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Managing dam hazard in changing urban spaces
Dam owners and regulators must be constantly vigilant for the creeping hazard of a changing urban landscape. With downstream housing and commercial development increasing the demands on inspections, and resources constrained, regulators are turning to new digital tools to help them reduce risks and keep people safe
FOR DAM OWNERS AND the regulatory agencies responsible for ensuring dam safety, risk isn’t a static problem but rather a dynamic challenge that must be regularly assessed as conditions change. So-called hazard creep can occur for a number of reasons, such as climate change or aging assets for example. However, hazards can also evolve over much shorter time scales.
One of the most significant issues facing dam owners and regulators in managing hazard creep is urban development. As the global population expands and homes and commercial spaces are developed in once-rural areas, new communities can start to encroach downstream of hydraulic structures and within floodplains or inundation zones. As Association of State Dam Safety Officials (ASDSO) President Bill McCormick notes: “Generally there is much more urbanization. That’s certainly an issue and I think it’s pretty common across the US.”
New buildings can spring up quickly and where these appear in the inundation area of a dam this may have a major impact on the hazard classification of such a structure. Indeed, the latest Infrastructure Report Card from the American Society of Civil Engineers notes that over the last 20 years the number of high hazard dams has more than doubled across the US as development encroaches on dams and reservoirs that were once rural but now feature downstream construction.
Changing dam hazard classification Dams are broadly categorised as low hazard, a
significant hazard or high hazard depending on their size and hazard potential, such as the risk of loss of life in the event of a failure. The management regime associated with these classifications is radically different. “Whereas previously, a dam could overtop and fail and the damages would be minimal. With risk to human life the owners have to invest in more protection for their dam which can be very costly. Then there’s certainly a heightened monitoring and surveillance requirement for high hazard dams with more frequent inspections or installation of monitoring instrumentation to keep a closer eye on things,” says McCormick. An inspection of a low hazard structure is required only once every five or six years whereas for high hazard structures the requirement is for an annual inspection and owners must also develop and share an emergency action plan (EAP). In the US these plans are mandated by State Dam Safety programs and the Federal Energy Regulatory
10 | June 2022 |
www.waterpowermagazine.com
Commission (FERC) among others and formally detail potential emergency conditions at high and significant hazard dams. EAP’s also detail specific pre-planned actions that are to be followed in the event of an emergency. They are designed to minimise damage to property and any loss of life. The EAP also contains procedures to assist the dam owner in issuing early warning and notification messages to downstream emergency management authorities as well as inundation maps to show the critical areas for action in case of an emergency. Of the more than 91,000 dams in the US, around 15,600 are currently classified as high-hazard structures. The number of high-hazard-potential dams has increased as has the overall percentage of these dams with an EAP. As of 2018, 81% of the country’s high hazard dams had a plan on file, a 5% increase from the 2015 figure according to the ASDSO. In addition, for high hazard structures, owners usually invest more on producing flood maps and recording inundation zones, while for low hazard dams this assessment is often a more ad hoc approach as the potential consequences are much smaller. Nonetheless, in order to ensure dams are
appropriately classified and that any necessary EAPs are in place some states require dam owners to self- report changes. In most states regulatory bodies are required to keep abreast of urban developments and any new houses or commercial buildings that have been constructed downstream of a dam and in a potential inundation zone. A critical step in the periodic inspection of low and significant hazard dams a review of new downstream development. When inspectors visit dams they also strive to identify any new structures or dwellings that could increase the consequences of failure, and therefore the hazard classification.
Walking the walk Historically, this assessment of urban development has
been conducted by the regulatory inspectors who are responsible for assessing dam conditions and safety. State Dam Safety Programs hold regulatory authority over nearly 70% of all the 91,000 dams listed in the US and are responsible for inspecting existing dams, overseeing remediation of deficient dams, and working with local officials and dam owners on emergency preparedness. Changes to dam classifications are assessed through physical inspections and by experts walking an inundation zone to determine if hazard conditions have changed. However, this potentially represents a knowledge
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