Washed Away: Assessing Bank Erosion Risk Along Highway 8 in B.C.

July 31, 2023
10:28 am

An extreme atmospheric river event in mid-November 2021 resulted in floods that caused extensive damage to transportation corridors, pipeline infrastructure, and communities throughout British Columbia. Although the damage was staggering in many areas—four bridges collapsed on the Coquihalla Highway (Highway 5) alone—nowhere was hit harder than Highway 8, a 60-km-long highway that travels along the Nicola River valley, connecting the towns of Merritt and Spences Bridge in BC’s arid interior.

Heavy rain fell throughout two tributary watersheds and the Nicola River peaked at an estimated discharge of 750 m3/s, which is more than double the highest discharge recorded over its 60-year record. Severe bank erosion damaged Highway 8—or entirely washed it away—in 25 locations as the river widened by over 50% (from 65 m to 100 m on average).

The destruction of Highway 8 disconnected communities for 361 days while the highway was repaired. During this time, the BC Ministry of Transportation and Infrastructure (MoTI) engaged BGC to assess the risk of highway closure due to hydrotechnical and geotechnical hazards along its length. The assessment supports MoTI’s highway reinstatement decisions in the context of other geohazard risks and informs mitigation design criteria (e.g., design flows for bank armouring). Building on data shared across BGC’s Transportation, Pipelines and Communities Sector clients in the same corridor, we considered a suite of geohazards: rock fall, soil slides, soil spreads, debris flows, debris floods, clearwater flood inundation, avulsion and bank erosion. After dividing the highway into smaller segments (10s to 100s of metres) we characterized the hazards to each segment, estimated their probability of impacting the segment, and quantified the associated consequence.

The risk associated with bank erosion was estimated based on:

The probability of a given flood magnitude occurring
The probability of the ensuing erosion spatially intersecting the highway
The consequence of that highway segment experiencing erosion (in terms of closure days).

As a first step in our bank erosion assessment, we identified all locations along Highway 8 that were potentially susceptible to bank erosion based on topography and proximity using pre- and post-flood orthophotos and BGC’s lidar change detection. We then used a probabilistic, physically-based model to estimate the probability that bank erosion could reach the road in flood events ranging from an annual exceedance probability (AEP) of 5% to 0.5% throughout the 47 km of the highway deemed susceptible to erosion. The bank erosion model assumes that erosion in gravel-bed rivers initiates when the coarse material on the riverbed is mobilized, causing the bed and banks to destabilize, and that erosion proceeds until this coarse material is redeposited. The primary inputs include shear stresses derived from hydraulic modelling and grainsizes estimated by our project team in the field. Observed bedrock outcrops, as well as armouring such as riprap, are also used to modify the predicted erosion.

Bank erosion was determined to pose the greatest risk to Highway 8 of the geohazards considered. During the “design” flood scenario of 1,950 m3/s (i.e., the climate change-adjusted flood with an AEP of 0.5%, or a return period of 200 years) we estimated that 17.5 km of the highway had a 25% or higher probability of being impacted by bank erosion. The total estimated closure duration of 670 days, or nearly double the closure time in 2021-2022. Moreover, among the twenty highest-risk sites identified, fourteen were related to bank erosion hazards.

BGC’s work on Highway 8 has helped MoTI adapt highway design criteria to a changing climate. The 0.5% AEP (200-year) flow on the Nicola River (1,950 m3/s) has a discharge of more than double the estimated flood discharge in 2021 (750 m3/s), due to the projected increases in atmospheric-river events related to climate change. Such high flows make it impractical—and ecologically undesirable—to apply MoTI’s traditional standard of a 200-year design flow with climate change. Supported by BGC’s communication throughout the project, MoTI has defined design criteria that target the flood of record, or a risk-informed approach to adjust criteria where designing to the flood of record poses unreasonable challenges. The process has enabled a shift towards risk-informed decision making that can still align with existing Ministry standards.

Many BGCers across the Surface Water, Geohazards, GIS, Cambio, Communities, and Transportation teams contributed to the Highway 8 risk assessment.