PIAHSProceedings of the International Association of Hydrological SciencesPIAHSProc. IAHS2199-899XCopernicus PublicationsGöttingen, Germany10.5194/piahs-377-83-2018Satellite images survey for the identification of the coastal sedimentary
system changes and associated vulnerability along the western
bay of the Gulf of Tunis (northern Africa)Spatial monitoring for coast vulnerability studyHzamiAbderraoufabderraoufhzami@gmail.comAmrouniOulaRomanescuGheorgheConstantin StoleriuCristianMihu-PintilieAlinSaâdiAbdeljaouadUniversity of Tunis El-Manar, Faculty of Science, Laboratory of Mineral Resource and Environment, Tunis, TunisiaLaboratory of Marine Environment, National Institute of Marine Science and Technology, Tunis, TunisiaUniversity Alexandru Ioan Cuza Iasi, Faculty of Geography and Geology, Department of Geography, Iasi, RomaniaUniversity Alexandru Ioan Cuza Iasi, Interdisciplinary Research Department – Field Science, Iasi, RomaniaAbderraouf Hzami (abderraoufhzami@gmail.com)16April2018377838913June201730July2017This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://piahs.copernicus.org/articles/377/83/2018/piahs-377-83-2018.htmlThe full text article is available as a PDF file from https://piahs.copernicus.org/articles/377/83/2018/piahs-377-83-2018.pdf
The aim of this study consists in testing the effectiveness of
satellite data in order to monitoring shoreline and sedimentary features
changes, especially the rapidly changing of Gulf of Tunis coast. The study
area is located in the Gulf of Tunis western bay (Southern Mediterranean Sea)
which is characterized by sandy beaches of Ghar Melah and Raoued (Medjerda
Delta area). The aerial photographs and satellite imageries were used for
mapping the evolution of shoreline. Diachronic data (satellite imagery,
aerial photography and topographic maps) were used to monitor and to
quantify, the evolution of the coastal areas. These thematic data were
digitally overlaid and vectorised for highlighting the shoreline changes
between 1936 and 2016, in order to map the rate of erosion and accretion
along the shoreline. Results show that the accretion and degradation are
related to the Medjerda: change of outlet in 1973 and impoundment of the Sidi
Salem dam in 1982. We found that the general trend of the coastal geomorphic
processes can be monitored with satellite imageries (such as Sentinel A2,
Spots 4 and 5), due to its repetitive coverage along the time and their high
quality concerning the spectral contrast between land and sea areas. Improved
satellite imageries with high resolution should be a valuable tool for
complementing traditional methods for mapping and assessing the sedimentary
structures (such as shoreline, delta, marine bars), and monitoring especially
the lowlands coastal areas (slightly eroded).
Introduction
The general characteristics of coastal erosion worldwide are
described in terms of geography by the types of erosion, the causes which
starts the erosion processes, and the effects generated by erosion processes.
A shoreline is defined as the linear interface between land and water areas
(Dolan et al., 1980). Shoreline is an element with a high spatial variability
which imposes a rapidly changing for coastal landforms (Mujabar and
Chandrasekar, 2013). Hence the efficient methods are needed to handle the
spatial and temporal variability of coastal shoreline using GIS techniques.
Thieler et al. (2009) developed an extension for ArcGIS software which allows
automatic measurements for shoreline changes. Many researches utilized remote
sensing data to analyse coastal environments: Louati et al. (2014), Oyedotun
(2014, 2017), Thinh and Hens (2017). Many studies in Tunisia have shown the
effect of coastal degradation related to different natural and anthropogenic
factors (Paskoff, 1988; Oueslati, 2004, 2010; Halouani et al., 2011, 2013;
Saïdi et al., 2012, 2013; Louati and Zargoun, 2013; Louati et al., 2014).
Study area
Gulf of Tunis is located in NE of Tunisia and is bordered to the east by
Mediterranean Sea, between 37∘10′ N–10∘16′ E (Cape
Farina) and 37∘55′ N–10∘18′ E (Cape Gammarth). The
western bay of the Gulf of Tunis regular coastline consists in a series of
lagoons disposed from north to south: such as Ghar Melah, Kalaât Andalous
and Ariana. The coastline has a length of 40 km and is characterized
by landforms such as coastlines, sandy features, sandy spit, river mouth
deposit and dunes covered by forests.
