According to data from the UBC Malcolm Knapp Research Forest, a total of 14 millimetres of rain fell in north Maple Ridge last Thursday, Nov. 1.
That was followed on the Friday by another 49 mm.
The rainfall triggered flooding of properties along the North Alouette River, near 132nd Avenue and 224th Street, with residents wondering why basements and garages were inundated after such a normal weather event for the West Coast.
“That’s not a lot of rain,” said Bruce Hobbs, who lives on 132nd Avenue.
“So something is not right here.”
Hobbs added that the water level gauge in the North Alouette River also was running high for seven hours.
“Which is very unusual.”
Hobbs said it’s difficult to pinpoint a cause for last week’s strange water flows. It’s possible the river was blocked somewhere, causing water to back up. Or, it could have been caused by the release of water from Alouette Lake reservoir into the South Alouette River, which would have also affected the North Alouette, Hobbs added.
“You can’t say it was the development [uphill in Silver Valley] because you just don’t know.”
But, Hobbs maintains, it was a strange event after just a few days of rain in a place of the world where it can rain for weeks.
“Really, there shouldn’t have been a flood.”
Municipal engineer Dave Pollock said that operations crews are surveying the river to see if there are any blockages on it.
The city is still trying to get rainfall amounts for those days from that particular area from Metro Vancouver. It’s possible that a storm cell produced a concentration of rain in a short period of time, causing the flooding.
But there could have been multiple factors involved, he added.
The city said earlier that high tides at about the time of the rain pushed water up into the Pitt River which, would have slowed the drainage from the Alouette system.
Pollock, though, pointed out that Silver Valley subdivision development in the hills above the North Alouette has been going on for years, without causing flooding, adding that the city hasn’t changed its development standards for storm-water management.
That continues to require post-development stormwater flows off a subdivision to match those from when the area was forested, he added.
As well, he said that the residential part of Silver Valley isn’t a significant part of the North Alouette River watershed. Silver Valley subdivision actually drains into three systems, Blaney Creek and North and South Alouette Rivers.
He added that the 45-square-kilometre watershed for the North Alouette extends well past Silver Valley, which is not a significant part of that.
Pollock said there hasn’t been any new source of storm-water feeding into the North Alouette. A new culvert at 232nd Street is only feeding Paradise Creek, he added.
“That was a pre-existing water course,” he added.
“There’s been no material changes in terms of how stormwater management has been handled,” Pollock said.
He said that operations crews are continuing to check the river to see what may have caused the flooding.
“If we can identify the source, we will.”
Jim Findlay, whose property on 132nd Avenue also was flooded, said the sewage system is a major issue in the area, noting that six riverfront properties had their septic tanks overflow into their homes. The high water brought spawning salmon into nearby farm fields, which homeowners put back into the river.