CGRG Bibliography of Canadian Geomorphology
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Author : Brooks, G.R.
Date : 2000.
Title : Catastrophic channel widening during extreme floods: Examples from the 1996 flooding in the Saguenay Valley.
Publication : Workshop on Geotechnique and Natural Hazards, 53rd Canadian Geotechnical Conference, Hotel Delta Centre-Ville, Montréal (QC), October 15-18 Octobre, 2000
Issue :
Page(s) :
Abstract
In July 1996, a severe rainstorm triggered widespread flooding in southern Quebec. Among the many impacts, the flooding caused catastrophic channel erosion along two rivers in the La Baie area, Saguenay Valley. Along the Ha! Ha! River, flooding from the rainstorm runoff was accentuated substantially by waters escaping from the Lake Ha! Ha! reservoir after an earthen dyke was overtopped and breached. Along the 35 km distance from Ha! Ha! Lake to the river mouth, the geomorphic effects of the flooding ranged from erosion, locally up to 280 m of channel widening, to channel aggradation, locally up to several metres. The occurrence of erosion or aggradation along the river is attributed to variations in flow energy above or below the general resistive (or geomorphic) threshold of valley bottom. Of the controlling variables, change in valley slope was the most important in determining whether or not the flow energy exceeded the resistive threshold. However, this does not explain all of the erosion and deposition. Local factors were also important, such as, the overtopping of the drainage divide between the main channel and a ravine that led to the avulsion and deep incision of the channel. In contrast to the Ha! Ha! River, flooding along the lower 10 km of the Mars River was generated entirely by rainstorm runoff. This reach experienced up to 400 m of channel widening through a combination of bank erosion and channel avulsions, transforming a narrow, meandering river into a wide, braided one. Aerial photograph analysis and empirical planform discriminate equations indicate that the pre-flood channel morphology of the river was transitional between the meandering and braided morphologies. Such transitional morphologies are vulnerable to large-scale erosion and channel widening during a severe flooding. Given a sufficient period prior to the occurrence of the next extreme flood, the channel should revert to its pre-flood morphology through sedimentation and the re-establishment of vegetation along the river.
Bibliography of Canadian Geomorphology