greg brooks geological survey of canada natural resources canada
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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 - PowerPoint PPT PresentationTRANSCRIPT
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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
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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
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Saguenay area
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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
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3500 km2
608 km2
660km2
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July 1996 Rainstorm
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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)
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Sables and Chicoutimi rivers
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Flood hydrograph
653 m3s-1
1100 m3s-1
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Flood hydrograph
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Jonquière dam - Sable River
Built in 1943
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Chute-Garneau dam - Chicoutimi River
Built in 1925
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and two additional dams!
Chute-à-Besy dam (1911)
Pont-Arnaud dam (1912)
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Other dams
Ville-de-Jonquière dam (1996)
Chicoutimi dam (1923)
Elkem-Métal Dam(1958)
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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
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Problems elsewhere• Primarily inundation of low-
lying areas• Scouring of vegetation along
steep bedrock reaches
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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
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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)
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Pre-floodPre-flood (May 1994)
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Post-flood
Pre-flood (May 1994)
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Change in total channel width
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Post-flood channel
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Impacts on infrastructure
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A transitional meandering planform
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Empirical planform discriminate diagram
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Empirical planform discriminate diagram
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Empirical planform discriminate diagram
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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
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Ha! Ha! River flood
La Baie
• 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
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Pre-flood Lake Ha! Ha!Inflow – 160 m3s-1
Max. spilling capacity – 250 m3s-1
Avail. spilling capacity – 86 m3s-1
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Post-flood Lake Ha! Ha!
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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
(est. 384)
- gauging station located about 7 km above river mouth
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Downstream geomorphic effects
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Downstream geomorphic effects
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Downstream geomorphic effects
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Downstream geomorphic effects
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Transition from deposition to erosion
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Erosive threshold
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Erosive threshold
Reach Valley slope
Specific weight of
water
(Nm-3)
Discharge
(m3s-1)
Width range
(m)
Unit stream power
(Wm-2)
km 33-27 0.0016 9800 900-1380 75-220 59-289
Below threshold
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Moderate widening
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Large-scale widening - lower reach
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Geomorphic effects - bedrock reaches
Downstream view Upstream view
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Large-scale avulsion
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Large-scale avulsion
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River mouth• 9.3M m3 of sediment
transported to tidal flats and Saguenay fjord
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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
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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
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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
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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!