By Phil Forzley, PE and Shannon Beaumont, PE
Bridges, culverts, and other water-conveying structures serve more than just people. These structures may serve motorists, bicyclists, and pedestrians; they must accommodate both normal stream flows and flood flows; and they must not negatively impact fish, animals, and plant life. Furthermore, the effects of climate change must be considered. Effective modern design requires a multidisciplinary approach.
Historically, rainfall data used to size structures were from the mid-1900s. During the past decade, the engineering community recognized how changing climate conditions have impacted rainfall amounts and intensity. Our design approach has to adapt to the changing climate and consider future conditions. Culverts and bridges are critical to our transportation infrastructure and serve an important role in public safety, ecological health, and climate resilience. This is not simply a coastal concern; inland communities need to consider the effects of increased temperature and precipitation as well. As such, many states and agencies are incorporating climate change impacts into design standards and hazard mitigation planning guidance.
Climate change impacts are just one component of the multidisciplinary modern design approach. Natural resource needs must also be understood and accommodated. Repair and replacement structure designs now provide fish and wildlife habitat connectivity. Designs include natural stream bottoms and banks that attract fish and wildlife and that facilitate passage of aquatic organisms. This is particularly important when watersheds upstream and downstream of crossings merit contiguous habitat and wetland functions such as groundwater recharge or discharge and nutrient retention. If improperly designed, engineered systems can actually endanger natural systems. Undersized hydraulic openings or openings that are not geometrically consistent with a stream corridor can result in loss of wetland and associated upland habitat. Poorly designed structures can also result in significant morphological changes and scour on new improvements.
For all needs to be considered, proper design requires a multidisciplinary approach. A modern design team typically includes structural engineers that consider design loads on the culvert or bridge; civil engineers that ensure slopes are stable and resistant to erosion from normal and flood flows; geotechnical engineers that understand the bearing capacity of soils or rock that will support the structure being designed; wetland and natural resource scientists who determine how much of a stream corridor should be spanned, what aquatic organisms are present that need to be accommodated, and what the impacts are and how to mitigate them. Teams may include hydrology and hydraulic experts who determine the magnitude of low, normal, and flood flows, as well as any impacts on those flows from climate change. They may develop hydraulic models that determine the size of any opening to accommodate flood flows, design low-flow channels for fish passage, and determine potential wildlife passage opportunities that animals may use during normal flows to pass through the structure.
So what does this mean for the overall project? Inclusive and considerate design. Structural capacity and safety are always paramount, and safety is never compromised. But the addition of viewpoints that are trained to think about how nature interacts with the structure and how the structure interacts with nature yields enhanced design.
The following projects demonstrate the power of collaboration and multidisciplinary design.
Brookside Drive Culvert Replacement, New London, NH
The existing stream crossings were constructed in the early 1980s and consisted of three 49” x 33” galvanized corrugated metal arch-pipes that had deteriorated and were categorized as structurally deficient by NHDOT. The design team consisted of civil engineers, transportation engineers, structural engineers, and natural resource specialists.
US Route 2 Culvert Replacement, Lunenburg, VT
Design of a 20-foot precast concrete arch on a mat foundation to replace a partially crushed 27” CMP upstream with a 4’ x 4’ box downstream. Due to the severely undersized structure, the roadway was overtopped during Tropical Storm Irene. The design team consisted of structural engineers, transportation engineers, surveyors, water and natural resource engineers, and civil engineers.
Phil Forzley, PE is a Vice President at Fuss & O’Neill with more than 35 years of experience designing, managing, and executing civil and environmental engineering projects throughout New England. Currently managing the firm’s White River Junction, VT office, Phil specializes in stormwater management, hydrology and hydraulic structures design, dam engineering, site planning and engineering, wastewater treatment and disposal, construction services, and regulatory compliance.
Shannon Beaumont, PE is a Senior Bridge Engineer in Fuss & O’Neill’s Manchester, NH office. Her 20+ years of technical experience include the design of steel, reinforced concrete, and precast concrete structures, bridge ratings, and bridge inspections. She is an experienced hydraulics engineer and is proficient at sizing bridges and culverts using accepted methods to calculate flood flows and elevations.