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The July 1996 floods in the Saguenay
Valley, Quebec, Canada: a case study of
the effects of extreme flooding
Greg Brooks
Geological Survey of Canada
Natural Resources Canada
Purpose of talk
• Overview the regional setting and cause of the 1996
Saguenay flood disaster
• Highlight geomorphic effects and impacts of
flooding along four Saguenay area rivers:
– Chicoutimi and Sables rivers - small dams
– Mars River - planform transformation
– Ha! Ha! River - dam breach–rainstorm flood
• Summarize mitigation implemented in response to
the flood disaster
Saguenay
area
Canadian Shield
• Bedrock terrain with thin, generally discontinuous
cover of glacial deposits
• Drainage courses:
– Deranged by Laurentide Glaciation
– Locally controlled by bedrock and glacial
deposits reflecting deglacial history
• Channels morphology vary locally from alluvial,
‘semi-alluvial’ and bedrock
• Channel gradients irregular (substrate changes)
• Drainage basins contain numerous small lakes
3500 km2
608 km2
660
km2
July 1996 Rainstorm
An extreme flood!
 Lake Kénogami - source of Sables and Chicoutimi
rivers
 Inflow into the reservoir (3390 km2)
 previous max. April 1941
– 997 m3s-1
 100-yr flow (1912-1995)
– 973 m3s-1
 10 000-yr flow (1912-1995)
– 1437 m3s-1
 max. inflow July 21, 1996
– 2364 m3s-1
(Source: Nicolet Commission Report 1997)
Sables and Chicoutimi rivers
Flood hydrograph
1100 m3s-1
653 m3s-1
Flood hydrograph
Jonquière dam - Sable River
Built in 1943
Chute-Garneau dam - Chicoutimi River
Built in 1925
and two additional dams!
Chute-à-Besy dam
(1911)
Pont-Arnaud dam (1912)
Other dams
Ville-de-Jonquière dam (1996)
Chicoutimi dam
(1923)
ElkemMétal
Dam
(1958)
Problems at run-of-the-river dams
• Inadequate spilling capacity at the dams:
– Flood exceeded maximum operating spilling
capacity of 6 of 7 dams
– Maximum operating spilling capacity not
available at 7 of 7 dams
• sluice gates not all opened (maintenance)
• sluice gates malfunction/damaged during
flood
• sluices obstructed by flooding debris
Problems elsewhere
• Primarily inundation of lowlying areas
• Scouring of vegetation along
steep bedrock reaches
Chicoutimi - Sables rivers conclusions
• ‘Older’ small dams may have an insufficient
maximum spilling capacity
• Maximum spilling capacity may not be available
during an extreme flood
• Uncontrolled overtopping of abutment areas can
result in reservoir breaching and loss of dam
function
• Erosion and overtopping flows can caused
‘collateral’ damage to nearby buildings even
where these are situated above flood levels
Mars River study area
• Alluvial, irregular
meander planform
(sinuosity 1.2)
• Gravel-bed channel
• Valley gradient
averages 0.012
• River occupies deep stream-cut valley 300 to 1200
m wide
• Storm-generated flood discharge (i.e, not
influenced by dam breaches)
Pre-flood
Pre-flood (May 1994)
Pre-flood (May 1994)
Post-flood
Change in total channel width
Post-flood channel
Impacts on infrastructure
A transitional meandering planform
Empirical planform discriminate diagram
Empirical planform discriminate diagram
Empirical planform discriminate diagram
Mars River conclusions
• Storm runoff caused large-scale valley bottom erosion
• Pre-flood channel represents a transitional meandering
planform
• Inferred from empirical Q-s equations that pre-flood
channel was at or close to the braided zone of
planform types
• Large-scale channel widening during 1996 flood
represents the transformation from a transitional
meandering to braided planform
• Expected that there would be a post-flood recovery of
the channel to a transitional meandering planform
Ha! Ha! River flood
• Most severe flooding in region
occurred along Ha! Ha! River
• Flood accentuated due to dyke
breach at Lake Ha! Ha!
• 35 km of valley affected by
resulting flood
• Important to consider breach
from the context of the problems
with dams along Sables and
Chicoutimi rivers
La Baie
Pre-flood Lake Ha! Ha!
Inflow – 160 m3s-1
Max. spilling capacity – 250 m3s-1
Avail. spilling capacity – 86 m3s-1
Post-flood Lake Ha! Ha!
Estimates of flood discharge
Method
Discharge
(m3s-1)
Comment
Empirical
relationship of
drained lake volume
and peak flow
7650
- based on the equation Qmax = 1730 V0.48
(modified from Costa, 1988).
Drawdown of
reservoir
1380
- based on reported rate of drawdown from
erosion of dyke.
Runoff modeling/
reservoir drawdown
900 at dam
1100 (lower 10
km)
- estimates from Nicolet Commission
Report (1997).
Slope-area method
1080-1260
- cross-section located 8.5 km above river
mouth.
Max. recorded
instant. discharge
114
- gauging station located about 7 km above
river mouth
(est. 384)
Downstream geomorphic effects
Downstream geomorphic effects
Downstream geomorphic effects
Downstream geomorphic effects
Transition from deposition to erosion
Erosive threshold
Erosive threshold
Below
threshold
Reach
km 33-27
Valley
slope
Specific
weight of
water
(Nm-3)
Discharge
(m3s-1)
Width range
(m)
Unit stream
power
(Wm-2)
0.0016
9800
900-1380
75-220
59-289
Moderate widening
Large-scale widening - lower reach
Geomorphic effects - bedrock reaches
Downstream view
Upstream view
Large-scale avulsion
Large-scale avulsion
River mouth
• 9.3M m3 of sediment
transported to tidal flats
and Saguenay fjord
Ha! Ha! River conclusions
• Flood accentuated by erosion of an earthfill dyke and
incision of underlying cohesive glacial deposits
• Estimates of flood discharge range between 900 and
1380 m3s-1
• 300 Wm2 is a useful threshold to discriminate between
reaches of large-scale erosion and deposition
• Pre-flood valley morphology and valley bottom
materials are important controls on channel widening
• An avulsion caused the bypassing of a bedrock control of
local base level resulting in the large-scale river incision
Mitigating a future disaster
 Many area dams were replaced or
upgraded to accommodate more
extreme discharges
 Changes were made to the
operating procedures of individual
dams
 Legislation (Bill 93 – Dam Safety
Act) passed that improves the
regulation, operation and
maintenance of dams in Quebec
 especially small dams (< 15 m)
Jonquière dam
Mitigating a future disaster
• Floodplains
– 65 km of floodplains ‘sterilized’
through buyouts/expropriations of
property owners
– floodplain re-mapped
incorporating the July 1996
discharge (20-yr flow; 100-yr
flow)
– floodplain zoning considered bank
erosion
Below Chicoutimi dam
Acknowledgements
• T. Lawrence, C. Bégin, D. Perret (GSC)
• Office of Critical Infrastructure Protection and
Emergency Preparedness Canada
• Canada Centre for Remote Sensing
• Quebec Ministry of Public Security
• Abitibi Consolidated Inc.
• SNC Lavalin
New dam and dyke
at Lake Ha! Ha!