Modeling a better, wetter swamp

Published 10:41 pm Friday, December 28, 2018

A new computer model will help manage water levels in the Great Dismal Swamp National Wildlife Refuge with more precision, helping reduce the risk and harm of wildfires and increasing biodiversity in the refuge.

Congress established the Refuge in 1974, after the Great Dismal Swamp was altered by centuries of draining, wildfires, construction and timber harvesting. The refuge currently encompasses 112,000 acres managed by the U.S. Fish and Wildlife Service in southeastern Virginia and northern North Carolina, according to a report prepared by the U.S. Geological Survey.

The swamp was drained in centuries past to create dry conditions for loggers, and an extensive network of roads and ditches was constructed. According to the report, this network fundamentally changed the swamp’s hydrology, which harmed the trees, lowered the land-surface elevation and increased the risk of severe wildfires.

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The peat soil became more flammable and less able to hold water as it dried up, so approximately 50 water control structures were replaced and installed in ditches across the refuge to restore the soil.

These act as adjustable dams, each made of corrugated aluminum culvert material, according to the U.S. Geological Survey. Six-inch-thick aluminum boards called “stop-logs” are placed in the structure for water level adjustments.

“They create a barrier to water movement in the ditch, and the height of the water in the ditch is controlled by the number of boards we put in the structure,” said Fred Wurster, U.S. Fish and Wildlife Service hydrologist.

Put simply, more stop-logs mean more water is kept in the swamp, and vice versa. This will remoisten the peat soil and reduce the risk of forest fires. Refuge wildlife and vegetation will also benefit from the improved wetlands.

“It allows us to manage water levels for forest health and to reduce the possibility of wildfires,” Refuge Manager Chris Lowie said. It will also reduce the overall impact of wildfires that do occur.

But there are ripple effects when directing water flow, and the flat and thickly-forested swamp poses another challenge for refuge staff.

“If you’re trying to understand how water moves in the swamp from the ground, it’s tough to get perspective,” Wurster said. “You can’t stand up high, look over the whole swamp and see where water is coming and going. To manage and understand water flow patterns in the Great Dismal Swamp, you need to step back and look at it at a larger scale.”

That scale is illustrated in a three-dimensional, numerical model developed through a U.S. Geological Survey study with the help of the Fish and Wildlife Service. U.S. Geological Survey first built a digital version of the swamp — including topography, peat thickness, ditches and roads — then ran a computer simulation to calibrate the model based on average spring hydrologic conditions from the years 2005 through 2015.

With this model, simulations of water-management scenarios can be used by refuge staff to better inform their water-management decisions.

“It provides that kind of geographic perspective to get a sense of the area that will be influenced by a particular management action,” Wurster said, adding that a single structure has the potential to influence water levels over miles.

It’s not a perfect representation, he said, but it’s still a major step forward in predicting the effects of a water-management action, rather than waiting several years to see the results.

“The model isn’t going to give us all the answers to manage this place, but it is an important tool that will improve our ability to manage the place,” he said.