VA Coop Unit Evaluates How and When Watersheds Regulate Flooding

USGS Cooperative Research Unit Corner

VA Coop Unit Evaluates How and When Watersheds Regulate Flooding


The USGS Virginia Cooperative Fish and Wildlife Research Unit recently 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. Led by Beatriz (Tiz) Mogoll?n, in collaboration with Drs. Paul Angermeier, Emmanuel Frimpong, and Amy Villamagna (all in the Department of Fish and Wildlife Conservation at Virginia Tech University), the analysis focused on approximately 30 watersheds distributed across Virginia and North Carolina, each with long-term (at least 20 years) records of river flow. The three separate studies will all be published this year.

Watersheds provide many benefits ? or ecosystem services ? to people. These include obvious benefits like the water itself, which we extract to use for all sorts of purposes such as drinking, swimming, irrigation, and industry. Some widely valued benefits of natural water bodies include their beauty and capacities to support recreation such as canoeing and fishing. Less recognized services of watersheds, such as purifying the water we drink or modulating the floods that follow rainstorms, stem from ecosystem processes that most of us don't see in action or understand very well. These processes result from complex interactions between precipitation and landscape features (e.g., topography, soil type, and land cover) within watersheds. That is, landscape features can strongly affect the paths taken and speeds reached by water as it flows across and through a watershed. These paths and speeds, integrated across an entire watershed, ultimately determine a watershed's capacity to provide services.

The first study (Mogoll?n, et al. 2016. An empirical assessment of which inland floods can be managed. Journal of Environmental Management 167: 38-48) showed that landscape features affect the magnitude and duration of only small floods (those with return periods ? 10-years). This result suggests that larger floods cannot be managed by manipulating landscape structure. Mogoll?n also found that urban watersheds exhibited larger, quicker floods than non-urban watersheds, but urban watersheds with more engineered features designed to regulate floods had lower flood magnitudes than urban watersheds with fewer flood-regulating features.

The second study (Mogoll?n, et al. 2016. Recent changes in stream flashiness and flooding, and effects of flood management in North Carolina and Virginia. Journal of the American Water Resources Association. 1-18. DOI: 10.1111/1752-1688.12408) found that although mean annual precipitation decreased over the study period, the flashiness (tendency to rapidly change) of river flow in some watersheds increased. Mogoll?n determined that watersheds that increased in flashiness gained more urban cover, lost more forest cover and had fewer best management practices (e.g., detention ponds)than watersheds that decreased in flashiness.

The final study (Mogoll?n, et al. 2016. Mapping technological and biophysical capacities of watersheds to regulate floods. Ecological Indicators 16: 483-499) demonstrates that watersheds with high biophysical and technological capacities to regulate floods, exhibited longer but smaller floods. Mogoll?n's new approach to mapping flood-regulation capacities revealed great differences among watersheds and improved on previous approaches by incorporating a) technological (rather than only biophysical) capacity, b) high spatial resolution of capacities, and c) relative importance of certain landscape features.

Mogoll?n's findings are germane to many residents of Virginia and North Carolina who live or own land near rivers, and to agencies and municipalities who invest in flood control measures. Her findings also can be applied to other areas with similar climate, topography, and land use, and can help ensure that investments in flood management are made wisely after considering the limitations of landscape features to regulate floods. For example, flood managers in urban areas might consider implementing more of the flood-control practices that seem to be especially effective, and those that provide valuable co-benefits (e.g., recreation or green space).

The ONB features articles from Cooperative Fish and Wildlife Research Units across the country. Working with key cooperators, including WMI, Units are leading exciting, new fish and wildlife research projects that we believe our readers will appreciate reading about. This article was written by Paul Angermeier, Assistant Unit Leader at the USGS Virginia Cooperative Fish and Wildlife Research Unit.

April 14, 2016