Thoughts From Engineers: Designing for the Next Storm
Civil engineers use a range of hydrologic methodologies and software—from the widely used Rational Method to U.S. Army Corps of Engineer’s HEC-HMS to the Environmental Protection Agency’s SWMM and countless others—to model the flow of surface runoff and design stormwater management systems suited to a specific level of acceptable flood risk. So long as rainfall patterns remained consistent with historical Intensity Duration Frequency (IDF) curves, our models—and the resulting infrastructure—held up.
With recent storm events deviating from historic norms, communities around the country are struggling with stormwater infrastructure that now falls short of design objectives. Even a well-planned stormwater system consisting of the most-advanced green infrastructure tools and other time-tested drainage elements can be quickly overwhelmed when baseline data no longer reflect current conditions.
Limitations of Current Stormwater Risk Methodology
In 2023 alone, multiple 24-hour precipitation records (bit.ly/ClimateEvents2023) across the United States were roundly broken. According to the National Oceanic and Atmospheric Agency (NOAA) National Center for Environmental Information, within a 48-hour period on April 12-13, 2023, Fort Lauderdale Airport was hit with more than 25 inches of rain. On July 18-19, 2023, parts of Kentucky and Illinois experienced heavy flash flooding, with Mayfield, Kent., receiving 11.28 inches within a 24-hour period. New York City was deluged with rain on Sept. 29, 2023, with JFK Airport breaking a new record when it recorded nearly 9 inches of rain in 24 hours.
Flood-risk assessment methodologies lose predictive power when baseline assumptions lose credibility. Even under stationary climatic conditions, use of the Rational Method—a simple but widely used three-variable hydrologic model—can be problematic. The methodology has never been effective at volume-based analyses because the focus is on determining design peak flow for a specific point in time. In many cases, the resulting analyses can both overestimate—and, more importantly, underestimate—total stormwater discharge that accumulates during longer durations.
If initial design parameters for a small-scale project are conservatively based on a 10-year storm event with minimal considerations given to influences of climate, land cover or larger-scale flood dynamics, the resulting drainage system will be overwhelmed when hit with a rain event more in line with a 100-year storm. To rely on historic IDF curves—in combination with a stormwater-modeling methodology that may yield oversimplistic results under certain circumstances—is to expose a community to an indeterminate level of flood risk. The release of NOAA’s Atlas 15 is still some years away. With few other alternatives, the default position is to stick with historic data and routine engineering protocols.
Rise of the Climate Model
All indicators point to a need to reconsider our design methodologies and forge a new way forward. Generalized Circulation Models (GCMs), the scientific term for climate models, work by simulating the movement of energy and tracking the impacts of radiative forcing—progressively more-intense levels of solar energy—on other vectors and materials in the atmosphere, ocean and on land. The quality of climate models is always improving, but the trickiest bit involves scaling coarse projections of GCMs to a scale and resolution that can be used by planners and other professionals.
The Climate Mapping for Resilience and Adaptation project makes data and maps from multiple climate models accessible via an interactive platform on the U.S. Climate Resilience Toolkit website (bit.ly/USClimateTools). It translates the projections of different models into a range of potential future climate scenarios; key metrics relating to the most-common climate-driven hazards—temperature, precipitation and others—change relative to the degree to which specific carbon-emission-reduction targets are achieved. Cities and larger metropolitan areas across the United States are using these forecasts and employing other strategies to initiate critical work on the ground, which can include revising engineering design protocols.
My hometown of Madison, Wis., is modifying infrastructure design specifications by increasing recurrence intervals from 25 to 50 years for certain types of stormwater practices. The City of Madison also requires planning for 500-year events (bit.ly/DesignStandardsMadison) in some situations involving new development. New York City has developed future rainfall projections (on.nyc.gov/434ATkO), which are core to new (2022) stormwater regulations and current infrastructure design criteria that are risk-based, particularly with respect to critical infrastructure. The City of Virginia Beach, Va., recently updated stormwater design standards (bit.ly/DesignStandardsCVB) to modify rainfall depth-duration values by 20 percent over NOAA’s Atlas 14 values.
Planning Considerations Going Forward
An American Society of Civil Engineers (ASCE) article by Hathaway et al., “A Synthesis of Climate Change Impacts on Stormwater Management Systems: Designing for Resiliency and Future Challenges” (bit.ly/HathawayASCE), offers guidance for communities in the thick of planning stormwater management systems for future climate conditions. The authors stress that although we clearly can’t eliminate flood risk (we never could), we can minimize it by building resilience and adaptability into our designs.
For example, watershed-scale planning is key to effectively managing hydrological regimes, and it should become standard practice to mitigate flood risk as well as protect water quality. We should try to build multi-purpose objectives into our stormwater systems, including recreational spaces serving as detention areas during wet periods. Use of IoT-enabled devices that monitor incoming floodwater and remotely activate backup systems is yet another consideration.
Obviously, research to fortify resilience in stormwater design needs to ramp up. How do we ensure all components of stormwater infrastructure demonstrate “adaptive design” and continue to operate at the highest levels over time? New York City’s Stormwater Manual explicitly mandates ongoing and continuous management of stormwater systems post-construction—from mulching and weeding plant installations to cleaning drains and performing other important maintenance. Finally, as Hathaway et al. point out, the need for additional guidance going forward can’t be overstated.
Highly variable and site-specific dynamics are at work with each model a hydrologist works on; in all honesty, at times it can feel more like we’re practicing an art rather than a science. Notwithstanding the complexities inherent in the field of hydrology, hydrologists and the communities they serve would benefit from clearer standards relating to future flood risks. Without such universal standards, a patchwork of projects designed and built with varying degrees of engineering rigor and climate-resilient planning will take shape across the country. We will then ask ourselves: How many of those projects will survive the next storm? More importantly, wouldn’t we want to be sure they do? 
About Chris Maeder
Chris Maeder, P.E., M.S., CFM, is engineering director at CivilGEO Inc.; email: [email protected].
