ABSTRACT: Geomorphological effects of an extreme flood event on an urbanized river system, Toronto, August 19th 2005.

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Abstract

Highland Creek is a highly urbanized (85% or the basin area) stream system in east Toronto with a drainage area of approximately 102 km 2 (see Ashmore et al. poster). The river is deeply incised into glacial deposits and many reaches have thin or no alluvial cover. Since urbanization in the 1970s the stream-flow regime shows very large increases in flood flows, especially of the annual instantaneous maximum discharge, and this is probably a major cause of ongoing erosion problems along the already-energetic stream system. On August 19th, 2005 Toronto experienced an extremely intense rainstorm with totals and short-term intensities well in excess of 100 year recurrence intervals. The centre of the storm tracked across the upper part of the Highland Creek basin. Short-term peak rainfall intensity was close to 200mm/hr. There was no operational stream gauge on the stream system at that time but a previously-calibrated runoff model for the basin indicates that peak flow in the east branch (drainage area 40 km 2) was of the order of 500 m3 s-1 and the flood wave passed through the system in only 4-5 hours. This peak flow is comparable with the extreme (99th percentile) instantaneous flows for all regions of the United States (O’Connor and Costa, 2004). Peak total and specific stream power were of the order of 50,000 W m-1 and 1000 Wm-2 in some reaches. Complete assessment of the geomorphic effects on the channel system are only just beginning, based on recently-available pre- and post-storm, large-scale aerial photography. Many reaches show considerable widening, channel migration and in one case a large meander cut off. Infrastructure damage included collapse of armour-stone bank protection and loss of a long section of sanitary sewer on the valley floor. Channel reconstruction and restoration is already underway in one severely-affected reach. Analysis of the geomorphic response to this event will allow better understanding of the dynamics of semi-alluvial, till-bed channels and potential improvements in erosion mitigation and channel restoration design in this type of channel in urbanizing basins.

Bibliography of Canadian Geomorphology