costal zone mapping using multitemporal ers sar

COSTAL ZONE MAPPING USING MULTITEMPORAL ERS SAR AND
ENVISAT ASAR DATA
Souléye Wade1, Kader Bâ2, Hervé Trébossen3, Isabelle Niang2, Jean Paul Rudant2
(1)
Laboratoire de Télédétection Appliquée, Institut des Sciences de la Terre, Université Cheikh Anta Diop de Dakar, BP
5396 Dakar-Fann; [email protected]; Tel: (221) 825 25 30/(221) 579 41 47 Fax : (221) 824 63 18
(2)
Département de Géologie, Faculté des Sciences et Techniques, Université Cheikh Anta Diop de Dakar (UCAD)
Sénégal. BP 15490 Dakar Fann; Tel: (221) 539 55 58 Courriel : [email protected]; Courriel :
[email protected];
(3)
Centre Régional AGRHYMET, BP 10111 Niamey, Niger, Courriel : [email protected]
(4)
Laboratoire G2I/Institut Francilien des Géosciences/Université de Marne la Vallée (FRANCE), Courriel : [email protected]
Abstract
This paper presents preliminary results obtained with ERS-2 SAR scenes applied to coastal zone mapping in the area of
the Langue de Barbarie (Saint Louis) and the Senegal river estuary. Saint Louis faces with severe and recurrent floods.
In 2003, such a situation appeared and justified the opening of an artificial channel or breach on the Langue de
Barbarie, in order to save the city from floods. The breach is thought to have caused environmental problems on the
estuary, the maritime zone and the coastal zone. An impact study was therefore launched, starting from satellite ERS
SAR data and GIS. Radar data allowed the identification and cartography of various themes, like vegetation and
geomorphological units, urban structures, hydrographic and road networks, coast lines and offshore bar, as well as
phenomena of sedimentation and erosion along the Langue de Barbarie. This information is considered as valuable
input for topographic and bathymetric map updating of the zone.
1. INTRODUCTION
Since their availability in 1992, radar SAR data are
used for mapping purposes, thus allowing
homogenisation of cartographic coverages at regional
and local levels [1][2][3]. Application of satellite
imageries in coastal zones studies, for example in
Guyana and Cameroon, allowed the SHOM
(Hydrographic and Oceanographic Service of the
Navy, France) to update several marine maps [4][5]. In
Saint-Louis, the presence of the sandy on-shore bar
called Langue de Barbarie, as well as the low
topography of the city (less than 2 m above the mean
sea level), constitute factors which favour recurrent
floods in the city and its surroundings [6]. In 2003,
following heavy rainfalls recorded in the Senegal river
basin, the water level in Saint-Louis strongly increased
to reach 2 m, thus involving overflows on all the low
points. Considering this situation, an artificial channel
or breach of 100 m length, 4 m large and 1.5 m deep
were dug on the Langue de Barbarie, 7 km downstream
from the city, as a preventive measure against floods.
The opening of the breach had certainly made it
possible to lower the water level of 1 m, but the
combined actions of the river and the sea thereafter
quickly modified its width (which is today more than
1000 m) and depth. Cartographic data and the support
of a GIS are necessary to better understand the
environmental impacts of the breach. Several authors
have shown an interest in the monitoring and
assessment of environmental issues in the Senegal river
estuary [7]
_____________________________________________________
Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland
23–27 April 2007 (ESA SP-636, July 2007)
[8][9][10], but little of them took account of the
potential of satellite imageries in general and radar
imagery in particular. It is to meet these needs that
CORUS/GESCAN and TIGER/TREES projects have
been launched. Their main objective is the assessment
of the contribution of ERS SAR ERS and ASAR
Envisat data to the monitoring and cartography of
coastal changes on the Langue de Barbarie and the
Senegal river estuary, related to the opening of the
breach. The specific objectives are: (1) to demonstrate
the cartographic potential of radar SAR ERS imagery,
in synergy with optical and aerial photography, for
coastal zone, coast line, vegetation units, sandy and
muddy benches and islands monitoring; (2) to use these
results in shallow depth bathymetric mapping; (3) to
elaborate a cartographic documentation taking into
account the Langue de Barbarie, river mouth and river
estuary dynamics. Analysis of multitemporal radar
scenes acquired at different seasons and different levels
of tides makes it possible to better understand the
phenomena of sedimentation and erosion, as well as
coastal landscape evolutions. Moreover comparison of
multitemporal images makes it possible to assess the
coast line displacement along the Langue de Barbarie
2. STUDY SITE
The Langue de Barbarie is a sandy on-shore bar located
on the delta of the Senegal river, in the area of SaintLouis, between the latitudes 16°23 and 16°35' N and
the longitudes 15°45' and 16°15' W (Fig. 1). It is a long
sandy bar, from 100 to 500m large, with heights
varying between 2 and 7m.
