Geoscience Reference
In-Depth Information
9.2.2
G RANDE -E NTRÉE L AGOON
The Magdalen Islands are located in the middle of the Gulf of St. Lawrence in
eastern Canada (Figure 9.2.1). They include several lagoons that have traditionally
sustained aquaculture activities. Two of these lagoons, Grande-Entrée Lagoon
(GEL) and Havre aux Maisons Lagoon (HML), are connected by a 60-m wide
and 7-m deep pass under a bridge. The geometry of their entrance passes makes
them leaky and restricted lagoons, respectively, and a tidal phase difference
between these entrances generates significant tidal currents in some parts of the
lagoons. 11 The focus of this study is GEL (Figure 9.2.1). Its bathymetry features
a 7.5-m-deep navigation channel leading from its entrance to a harbor in its
northern part. This channel separates a deeper basin (5-6 m) to the east from a
shallower region (2-3 m) to the west leading to HML. The eastern deep basin
sustains considerable mussel aquaculture activities. Tides in GEL are mixed and
mainly diurnal with an average tidal range at the entrance of 0.58 m, reaching
0.95 m during spring tides. 11 The lagoon surface area is about 68 km 2 such that
the tidal prism for normal tides is about 74.4
×
10 6 m 3 . Assuming that a fraction
β
of this volume remains inside the lagoon during each tidal cycle and is completely
mixed with ambient waters, 14 a lower limit for the GEL flushing time of 6 days
during strong wind conditions (
=
0.25) is estimated. GEL has no freshwater river run-off and evaporation is negligible.
β
= 1) and 23 days during calm wind conditions (
β
N
62 ° 00
61 ° 40
61 ° 30
Harve aux Maisons
Lagoon
47 ° 30
20 m
Grande-Entrée
Lagoon
47 ° 20
47 ° 35
0
15 km
2 m
5 m
QUEBEC
10 m
GULF OF
ST. LAWRENCE
47
°
30
0
5 k m
Bridge
0
20 0 km
61
°
40
U.S.A.
FIGURE 9.2.1 Grande-Entrée Lagoon bathymetry in the Magdalen Islands (upper left inset),
in the Gulf of St. Lawrence, Canada (lower right inset).
 
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