Most Fraser River dikes upstream of Langley should be built higher to withstand the potential for more severe floods that are now expected more often.
That’s the finding of a new report released by the province that recalculated the appropriate height of 15 dikes from Mission to Hope based on the latest changes to the river’s channel, which constantly shifts due to sedimentation, erosion, the effect of bridges and other factors.
“Most of the dikes were found to have inadequate freeboard and are at high risk of over topping during a design flood event,” the report said, noting only the Matsqui B dike met the latest requirements.
The analysis didn’t look at City of Chilliwack dikes, which were the subject of a 2013 update.
Many of the Fraser Valley dikes were first built in the 1970s and 1980s to design standards that have since been revised using new data from the 2012 freshet.
A second provincial study projects large floods on the lower Fraser River will be worse and strike more frequently as a result of sea level rise and climate change.
By the end of this century, the report said, a 50-year flood could be similar in magnitude to floods that have occurred only every 200 to 500 years previously.
A moderate climate change scenario would mean a flood on the Fraser similar to the devastating 1948 flood could then be expected every 50 years, instead of every couple of centuries.
If climate change is intense, the report indicated, the 50-year flood could instead be worse than the more severe flood of 1894 and close to what would until now have been considered the 1,000-year flood.
The underlying climate change projections “are subject to large and unquantifiable uncertainty,” the report said.
The analysis was conducted by the forests ministry’s flood safety section and Northwest Hydraulic Consultants.
It also outlined three scenarios for an extreme flood of the lower Fraser.
The first is widespread Interior rainfall on snow in the watershed north of Hope – the scenario that caused the record flood of 1948 that inundated low areas of the Fraser Valley.
Another threat is the potential for a landslide to temporarily block the river or one of its tributaries and then release an “outburst flood” that could devastate communities downstream.
A slide in 1880 blocked the Thompson River and quickly formed a 14-kilometre long lake, but it drained gradually avoiding an outburst flood.
The geologic record shows outburst floods triggered by slides have happened in the Fraser system, but researchers found no indication of how large they were.
Another extreme flood risk is dam failure.
BC Hydro has two dams – Lajoie and Terzaghi – that hold back the Carpenter and Downton reservoirs for power generation on the Bridge River, which flows into the Fraser near Lillooet.
“A breach of the upstream dam, causing subsequent failure of the downstream dam, would result in a catastrophic flood at Hope significantly larger than the estimated 10,000 year return period event,” the report said.
“The time to the flood peak at Hope would be about 11 hours following the start of the breach.”
Large Fraser River floods
June 2012 – Peak flow 11,700 cubic metres per second. Fifth largest on record. 10-25 year flood. Caused by widespread rainfall on a ripe snowpack.
June 2007 – Peak flow 10,800 cubic metres per second. 10-year flood.
May 1948 – Peak flow of 15,200 cubic metres per second. 200-year flood. Damaged or destroyed 2,000 homes and resulted in the evacuation of 16,000 people.
1894 – Largest flood of the Fraser on record, with an estimated flow of 17,000 cubic metres per second, caused flooding from Harrison to Richmond. Damage was less than 1948 because of sparser development.
Future – New estimates suggest that by 2100 the 50-year flood will have flows of 15,500 to 17,100 cubic metres per second, depending on the severity of climate change, instead of 13,400 historically. 200-year flood flows would be 18,100 to 20,800 under the model, instead of 15,200 historically.