Unpacking Abbotsford’s flood risk

This is the second of three stories on the annual flood risk in the Fraser Valley’s biggest cities. We also examined the Fraser’s flood danger in Langley and Chilliwack. Find those stories here: Langley | Chilliwack. We also wrote about the threat to Mission’s waterfront here.

The Nooksack’s threat to Sumas Prairie might now be clear, but it’s only one of three rivers that could pose a flooding threat to Abbotsford.

When last November’s rains came pouring down, the flood seemed to catch many—including those in government—off guard.

Now, as The Current goes city-by-city analyzing local flood risks, we are looking at the potential danger the Fraser and Vedder rivers pose to Abbotsford.

Weather conditions mean the Fraser has the potential to flood this year, but even if it stays below its banks, experts say knowing one’s own local flood risk is key to protecting oneself and preparing for an emergency.

Having already looked at Langley’s flood risks (see that story here), we’ve unpacked Abbotsford’s other main flood threats below. We will tackle Chilliwack in the near future.

Matsqui Prairie

Despite the consequences of November’s flood, the Fraser River poses the single largest threat to Abbotsford in terms of flooding. Only the Fraser can inundate both Sumas and Matsqui prairies.

When it does rise, it is Matsqui Prairie that is most vulnerable.

The prairie’s vulnerability is complex and largely dependent on the structural integrity of the kilometres of dikes that protect it. Unfortunately, there are more than a few indications that those flood protections may be insufficient to stop a serious flood.

Compared to many of the dikes in the region—including those directly across the Fraser that protect Mission’s waterfront—the Matsqui dike is relatively high. If floodwaters were to rise as high as they did in 1894, for instance, Mission’s waterfront, all of Hatzic, and Greendale could all see flooding before Matsqui Prairie became inundated.

When water does start to elude the Matsqui dike, it is expected to do so at its westernmost and easternmost points. The area around the northernmost extent of Mt. Lehman Road could see some water, as could scattered properties at the base of Sumas Mountain. In general, the scale of the flooding might pale in comparison to scenes on the northside of the river, or in Glen Valley.

But most of those calculations assume the dike will hold. And there is an array of recent evidence to doubt that assumption.

For years, the Fraser’s current has been eating into the shoreline adjacent to its eastern segment, creating large erosion arcs where the river is beginning to encroach on the dike. The problem has already demanded more than $10 million worth of work to create spurs in the water to attempt to guide the current away from the shoreline and prevent further erosion. And the issue shows how the triangular tip of Matsqui Prairie is particularly vulnerable to erosion.

As a river rises, so too does its speed, energy, and ability to erode its banks—as last November’s storms demonstrated. The deepest, fastest-flowing part of the river is also shifting course, moving closer to the river’s southern bank.

An assessment of the weakest part of the Matsqui dike declared that while its crest was better than in many places, the dike is “seismically unstable” and “may fail due to erosion.”

Further west, a large sinkhole was detected in the Matsqui dike. Fixing it cost more than $1 million. Erosion or any other lurking structural issues could lead the dike to fail before flood waters start rushing over its top. And already, the river has eaten into the shore in that area, costing the city thousands to repair.

Another thing to watch: that sinkhole was located next to the wastewater treatment plant that serves both Abbotsford and Mission. Called the Joint Abbotsford Mission Environmental System (or JAMES, for short) wastewater treatment plant, the large facility is located south of the Abbotsford-Mission bridge. If the facility were to flood, it would be catastrophic for residents across the city. In November, Merritt’s entire population was evacuated because its wastewater facility was flooded.

We reported on the threat flooding poses to Fraser Valley wastewater plants in this story.

Sumas Prairie

Although the Nooksack will continue to be the most likely cause of future flooding in Sumas Prairie, it’s not the only watercourse that threatens in the area.

As geography would suggest, the adjacent Vedder Canal/River also can affect nearby property owners. The canal was created to re-route the Vedder/Chilliwack River away from Sumas Lake when it was drained, and its banks have largely met that task over the years.

The dike separating Sumas Lake bed from the canal is also thought to be high enough to prevent most floods, and even during November’s historic rainfall event, the water didn’t threaten to overtop the banks.

While better than many in the region, the dike is far from perfect and during flood conditions, “minor stability problems, such as seepage, can be expected.” Like most around the region it is also seismically unstable, which means it could crumble in the event of an earthquake. That might be particularly worrying in the case of the Vedder dike, given the sheer size of the dike, and the comparable low elevation of the former lake bed of Sumas Lake. Even before it reaches flood stage, the Vedder canal is often significantly higher than the dry land just to its west.

The largest Vedder-related threat to Abbotsford and Sumas Prairie likely comes not from water flowing down the river, but from water from the Fraser flowing up it and raising its level above that of the surrounding dikes. The threat is expected to rise over the coming decades with climate change and more sudden warming of snowpacks in the BC Interior.

Sumas Prairie is also vulnerable in the short-term from any storm event that could knock out power to Barrowtown Pump Station over the winter. The station is responsible for draining water from Sumas Lake, but does not yet have its own generator. The City of Abbotsford is working to change that, and has already designated money to buy a generator, and renovate the pump station to accommodate it. The pump station can draw power both from the west and, as was needed during November’s storms, the east. Fortunately, it’s unlikely that power lines in both directions will fail for a prolonged period before a new generator is in place. Unfortunately, unlikely does not mean impossible.

City-wide flooding

November’s downpours demonstrated why Abbotsford is looking to upgrade its drainage infrastructure and storrmwater management plans.

Water inundated basements across the city, including neighbourhoods in areas above the floodplain and not normally susceptible to flooding.

Abbotsford created a new drainage plan in 2018 that catalogued the various challenges facing the city’s drainage system. They include localized flooding risk at the north end of Fishtrap Creek on the south end of Mill Lake, and in various other areas.

Such flooding is relatively rare and a short-term problem. But with more storms, more people, and more development, the city will need to spend money to upgrade infrastructure to smooth the flow of the water into ponds and away from homes.

Clayburn Village on Matsqui Prairie is the most susceptible to sudden storm-linked floods. While normally docile, Clayburn Creek drains water from a broad expanse of Sumas and Eagle Mountains. When those hillsides see large amounts of rain, all that water ends up draining through Clayburn Creek at the same time, turning it into a large river. And when that torrent hits the valley bottom, it rapidly slows and collects in the Clayburn village area.

Some work has already been done, but in 2018, the city estimated that further upgrades to the infrastructure in the area is expected to cost upwards of $8 million.