Investigating the Cause of Flooding in a Western Canadian Watercourse

Guichon Creek Project Profile
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Project Info

BC Ministry of Transportation

British Columbia, Canada

In the recent past, Guichon Creek has been a relatively benign and quiet stream, typical of mountain watercourses in the interior of British Columbia. Normally, this watercourse has its peak water flows between April and June each year, as the warm air of spring melts the snowpack in the surrounding mountains. Records taken since 1962 show only four years in which peak flows in Guichon Creek topped 20 cubic metres per second.

But in 2017 and 2018, peak flows more than doubled – estimated at over 50 cubic metres per second.

This increased flow brought flooding that caused property damage in 2017. In 2018, the much higher sediment and debris load carried by the flood water blocked a bridge on a major road – Highway 8 — intermittently closing it to traffic for over 10 days until the blockage could be cleared with heavy equipment.

Concerned that the problem would recur, the BC Ministry of Transportation asked Golder to investigate. (Also involved were the Thomson-Nicola Regional District in which Guichon Creek lies, and the Lower Nicola Indian Band indigenous group. The lower portion of Guichon Creek runs through the lower Nicola Indian Reserve). The two main questions to be answered were: What changed, if anything, to cause Guichon Creek to flood so high in those two years? And, is this problem likely to continue?

From this, the Ministry hoped to get guidance to plan its response.

One reason for the urgency was the economic importance of Highway 8. The road connects the Trans-Canada Highway and the Coquihalla Highway, which run roughly parallel, carrying traffic between Vancouver and the eastern part of the country. Roadways in the province are seasonally at risk of being shut down due to rockfall, landslide, avalanche, or traffic blockage. Highway 8 provides a safety valve, carrying traffic between the two main highways in case either of them is closed.

Assembling a picture from many different sources

A watershed study like this can be something like putting together a jigsaw puzzle – pulling pieces of information from many different places to compile a complete picture. As with many such studies, the Golder team started with currently available sources of information.

First, the team looked at aerial and satellite images to see if there had been any changes to the vegetation cover, such as forest fires or timber harvesting, expansion of rangelands or extensive road building. Sometimes loss of trees and other vegetation can cause precipitation runoff to increase and to flow more quickly into watercourses. It was revealed that clear cuts from timber harvesting expanded over the last half century to occupy about half the watershed, a significant change in cover that occurred slowly. Although it likely contributed, this alone could not explain the significant flow increases observed in 2017 and 2018, when considered separate from the flows of the last few decades.

Then, the team dug into the available meteorological data, site measurements of stream flow, and measurements of snowpack depths in the area. Some of the data extended back to 1911, being seasonally recorded in the early years of measurement and consisting of more complete, year-round records in recent years. But it all built a picture of a watercourse that was largely the same, year after year.

Finally, the team noted that in 2017 and 2018 there was some unusual weather within the Guichon Creek watershed, which could have contributed to the peak flows. This was partly due to the snowpack thickness in the watershed during those two years. Both snow survey stations, located high up in the valley at over 1500 metres above sea level, recorded much thicker than usual snowpack in those two years. Gnawed Mountain Station showed snowpack in 2017 and 2018 that was almost 300 to 400 percent greater than normal. The Highland Valley station recorded snowpack that was 500 to 900 percent greater than normal. These are very rare thicknesses of accumulation when compared with statistical analyses of typical snowpack thicknesses. In addition, high temperatures recorded during peak flow events likely contributed to a faster than normal snowmelt rate at the time those peaks occurred.

Another contributor to the blocked-bridge problem was the amount of sediment available in and near the stream. At some points along its journey, Guichon Creek runs beside bluffs of aggregated sand and gravel deposited as outwash from the end of the last major ice age. At high water, the creek rises to a level that it erodes these bluffs, and this can add to the sediment load available to be carried along by the force of water. The team found several failure areas along the bluffs where the stream is likely to pick up more sand and gravel. Due to decades of relatively modest flood flows, there had been a gradual buildup of sediment from these flow-eroded bluffs, ready to be carried downstream whenever there was a high enough volume of peak water flow — as there was in 2017 and 2018.

Additionally, the Highway 8 bridge over Guichon Creek is located on an alluvial fan, where sediment naturally accumulates. So, when the high volume of freshet water reached the bridge, carrying an unusually high sediment load, excess sediment deposition was all too likely to happen.

Although a detailed hydrologic assessment nor runoff modelling were undertaken to fully understand the causes of the unusual flood events, the team was able to narrow down the factors that combined to set up the situation just waiting to happen at Highway 8: anytime there was high enough water flow, the sediment buildup would get washed downstream.

Looking to the future

One of the questions the Ministry of Transportation asked was whether floodwater and sediment of the scale seen in 2017 and 2018 will continue, and what this means for the future of the bridge over Guichon Creek. Analysis showed that the sediment supply in the stream will likely be higher over the next five to 10 years due to the abundance of remaining sediment stored upstream of the bridge due to the recent flood events.

Golder’s team has advanced the second Phase of this study, which includes traversing Guichon Creek channel this fall from the Highway 8 bridge to the channel apex, to document and rank sediment sources and identify potential sediment management options.

Another issue to resolve is the impact that may come from climate change. A full-scale climate impact study has not been completed and, even with that level of assessment, the accuracy of climate change projections is still largely unknown. But, in general, climate change can be expected to involve an increase in winter precipitation and an increase in mean annual temperature, potentially leading to an increase in overall runoff. This means that we may see an increase in frequency of occurrence of large and rare hydrological events such as the high flow events that caused Guichon Creek to flood in 2017 and 2018. This indicates that further investment in mitigation measures may be required in Guichon Creek and other streams that cross roadways, to protect bridges.

To the geomorphologists, hydrologists and other professionals on Golder’s team, the factors that led to the floods of 2017 and 2018 were part of natural processes, on a geological time scale. Having a better understanding of how these processes can combine to impact Highway 8, enables the BC Ministry of Transportation to better plan and respond.

Project Info

BC Ministry of Transportation

British Columbia, Canada

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