Dam Safety 101: What Hazard Potential Really Means

Dam Safety 101: What hazard potential really means

Every dam in the United States that meets the National Inventory of Dams inclusion criteria carries a hazard potential classification. This classification is one of the most important and most misunderstood concepts in dam safety. It does not describe the condition of the dam. It does not predict whether the dam will fail. It describes what would happen downstream if the dam did fail. Understanding this distinction is essential for communities, dam owners, emergency managers, and anyone who wants to make sense of dam safety data.

This guide explains the three levels of hazard potential classification used in the United States, how they are determined, how they differ from condition assessments, and what they mean for the communities that live downstream of dams.

Hazard potential versus risk

Before diving into the classification levels, it is important to understand the difference between hazard potential and risk. In dam safety terminology, these are distinct concepts that are often confused.

Hazard potential describes the consequences of dam failure. It answers the question: "If this dam were to fail, what would happen downstream?" The answer depends on what exists in the area that would be flooded, including homes, businesses, roads, utilities, and people. A dam with high hazard potential is one where failure would probably cause loss of human life.

Risk combines the probability of failure with the consequences of failure. It answers the question: "How likely is this dam to fail, and how bad would it be if it did?" Risk assessment requires detailed engineering analysis of the dam's condition, structural integrity, hydrologic capacity, and seismic vulnerability, in addition to the downstream consequence analysis that drives hazard potential classification.

The NID records hazard potential, not risk. This means that a dam in pristine condition and a dam with serious structural deficiencies can carry the same hazard potential classification if the downstream consequences of failure would be similar. The hazard potential classification tells you about the stakes. The condition assessment and engineering analysis tell you about the probability. Both are needed to understand the full picture of dam safety.

The three NID hazard levels

The Federal Guidelines for Dam Safety, published by the Federal Emergency Management Agency, define three levels of hazard potential. These definitions are used by the NID and by most state dam safety programs, although some states have additional subcategories or slightly different terminology.

High hazard potential

A dam is classified as high hazard potential when failure or mis-operation is expected to cause loss of human life. This is the most consequential classification a dam can carry, and it triggers the most stringent regulatory requirements.

The classification is based on the presence of people in the dam-breach inundation zone, which is the area that would be flooded if the dam failed. If there are permanent residences, hospitals, schools, or other occupied structures in the inundation zone, the dam will almost certainly be classified as high hazard potential. Frequently traveled roads in the inundation zone can also drive a high hazard classification if people could be caught in floodwaters before they could evacuate.

High hazard potential dams are subject to the most rigorous regulatory requirements in most states. These typically include:

• More frequent inspections, often annually or every two years
• Mandatory emergency action plans (EAPs) that describe notification procedures, evacuation routes, and coordination with emergency management agencies
• Requirements for instrumentation and monitoring, such as seepage measurement devices, survey monuments, and in some cases, real-time monitoring systems
• Higher design standards for spillway capacity and structural integrity
• More detailed engineering analyses, including dam-breach inundation studies and probable maximum flood (PMF) analyses

As of the most recent NID data, there are more than 15,000 high hazard potential dams in the United States. This number has been steadily increasing, not because more dams are being built, but because downstream development continues to expand into areas below existing dams. A dam that was classified as low hazard when it was built 50 years ago may now be high hazard because a housing development, school, or highway has been constructed in the area that would flood if the dam failed.

For a deeper look at the implications of high hazard classifications, including specific examples and community response strategies, see our detailed article on understanding high hazard potential dams.

Significant hazard potential

A dam is classified as significant hazard potential when failure or mis-operation would not be expected to cause loss of human life but could cause significant economic loss, environmental damage, or disruption of lifeline facilities. Lifeline facilities include roads, railroads, water treatment plants, power plants, and communication infrastructure that communities depend on.

The significant hazard classification occupies a middle ground between high and low hazard. Dams in this category may have commercial or industrial property, important transportation routes, or environmentally sensitive areas in their inundation zones, but not occupied residential structures or other locations where people would be at risk.

Regulatory requirements for significant hazard potential dams vary by state but are generally less stringent than for high hazard dams. Many states require inspections every three to five years for significant hazard dams and may require emergency action plans, although this varies. Design standards for spillway capacity and structural integrity are typically lower than for high hazard dams but higher than for low hazard dams.

The significant hazard category is important because it captures dams that could cause serious harm even though they are not expected to kill anyone. A dam failure that destroys a major highway, contaminates a drinking water supply, or floods a commercial district can cause millions of dollars in damage and significant disruption to the affected community, even without loss of life.

Low hazard potential

A dam is classified as low hazard potential when failure or mis-operation would result in no expected loss of human life and only low economic or environmental losses. Losses are generally limited to the dam owner's property and to rural or undeveloped land in the inundation zone.

Low hazard dams receive the least regulatory attention. In many states, they are inspected infrequently or not at all. Some states exempt low hazard dams from their dam safety programs entirely. Emergency action plans are generally not required for low hazard dams. Design standards are the least stringent of the three categories.

