Environmental Engineering Reference
In-Depth Information
Fig. 5
Enlargement of the
breach and elongation of the
north spit between October 2003
and October 2012 as measured on
a series of LANDSAT images
(
www.usgs.gov
)
8
The breach
The North arrow
7
6
5
4
3
2
1
0
04-oct.-03
04-oct.-05
04-oct.-07
04-oct.-09
04-oct.-11
deepened
its
initial
depth
of
1.5 m
on
4
October
and
arrives, water is quickly discharged to the sea. Indeed, the
river flow dynamics have been reinforced by the currents
and marine dynamics, and this new situation has raised
serious concerns about the nature of the flood hazard in the
region which could now come from the sea (Durand et al.
2010
). More particularly, much of the country around St.
Louis is now below sea level (-0 m IGN). The island of
Doun Baba Dieye, located to the right of the breach, which
threatened the early months of operation of the new mouth,
is now almost engulfed because of the tides and the high
penetration of marine waters into the estuary. This possible
disappearance of the Doun Baba Dieye Island has already
been studied by Dieng (
2010
) and resulted in increased
vulnerability of the population of the island, which forced
them to relocate on the mainland.
Today with two new openings south of the channel of
2003, the entire lower estuary is threatened with destruc-
tion. The establishment of a monitoring mechanism for the
Langue de Barbarie is a necessity in the context of the
development of the lower estuary, in particular with a view
to safeguarding human settlements. The challenge of man-
aging this environment revolves around the evolution in the
short and medium term of these three openings which have
tripled the contacts with the Atlantic Ocean (Box 1).
The breach of the Langue de Barbarie in October 2003
established the new position of the Senegal River mouth. In
addition to shortening the transit time of water from the
Senegal River to the sea, this has also resulted in a change
in the flow conditions upstream and downstream of Diama.
At the scientific level, the first reports indicate a very
alarming diagnosis of the morphology of the littoral spit, the
hydrodynamics, the biological diversity and environmental
changes, which in the long term may result in a disturbance
of this region that is without precedent.
increased to 6 m by 26 October.
These data are confirmed by measurements from a series
of more than 130 LANDSAT images between October 2003
and October 2012 (Fig.
5
). In contrast to the field data,
which showed an exponential growth of the breach, the
measures using LANDSAT imagery show that the width of
the breach showed high intra-seasonal and inter-annual
variations, probably related to oceanic forcing (tides, sea
level pressure) and winds (zonal winds and southern winds
to 800 HPA, winds to 10 m). This assumption needs to be
confirmed with different correlations between the width of
the breach and the evolution of these different elements.
After an almost exponential trend between 2003 and
2006, with a width increasing from 4 m to about 2 km in
September 2006, the breach experienced a period of
decline, with widths ranging between 700 and 900 m. A
new phase of exponential growth then began, with
increasing widths reaching a record 2.7 km in July 2009.
The width measured on the image of 20 September 2012
was 2.38 km. Starting from September 2010, with the
appearance of a small island which resulted in the creation
of a second passage way, the breach behaved as a real
mouth similar to the old mouth, moving slowly but surely
towards the south.
The speed of progression of the sandy spit seems syn-
chronized with that of the widening of the breach (Fig.
5
).
The length of the northern portion of the spit has rarely
exceeded 7.5 km since 2003. On 6 October 2012, the length
of the northern portion of the Langue de Barbarie was 7.3 km.
The gap functions as a real mouth, wide open to the sea,
very deep and subjected to a strong agitation. Since 2003,
the city of St. Louis, in particular in its island component,
has
been
protected
from
river
flooding.
When
a
flood
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