Adapting to Flooding in Norfolk
As Virginia’s Hampton Roads faces significant challenges due to a high rate of relative sea level rise, which will require innovative long-term strategies for resilient design, a new residential project at Naval Station Norfolk provides a viable solution for managing stormwater through exfiltration basins.
By Christopher G. Ions, AIA, LEED AP, M.SAME, and Christopher Cook, P.E., LEED AP, M.SAME
PHOTOS COURTESY CLARK CONSTRUCTION GROUP
Residents are accustomed to living with nuisance flooding in Norfolk, Va. When rain falls too hard and too quickly, low-lying areas become waterlogged. Roads close, schedules get disrupted, and people find higher ground until the waters subside. Even during an average rainstorm there are inconveniences. The real problem, however, is the flooding continues to grow more disruptive. And the disruptions are becoming more frequent.
Though Norfolk is not unique in facing problems due to climate change, the area is uniquely positioned to feel substantial impacts due to the confluence of three major factors: rising seas, sinking land and shifting currents. Positioned on the Atlantic coastline in Virginia’s Hampton Roads region, Norfolk is experiencing steady land subsidence caused by the area’s unsettled geology and decades of pumping groundwater from the coastal aquifer. Simultaneously, the Atlantic current is shifting and the effects of this are most noticeable at “hot spots" for rising waters along the mid-Atlantic coast. This trifecta of issues means that relative sea level rise in Norfolk is occurring at more than twice the global average. The area is experiencing the highest rate of relative sea level rise on the East Coast. Though predications vary, the Virginia Institute of Marine Sciences predicts that waters in Norfolk could rise between 1.5-ft to 7.5-ft by the end of this century, with 5.5-ft being a likely scenario. Sea level rise on this scale would have a profound impact on infrastructure, marine ecosystems and quality of life for residents. Add in the probability of additional issues experienced during hurricanes, nor’easters and tidal surges, and the concern quickly becomes critical.
It is within this geographic context that sits Naval Station Norfolk, the largest naval base in the world and the home and support system for a large portion of the Atlantic fleet. Like other low-lying areas on Chesapeake Bay, Naval Station Norfolk is prone to tidal, storm and precipitation flooding. Because maintaining efficient operations is paramount, avoiding disruptions caused by flooding is a serious concern.
Home to 75 ships and more than 130 aircraft, Naval Station Norfolk lies within a dense urban environment with little topography, a high water table and significant amounts of impervious pavement.Providing safe, comfortable and efficient housing for sailors is a key goal in attracting and retaining the highest caliber personnel. In support of this, the U.S. Navy commissioned a residential facility for unaccompanied sailors, the P-123 Bachelor Enlisted Quarters Homeport Ashore. The project was awarded to the design-build team of LS3P and Clark Construction Group along with ADC Engineering Inc. The goal was to provide market-style housing for 900 sailors, with all the comforts of an off-base apartment complex in close proximity to ships and on-shore responsibilities.
Creating a residential atmosphere in the middle of a military base was challenging. But the project provided opportunities for a variety of floor plans and amenities. The design emerged from a charrette conducted in whole-team integrated project delivery with all design disciplines, the contractor and major subcontractors. The solution takes advantage of east-west building orientation and geothermal water source heat pump technology for optimum performance, energy usage and operating cost.
The project site is bordered by industrial, administrative and housing districts. The layout features sheltered courtyards to offer the most desirable lifestyle for sailors ashore. Parallel apartment wings connected by a central element create an “H” shaped layout with two distinctive courtyards for an urban residential feel. The courtyards, enclosed within Anti-Terrorism/Force Protection barriers, provide amenities that would otherwise be precluded by clear stand-off requirements. Additionally, the courtyards function as a central part of the stormwater management strategy.
The building elevation for the new P-123 Bachelor Enlisted Quarters is well-above existing facilities on the base and the flood zone established by the Federal Emergency Management Agency. This will reduce the amount of runoff and improve the quality of the water that is discharged.
The project was required to achieve LEED Silver certification, including limits for rate, quantity and quality. LEED mandates removal of 80 percent of total suspended solids for a 90th percentile storm. The Virginia Stormwater Management Program required a 10 percent reduction in phosphorus and management of a 10-year/24-hour storm. Section 438 of the Energy Independence and Security Act enacted a goal of retaining and infiltrating, or re-using on-site, the volume of the 95th percentile storm as a measureable way of meeting pre-developed hydrology. The 95th percentile storm event for Norfolk is 1.68-in over 24 hours. The maximum volume to be stored is based on the one-year/24-hour rain event, which is 2.93-in over 24 hours. The water table is estimated to be approximately 3-ft to 5.5-ft below existing grade. The infiltration rate provided in the request-for-proposal was 3.5-in/hour.
Particular challenges included a nearly 100 percent paved site, a high water table, low-moderate infiltration rates and existing grades 8-ft to 10-ft above mean sea level. As with all residential projects, the design had to include adequate parking to accommodate the high-density development.
As a primary strategy to help meet stringent Low Impact Density requirements, the team implemented an innovative stormwater management system. The team raised the entire footprint of the building and courtyards 3-ft. This additional height was used to create large exfiltration basins overlain by pervious pavement within the courtyards. The 6-ft deep basins are filled with crushed stone down to the water table. This satisfies stormwater management criteria for quantity and quality while mitigating flooding potential and meeting the project’s entire Low Impact Density component.
The team implemented a number of best management practices to meet all stormwater requirements, using pervious pavement wherever possible, and specifying that roof drains are not connected to storm drainage piping and, in most cases, flow across permeable pavers. In addition to the courtyard exfiltration basins, the design manages stormwater through four additional components. Fire lanes, which serve as pedestrian circulation around the building, are composed of permeable pavers above storage rock to handle runoff from roof drainage and the surrounding landscape. Parking lots use the same permeable paver and storage rock system, while grassed swales around an existing asphalt lot improve water quality and enhance the approach to the building. Pervious pavers serve parallel parking spaces between the building and waterfront. They allow infiltration of any initial rainfall. This benefits adjacent streets, since most of the rainwater flows across these parallel spaces.
A Safety Factor of 2 was applied to all field-tested infiltration rates to account for actual variations, future restrictions and any potential clogging. All areas used for storage and infiltration (courtyards, private owned vehicle parking areas, and fire lane/pedestrian areas) include additional surface runoff capacity through surface slopes, catch basins and overflow areas to handle runoff even in the event of complete backup of the storage and infiltration systems. The low-maintenance, highly articulated building complex also incorporates an optimal solar solution and energy efficient design, including geothermal, solar and photovoltaic renewables.
A RESILIENT RESIDENCE
The stormwater management system at the P-123 Bachelor Enlisted Quarters offers an innovative design for resilient facilities in challenging low-lying environments.
In addition to providing comfortable and convenient housing for sailors living on base at Norfolk, the project is intended to mitigate or eliminate problems associated with nuisance flooding while providing a workable strategy with implications for wider use at other military installations.