New mapping approach reveals benefits, shortcomings of flood-control strategies
It should come as no surprise that urban areas, with impenetrable rooftops and parking lots, contribute to flooding. But natural and manmade structures within the watersheds that serve urban and rural areas can influence the path and speed of water, for better or worse.
Landscape features, such as vegetative cover, soil type, and the steepness of hillsides, affect the magnitude and duration of only small floods, according to research by Beatriz “Tiz” Mogollón of Bogota, Colombia, who earned her master’s degree in fish and wildlife conservation from Virginia Tech’s College of Natural Resources and Environment in 2014. To regulate flooding, urbanized watersheds require engineered features, such as storm-water ponds, buffer strips, and other strategies for holding water.
Watersheds, as the name describes, are landforms that direct water towards streams and rivers. They can be huge, such as the six states that drain into the Chesapeake Bay, or only a few square miles, such as ridges that shed snow melt into a creek and from there into a river.
For her thesis research, Mogollón examined the landscape processes that influence river flooding to better understand the circumstances under which watersheds regulate flooding, as well as the prospects for managing floods by manipulating landscape structure. In collaboration with Professor Paul Angermeier, Associate Professor Emmanuel Frimpong, and Research Scientist Amy Villamagna, all in the Department of Fish and Wildlife Conservation, she focused her analysis on 31 watersheds across Virginia and North Carolina that had at least 20 years of records about river flow.
Mogollón’s first study, published in the Feb. 1, 2016, Journal of Environmental Management, showed that larger floods cannot be managed by manipulating landscape features. She pointed out that urban watersheds have the potential for larger, quicker floods, but engineered features, such as constructed wetlands, grass swales, and storm-water ponds, can lower flood magnitudes in these areas.
In a second study, published in the March 16, 2016, Journal of the American Water Resources Association, Mogollón’s research showed that although annual precipitation has decreased since 1991 in the areas she studied, the river flow in some of the watersheds has had a tendency to increase rapidly. This “flashiness” has increased in urban areas that have lost forest cover and have few flood-management strategies. “Impervious cover pours water rapidly into creeks and rivers,” explained Angermeier, who is an affiliate of the Fralin Life Science Institute.
In a third study, published in the February 2016 issue of Ecological Indicators, Mogollón and colleagues mapped the capacities of technological and landscape attributes of watersheds to regulate floods. The research showed that strategic use of engineered and landscape structures results in smaller but longer-lasting flooding.
“Flooding doesn’t happen as quickly, so people and property are less likely to get caught in it and its high point is lower — it doesn’t reach as much property — so it is less damaging,” Angermeier said. “It’s safer, but the tradeoff is that it lasts longer because the same amount of water still has to go through the system.”
Mogollón’s new approach to mapping flood-regulation capacities incorporates technological capacity, high spatial resolution across watersheds, and the changes in and importance of such landscape features as cropland, grassland, wetland, forest, and urban development, which vary significantly among watersheds.
“Watersheds provide many benefits, from water itself, to beauty and recreation,” Angermeier said. “The less obvious benefits are the processes that purify water and control flooding. Mogollón’s findings are relevant not only to residents of Virginia and North Carolina and the agencies that invest in flood control, but can also be applied to other areas with similar climate, topography, and land use.”
The more finely tuned mapping approach can help ensure that investments in flood management account for the limitations of landscape features, he said. “For example, flood managers in urban areas might consider implementing more of the flood-control practices that seem to be especially effective, such as storm-water ponds and grassy swales, which can also provide recreation and green space.”