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From trickling creeks to raging rivers

There are roughly 475,000 kilometres of pipeline in the Canadian province of Alberta alone. These pipelines cross creeks, rivers, and streams. These waterways can be as small as a walkover creek or as wide as the Athabasca River.

Water moving through a river cross-section exerts both frictional and buoyant forces on the bed. Those forces in turn causes silt, sand, gravel, and cobbles to move, creating what we refer to as the mobile bed. The depth of the mobile bed (the amount that is moving) increases with more water, during a flood for example. This is commonly referred to as flood scour.

Pipelines are buried well beneath a watercourse to avoid interaction with this mobile bed, thereby reducing the potential vulnerability of the pipe. However, over time the soil over a pipeline, also known as the depth of cover (DOC), is often reduced as rivers transport more sediment away from a crossing than to the crossing. Eventually, the pipeline itself may be within the mobile bed and could be impacted or even exposed during a flood.

This means the pipeline may be subject to hydrodynamic forces, local scour, impact forces (from grains moving in the river flow), or vortex induced vibration as water is forced past the pipeline. For these reasons, pipeline companies assess their watercourse crossings carefully to try to understand the extent to which they may be subject to hydrotechnical hazards, and to prioritize the pipelines segments that may need mitigation or monitoring.

Our team developed a geohazard screening tool that accomplishes much of this task quickly and efficiently¡ªsaving time and money¡ªby eliminating non-credible threats when we only know the location and the DOC of a pipeline.

How do companies currently assess their hydrotechnical hazards?

Currently, pipeline operators engage a company like Â鶹´«Ã½ to assess watercourse crossings, ideally determining both the DOC and the potential geohazards at the site. Our team, including pipeline surveyor and a geohazards specialist, makes these determinations.

When the pipeline DOC is less than 1.2 metres, or the site conditions warrant it, the site receives a full survey followed by scour analysis using the US Army Corps of Engineering tool HECRAS.

After hundreds of detailed investigations, it became obvious that many of the sites simply don¡¯t need that level of effort, even when the pipeline is shallower than 1.2-metre DOC. But without a defensible method to support those observations, the expensive route was the only route. Until now.

How can we assess pipelines now, using our new tool?

Our geohazards team realized that there are credible limits to scour depth under all conditions and river configurations. In many cases, small rivers in flood simply don¡¯t have the power to reach the pipeline depth, regardless of the river cross-sectional shape or bed material. In these cases, we developed a screening tool to assess these small rivers in a more efficient, cost-effective manner.

We have performed detailed analysis of hundreds of pipeline water crossings, including channel size, shape, and sediment moving through it, at a variety of flood return periods. Using this data, we plotted the predicted scour depth for the range of river cross sections and a range of floods up to the 100-year event. The results provide a screening tool that will predict the maximum amount of scour at a site, below which the true value is expected to lie. Using this tool, and knowing the DOC, we can identify pipelines that simply aren¡¯t going to be affected by flood scour and focus detailed assessments on those watercourse crossings that really matter to our client. This saves them time and money without exposing them to unnecessary liability.

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Our new Pipeline Geohazard Screening tool will help us focus field work where it can best be used¡ªwhere the hazard and the consequences are likely to be the highest. This ultimately results in keeping pipelines secure and communities and the environment safe.

Stantec¡¯s new Pipeline Geohazard Screening tool is the result of the excellent work of the Geohazards team (including co-authors Julia Ryherd, Colleen Small, and Song Ling), the Geomatics team (kudos to Mark Strickland, Chris Boettcher, and all the survey staff), and the excellent work on detailed flood scour analyses by the Alberta South Water group (kudos to Matt Wood, David Jones, and Michael Uniake). We will keep testing and updating the tool to ensure it¡¯s used appropriately.