Inadequately sized channels passing under superhighways can seriously disrupt roadside wetland ecosystems by interfering with the natural flow of water, a Duke Wetland Center study has shown.
The first phase of a study funded by the North Carolina State University-based Center for Transportation and the Environment investigated the effect of highway construction on wetlands. DUWC researchers found that trees, plants, and soils in two wetland systems were significantly changed by the way underlying culverts altered water levels on either side of a stretch of Interstate 40 in eastern North Carolina.
The extension of I-40 from Raleigh to Wilmington in the late 1980s meant cutting through miles of wetlands. The highway’s effects on drainage, water flow, and species habitats posed many important questions. According to Duke Wetland Center Director Curtis J. Richardson, state and federal transportation officials now recognize a vital need for new construction standards in wetlands. "But they really don't have the data to say how they should design when crossing wetland areas," he added. "So that is what we are trying to provide."
Dr. Richardson and fellow investigator Kevin Nunnery, now a Nicholas School post-doctoral researcher, faced an immediate hurdle when they began their study in 1995. They had to assess construction impacts of a highway that had been built seven years before their study began.
They chose two adjacent wetland corridors, Beaverdam Swamp and Kill Swamp, which cross Interstate 40 about 1.5 miles apart near the Sampson County town of Newton Grove in North Carolina's coastal plain. Both swampy creeks had similar water flow rates and kindred upstream and downstream environments. Their upstream land use patterns–agriculture and livestock production–were also similar. And both passed under I-40 through the same type of conduit systems–a central box-shaped culvert and two smaller stream overflow pipes. The locations were deemed the best sites available in North Carolina for studying a recently constructed highway crossing's effects on wetlands.
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In a move to mimic how the affected wetlands might have appeared before the superhighway's construction, Dr. Richardson and Dr. Nunnery selected an undisturbed "reference area" about 320 yards upstream from the Beaverdam Swamp study site. |
| DOT Highway Study "reference area." |
They also obtained highway department pre-construction aerial photos of the wetland crossing sites to gauge what the tree cover was like before I-40 was completed.
| Their study showed that the box culvert systems ended up acting more like bottlenecks than conduits, especially at the Beaverdam Swamp crossing. "During the wet periods water backed upstream significantly," Richardson said in a recent interview."That significantly changed the areas that were wet, and it quite often killed upstream vegetation there. |  |
| DOT Highway Study "impacted site." |
More than that, we found after a period of time that a lot of the wetlands were converted into new types of wetland. And some uplands also were converted into wetlands."
In a final report on the 18-month field study, conducted from March 1995 until October 1996, the researchers said the culvert bottleneck raised Beaverdam Swamp's mean surface elevation more than 7 inches upstream of the highway crossing. Resulting ponding extended more than 100 yards upstream. And the soil decomposition rate was significantly lower upstream as well, "suggesting that ponding was inhibiting decomposition there," the report said.
While Kill Swamp's water levels were altered less dramatically, there were significant changes in vegetation within both wetlands. Plants downstream of both crossings were 30 percent to 50 percent thicker below the crossing than in upstream sections. Trees also grew 10 percent to 40 percent less prolifically upstream than downstream.
The reference area, which had escaped any impact from I-40, contained "several less flood tolerant species" than the affected areas, "suggesting that stream hydrology had been disturbed near the highway," the report said. Trees in the reference area were also significantly leafier than in the affected areas.
The report described a general "paucity of information on what effects highways may have on wetland ecological functions, and how severe and geographically far-reaching those effects are." There is also "no accepted, reliable procedure for quantifying and assessing" those impacts, it added.
In the study’s second phase the center studied how a new U.S. 17 bypass project near the Camp Lejeune Marine Corps base in the vicinity of New Bern will affect coastal wetlands in the adjacent New River Estuary. This time the scientists weren able to begin their work before construction began.