Tidal range at the study area is low with amplitude of approximately
35 cm (Oueslati, 1993). Mean amplitude of semi-diurnal micro-tidal
activities in the Gulf of Tunis measures 12–30 cm (El Arrim, 1996;
Saïdi et al., 2012). The Western bay coastal system includes three main
sedimentary systems with followed entities: Lagoons of Kalaât Andalous,
delta of Medjerda and the coastal foredune of Raoued beach (Fig. 1). The
hydrologic regime of the coastline is controlled by the Medjerda river which
represents the most important river in Tunisia. During the last century, in
the study area, the relative sea level has reached
11.5 mmyear-1; part of this value, such as
1.5 mmyear-1, has been attributed to eustacy (Pirazzoli, 1986).
The spatial variability of sedimentary activities for the coastal system has
a significant decline due to erosion processes: in the Medjerda Gammaret area
between years 1887–1974, the coastline was registered a retreat rate of 0.12
to 1.11 myear-1, and a decline of 3.73 to 9 myear-1
between 1974 and 2000 (Saïdi et al., 2014).
(A) Location map of the study site: the bay of Gulf of
Tunis (Bay western). (a) Spit of Kalaât Andalous,
(b) New mouth of Medjerda river and (c) Raoued beach.
The Medjerda river (western bay) and Méliane river (eastern bay) supplies
the most of continental supply for Gulf of Tunis coast. In this case, the
impacts of climate change on the sedimentary dynamics of the catchment areas
are subject to a very active research for a decade and which emphasise the
reduction of fluvial fluxes of the river to the coastal environment of Gulf
of Tunis (Arnell, 1999a, b; Oueslati, 2004; Saïdi et al., 2014).
Material and methods
In this study, three cloudless satellite images were used (SPOT1, SPOT4, and
Sentinel A2). The shooting time, satellite information, and image resolution
of each image are summarized in Table 1. As same resolution data is not
available for the desired period but the multi temporal capabilities allow
tracking of changes over a long time (80 years).
Pretreatment of imageries consisted in radiometrically and geometrical
correction in order to minimize weathering effects on radiometric values
(Fig. 2). The satellite imageries (SPOT1, SPOT4, Sentine A2), topographic
maps and aerial photographies used in this study were ortho-rectified. The
datum of cartographic and satellite and aerial imageries is World Geodetic
System (WGS84), and the projected system is Universal Transverse Mercator.
Firstly, the atmospheric effect where corrected by providing the dark object
subtraction model (Chavez Jr., 1996), to compensate differences in shooting
conditions and to calibrate the sensors.
The workflow of methodology.
The photo interpretation methods were used in order to draw line features (in
vector format) for delineating the coastlines for each year corresponding to
products listed in Table 1. Synthetic maps were obtained using ArcGIS 10.2
software. The diachronic synthesis based on shapefiles that contains
coastlines had needed to evaluate the coastal dynamics (Robin, 2002).
The all graphical data (vector format) were used to calculate the spatial
shoreline evolution using statistics provided by Digital Shoreline Analysis
System (DSAS) (Thieler et al., 2009) in order to highlighting the differences
between coastline's positions. DSAS tool was used to create transversal
transects over coastlines needed to analyse and calculate, in different
points, the change rate at the specified time interval (Thieler and Danforth,
1994). For each transect, DSAS provides a calculation of erosion and
accretion of shoreline (Bush and Young, 2009). Transects lines were generated
automatically using DSAS with followed characteristics: lines with 1 km
length placed at 200 m distance between them to study the changes that
occurred along the delta of the Medjerda, and the western bay of the Gulf of
Tunis (Fig. 3). Baselines can be placed ofshore or onshore of shorelines
limit, however a baseline cannot be placed between shorelines. However in our
study, the baseline was built offshore and parallel to the general trend of
the coastline of gulf of Tunis. Transects will be cast perpendicular to this
baseline (200 m spacing) and intersect the shorelines to establish
measurement points (Himmelstoss, 2009).
