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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.

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Presentation on theme: "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."— Presentation transcript:

1 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

2 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

3 Saguenay area

4 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

5 3500 km km km 2

6 July 1996 Rainstorm

7 An extreme flood! Lake Kénogami - source of Sables and Chicoutimi rivers Inflow into the reservoir (3390 km 2 ) previous max. April 1941– 997 m 3 s yr flow ( ) – 973 m 3 s yr flow ( ) – 1437 m 3 s -1 max. inflow July 21, 1996 – 2364 m 3 s -1 (Source: Nicolet Commission Report 1997)

8 Sables and Chicoutimi rivers

9 Flood hydrograph 653 m 3 s m 3 s -1

10 Flood hydrograph

11 Jonquière dam - Sable River Built in 1943

12 Chute-Garneau dam - Chicoutimi River Built in 1925

13 and two additional dams! Chute-à-Besy dam (1911) Pont-Arnaud dam (1912)

14 Other dams Ville-de-Jonquière dam (1996) Chicoutimi dam (1923) Elkem- Métal Dam (1958)

15 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

16 Problems elsewhere Primarily inundation of low- lying areas Scouring of vegetation along steep bedrock reaches

17 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

18 Mars River study area Alluvial, irregular meander planform (sinuosity 1.2) Gravel-bed channel Valley gradient averages River occupies deep stream-cut valley 300 to 1200 m wide Storm-generated flood discharge (i.e, not influenced by dam breaches)

19 Pre-flood Pre-flood (May 1994)

20 Post-flood Pre-flood (May 1994)

21 Change in total channel width

22 Post-flood channel

23 Impacts on infrastructure

24 A transitional meandering planform

25 Empirical planform discriminate diagram

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28 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

29 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

30 Pre-flood Lake Ha! Ha! Inflow – 160 m 3 s -1 Max. spilling capacity – 250 m 3 s -1 Avail. spilling capacity – 86 m 3 s -1

31 Post-flood Lake Ha! Ha!

32 Estimates of flood discharge MethodDischarge (m 3 s -1 ) Comment Empirical relationship of drained lake volume and peak flow based on the equation Q max = 1730 V 0.48 (modified from Costa, 1988). Drawdown of reservoir 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 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

33 Downstream geomorphic effects

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35

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37 Transition from deposition to erosion

38 Erosive threshold

39 ReachValley slope Specific weight of water (Nm -3 ) Discharge (m 3 s -1 ) Width range (m) Unit stream power (Wm -2 ) km Below threshold

40 Moderate widening

41 Large-scale widening - lower reach

42 Geomorphic effects - bedrock reaches Downstream viewUpstream view

43 Large-scale avulsion

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45 River mouth 9.3M m 3 of sediment transported to tidal flats and Saguenay fjord

46 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 m 3 s Wm 2 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

47 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

48 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

49 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!


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