FINAL REPORT - International Joint Commission

The Lower St. Lawrence River
In comparison to the upper portion of the study area, the waters downstream of the control dam are
differentiated by the influences of river flows, ship wakes and the response of shorelines composed,
for the most part, of marine clays.
ANNEX 2
An extensive digital data collection of bathymetry, topography, flow conditions, aerial photography and
databases relating the land-use and erosion processes was compiled in a data warehouse based in the
Ste-Foy, Quebec offices of Environment Canada. This dataset formed the backbone of the performance
indicator analysis for the lower St. Lawrence.
Application of regional-scale computer modeling to the shorelines of the lower St. Lawrence River from
Cornwall, Ontario to Trois-Rivières, Quebec was developed by Pacific International Engineering to create a
new and clear understanding of the relative importance of river currents, wind waves and ship-generated
waves and how they interact with water levels (Pacific International Engineering, March 2004). This enabled
the development of simplified predictive tools for erosion and an assessment of economic impacts.
Flooding stage-damage relationships were developed for 42 municipalities on the lower St. Lawrence River
from Cornwall to Trois Rivières by Environment Canada-Quebec Region based on parcel and assessment
data of the affected properties and a fine-resolution digital elevation model developed for the Coastal
Technical Work Group by the Common Data Needs Technical Work Group (refer to that section).
Performance Indicators
Lake Ontario–Upper St. Lawrence River
The impacts of water level fluctuations on Lake Ontario and upper St. Lawrence River shoreline
communities were categorized by three primary performance indicators, specifically: flooding, shoreline
erosion of developed properties, and existing shoreline protection maintenance. The Coastal Technical
Work Group also examined sediment budgets, beach access, and barrier beaches and dunes as possible
performance indicators.
Flooding
Over 3,000 shoreline property parcels are located below elevation 76.2 m (250 ft) and could be at risk of
flooding on Lake Ontario and the upper St. Lawrence River. Flood damages to these properties generally
occur during periods of high lake levels and severe storms. The flooding performance indicator on Lake
Ontario quantifies the impacts of flooding due to inundation of structures and the force of waves striking
buildings. Economic damage calculations are made for individual property parcels at risk of flooding.
The flooding performance indicator algorithm was developed, tested, calibrated and verified in the Flood
and Erosion Prediction System (FEPS). The flooding performance indicator algorithm is applied to the
entire parcel database for the duration of a simulation in the Shared Vision Model, commonly 101 years.
However, the wave database was based on 40 years of data statistically hindcast to represent the 101-year
simulation period. An analysis determined that this could significantly affect plan results as the happenstance
nature of waves meant that a plan that shifted levels might avoid or enhance damage depending on the wave
sequence. To address this, a number of statistically generated wave sequences were derived based on the
quarter month maximum of the maximum, the average of the maximums and the 1st and 2 nd standard
deviation from the maximum. The results could be tested for all of these sequences. Using this method,
it was determined that the 1st standard deviation was probably the best wave-sequence representation for
ensuring that the numbers were not being underestimated. The 40-year hindcast wave data was used in
the 50,000 stochastic analyses since the shear number of combinations of levels and waves would dismiss
any bias in the data. For more information on the flooding performance indicator refer to the report
Flooding Performance Indicator: Methodology and Shared Vision Model Application (Baird, 2004a).
Options for Managing Lake Ontario and St. Lawrence River Water Levels and Flows
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