Atlantic Ocean
Senegal River
Langue de Barbarie
Figure1. The study area
It separates the Senegal river from the atlantic ocean on
about thirty kilometres, in the south of the city of Saint
Louis. Its littoral environment, located between ground,
sea and river, is characterized by a very marked
sedimentary dynamics. Indeed, the river which joins
the ocean 30 km in the south of the city mobilizes
approximately 2.000.000 tons of sediments, deposited
in the estuary, whereas the littoral drift deposits
between 600.000 and 1.000.000 tons/year of sand
along the littoral. These fluvio-marine sediments
generate disturbances such as the retreat of the external
shore, deposits of sand banks and silting of the river
mouth, which make difficult navigation on the river.
For a long time, the operation of the littoral arrow had
remained natural and marked by a migration of its
distal end towards the south, determining the position
of the river mouth, the retreat of its external shore and
periodic cuts. It is from the 20th century that it started
to undergo morphological and hydrological
modification, with the construction of the Diama dam
in 1987 and very recently with the opening of a breach
7 km, in the south of the city, on October 4, 2003.
3. MATERIALS AND METHODS
Tableau 1. : Characteristics of SAR ERS-2 scenes of the
database (Frame: 3285; Mode: descending; PRI: Precision
Image; AT: Ascending Tide; DT: Descending Tides).
N Orbit
Date
Time
Water height
Tide
1 41202
08 03 03 11h32
1,3 m
AT
2 42705
21 06 03 11h33
1m
DT
3 46212
21 02 04 11h33
1,5 m
AT
4 47214
01 05 04 11h33
0,9 m
DT
5 50721
01 01 05 11h32
1,1 m
AT
6 53226
25 06 05 11h33
1,7 m
DT
7 56232
21 01 06 11h32
1,1 m
AT
neighbourhoods and were taken between March 8,
2003 (before the opening of the breach) and January
21, 2006 (after the opening of the breach). They were
acquired within the framework of the project
TIGER/TREES
3.2 Methodology
The methodology includes four steps: (1)
georeferencement of the radar scenes; (2) radiometric
corrections, (3) photo-interpretation i.e information
extraction, and (4) information fusion.
3.2.1 Georeferencement
Seven radar scenes were georeferenced in the geodetic
system WGS84 and UTM projection, zone 28 North.
This was carried out without ground control points and
via physical modelling of the ERS-2 radar sensor
[11][12], integrating precise orbits of Delft Technology
[13], and taking into account the geoid model EGM-96.
3.2.2 Radiometric corrections
To reduce the speckle and enhance images, the
amplitude radar scenes already georeferenced
underwent multitemporal filtering. The principal
advantage of this multitemporal filter implemented by
[14] lies in its capacity to reduce the speckle on the
zones macroscopically homogeneous, while taking
globally into account the changes occurred locally at
the different dates of acquisition. This operation allows
a better discrimination of the targets and facilitates the
automatic segmentation of the images, as well as the
detection of contours to optimize classification.
The satellite database exploited in this study includes
seven radar SAR PRI images taken between 2003 and
2006 (table 1). These images underwent a geometric
and radiometric corrections before being georefereced
in the projection system WGS 84. These corrections
which is possible without ground control points. This
software presents the originality to carry out together
and in once georeferencement and multitemporal scene
data filtering.
3.1 Database
SAR ERS-2 PRI scenes were acquired at ascending
and descending tides; that is to say two images each
year. (Tab. 1). Images cover the town of Saint-Louis
and its
a
g
b
Figure 2. Speckle reduction by multitemporal filtering:
a- raw image; b-filtered image
3.2.3 Photo-interpretation
This step makes it possible to extract automatically
various information about contours, in particular coast
lines, hydrographic network, urban and road structures,
vegetation units, but also to locate sedimentation and
erosion zones. It also makes it possible to establish
landcover maps. Field trips are scheduled in order to
refine working hypotheses and to verify certain
interpretations.
3.2.4 Information fusion
The objective of this information fusion is to achieve a
partial bathymetric map updating in a known geodetic
system (WGS84). Data gathered have been integrated
as thematic layers into a GIS, using ArcView 3.2. This
software makes it possible to integrate several data
acquired in the same system and at different times, in
order to facilitate their photo-interpretation and to
generate maps containing several information, in
particular information from ancient maps and
information recently derived from radar images
(current coast line, positions of sand banks, status of
the ancient river mouth.)
image of January 1, 2005, one distinguishes wetlands
(clay and vase) which appear with a very weak
backscatter signal, close to that of the river,
hydrographic network and less wet or dry
surfaces(sand dunes, offshore bars) with a strong
backscatter. This scene gives an idea (figure 5) about
the quaternary surface geological formations of the
delta. These marine, fluvial or eolian sediments are
constituted of of sands, clays and silts. On the radar
images, they appear with different radiometric values,
depending to their fine texture for clays and silts,
intermediate to coarse grained for sands.
