By Robert Barrett, M.SAME, Anhthu Nguyen, M.SAME, and Kandi Brown, M.SAME
Located alongside the Chesapeake Bay, adjacent to Newport News and Hampton Roads, Va., Joint Base Langley-Eustis serves as the headquarters of Air Combat Command. The base, in its original forms, is also more than a century old (its forerunner installations—Langley Airfield and Fort Eustis—were established in 1916; they merged as part of the 2009 Base Realignment and Closure).
Because of the dueling realities of the installation’s modern mission, historic legacy, and geographical location, it is confronting a unique set of challenges: primarily, how to preserve its natural and built infrastructure while facing the uncertainty of climate change.
Over the last 10 years, Joint Base Langley-Eustis has endured damage from six separate climate-related events. From 2003 to 2011, the base experienced three storms with surge exceeding 7.5-ft. In response to these storm and flooding events, numerous best management practices (BMPs) have been installed, including a living shoreline; changing first floor building code elevations to 10.5-ft; purchasing door dams; installing bathtub basements; providing vacuum sanitary sewers; elevating all critical infrastructure; and installing a new airfield drainage project. And while these measures reflect a cumulative approach to stormwater management, important questions remained on the suitability and performance of further proposed practices.
This effort identified the need for additional BMPs—including five seawalls and two tidal gates—to address the anticipated sea level rise over the next 10 years and the current nuisance flooding it will exacerbate.
Predictive modeling identified sea level rise as the most challenging impact within the next 10 years. The area around Langley-Eustis has seen a relative sea level rise of 1.45-ft over the past 100 years—one of the largest documented changes in the world, and one that also shows no signs of diminishing in the foreseeable future.
In 2014, a study documented that the nearby city of Hampton is second only to New Orleans in its potential to experience impacts from flooding. So it is not surprising that the anticipated 1- to 1.7-ft of sea level rise and the current nuisance flooding will exacerbate the potential to impact operations. The base will not only be impacted by sea level rise directly, but also be adversely impacted by neighboring responses to sea level rise.
The vulnerability of Joint Base Langley-Eustis to future climate change impacts was assessed through a 2D unsteady state model, which was developed using the River Analysis System (RAS) model of the U.S. Army Corps of Engineers’ Hydrologic Engineering Center (HEC). The model is capable of simulating surface water flow generated by rainfall and tidal surge events over land surfaces and through complex systems of natural or man-made open channels and floodplains. It also allows for the modeling of numerous BMPs, including pumping systems, and can produce scenario-based time series predictions of flood depth, water surface elevation, and inundation boundaries.
Ensuring accurate modeling. The accuracy of the HEC-RAS model was assessed by comparing results to other predictive models previously developed for the base, including the Storm Surge and Inundation Model of the National Aeronautics & Space Administration (NASA). Specifically, the HEC-RAS output was compared to the water surface elevation in buildings during Hurricane Irene predicted by the NASA model. The NASA model forecasted a water surface elevation of approximately 5.93- to 5.95-ft inside select buildings during Hurricane Irene; the HEC-RAS model predicted elevations of 5.92-ft for the same buildings, showing good correlation.
The spatial extent of the model was based on the U.S. Geological Survey Hydrologic Unit Code 12 Watershed Drainage Boundaries (Northwest Branch Back River and Southwest Branch Back River) that encompass the installation. The boundaries were modified to capture only areas that flow toward and around the base. To create the model, the spatial extent was generally discretized into 100-ft by 100-ft grids. In key hydrological/hydraulic areas, grid sizes as small as 5-ft by 10-ft were used. To add in on-base and surrounding area land use, elevation data from two sources were used: a 0.25-m LiDAR of the base from 2017 and a 1-m LiDAR of the region from 2019.
Rainfall data was obtained from amounts published in the National Oceanic & Atmospheric Administration Atlas 14. However, city design standards have found that the published amounts underestimate the measured totals. Therefore, published values were increased by 20 percent. The rainfall totals were distributed based on the Type II rainfall distribution at 6-min intervals.
Tidal/surge data was obtained from the Sewell Point Station (part of the National Ocean Service Water Level Observation Network). Data was extracted at hourly time intervals using NAVD88 (ft) datum.
PUTTING IT INTO PRACTICE
Once created, the HEC-RAS model was used to evaluate various flooding scenarios under the existing conditions and with proposed BMPs installed. Based on observed surface flow and drainage patterns, the base was divided into five Hydraulic Management Units for analysis of modeling scenarios and performance. The research considered several short- and long-term scenarios of tidal flooding; tidal flooding coupled with rain events (2-, 100-, and 500-year); future tidal flooding coupled with sea level rise; sea level rise coupled with rain events; storm surge; and storm surge coupled with rainfall events.
The suitability and performance of the proposed BMPs were evaluated by modeling the base with all existing and proposed measures installed. This effort identified the need for additional BMPs—including five seawalls and two tidal gates—to address the anticipated sea level rise over the next 10 years and the current nuisance flooding it will exacerbate. Once the proposed BMPs were modeled, a team of experts conducted a facilitated focused conversation and reviewed the efficacy of each one. The group used three distinct ranking exercises within and across the Hydraulic Management Units to build consensus. The group’s preferences may be correlated to future funding priorities.
Designing for adaptability. The proposed BMPs will be conservatively designed. This should reduce the potential for waste, support future expansion to deal with climate change uncertainty, and align with adaptive design and risk management principles. Near-term design recommendations focus on low-regret strategies that work well for both current and uncertain future conditions, allowing for updates over time as conditions change. This also will allow the base to assess the impact of regional BMPs planned nearby.
In total, the proposed BMPs range from $4 million to $4.2 million and will increase the installation’s wetlands inventory by 27-acres without impacting operations. This investment, along with strategic plantings and rehabilitation of the existing BMPs, will reap a significant return over the next 10-year horizon.
A COLLECTIVE APPROACH
Looking just to what is projected to occur within the boundaries of Joint Base Langley-Eustis will not be enough. Leadership must work collaboratively with regional partners to assure that off-base plans for stormwater management complement the installation’s approach.
Use of a common platform to evaluate all regional BMPs would be instrumental in establishing shared knowledge and collectively forecasting the effectiveness of mitigation approaches. Expansion of the HEC-RAS model to include off-base, regional partners, and their projected BMPs could achieve this goal. Once established, such a model could be used to support multi-party focused facilitation that addresses the benefits and tradeoffs of regional BMPs, strengthening the resilience of all.
Robert Barrett, M.SAME, is Engineering Flight Chief, and Anhthu Nguyen, M.SAME, is General Engineer, Joint Base Langley-Eustis, Va. They can be reached at robert.barrett@ us.af.mil; and firstname.lastname@example.org.
Kandi Brown, M.SAME, is President, NewFields Government Services; email@example.com.
Brian Wellington, Ph.D., P.E., Daphne Williams, M.SAME, Isabella McLerran, and Adam Saslow contributed to this article.
[This article first published in the January-February 2021 issue of The Military Engineer.]