FINAL REPORT - International Joint Commission

Contrecoeur
Longueuil SSPI
Île-Perrot
Longueuil
Dorval
Lachine
La Prairie
Bécancour
RE Île-Perrot
RE Varennes
Candiac
St-Lambert
Pointe-Claire
Verchères
Montréal
Lavaltrie
Water level in Pointe-Claire (m)
ANNEX 2
Figure F-1: Critical water levels (red performance indicator) referenced to Pointe-Claire.
Red performance indicators describe the minimum water level required at
Pointe-Claire to supply nominal capacity using all intakes available (IGLD 85)
A special black performance indicator was defined for the intake structure of Montreal’s Atwater and
DesBaillets WTPs. Contrary to the other cities, they are in a unique situation in which capacity is lost
gradually. Their red performance indicator means that the nominal capacity can no longer be supplied but
91% of the production can still be distributed. The black performance indicator corresponds to the level
at which the Atwater plant is lost completely; at this point, a large portion of the distribution system would
be unpressurized.
The impact of water levels on plant capacity can also be expressed with respect to the total number of plants
or the total population affected. This information is presented in Figure F-2. The representation highlights
the relative weights of each treatment plant with respect to the total population of the study zone.
Municipal and industrial water use has generally not been vulnerable to water level changes, except in
2001, when the critical level of 20.53 m (67.36 ft), with reference to Pointe-Claire, was reached temporarily
in Montreal; almost the same situation occurred in 1999 (20.54 m) (67.39 ft). The Montreal main water
intake pipe, located just below Lac St. Louis, is relatively shallow and situated in uneven, mildly sloping
marine topography. Access to deeper water is possible about 400-500 m (437-547 yd) from the actual
intake site, but with major constraints to be overcome (strong current, bedrock to dynamite, large pipe) to
gain little depth. The City of Montreal rebuilt its emergency intake in 2003 to support its main intake
(Atwater and DesBaillets), allowing an increase in total adduction capacity of 21%. Under projected
normal conditions, proper operation should be maintained until the 20.53-m (67.36-ft) level is reached,
potentially causing a 9% drop in nominal capacity. However, during the next 50 years, with climatic
changes, the Montreal major intake (main and emergency) could still be at risk if the chart datum level is
reached at 20.35 m (66.77 ft) or lower, with reference to Pointe-Claire.
128 Options for Managing Lake Ontario and St. Lawrence River Water Levels and Flows