The majority of dams in the NID are classified as low hazard potential. This reflects the fact that most American dams are small earth structures in rural areas, often built for farm ponds, stock watering, or small recreational lakes, where there is little or nothing downstream that would be damaged by a failure.

How hazard potential is determined

Hazard potential classification is based on an analysis of what exists downstream of the dam in the area that would be flooded if the dam failed. This analysis has two main components: dam-breach analysis and downstream consequence assessment.

Dam-breach analysis

A dam-breach analysis models what would happen if the dam failed suddenly and completely. Engineers use computer models to simulate the resulting flood wave as it moves downstream. The models calculate the depth, velocity, and timing of floodwaters at various points along the downstream valley. The output of the analysis is a dam-breach inundation map that shows the area that would be flooded and the depth of flooding at each point.

The inundation map is the foundation of hazard potential classification. If the map shows that floodwaters would reach occupied structures, the dam will be classified as high hazard potential. If the map shows that floodwaters would reach significant infrastructure or property but not occupied structures, the dam will be classified as significant hazard potential. If the map shows that floodwaters would be confined to undeveloped or low-value land, the dam will be classified as low hazard potential.

Downstream consequence assessment

Once the inundation zone has been mapped, the downstream area is assessed to determine what would be affected by the floodwaters. This assessment considers:

• Residential structures, including permanent homes, mobile homes, and seasonal cabins
• Commercial and industrial facilities
• Public buildings such as schools, hospitals, and emergency facilities
• Transportation infrastructure including roads, bridges, and railroads
• Utility infrastructure including water treatment plants, power plants, and pipelines
• Environmental resources including wetlands, endangered species habitat, and water bodies
• Agricultural land and livestock
• Recreational areas including parks and campgrounds

The assessment also considers the population at risk, which is the number of people who would be in the inundation zone at various times of day and year. A dam above a campground, for example, might have a different population at risk during summer than during winter.

Inundation mapping

Inundation mapping is a critical tool for hazard potential classification and emergency planning. Modern inundation maps use digital elevation models, geographic information systems, and hydraulic modeling software to produce detailed maps of the areas that would be flooded under various dam failure scenarios.

These maps serve multiple purposes beyond hazard potential classification. They are used to develop emergency action plans, including evacuation routes and notification procedures. They inform land use planning by showing developers and zoning officials which areas are at risk from dam failure. And they help emergency managers prepare for dam incidents by identifying which roads, bridges, and facilities would be affected.

Not all dams in the NID have current inundation maps. For some older, smaller dams, the hazard potential classification may have been assigned based on a desktop review of downstream conditions rather than a detailed engineering analysis. As state dam safety programs update their inventories, many of these dams are receiving updated analyses and, in some cases, reclassified hazard ratings.

How hazard potential differs from condition assessment

This is perhaps the most important concept in dam safety, and it bears repeating: hazard potential and condition assessment are completely independent classifications that measure different things.

Hazard potential measures the consequences of failure. It is driven by what exists downstream. A brand-new dam built to the highest engineering standards can still be classified as high hazard potential if there are people living below it.

Condition assessment measures the current physical state of the dam. It is driven by the results of inspections and engineering evaluations. A dam can receive an unsatisfactory condition rating, indicating serious safety deficiencies, while being classified as low hazard potential if there is nothing downstream at risk.

The combination of the two classifications provides a more complete picture of dam safety:

• A dam with high hazard potential and satisfactory condition is one where the stakes are high but the probability of failure is low. These dams need continued monitoring and maintenance to keep them in good condition.
• A dam with high hazard potential and poor condition is the most urgent safety concern. The stakes are high and the probability of failure is elevated. These dams need immediate attention.
• A dam with low hazard potential and poor condition may need repair, but the urgency is lower because failure would cause limited damage.
• A dam with low hazard potential and satisfactory condition requires the least attention from a safety perspective.

The distinction matters enormously for resource allocation. Dam safety programs have limited budgets and staff, and they must prioritize their efforts. The combination of hazard potential and condition helps them focus on the dams that pose the greatest risk to public safety.

Why hazard potential changes over time

Hazard potential is not a permanent classification. It can change over time as conditions downstream of the dam change. The most common reason for hazard potential reclassification is downstream development. When new homes, businesses, roads, or other structures are built in the area that would be flooded if the dam failed, the consequences of failure increase, and the hazard potential classification must be updated to reflect the new reality.

This phenomenon is one of the most significant challenges in dam safety. Dams that were built in rural areas decades ago may now have subdivisions, shopping centers, and schools downstream. The dam has not changed, but the world around it has. When the hazard potential classification is updated from low to significant or from significant to high, the dam owner suddenly faces new regulatory requirements, including more frequent inspections, mandatory emergency action plans, and potentially expensive structural upgrades to meet higher design standards.

