By Capt. Jedidiah Langlois, USAF, and Christopher Chini, Ph.D.
The U.S. Air Force relies on installations to project air power in support of foreign policy. Mission requirements, logistics, and geographic proximity require many air bases to be located adjacent to bodies of water. Whether ocean, river, or lake, these bodies of water carry inherent risks to the neighboring bases and airfields, with an acutely escalating risk being sea level rise.
Warmer temperatures cause sea level rise as ice melts in the regions surrounding the Earth’s northern and southern poles. The United Nations Intergovernmental Panel on Climate Change predicts that by 2100 the global mean sea level will rise between 0.25-m and 1.1-m, based on different climate scenarios. However, there are regional differences in how this global mean sea level will be observed, with some locations projecting higher impacts due to local conditions. The amount of sea level rise is dependent on the global emissions scenarios; faster reduction of greenhouse gases will mitigate overall sea level rise.
To explore the effect of a rising sea on airfields, a case study was conducted of Tyndall AFB, Fla. The installation’s stormwater system is currently under redesign, with the planned systems intended to service the entirety of the airfield and be resilient to extreme sea level rise and rainfall.
By using the Environmental Protection Agency’s stormwater management model to develop a planned system on a portion of the airfield currently without a stormwater system, the future impact and resilience of the system could be explored.
SYSTEM CHARACTERISTICS
Located on a peninsula bounded by the Gulf of Mexico and East Bay, Tyndall AFB has high projections of sea level rise possible by 2100 and already a history of significant exposure to strong storms, such as Hurricane Michael in 2018, which caused hundreds of millions of dollars in damages and called attention to the base’s potential vulnerability to climate change. Tyndall has become a focal point for the Defense Department’s push to strengthen resilience and sustain mission readiness against more severe and frequent extreme weather.
Relative sea level rise projections for Tyndall AFB were compiled in 2016 by the Strategic Environmental Research & Development Program and the Environmental Security Technology Certification Program. Under the low scenario, only 0.1-m of sea level rise is anticipated by the end of the 21st century. However, this projected amount is extremely variable, with high scenarios predicting up to 2.1-m of sea level rise by 2100, and 0.9-m by 2065.
The stormwater system chosen for modeling would service the majority of the Bravo Taxiway at Tyndall and a portion of one of the main runways. Stormwater flows from the terminus of the system, a 6-ft by 3-ft box culvert at a height of 10-cm above sea level, and into 1.2-km of ditch and canal before emptying into East Bay through Fred Bayou.
The planned system was modeled in the stormwater management model using construction drawings of the overall airfield stormwater system renovation. The culverts/conduits, inlets, and outfall of the system were built using these drawings, with 27 conduits and 28 inlets created. Inlets are referred to and labelled as nodes. Drainage subcatchment areas and boundaries were estimated using the drawings, with 51 subcatchments created.
TESTING AND RESULTS
To test the model in conjunction with the projected sea level rise values, model storms were generated. Intensity-duration-frequency curves and mass rainfall curves from the Florida Department of Transportation Drainage Manual were used to create 10-, 25-, 50-, and 100-year 24-hour return storms for the area of the base. Furthermore, the 100-year storm was scaled in increments of 10 percent to create stronger return storms to test the model, up to 50 percent higher than the current 100-year storm. Finally, sea level rise projections were applied to the model as a fixed outfall height in order to represent standing water at the system terminus. Every return storm was applied to each sea level rise projection (from no sea level rise to 2.1-m, moving in 0.2-m increments), for a total of 153 simulations.
Simulations using the stormwater management model produce many indicators of system performance throughout the duration of a simulated storm. Of the properties available to monitor, duration and volume of flooding from inlets, duration of inlet surcharge, and duration of conduit limitation from surcharge were chosen to interpret the overall performance and potential failure of the system. These properties were monitored at the last several conduits and inlets of the stormwater system. These nodes were selected due to their relatively early failure and proximity to airfield pavement.
Throughout the simulations, the system experienced limitations even with no sea level rise, with downstream conduits experiencing higher impacts at lower return storms. However, 1-m of sea level rise was a limiting factor in nearly every simulation, with 100-year storm events and greater posing limitations on the system performance.
A major indicator of system performance (node surcharge) indicates significant impacts on the stormwater system from high sea level rise and in more extreme storms. The three most downstream system inlets are representative of the overall system performance and were surcharged in all storms at the 1.2-m sea level rise mark, with surcharge occurring at 1-m of sea level rise in higher storms.
Using the results of the analysis, a risk matrix was generated to assess the threat of sea level rise against storm events. Utilizing rankings of minimal, low, moderate, and high, the risk to flooding was highlighted for each combination of return storm and sea level rise. Overall, 1-m of sea level rise can be seen as the system threshold before the system becomes stressed by lower return year storms. Widespread system stress could be expected at sea level rise of over 1-m, which correlates to 2065 under highest emissions scenarios and 2100 under medium emissions scenarios. Significant system stress and failure could be expected at sea level rise of 1.4-m and higher, which correlates to 2100 under moderate to high emissions scenarios.
In general, sea level rise influences the system performance more than rainfall.
ADDRESSING VULNERABILITIES
Stormwater system inundation from sea level rise would have many effects. Loading from surcharge places stress on the system and could result in premature degradation or failure. The salinity of the seawater also could accelerate system component degradation. Recurring ponding would hinder airfield vegetation cutting and could attract wildlife dangerous to airfield operations. Ponding from inlets on interior grass could spread to airfield pavements and hinder or halt airfield operations.
To avoid these negative effects, Tyndall could install flap gates on system outfalls to prevent backflow from high tailwater. The base could consider offshore pumps as sea levels continue to facilitate airfield drainage under rainfall events. There is precedent for this stormwater management tactic within the Air Force. A series of stormwater pumps were installed at Langley AFB, Va., following Hurricane Isabel in 2004.
Overall, this study serves to indicate timeframes and emissions scenarios in which the proposed stormwater system at Tyndall could be vulnerable, and when mitigation could be considered for implementation. Future work will entail the installation of stormwater meters at existing outfalls and collaboration with the Air Force Civil Engineer Center’s Program Management Office to ensure that the model matches existing site conditions.
Additionally, this study can be enhanced with rainfall amounts specific to the base from the National Oceanic & Atmospheric Administration, and using shorter duration storms. Work also can be furthered through the incorporation of elevation data to analyze the resulting flow of ponded stormwater.
Capt. Jedidiah Langlois, USAF, is Graduate Student, and Christopher Chini, Ph.D., is Assistant Professor of Engineering Management, Air Force Institute of Technology. They can be reached at jedidiah.langlois.1@us.af.mil; and christopher.chini@afit.edu.
[This article first published in the July-August 2022 issue of The Military Engineer.]
