US dam break scenarios

Dams provide multiple societal benefits – the ability to store water from high flow events and slowly release it over time allows flood mitigation, as well as drought resilience, provision of water for hydroelectric power generation, agriculture, and water supply. 

Historic events have shown that dams can fail for a wide range of reasons. The critical infrastructure has multiple associated risks, including the failure or break of a dam in response to extreme flow events, natural disasters, or hostile actors. When this occurs, the impact of the resulting downstream flood wave on surrounding habitat and communities is profound. Understanding the full extent of this exposure through in depth and data-driven risk assessments is essential to identify potential failures, their likelihood of occurrence and commence resilience strategies to mitigate the consequences of such failure. In this article, we use data from Fathom’s US Flood Map to explore several potential dam failure scenarios.  

Understanding dam failure

While quantifying the probability of any individual dam failing is very difficult even with the aid of detailed engineering studies, it would be remiss to assume that such failures are so implausible as to be ignorable.

In addition to the possibility of failure due to ‘natural’ causes such as extreme weather events, erosion or  tectonic activity, dams also remain a major national security concern for governments facing possible hostile actors. The US has flirted with disaster a few times in living memory, with the near-failure of the vast Glen Canyon Dam that holds back Lake Powell in 1983 offering the closest brush with a truly apocalyptic dam break the modern US has experienced

In this instance, cavitation in the dam spillway tunnels led to complete erosion of their 3 foot thick concrete linings in places and, had this continued unabated, further erosion may have caused a connection to the reservoir floor resulting in the complete uncontrolled emptying of Lake Powell. What is perhaps most alarming about this event is that it occurred during a relatively small flood along the Colorado River which had an estimated return period of just ~1 in 25 years.  While subsequent engineering works have mitigated against this risk at Glen Canyon, one only has to look back to 2017 and the Oroville dam crisis in California for another example of near-catastrophe.

To give some context to the scale of the impact of dam failures we therefore felt it would be an interesting exercise to compile a database of all large dams in the US and simulate the potential flood footprints associated with a catastrophic failure of each.

The largest dams required unbroken model domains extending many hundreds of kilometers, which at a simulation resolution of 30 meters provided an interesting technical challenge for our hydraulic model frameworks. This completed dataset provides our clients with an interesting and valuable tool for assessing Probable Maximum Losses (PMLs) and other ‘worst case’ realistic disaster scenario metrics.

Now that we’ve seen the impact of dams and dam failure, why is this important for your risk assessment and investment decision-making? Dams play a critical role in water resource management, energy production, and infrastructure development, making them crucial indicators for market players. With water involved in nearly every stage of a business’ supply chain, understanding the impact of potential dam failures is vital for sound investment decisions.

65% of dams in the US are privately held, but frequently asset owners lack the financial resources necessary for adequate dam maintenance. If a dam fails, assets and businesses in every state are at risk.

Therefore, there is significant demand from banks, lenders and investors for granular flood-risk data, including extreme scenarios, for vulnerability assessment, impact analysis and scenario testing. Fathom’s new US Flood Map is the most complete and technologically advanced flood map for the United States. Fathom has deployed a machine learning model trained on all available observations to estimate the influence of all dams on extreme flows nationwide. With all this available at 10 meter resolution across the States, the latest US Flood Map provides the most complete view of flood risk to date. 

Dams, in reservoir form, store water from high flow events and release it slowly over time. This can result in significant reduction of downstream peak flows, therefore reducing downstream flood risk. Therefore, representing these structures is very important in simulating an accurate view of risk. To date, there are no comprehensive datasets defining their impact. 

In our updated US model, we have simulated the impact of major dams in the US on extreme flows downstream of them. This involved using freely available records of flows upstream and downstream of dams, calculating the reduction in extreme flows, and deriving a regression model to relate this reduction with dam characteristics. 

This model could then be applied to any dam in the US with identified dam characteristics, and the degree of extreme flow reduction, or “dam reduction factor” could be estimated. For a given dam, the dam reduction factor was then propagated downstream of the dam to indicate the degree of peak flow reduction, and thus flood defense, provided downstream of the rivers.  

Map results show that high capacity dams on small rivers would reduce peak flows significantly, whereas smaller capacity dams on smaller rivers would have less impact on peak flows. The degree of flow reduction decays as one tracks further downstream of the dam, due to inflows from other tributaries. 

For the largest dams, Fathom’s new US model simulates the impacts of dams on peak flows many 10s of kilometers away from the dam itself. In total, Fathom’s new US model simulates the impact of 1716 dams, nationwide. 

This allows investment, portfolio and asset managers the ability to conduct thorough risk assessments into an asset’s proximity and exposure to dams; highlighting areas that might not be obviously at risk in the event of a dam failure.