1-, 2- AND 3-D HYDRODYNAMIC AND WATER QUALITY MODELING OF NORTH CAROLINA'S ROANOKE RIVER AND FLOODPLAIN

The downstream-most 161 kilometers of the Roanoke River basin, from Roanoke Rapids Dam to Albemarle Sound (Lower Roanoke River), supports a large and diverse population of nesting birds, waterfowl, freshwater and anadromous fishes, and wildlife, including threatened and endangered species. Much of the Lower Roanoke River basin is forested swampland in conservation by federal (including the Roanoke River National Wildlife Refuge), state, and non-profit entities. Streamflow, floodplain inundation, and the corresponding water quality are primarily governed by upstream reservoir releases.

The U.S. Geological Survey has been collecting hydrologic data in the Lower Roanoke River since the early 1900s. A combination of streamflow, instream and floodplain water level, and water quality data at 14 streamgages and 9 floodplain gages were used to calibrate and validate coupled hydrodynamic and water-quality models of the Lower Roanoke River. Model development was bolstered by a digital elevation model with 3-m by 3-m cells and a vertical accuracy of approximately 20 cm derived from Light Detecting and Ranging data that facilitated the establishment of one-dimensional (Hydrologic Engineering Center’s River Analysis System (HEC-RAS)), two-dimensional (CE-QUAL-W2), and three-dimensional (Environmental Fluid Dynamics Code (EFDC)) hydrodynamic models, and two different water quality models (Water Quality Analysis Simulation Program (WASP) and CE-QUAL-W2). Results show the depth, duration, and extent of floodplain inundation and the corresponding water quality over a 10-year simulation period.

To date, the monitoring data and models have been used to investigate the relation between hydropower operations and large storm events on dissolved oxygen (Wehmeyer and Bales, 2009a), the relation between floodplain water levels and instream dissolved oxygen (Bales and Walters, 2003), cropland inundation during various sustained high flows, salinity movement from Albemarle Sound up into the Roanoke River, and the distribution of biochemical oxygen demand loading between point and non-point sources (Wehmeyer and Bales, 2009b). Ongoing work includes three-dimensional hydrodynamic and water quality modeling to evaluate the implications of simulated operational changes to upstream reservoir releases as part of the John H. Kerr Dam & Reservoir (Section 216) Study. Future applications of the 3-d models that have been discussed include assessing streambank erosion, defining the effects of sea level rise on saltwater intrusion, and evaluating the relation between floodplain water quality and tree growth.

References Cited

Bales, J.D., and Walters, D.A., 2003, Relations Among Floodplain Water Levels, Instream Dissolved-Oxygen Conditions, and Streamflow in the Lower Roanoke River, North Carolina, 1997 –2001: U.S. Geological Survey Water-Resources Investigations Report 03 – 4295, 81 p.

Wehmeyer, L., and Bales, J.D., 2009a, Relation Between Flow and Dissolved Oxygen in the Roanoke River Between Roanoke Rapids and Jamesville, North Carolina, 1998–2005: U.S. Geological Survey Scientific Investigations Report 2009-5238, 42 p.

Wehmeyer, L., and Bales, J.D., 2009b, Simulated Hydrodynamic and Water Quality Effects of Dam Releases and Permitted Discharges on an Estuarine-Influence Portion of the Roanoke River, North Carolina, 1997-2007. Proceedings of the 2009 Annual American Water Works Association-Water Environment Association Conference, November 16, 2009, Raleigh, NC, 10 p.

Acronyms

1-d one-dimensional

2-d two-dimensional

3-d three-dimensional

EFDC Environmental Fluid Dynamics Code

HEC-RAS Hydrologic Engineering Center’s River Analysis System

WASP Water Quality Analysis Simulation Program