Marine Corps Air Station (MCAS) New River, NC, has experienced significant growth in infrastructure, which has stressed its ability to meet the requirements for management of stormwater quality and quantity under current state and federal rules. In the past, the air station operated under a system that required a discrete permit for each new project that added to the impervious surface at the station. Already burdened with more than 90 stormwater permits issued by the North Carolina Department of Environment and Natural Resources (DENR), the air station was challenged to find more usable space for each new project best management practice (BMP). To address the challenge, the air station elected to develop a comprehensive stormwater management plan that would provide for future growth under one master permit that covers the entire air station in a logical fashion. This stormwater plan and master permit approach would also be designed to minimize impacts to ongoing and planned future operations at the air station.
Ultimately, the base sought to maximize flexibility in using limited available space for stormwater BMPs by developing a regional strategy. The master plan and permit was developed to codify that approach. Key components of the plan were to improve the air station’s ability to satisfy stormwater treatment requirements while safely executing mission-critical base operations. The stormwater management plan that was finally developed accounts for stormwater quantity and quality impacts of 190 acres of new built upon area (BUA) planned for the air station over five years. That stormwater management plan:
- protects and improves water quality,
- satisfies North Carolina coastal and federal stormwater regulations,
- provides flexibility for current and future development and redevelopment,
- promotes stormwater reuse and low-impact development (LID), and
- maintains flight line safety.
The mission of MCAS New River is to support and enhance the combat readiness of the Marine Corps Aviation Combat Element and Department of Defense (DOD) units while improving quality of life of military personnel, their families, and the workforce assigned to the air station. MCAS New River, established in 1945 in eastern North Carolina adjacent to Marine Corps Base (MCB) Camp Lejeune, is home to the 2nd Marine Aircraft Wing rotary and tilt-rotor aircraft. There are two runways, each roughly 5,000 feet in length.
“Grow the Force” and “Marine Corps Aviation Transition Strategy” initiatives resulted in two additional squadrons being located on MCAS New River, resulting in a significant increase in the number of aircraft. The footprint of the V-22 Osprey tilt rotor vertical takeoff and landing aircraft is larger than that of its predecessor, the CH-46 Sea Knight helicopter. Current hangars are not tall enough or deep enough to comfortably perform maintenance activities. Additional ramp area for aircraft staging is also required. In addition to new hangars, apron, and runway extensions for these aircraft, new barracks, family housing, mess halls, and other supporting infrastructure are required to house the personnel to fly and maintain the aircraft. These improvements will ultimately result in 190 acres of new impervious surface. Figure 1 shows the permit limits and proposed development by year.
Before development of the stormwater management plan, stormwater BMP criteria were developed in collaboration with MCAS New River staff. BMPs were considered with respect to satisfying regulatory, physical, and operational constraints onsite. Safety and flexibility of airfield operations had to be considered in addition to water-quality improvement. The primary criteria used in creation of the plan were as follows:
- Satisfy stormwater quality requirements.
- To the maximum extent practicable, avoid open water BMPs to minimize Bird/Wildlife Aircraft Strike Hazards (BASH).
- Avoid impacts to operations in the flight line.
- Due to limited available buildable area, maximize opportunities for development and redevelopment of the air station infrastructure.
- Promote green infrastructure and LID.
- Be flexible.
Regulatory and policy drivers include the North Carolina Coastal Stormwater Rules (15A NCAC 02H.1000 and Session Law 2008-211), the Energy Independence and Security Act (EISA) Section 438, DOD Stormwater Policy, DOD Unified Facilities Code on LID, BASH Program Implementation Guidance (Section 5.3.6.1 Stormwater Management), and DOD Antiterrorism Force Protection.
Because of the air station’s location in the coastal plain along a tidally influenced river, physical constraints for successfully locating BMPs include extensive wetlands areas, flat topography, poor soil infiltration, elevated seasonal high water table (SHWT); and the 100-year floodplain. Figures 2 and 3 illustrate limits of physical constraints on site. Lastly, operational constraints due to internal encroachment issues eliminated certain areas from consideration. These areas include the flight line, ordnance handling areas, and installation restoration sites.
Once BMP criteria were established, maps were reviewed and GIS layers were updated based on all existing stormwater permits issued for the air station. GIS updates required access to the existing GIS data from the air station, information on permit limits from the public works archives at MCB Camp Lejeune, and access to permit information at the NC DENR Wilmington Regional Office. From existing mapping, the drainage areas within the permit boundary were delineated then refined based on permit limits and site visits to ground-truth the accuracy due to the flat topography. Because EPA guidelines developed to satisfy EISA Section 438 are more restrictive than the North Carolina Coastal Stormwater Rules, the solution was sized to treat the 95th percentile storm rather than the first 1.5 inches of rainfall as required by the state. A hydrologic assessment was conducted to determine time of concentration and peak flow from each sub-basin within the permit boundary.
From the GIS data and hydrologic assessment, the team identified potential areas to locate BMPs, which were investigated in the field. At the sites that looked promising, a soil scientist performed SHWT determinations and infiltration tests. Figure 4 identifies the locations of infiltration tests. In many instances, the seasonal high water table was too close to the surface or the soil infiltration capacity was too poor to effectively incorporate stormwater BMPs that avoid open water features to minimize BASH concerns.
From this information, three options were developed and presented to the commanding officer at MCAS New River. The options ranged from multiple small BMPs to regional BMPs to a combination of both approaches. The strategy chosen includes a combination of LID approaches to stormwater management and a regional BMP to act as a bank for credits as needed.
Several scoping meetings were held with the NCDENR Stormwater Permitting Unit in the Wilmington Regional Office. During multiple discussions, ratios were identified for treatment credit offsets. This approach uses a regional BMP to treat stormwater runoff from a 273-acre industrial area, much of which was developed prior to 1988 when stormwater rules were applied to new development. This regional BMP improves water quality by treating grandfathered areas and using credit for this treatment to offset credits needed in other areas of the air station where site conditions limit stormwater BMPs or affect safe operations in the flight line. This approach allows for transitioning to the master permit from the current permit method and it allows for flexibility for future development. It also protects Marines and aircraft from BASH to the greatest extent possible by limiting open water.
Figure 5 illustrates the credit ratios negotiated with NC DENR. Of note, the area draining to the canal through the industrial area that discharges to a regional infiltration basin received a 1:1 treatment credit. The area draining east toward New River from the north-south runway was set to trade at a 2:1 treatment ratio. The remaining area received a trade treatment credit at a 1.5:1 ratio. Figure 6 shows the final solution presented as part of the Stormwater Management Plan Master Permit.
In summary, the stormwater permit strategy successfully captured all 92 existing stormwater permits into one master permit. It maintained valuable real estate for future growth on base, maximized flexibility, improved water quality by treating previously untreated areas, and used existing features where possible. New development that is not able to meet all stormwater requirements using onsite BMPs has the option of using available credits from a regional BMP to satisfy water-quality treatment requirements. This solution satisfies the most restrictive stormwater rules and maintains flexibility for the base to execute its mission.
