Greg Brooks Geological Survey of Canada Natural Resources Canada

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 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 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)

8. Sables and Chicoutimi rivers

9. 1100 m3s-1 653 m3s-1 Flood hydrograph

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

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. Erosive threshold Below threshold

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!