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Greg Brooks Geological Survey of Canada Natural Resources Canada

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Presentation on theme: "Greg Brooks Geological Survey of Canada Natural Resources Canada"— Presentation transcript:

1 Greg Brooks Geological Survey of Canada Natural Resources Canada
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 km2 608 km2 660 km2

6 July 1996 Rainstorm

7 An extreme flood! Lake Kénogami - source of Sables and Chicoutimi rivers Inflow into the reservoir (3390 km2) previous max. April – 997 m3s-1 100-yr flow ( ) – 973 m3s-1 yr flow ( ) – 1437 m3s-1 max. inflow July 21, – 2364 m3s-1 (Source: Nicolet Commission Report 1997)

8 Sables and Chicoutimi rivers

9 Flood hydrograph 653 m3s-1 1100 m3s-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 0.012 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 (May 1994) Pre-flood

20 Pre-flood (May 1994) Post-flood

21 Change in total channel width

22 Post-flood channel

23 Impacts on infrastructure

24 A transitional meandering planform

25 Empirical planform discriminate diagram

26 Empirical planform discriminate diagram

27 Empirical planform discriminate diagram

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 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

30 Pre-flood Lake Ha! Ha! Inflow – 160 m3s-1
Max. spilling capacity – 250 m3s-1 Avail. spilling capacity – 86 m3s-1

31 Post-flood Lake Ha! Ha!

32 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 - 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

34 Downstream geomorphic effects

35 Downstream geomorphic effects

36 Downstream geomorphic effects

37 Transition from deposition to erosion

38 Erosive threshold

39 Specific weight of water
Erosive threshold Below 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 75-220 59-289

40 Moderate widening

41 Large-scale widening - lower reach

42 Geomorphic effects - bedrock reaches
Downstream view Upstream view

43 Large-scale avulsion

44 Large-scale avulsion

45 River mouth 9.3M m3 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 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

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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