Elk Valley Flood has Lessons for Dike Authorities
In a paper presented to the Canadian Water Resources Association in October 1995, Peter Woods (Inspector of dikes for British Columbia) reported that flooding in southeast B.C. serves lessons to operators of flood protection works throughout the province.
Just when it seemed that the snowmelt freshet would not cause problems, intense rain on June 6 and 7/95 brought streams in the Elk River valley to record levels resulting in some $20 million in flood damages. In addition to causing extensive damages to transportation systems, storm flows breached the Coal Creek training berm at Fernie and nearly failed the Michel Creek Mountain View Mobile Home Park dike in Sparwood.
At Fernie the failure of the training berm was witnessed by public works staff who were actively flood-fighting at the time. Fernie Public Works Superintendent, Fred Lightfoot, says that by 4:00 PM on June 6 he was quite concerned because of high velocity flows and debris in the creek, and had deployed men and equipment to shore up banks and clear debris. The creek was described as a raging river...propelling tall trees, logs and bridge timbers like battering rams.
Suddenly about 7:30 PM, the training berm began to unravel at a sharp dogleg corner about 130 m above a bridge with peeling away of riprap by high velocity flows. The failure progressed downstream in a matter of about a minute and one-half followed by incremental chunks of exposed dike fill collapsing one foot at a time and being swept away. Flows behind the riprap then proceeded to wash out an additional 160 m downstream causing flooding through the airport subdivision. City crews, aided by volunteers, managed to limit damages by confining flows down streets with the aid of sandbags. At height of the flooding some 750 people were evacuated.
Meanwhile at Sparwood, the Search and Rescue Club observed a buildup of debris hanging up and accumulating on the center pier of the CP Rail bridge about 1 AM June 7, prompting the evacuation of some 90 families. Trees with reported 8-9 foot diameter root wads crashed against the bridge, causing it to þring like a gong. By 4:30 AM, the clear opening was reduced to less than about one-half which increased velocities around the bridge abutment causing it to collapse. The failed bridge then diverted water towards the dike and immediately eroded out some 20 lineal feet of fill.
There was initial confusion as to ownership and responsibility for the private dike, necessitating that the District of Sparwood step in to make emergency repairs with technical direction from MELP. Flooding of the mobile home park was narrowly averted.
Fortunately, no one was injured in these incidents and damages were at least partially covered by Disaster Financial Assistance from the Provincial Emergency Program. City crews and volunteers in both communities were later commended for their actions.
Diking authorities must be aware that one of the first questions being asked by disaster officials prior to extending disaster assistance is:
Are the flood protection works being properly maintained to minimize flood damages?
Fortunately we can say that this is true in the case of Fernie; at the private Sparwood dike, it was only volunteers and the community stepping in at a crucial time that averted disaster.
There were many lessons learned including:
- early deployment of local community resources is the most effective initial flood response;
- debris jamming and battering can damage even heavy riprap and well designed flood protection works;
- the effectiveness of bank protection can decrease over time as some rock types weather and break down;
- there is no substitute for a regular operation and maintenance program of flood protection works, including advance emergency contingency planning.
More information on the B.C. Dike Safety Program is available from the Inspector of Dikes in Surrey.
Neil J. Peters, M.A.Sc., P.Eng.
Inspector of Dikes
Ministry of Environment
Neil.Peters@gov.bc.ca
Contact Us: E-Mail: Water.Stewardship@gov.bc.ca
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