Radar images radar of the multitemporal series have
been processed with the CYGWIN multitemporal filter,
and then compared between them. They show up the
progressive closing of the ancient river mouth with
sand sediments and the regular increase in width of the
breach, between 2003 and 2006, as well as the presence
of sand bank surrounding the river mouth (Fig. 3).
Breach evolution
4. RESULTS
Analysis and interpretation of processed radar data
provided results which gave an idea on various
landscape units and coastal themes of the zone
4.1. Analysis and interpretation of the landscape
The visual improvement made by multitemporal
filtering is quite clear on the image of January 21, 2006
It is possible to observe :
- homogenisation of textures allowing a clear
distinction between various vegetation formations,
urhan habitats, different soil types ;
- good visualization of hydrographic and road
networks, as well as installations (breach, runway, etc).
The image scene of January 1, 2005 made it possible to
observe at large the presence of waves or tides oriented
NW. Their refraction at depth, on the continental shelf
and in particular on the coast where they lose most of
their energy, is quite visible along the breach and the
river mouth These swells orientated NW cause,
because of their obliqueness to the coast, a current of
littoral drift and an important transport of sand
sediments parallel to the coast, in the north-south
direction The colour composite image from the three
ERS scenes dates acquired with low water heights
shows important radiometric changes with the dates of
acquisition. Comparison of this colour composite
image with the land-cover map of the zone makes it
possible to identify various vegetation formations: in
green are zones of sparse vegetation, in light blue and
red is aquatic vegetation, in dark blue are basins, and
marshes and in purple is mangrove. On the subset
Breach
Mouth
River mouth evolution
Figure 3. Evolution of the breach and the river mouth
from 2003 to 2006
4.2. Extraction of the coast line
At the Langue de Barbarie, the retreat of the shore line
and the displacement of the sand banks which surround
the breach and the river mouth are easy to map from
the filtered radar images (Fig. 4). The limits between
the sea, the sandy bar, and the river are quite visible.
The sandy bar presents a very weak backscattering,
which contrasts with the intermediate and strong
backscatter of the river and the ocean. The sandy banks
(in black) which surround the breach and mouth can be
distinguished from the zone marine and the river
characterized by a strong backscatter. Globally, ERs
images ensure a good cartography of the different units
of the Langue de Barbarie, underlined here by the
differences in contrast between marine and river waters
photographs for the former authors; optical images and
radar scenes for this present study.
5.2 Evolution of sand banks
The phenomena of migration of sand banks at the river
mouth can also be evaluated with the multitemporal
series of radar images radar of 2003- 2006. The width
of the river mouth changed from 500 m in 2003 to 320
m in 2004 and 5 m in 2004, before being almost
completely closed in 2006; that is to say an average
closing speed of about 150 m/an (Fig. 3).
6. CONCLUSION
Figure 4. Evolution of the coast line in the vicinity of
the breach. Background image: ERS-2 scene of June
21, 2003
(average backscatter),
backscatter) and the
backscatter).
the sandy
coast lines
bars (weak
(intermediate
5. DISCUSSIONS
5.1 Evolution of the shoreline
The displacements of the coast lines and the
phenomena of migration of the sand banks in the
vicinity of the breach of the Langue de Barbarie from
2003 to 2006 can be evaluated quantitatively by
comparing the different ERS scenes. Figure 8 presents
a retreat of 40-290m of the coast line in the vicinity of
the breach and of 40-90 m (that is to say a rate from 13
to 30 m/an) on the central part of the Island Babagueye,
between 2003 and 2006. This island experienced,
during the same period 2003-2006, an accretion of 40120 m at its bottoms, corresponding to rate of 13-40
m/an. The annual rates of shore line retreat and
accretion of the shore are measured perpendicular to
the coast. Our results differ from those given by [7]
[15][8][9] and [10] who used beach profiles and aerial
photographs and tried to evaluate the evolution of the
coast line. They recorded 1-2 m/year of coastline
retreat. This difference can be related to the nature and
the sources of the data used: beach profiles, aerial
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Acknowledgments
The authors address their grateful thanks to the French
Ministry of Foreign Affairs for its support within the
framework of the CORUS/GESCAN project
02317047, as well as the Francophone University
Agency (AUF), who supported the project ARR
N°P2.2015RR405 in the framework of the Remote
Sensing Network. The European Space Agency is also
thanked for the provision of radar images within the
framework of project TIGER/TREES. Lastly, Centre
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accommodated the research stay of one of us in
Niamey, with an important logistic and scientific
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