Afterwards, all the shorelines specifically for each studied year were
overlaid and spatially compared. During this stage can be emphasis the
evolution of the coastline in the studied period. The quantitative and
qualitative analysis is based on the statistics methods provided by DSAS,
such as Net Shoreline Movement (NSM), End Point Rate (EPR) and Linear
Regression (LRR).
Shoreline extraction with transect and baseline using satellite
imagery and maps in studied area.
The spatial error specific to georeferencing process for different products
listed in Table 1 is estimated by the root mean square (RMS) and the maximum
value is 2.33 m (Table 2). To calculate the margin of error of our
photo-interpretation is based on the report from the USGS (2006) which
expressed the calculation of the margin of error Eqs. (2), (3) and (4).
However, we will not take into account the error of digitization for the
development of the margin of error. In our graphics, we use a margin of error
of ±0.15 myear-1. We take into account this type of error
for a better comparison.
Margin of error of photo-interpretation.
Margin of error in meter Years19361974 198819992016RMS error< 0.5 Pixel error1.81.62 1.421.21Total error2.32.12 1.921.71Annual error(myear-1)0.2 EPR: EAnnual error(myear-1)0.12 LRR: UEquation (1)Esp=Eg2+Ed2+Et2+Ep2Equation (2)Ea=Esp12+Esp22timeEquation (3)U=∑i=1nCi1n+∑i=1nBinResults and discussion
The temporal data comparison concerning the evolution of shoreline's
positions using the statistic's variables, such as NSM and EPR, shows the
importance of taking comparison of the temporal series of the evolution of
the shoreline position using NSM and EPR statistical technics shows the
importance of the different periods in this study.
Based on results it can be emphasis in the study area the presence of three
dynamic zones:
Kalaât Andalous spit;
New mouth of Medjerda river;
Raoued beach.
The analyzing of the shoreline changes during 1936 to 2016 can be highlighted
that erosion is significant, especially at the Kalaât Andalous spit with
a severe erosion of up to -25 ± 0.15 myear-1
(-2 km) (Figs. 4, 5 and 6). Similar observation is reported along
this zone coast by Saïdi et al. (2014) and Louati et al. (2014) with
maximum rates of retreat respectively -17 ± 2.4 and
-20.7 ± 3 myear-1. The decreasing of sediment flow at
the old mouth of Medjerda river is generated by deviation of the new channel
by hydrotechnical works in 1973. These hydrotechincal works stimulates the
waves to erode and to carry out offshore the fluvial sediments from the old
delta. The sediments under the effect of the coastal drift disposed on SE/NW
direction facilitates the construction of barrier bar i.e. the sand spit of
Kalaât Andalous.
Multitemporal shoreline evolution of Kalaât Andalous.
The rates of shoreline changes (EPR) in Kalaât Andalous.
The rates of shoreline changes (MSN) in Kalaât Andalous.
During this period (1936–2016), the accretion is always occurred in the new
mouth with a maximum rate of 320 m
(+4 ± 0.15 myear-1) (Figs. 7, 8 and 9). During the
floods of March 1973, Medjerda river had resorted to a change of bed choosing
the channel of Henchir Tobias as an outlet.
Multitemporal shoreline evolution of new mouth of Medjerda river.
The rates of shoreline changes (EPR) in new mouth of Medjerda
river.
The rates of shoreline changes (MSN) in new mouth of Medjerda
river.
The river's spillway is no longer powered by the alluvial contribution of
Medjerda river. The results are shrinking beaches and coastal erosion. This
phenomenon is evident in the aerial photographs of 1962 (Paskoff, 1985). On
the other hand, the rate of accretion of the littoral has been eroded, at the
mouth with an average of -1.64 ± 0.15 myear-1. This rate
of erosion located at the river's mouth is explained by building of dam
construction (El Aroussia in 1957, Bou Heurtma in 1976 and Sidi Salem in
1982) across the new Medjerda river restricts the flow of terrigenous
sediments at the level of new mouth of Medjerda river. The Sidi Salem is
characterized by a volume capacity of 555 millionm3, a surface of
4.3 ha, also being the largest dam in the Tunis. Dam retain the
terrigenous sediment discharge to the sea.
