FERC releases new water conveyance guidelines, keyword is ‘proactive’
Failures of water conveyance structures have become too frequent and occur with an increasing degree of consequences. We’ve learned that the key to minimizing the impact of these incidents is proactive action by dam owners and operators. While each type of conveyance has specific failure modes and prudent defensive measures, a common theme is the need for personnel to be able to identify a failure and act quickly to minimize the consequences.
In August, the Federal Energy Regulatory Commission released Chapter 12 – Water Conveyance of the Engineering Guidelines for the Evaluation of Hydropower Projects. This new chapter provides owners and operators of hydropower water conveyance structures – penstocks, canals, flumes or tunnels – with guidance for the inspection, monitoring and evaluation of the safety of these structures. Below are a few key points.
To mitigate internal pressure build-up and vacuum failure, implement surge tanks, air valves and/or long valve or gate closure times. Also, in the event of a failure, are the controls to the upstream inlet easily accessible? For example, if the access road is washed out due to the failure, it will be difficult to manually close the gate.
The FERC recommends that above ground penstocks be inspected at least once a year. Inspectors should look for leakage, missing rivets, settlement and missing steel bands. Additionally, ultrasonic shell thickness measurements should be conducted every 10 years. At Mead & Hunt we recommend using drones to monitor inaccessible areas of penstocks.
Even if your dam is classified as low hazard, you may have a high hazard power canal. A high hazard canal is contained by an embankment or wall with water elevations more than the surrounding area that contain population. The canal freeboard should be evaluated under all conditions including extreme flood events. Piping through canal embankments may be caused by tree roots or animal dens.
The FERC recommends that low hazard canals be inspected visually at least once per quarter. High hazard portions of the canal should be inspected more often. Residents downstream of the canal should be provided information about issues related to an upstream canal upstream and the consequences of a potential failure. I typically communicate this information during the Emergency Action Plan exercise.
Instrumentation used on dam embankments may be appropriate for canals. These canals should be included in the hydropower project’s EAP.
Flume foundations are susceptible to erosion and landslides. Wooden flumes are also susceptible to fire, insects and wood rot.
The FERC recommends that access throughout the flume alignment is necessary for inspection. This the inspection should occur annually. However, if the flume is supported on weak foundations and there are residents nearby, those foundations should be inspected more frequently.
If the tunnel is dewatered too quickly, the hydrostatic pressure surrounding the tunnel can result in failure of the tunnel walls. Mitigate this situation with proper air venting that is inspected and maintained, and dewatering should be done slowly under controlled conditions. Conversely, sometimes dewatering of the tunnel can cause more harm than good. Consider the use of a remotely operated vehicle for inspection prior to dewatering.
Of the four types of water conveyance structures, flumes and canals pose the least risk because they are mostly visible and accessible for inspection. However, exterior influences such as land slides or rock falls can increase the probability of failures. Because they are typically under pressure, penstocks can pose the greatest risk and most significant damage and should be closely monitored.
The February 2017 Oroville incident taught us that dam safety must also include appurtenant structures including water conveyance. Proactive safety measures will mitigate the risk, and this new FERC document will help hydropower owners and operators with some guidance for just that.
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