during the production of the high
resolution DEM of the Dover Strait, where surveys
of subaqueous dunes and sandbanks were collected
by the French hydrographic office (SHOM) for the
last decades.
2. WORK CONTEXT
The present work is undertaken within the
framework of the TANDEM project (2014-2017),
dedicated to the appraisal of coastal effects of
tsunami waves on civil nuclear facilities along the
French coastlines (Owen and Maslin, 2014). A
special focus is brought on the Atlantic and The
English Channel coasts where French civil nuclear
facilities have been operating since about 30 years
(Hébert et al., 2014). For this project, seamless
integrated topographic and bathymetric high
resolution (20 m) coastal DEMs (Maspataud et al.,
2015) are generated for specific coast
configuration (embayment, gulf, estuary, harbor,
island…). These sites are used to simulate
expected wave height at regional and local scale on
the French coasts, for a set of defined Tsunami
generation scenario, such as earthquake (Roger
High resolution coastal DEM of the Dover Strait: managing
dynamic bedforms
A. Maspataud, SHOM, Brest, France – aurelie.maspataud@shom.fr
T. Schmitt, SHOM, Brest, France – thierry.schmitt@shom.fr
L. Biscara, SHOM, Brest, France – laurie.biscara@shom.fr
R. Créach, SHOM, Brest, France – ronan.creach@shom.fr
ABSTRACT: The Dover Strait has a high diversity of dynamic seabed sedimentary features. In this paper
we present recent work undertaken to compile a high resolution 20 m grid spacing bathymetric digital
elevation model extending along British and French coastlines. At this resolution, it is important to take
particular precautions on the precision of the datasets (both vertically and horizontally). More
importantly, in a highly dynamic environment, we had to take care of the temporal aspect of the dataset
and their potential overlap to ensure the best continuity of the bedforms. In order to get the best coverage,
a variety of dataset from different data providers, from different surveying systems, referenced at
different vertical and horizontal datums, had to be harmonized prior to be interpolated. This DEM is
intended to be used by hydrodynamic modeler in the context of the assessment of tsunami wave
propagation.

Marine and River Dune Dynamics – MARID V – 4 & 5 April 2016 – North Wales, UK
XXX
and Gunnell, 2012 ; Garcia-Moreno et al., 2015) or
landslide triggering.
3. PRESENT KNOWLEDGE ON
STUDY AREA
The Dover Strait area, located between the North
Sea and the English Channel, is submitted to semi-
diurnal macrotidal regime, with tidal currents with
strong alternative character enhanced by the strait
configuration. Soft sands along with the tidal
regime are responsible for the local complexity of
the study composed of superimposed bedforms.
Figure 1. Location of (A) the studied area and (B) the
coastal topo-bathymetric DEM prototype on the Dover
Strait.
Active sand dunes as part of giant dune fields, or
as associative bedforms usually well developed on
the flanks of sandbanks, are particularly well
organised and dynamic. In the French side, SHOM
performs regular surveys to detect the exact
position of dune crests, especially in the Dover
Strait (Garlan et al., 2007, 2008). 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), and 3) the mean rates of
displacement, or those of particular structures (Le
Bot et al., 2000 ; Garlan et al., 2008).
In this area there is a general tendancy of dune
migration imitating the tidal scheme: SE isolated
dunes commonly undergo the direct influence of
the dominant ebb with SW migrations at the rate of
1 to 12 m/yr, while NW dunes migrate slowly in
the opposite direction (to the NE) at an average
rate of 3 m/yr (Le Bot et al., 2000).
4. KEY POINTS IN COASTAL DEM
DEVELOPMENT
4.1. Strategy and DEM development process
Challenges in coastal DEMs development deal
with good practices throughout model
development that can help minimizing
uncertainties (Eakins and Grothe, 2014). In details,
the main tasks to face were (1) making sure of the
best detailed data coverage throughout a detailed
inventory of available data for the area, (2)
ensuring 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, (3)
minimizing overlap between redundant dataset
collected at different periods in time, (4) horizontal
and vertical datum conversions, on the basis
(national and/or local) datum conversion grids
based on known measurements (Maspataud et al.,
2015) and (5) interpolating the entire dataset in
order to fill gaps and get a continuous and regular
surface. Locally, gaps between marine and
terrestrial data have also required the introduction
of new methods and tools to solve interpolation.
Through these activities the goal was to improve
the production line and to enhance tools and
procedures used for the improvement of
processing, validation and qualification algorithms
of bathymetric data and merging of bathymetric
data with a special focus on dataset collected in
dynamic areas.
4.2. Data assessment
Data collection work required a substantial and
precise effort to build a consistent dataset prior to
DEM interpolation. This is particularly true as
scattered elevation data with variable density, from
multiple sources and from many different types
(paper fieldsheets to be digitized, single beam echo
sounder, multibeam sonar, airborne laser data…)
were gathered (Figure 2).
In addition to available surveys from the French
bathymetric database in this area (from SHOM and

Marine and River Dune Dynamics – MARID V – 4 & 5 April 2016 – North Wales, UK
XXX
French harbour authorities), external data were
also gathered: from the United Kingdom
Hydrographic Office (UKHO) in the British waters
and the Belgium hydrographic office (Vlaamse
Hydrografie) in French waters. SHOM digitized
Belgian paper fieldsheets (single beam surveys)
available in French waters. Topographic data for
the French, British and Belgium coastal parts of
the DEM were made available by national
topographic offices or international databases.
Figure 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.
Figure 3. Examples of morphological artefacts of
sedimentary bedforms encountered during bathymetric
data assessment in the Dover Strait area.
Consequently, 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 the datasets. 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.
4.3. Managing with complex morphology and
bedforms mobility
Sediment dynamics can locally induce artifacts on
the resulting DEM or poor temporal accuracy of
the representation of these bedforms. In some
cases, it leads to unproper shifting of the crests on
dune fields (Figure 3 B ; Figure 4 A) or of the
overall shape of sandbanks (Figure 3 A, C, D).
Figure 4. (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.

Marine and River Dune Dynamics – MARID V – 4 & 5 April 2016 – North Wales, UK
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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 DTM. Locally, differences with
previous bathymetric surveys, on sandbanks and
dune fields, are sometimes larger than the grid
spacing. In the case of the Dover Strait,
considerations on 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 Figure 4B ;
- dunes movements could reach several meters to
several dozen 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).
5. CONCLUSIONS
The navigation channel of the North Sea is unique
in hydrographic surveys conducted by SHOM
because it is the only environment where surveys
are repeatedly carried out, excepted in harbor
jurisdiction areas for the French coasts. The data
therein acquired are rich in information on the
dynamics of sedimentary bedforms but required, in
our high resolution DTM production line, a large
and attentive work in deconflicting numbers of
surveys (varied extent, age and type) for a coherent
and realistic modeling of sedimentary seabed.
In particular, the heterogeneous ages of the input
data stress the importance of taking into account
the temporal variability of bathymetric features,
and their migration rates, especially in the most
active areas.
6. ACKNOWLEDGMENT
This work is supported by a French ANR program
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 and
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 work was greatly appreciated.
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