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Tuesday, October 7, 2008 |
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Temp:
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9°C
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Feels like:
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7°C
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Humidity:
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66%
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Update provided on flood control study |
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Written by MARK NIELSEN
Citizen staff
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Tuesday, 08 July 2008 |
Consultants remained largely uncommitted to any one option over another when a progress report on efforts to come up with flood control measures for the Nechako and Fraser Rivers was presented to city council on Monday night.
Led by Vancouver-based Northwest Hydraulic Consultants Ltd., the group was hired by the city following the Nechako ice jam this winter and the high water levels along the Fraser and Nechako last spring and summer.
So far, a risk analysis has been completed, NHC senior engineer Monica Mannerstrom told council, and the next step will be to carry out a detailed technical investigation of possibilities before they're taken to the public for comment this fall.
Mannerstrom also said the city's existing floodplain map, completed in 1997, needs to be updates using newer data and modeling.
"Previously, 1997 floodplain mapping was undertaken using 1979 and 1995 channel cross sections," she said. "We suspect the sections have changed so there's good reason to update this work.
"At that time no ice modeling was done which we're proposing to do and also I think the design flows are need of revision."
Perhaps the most concrete development so far was identification of 14 areas as susceptible to flooding.
Seven are along the Nechako: south bank at confluence; north bank east of John Hart Bridge; north bank near confluence; Morning Place; south bank at Foothills Bridge; south bank between John Hart and Foothills bridges.
Seven are along the Fraser: west bank at Yellowhead Highway; South Fort George; South Fort George west of Queensway; Lansdowne south end; west bank at Cottonwood Island; Northwood Pulpmill Road; across river from Shelley.
The Fraser's level at South Fort George peaked at 9.86 metres during the 2007 freshet and at 9.7 metre in May this year.
"We've had fairly large floods recently but nothing catastrophic like say in 1972, when the water level read about 10.4 or in 1948 when it reached 10.5," Mannerstron said. "We have found some evidence that in 1894, the flood level was up around 11.5 or nearly two metres higher than this year."
As for ice jam flooding, Mannerstrom said they're caused by a combination of high flow and cold weather.
"It's not unusual to get a fairly high flow in November, December, January but luckily these flows don't always coincide with freezing temperatures of a longer term," she said.
It was noted that over the years, several of the Nechako's side channels in the River Road industrial area have been filled in, eliminating alternative floodways for the water in the process.
To the extent that any options were favoured, set-back dikes and land-use amendments have so far been favoured over river dikes because they're more exposed to erosion and have a greater effect on riparian habitat.
Regarding dredging, Mannerstrom said more work needs to be done before any firm proposals are developed.
Other options under consideration are upstream diversion and further regulation, improved drainage and various flood-proofing measures.
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Last Updated ( Sunday, 07 September 2008 )
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