By David Broadland, Focus on Victoria
LET’S SURVEY SOME OF THE DAMAGE and the circumstances that led to the Lower Mainland being cut off from the rest of Canada and the flooding of Merritt and Princeton in mid November.
The Tank Hill Underpass, just east of Lytton, was built in 1957 to allow the newly widened Trans Canada Highway to pass underneath the CPR Railway. In all the years since, there’s no record of the culvert below the highway not having enough capacity to safely transport water under the structure. In February of 1963, for example, the structure survived a 24-hour rainfall of 69.9 millimetres (2.8 inches) that washed out both the highway and railway at a point closer to Lytton. In the “Great Coastal Storm” of 2007 that hit BC, Washington and Oregon in early December, Lytton recorded a 24-hour rainfall of 106.9 millimetres and the underpass was unscathed. But on November 14, 2021, the structure washed out after the fall of 61.9 millimetres of rain in the previous 24-hour period. What had changed?
The 807-hectare watershed above the underpass was severely burned during last summer’s 84,000-hectare Lytton Creek Fire. The ministry of forests’ historical record of forest fires shows that watershed had no previous record of fire. This summer’s fire, rather than the quantity of rain that fell, may have determined the underpass’s fate.
The hydrological impact of forest fires is well understood.
A 2011 study, conducted by US Forest Service scientists, noted: “Basins with high-burn severity, especially those with steep, previously forested terrain, have flashier hydrographs and can produce peak-flows orders of magnitude greater than pre-fire conditions.” (See report attached at original article.)
Note the scientists’ use of the expression “orders of magnitude greater.” As you know, one order of magnitude means 10 times greater. Two orders of magnitude means 100 times greater. And so on.
Why would a forest fire have such a large impact on the hydrological function of a forest? Here’s the short answer from those scientists: “This is due to fundamental changes in the hydrology of burnt watersheds, especially in the short term (1-3 years). Consumption of the canopy and forest-floor organic horizon that formerly intercepted precipitation, moderated infiltration, and protected mineral soil, results in decreased evapotranspiration and infiltration, and increased runoff. Further, newly exposed soil surfaces are subject to rain-drop erosion, which may be exacerbated by fire induced soil-water repellency. Though the hydrologic impacts of high-severity wildfire have been well documented in the scientific literature, the socio-political ramifications of a latent, continuous, and highly unpredictable disturbance regime (i.e. post-fire flooding and sedimentation) has not been addressed.”
At about the same time as the Tank Hill Underpass was washing out, more serious trouble was brewing 50 kilometres to the east.
The Coldwater River began to surge over its banks where it joins the Nicola River at Merritt. The west end of the town was flooded above the level that hydrologists had determined would likely be the worst case scenario—the 200-year flood plain—for future floods from melting snowpack. No one foresaw Merritt being flooded by a mid-fall rainstorm. As many as 7000 residents were forced to evacuate. Yet Merritt itself recorded only 31.4 millimetres of rain (1.2 inches) in the critical 48-hour period on November 14 and 15 …