Between 1974 and 1988, the speed of advance of the shoreline continued during
this period, especially in the mouth area with a maximum of 290 m.
El Arrim (1996) reveals that the speed of the shoreline continued at a slower
pace which gains of almost 270 m between 1977 and 1987
(El Arrim, 1996 in Oueslati, 2004).
Raoued beach, situated at the south east of the Medjerda delta, is an
exceptional case in terms of fragility and management. The status of this
area is favored, especially in its western part, by a large alluvial
contribution occured by the new mouth of Medjerda river supply.
In this sector, we observe that the NSM is different for the two periods
(1936–1974 and 1974–2016). Before 1974, this is a zone of erosion range
with an average rate of -300 m
(-8 ± 0.15 myear-1) (Figs. 10, 11 and 12). This erosion
is due both to the effect of the longshore coastal drift from the SE to NW
direction and this zone is highly erosional because of its exposure to the
strong waves, winds and rip currents of the E to SE in summer, N to NE in
winter. After 1974, this sector tends to be deposited, the values of erosion
decreased significantly on all zone of Raoued
(-2.3 ± 0.15 myear-1). This decrease in erosion rates is
due to the deviation of Medjerda river in 1973. The natural river of Medjerda
was abandoned, and the entire terrigenous flow now passes through an
artificial canal of Henchir Tobias (2 km northern ward Raoued beach).
The main causes of erosion during this period are both related to natural
and/or anthropogenic factors, especially by significant reduction of sediment
supply caused by construction of numerous dams located in the catchment area
of Medjerda river.
Multitemporal shoreline evolution of Raoued.
The rates of shoreline changes (EPR) in Raoued.
The rates of shoreline changes (MSN) in Raoued.
Validation of Digital Shoreline Analysis System (DSAS)
The LRR is determined by adjusting a least squares regression line at all the
coastline points for a particular transect (Thieler et al., 2009). In the
transect 116, the linear regression is the slope of the line with y=7.720x+15953myear-1. This calculation provides the standard error
of the slope with the confidence interval of 95 %. Confidence in the
analytical results is validated also by comparing this study with other
researchers (Oueslati, 2004; Saïdi et al., 2014 and Louati et
al., 2014), which indicated a good correlation. In the present study, spatial
resolution (20, 10 and 2 m) is acceptable compared to previous
studies (Louati and Zargouni, 2009; Louati et al., 2014) in this area using
Landsat scenes with a spatial resolution of 30 m, which this
increases the error in coastal areas.
Linear regression of transect 116.
Conclusion
The assessment of erosion and accretion processes
through the transect lines using DSAS tool, applied on western bay of the
Gulf of Tunis, provide valuable statistics upon coastline dynamics in terms
of positional changes and in identification of depositional and denudational
areas. Based on this study it can be concluded that DSAS will be useful for
long-term (1936–2016) qualitative monitoring of shoreline evolution pattern
in case lack of field data sources. In the studied period most of the beach
underwent erosion (-25 ± 0.15 myear-1 in Kalaât
Andalous sandy spits) while some part of the beach follow accretion trend
(+4 ± 0.15 myear-1 in new mouth of Medjerda river). The
variation of the morphology in the recent Medjerda river mouth was
significant as well. The observed patterns of erosion and accretion along the
bay shorelines resulted from both natural and human impacts strongly managed
by human activities start to be more sensitive and vulnerable to natural
erosion processus. Most of the shoreline was exposed to natural erosion
processes induced by waves, tides and periodic storm surge. This study
extends the analysis period by three years compared to the study conducted by
Louati et al. (2014).
Spot image: two images were provided by the laboratory of
hydroscience Montpellier, in the project RYSCMED and were used in this work
after the acceptance of the “ISIS” file.
Sentinel image: the Copernicus Open Access Hub (previously known as Sentinels
Scientific Data Hub) provides complete, free and open access to Sentinel A2
(https://scihub.copernicus.eu/).
The authors declare that they have no conflict of
interest.
This article is part of the special issue “Water quality and
sediment transport issues in surface water”. It is a result of the IAHS
Scientific Assembly 2017, Port Elizabeth, South Africa, 10–14 July 2017.
Edited by: Gil Mahe
Reviewed by: two anonymous referees
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