Environmental Engineering Reference
In-Depth Information
of St. Louis and the downstream part, mainly open to the
estuary, is bordered by a modest mangrove swamp;
• Backwaters Khor and Marméal located upstream of St.
Louis and which join the river on the south,
• The short arm of the river between the Langue de Bar-
barie and the island of St. Louis, which is in communi-
cation with the main river,
• the lagoon complex on the left bank, between St. Louis
and Gandiole, which includes Leybar and Ndel backwa-
ters, shallow channels, temporarily flooded by the tides,
partially lined with mangroves.
The estuary also includes the National Park of the Lan-
gue de Barbarie (PNLB) located about 12 km downstream
of St. Louis covering an area of about 2,000 ha. The PNLB
extends, partly, onto the coastal zone with the presence of
some brackish lagoons and mangrove relic. Nesting areas
for thousands of birds, but also spawning fish, and for
shrimp
of drought, these two dams were essential to ensure the
sustainability of water resources but did not take into
account factors such as the sedimentology and ecology of
these environments. Indeed, the reduction of sediment yield
has transformed the estuarine system and introduced dis-
turbances that have altered and/or perpetuated the natural
instability of the estuary, particularly the mouth. The
hydrological functioning of the estuary has changed sig-
nificantly in recent years from the effects of these dams and
their very important transformations of the environment.
Langue de Barbarie and the Senegal River Mouth
The Langue de Barbarie is a long sandy spit of about
30 km, oriented NNW-SSE; it separates the Senegal River
from the Atlantic Ocean and is divided into three sectors
(Sall 1982 ): an external or shoreline maritime sector swept
by the swell; a dune area under the influence of the wind;
and a fluvial sector or internal shoreline subjected to fluvial
dynamics. The Gandiolais, which is the hinterland of the
Langue de Barbarie, is an area of dunes punctuated by
mangroves, mudflats and saline areas. It also corresponds to
the northern part of the Niayes region (Niang 2002 ).
At the maximum, the Quaternarian Nouakchottian trans-
gression between 5500 and 4000 B.C., the sea invaded the
Senegal River delta and built an estuary which then extended
to Boghé, 250 km from the coast (Monteillet et al. 1981 ).
Between 4000 and 1800 B.C., a significant littoral drift
resulted in the formation of sandy offshore bars from east to
west along the coast of Senegalo-Mauritanian sedimentary
basin. The Langue de Barbarie, whose construction continued
until the sub-actual period, was the result of North-South-
South currents resulting from the reflection of northwest
swells along the coast. These currents, very strong, cause an
intense marine sedimentation and force the river to erode the
coast and then elongate the Langue de Barbarie. The strong
power of the swells and the currents makes the Senegal River
mouth one of the most agitated in the world (Coleman and
Wright 1975 ): the power of the swell there,
112.42.10 7 ergs.s -1 , is 550 times that at the mouth of the
Mississippi and 5 times that at the mouth of the Nile.
Very fragile and morphologically unstable, the Langue
de Barbarie plays an important role in the dynamics of the
Senegal River. Indeed, its end has determined, since its
formation, the position of the Senegal River mouth which
has always been subject to high spatial and temporal
mobility (Gac et al. 1982 ; Kane 1997 ; Dia 2000 ; Lamagat
2000 ). The location of the mouth south of St. Louis prob-
ably dates back from the mid-seventeenth century. Between
the Senegal River and the Atlantic Ocean, the Langue de
including
the
park's
environment
are
strongly
threatened by sewage from St. Louis.
Before constructing the Diama dam, saline intrusion was
a major determinant of all socioeconomic activities in the
Senegal River Delta. In normal hydrological conditions,
saline water reached Podor, about 300 km from the mouth.
During extreme years, sea water intrusion reached Dioudé
Diabé, 150 km upstream of Podor. In the estuary, the river
flooded many areas occupied by mangrove mudflats and
halophytic steppes. Between Gandiole and St. Louis, the
landscape is composed, in the area between the road and the
river, of Rhizophora, Laguncularia racemosa and Avicennia
africana. The hinterland is mainly occupied by clumps of
vegetation of different varieties. In sandy areas, we note the
dominance of Sporobolus spicatus.
The beginning of the major hydraulic structures of
OMVS with the Diama dam in 1985 resulted in a new
balance in the evolution of the estuary of the Senegal River.
From the first months of operation of the dam, the hydro-
dynamic conditions were highly modified leading to a
continuation of morphological, sedimentological and eco-
logical changes.
Since 1985, the estuary of the Senegal River has been
confined to a very narrow zone between the Diama and the
Langue de Barbarie sand spit. The periodic opening of the
dam to prevent hyper-salinization of water due to evapo-
ration also led to a decline in mangrove mudflats, which
have been converted mainly into residential areas. Vulner-
ability to flooding increased, particularly in low-lying areas
surrounding the city of St. Louis.
The change in the hydrological regime of the Senegal
River related to the dams of Diama (downstream) and
Manantali (upstream) has had a strong impact on the mor-
phological evolution of the region. Designed in the context
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