For private dam owners, this reclassification can be a financial shock. A farmer who built a small earth dam for a stock pond in the 1960s may discover that the subdivision built downstream in the 2000s has turned the dam into a high hazard potential structure with all the regulatory requirements that entails. Some states have programs to help private dam owners with the costs of compliance, but many do not.

Hazard potential can also change in the other direction, although this is less common. Dam removal, changes in land use that reduce downstream population, or the construction of flood-protection infrastructure downstream can all lead to a lower hazard potential classification.

Emergency action plans

One of the most important consequences of a high hazard potential classification is the requirement for an emergency action plan (EAP). An EAP is a document that describes the actions to be taken if a dam-safety emergency occurs, including notification procedures, evacuation plans, and coordination with emergency management agencies.

What an EAP contains

A well-developed EAP typically includes:

• A notification flowchart showing who should be contacted and in what order when a dam emergency is detected
• Contact information for the dam owner, dam operator, state dam safety officials, local emergency management agencies, law enforcement, and downstream communities
• Inundation maps showing the areas that would be flooded under various failure scenarios
• Estimated flood arrival times at key downstream locations
• Descriptions of pre-planned actions to be taken at the dam during various emergency levels
• Evacuation routes for downstream communities
• Procedures for coordinating with the National Weather Service and other agencies

EAP gaps

Despite the critical importance of emergency action plans, not all high hazard potential dams in the NID have them. As of the most recent data, a significant percentage of high hazard potential dams still lack EAPs. This is a persistent challenge that state dam safety programs are working to address, but progress has been slow in some states due to limited staff and funding.

The absence of an EAP does not mean the dam is more likely to fail. It means that if the dam does fail, the emergency response is likely to be slower and less coordinated, which could increase the loss of life. EAP development is one of the most cost-effective dam safety investments because it improves emergency response without requiring any physical modifications to the dam.

What communities should know

For people who live, work, or travel in areas downstream of dams, understanding hazard potential is an important part of personal preparedness and community resilience.

Know your dams

The first step is knowing which dams are upstream of your community. The NID provides a searchable database that allows you to find dams by location, and many state dam safety programs maintain their own databases with additional details. If you live downstream of a high hazard potential dam, there should be an emergency action plan that includes information about how you would be notified and where you should go in the event of a dam emergency.

Understand the warning signs

Dam failures can happen suddenly, especially in the case of earthen dams that fail by internal erosion or overtopping. Warning signs that dam owners and downstream residents should be aware of include:

• Unusual seepage or wet areas on the downstream face of the dam or at its base
• Sinkholes, depressions, or cracking on the dam crest or slopes
• Slides or slope failures on the dam embankment
• Unusual sounds such as rushing water where none should be present
• Sudden changes in the reservoir level without explanation
• Muddy or discolored seepage, which can indicate internal erosion

If you observe any of these signs at a dam, contact the dam owner, your local emergency management agency, or your state dam safety program immediately.

Be prepared

Communities downstream of high hazard potential dams should treat dam failure as a potential emergency, similar to a flood, tornado, or earthquake. Preparedness measures include:

• Knowing your evacuation route and having a plan to move quickly to higher ground
• Signing up for local emergency notification systems
• Keeping an emergency kit with essential supplies
• Participating in community exercises or drills related to dam emergencies
• Staying informed about the condition of dams in your area through NID data and state dam safety resources

How the NID records hazard data

The NID records hazard potential classification as a single field for each dam, using the three-level system described above. The classification in the NID reflects the most recent determination by the responsible regulatory authority, which may be a state dam safety program, the Federal Energy Regulatory Commission, or a federal dam-owning agency.

The NID also records whether each dam has an emergency action plan. This binary field (yes or no) is important for identifying gaps in emergency preparedness across the national dam portfolio. Additionally, the NID records the date of the most recent inspection and the condition assessment, which provide context for interpreting the hazard potential classification.

For more information about how to access and interpret the full range of data available in NID dam records, see our guide on reading and understanding dam records.

The evolving landscape of dam safety

Dam safety in the United States continues to evolve as the profession develops better tools, methods, and standards. Risk-informed decision-making, which combines hazard potential with detailed probability analysis, is increasingly supplementing the traditional hazard-potential-based approach. New technologies for dam monitoring, including remote sensing, automated instrumentation, and data analytics, are improving the ability to detect problems before they become emergencies.

At the same time, the challenges are growing. Downstream development continues to increase the consequences of potential dam failures. Climate change is altering the hydrologic conditions that dams were designed to handle. And the aging of the national dam portfolio means that more dams are reaching the end of their design lives and need rehabilitation or replacement.

Understanding hazard potential is the first step in navigating this complex landscape. It provides a clear, consequence-based framework for prioritizing safety efforts and communicating risk to the public. Whether you are a dam owner, an emergency manager, a policymaker, or a community member, knowing what hazard potential means and how it affects the dams in your area is fundamental to dam safety literacy.