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

660km2

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

653 m3s-1

1100 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)

Elkem-Métal 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 low-

lying 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-floodPre-flood (May 1994)

Post-flood

Pre-flood (May 1994)

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

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

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

(est. 384)

- gauging station located about 7 km above river mouth

Downstream geomorphic effects

Downstream geomorphic effects

Downstream geomorphic effects

Downstream geomorphic effects

Transition from deposition to erosion

Erosive threshold

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

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!