Pipeline Geohazards Screening: Using Results of Flood Scour Assessments to Provide a Simple Screening Tool for Pipeline Watercourse Crossings for Western Canada

Pipeline watercourse crossing assessments typically require field investigations, river surveys, and detailed scour analyses to predict whether or not a pipeline may be subject to flood scour deeper than their depth of cover (DOC). Flood scour algorithms rely on discharge, median grain size, and some measure of channel cross-sectional area to determine the tractive force of water on the stream bed. These algorithms are applied to non-cohesive sediments typical of fluvial systems. To better define pipeline threats at a screening level, reducing unnecessary field and analytical expenses, and focusing effort on credible hazards, we developed a flood scour screening tool that uses return period discharge (Q) as the only input requirement. In order to develop the tool, we plotted the results of over 400 detailed scour assessments for several grain sizes (1100 data points) completed in Alberta and British Columbia, in 2017, 2018 and 2019. The results clearly show the importance of channel variability and grain size, but also show definable discharge related trends. We compared the results of the National Engineering Handbook (NEH) and the United States Bureau of Reclamation (USBR) methods, both of which use industry accepted algorithms. We developed, and provided herein, relationships that can be used to screen out scour assessments at watercourse crossings where DOC is already known, or to support and expedite field programs where DOC is being obtained. If only Q is known, then a single graph, or single equation is used for a given region using fine sand as the assumed median grain size. If both Q and median grain size are known, then the user can determine a slightly less conservative result from a series of complementary equations. In all cases, we propose using the mean result of the USBR method, originally intended for design, to fully capture the potential variability in the calculated NEH flood scour. While conservative, the tool is easy to use, and we expect it will substantially reduce the assessment effort on smaller, or less erosive streams.