By D.J. Rasmussen, Ph.D., Aff.M.ASCE

Despite the increasing threat of higher sea levels and intense storms, the allure of coastal living continues to drive population growth along the U.S. coastline. The American desire for seaside living, particularly in the warmer southern coastal regions, appears unshaken by more frequent high tide flooding, repeat damaging storms, and rising insurance premiums.

The elevation of a hospital, school, data center, or other facility is a key parameter in determining its flood risk. The higher the elevation, the lower the probability a structure will flood at some point over its intended lifetime. Conventional approaches for selecting a facility design elevation include designing to the Federal Emergency Management Agency-defined elevation that has 1% chance each year of getting wet from floodwaters (the base flood elevation). Often an additional safety buffer is added, sometimes referred to as freeboard. For many, the 1-in-100 annual chance event probably seems like it is very unlikely to occur, but they might be surprised to learn that over time, probabilities can compound and seemingly low-probability events can reach eyebrow-raising likelihoods.

Consider the growth of $1,000 over a 50-year term. At an interest rate of 4% and compounded annually, by the end of the term this amount grows to over $7,100. The same compounding principle applies to the 1% annual chance flood event; over 50 years, the probabilities compound, and ultimately there is a 39.5% chance of at least one event where floodwaters reach the base flood elevation. Risk-averse stakeholders may want to select a design flood elevation that results in a much lower likelihood of floods.

But that’s not all. For some hazards, climate change is making the historical 1% annual chance event more likely over time. For example, rising sea levels raise the baseline from which floods have historically occurred. For some cities along the eastern U.S. coastline, like Norfolk, Virginia, sea-level rise is expected to make the historical 1% annual chance event 60% more common in 50 years, the end of the intended life for many new structures being built now.

Returning to our compound growth example, if the interest rate of our initial $1,000 investment increased over the 50-year term from 4% to 10% (i.e., the rate increasing each year, like the infamous adjustable-rate mortgages that contributed to the 2008 financial crisis), our final amount would be $10,000 more than if the interest rate had stayed at 4% (a final total of over $17,000).

Analogously, rising background probabilities of hazard events that compound can result in uncomfortably high likelihoods. In Norfolk, when considering a background 3 ft of sea-level rise, a facility designed to the historical 1% annual chance event would have roughly a 50-50 chance of experiencing at least one flood over its 50-year design life. A coin flip is surely an unacceptable risk for most decision-makers.

How to select a design flood elevation in this dynamic landscape of changing probabilities? Fortunately, engineers and climate scientists have teamed up to develop ways to account for changing hazard likelihoods over time — termed “non-stationarity.”

In one approach, researchers Jose D. Salas and Jayantha Obeysekera derived a simple equation that can be used to calculate the probability of at least one hazard event over a given number of years. They call it the “risk of failure” equation. Assuming future flood probabilities are available, practitioners can input multiple design flood elevations to see how the likelihood of one or more flood events changes. Seeing a list of multiple hypothetical design flood elevations and the associated likelihood of at least one flood event can provide intuitive risk-informed decision-making.

The risk of failure equation also appears in a U.S. government interagency report on sea-level rise to provide guidance on how to incorporate sea-level rise into engineering design. While this approach is limited in that it does not account for uncertainty in the rate of local sea-level rise, it provides a practical way forward that is accessible to a diverse field of practitioners — a key prerequisite for change in engineering design procedures.

University curricula, professional organizations, and licensing exams are all key avenues for encouraging the adoption of non-stationary design considerations. ASCE is already upgrading its design standards to better protect structures from rising sea levels in its flood supplement to ASCE 7.

While Americans continue to flock to coastlines, engineers, planners, and other practitioners should adopt non-stationary design approaches to minimize damage, ensure operational continuity and quick recovery, and improve overall societal well-being.

D.J. Rasmussen, Ph.D., Aff.M.ASCE, is an applied climate scientist in the Earth & Environment hub in the San Diego office of WSP USA.

This article is published by Civil Engineering Online. It is based on a conference paper, “A non-stationary, risk-based approach for determining design flood elevations under sea-level rise” presented at the 2023 ASCE INSPIRE Conference.

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