Flood Monitoring: Evaluating Action Response Time Relative to Warning Time

Flood monitoring is one method currently being used by the pipeline industry to provide alerts when flooding is approaching, or has exceeded, levels that could create hydrotechnical conditions that threaten pipeline integrity. Flood monitoring does not provide protection from hydrotechnical hazards or reduce the probability of failure, but can lower risk by providing advanced warning, allowing operators to initiate actions that reduce the consequences of failure in the rare event that pipeline integrity is threatened by hydrotechnical forces. Pipeline pressure reduction, shut-in, purge, and spill response mobilization are all examples of actions commonly used to reduce failure consequence. However, these actions require time to execute, ranging from a number of hours to a number of days, depending on factors such as site location, valve spacing, and product type. The effectiveness of flood monitoring as a consequence reduction strategy is contingent on having sufficient time to implement the flood response action. In designing a flood monitoring program, it is necessary to ask: can flood monitoring provide sufficient advanced warning for an action plan to be fully executed before pipeline integrity is compromised? The present study evaluated 35 high priority pipeline watercourse crossings, to estimate the flood return periods at which actions could be taken that correspond to warning times of 12, 24, 48, and 72 hours before the critical flood (i.e., a conservative estimate of the flow at which fatigue failure is considered possible) and to evaluate the feasibility of flood monitoring as a short-term risk management strategy prior to mitigation. The 35 crossings are currently scheduled for mitigation and rely on flood monitoring as an interim risk management tool. The rate of increase in flood discharge during all previously recorded flood events at each real-time monitoring gauge was first obtained to estimate the rate of flow increase during the critical flood event. Of the 35 crossings, 33 had a maximum warning time of less than 48 hours. Using a 24-hour warning time, 10 of the 35 crossings have a warning flow of less than a 1 in 5-year flood. The results show that the ‘action initiation flood level’ for more than 90% of the most susceptible watercourse crossings may be too low to be practical; at crossings where more than 48 hours of response time is required, flood monitoring may not significantly reduce hazard consequence as the action response plan may not be fully executed prior to pipeline failure. Pipeline failures are rare, and flood monitoring provides a useful monitoring approach for short-term management in many watercourses. However, these results demonstrate the importance of evaluating the required action response time relative to the available warning time for each watercourse crossing to confirm that flood monitoring will achieve the risk reduction expected by the operator. If flood monitoring is determined to be impractical because the action initiation flood is too low, it may provide justification for initiating other management actions (e.g., flood forecasting, purging prior to the flood season, or elevating such sites on the priority list for physical repairs).

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