Innovative Grade Controls for Restoring an Incised Urban Channel at Millbrook Exchange
Barbara A. Doll, PhD, PE
NC Sea Grant and Biological and Agricultural Engineering Department
NC State University
Raleigh, NC
Authors: Barbara A. Doll, PhD, PE, Department of Biological and Agricultural Engineering, North Carolina State
University and North Carolina Sea Grant, Raleigh, NC
Jonathan Page, Dan Line, Cameron Jernigan, Jack Kurki-Fox, Department of Biological and Agricultural
Engineering, North Carolina State University, Raleigh, NC
Kevin Boyer and Heather Dutra, City of Raleigh, Engineering Services Department, Stormwater Management,
Raleigh, NC
In cooperation with the City of Raleigh (COR), NCSU has conducted monitoring, modeling, restoration design and construction oversight for an innovative restoration project on 1,000 linear feet of an unnamed tributary to Perry Creek located within the Millbrook Exchange Park. The purpose of this project is to test the use of innovative grade controls for repairing an incised and eroding urban stream. The project reach is located within the former extents of an impoundment created by an earthen dam, which breached in the 1980s. This reach receives stormwater runoff from a 180-acre, 85%-developed drainage including most of the park. When the embankment breached, the channel became severely incised into the legacy material that accumulated in the former impoundment, exporting large quantities of sediment, and degrading aquatic habitat within the reach and downstream. On average, the channel bed incised 5 to 6 feet below its probable pre-disturbance bed elevation. Along approximately 150 LF of this reach, the channel had incised up to 12 feet.
The restoration project features a low-cost and unique design that included installing 11 controls configured with boulder crossvanes constructed atop a rock pyramid foundation. The structures are intended to capture sand and gravel over time in order to raise the stream bed by five feet. The objectives of the project are to reduce the amount of sediment washed downstream, transform the energy and flow of the stream, and prevent future streambank erosion. This approach was selected to be cost-effective and minimally invasive to existing riparian vegetation when compared to re-meandering the channel and excavating a new floodplain using a more traditional natural channel design approach or filling the channel with imported sand and wood chips if a Regenerative Stormwater Conveyance approach was taken.
Construction was completed in February of 2019. NCSU monitors flow, TSS and sediment concentration upstream and downstream of the reach. In addition, bi-monthly survey of the longitudinal profile and 10 permanent cross-sections is conducted to document the sediment accumulation rate in the bed of the tributary. Initial data indicates the project reach has been effective at reducing sediment loading in the stream.
About Barbara A. Doll, PhD, PE
Barbara Doll is an Extension Associate Professor in the Biological & Agricultural Engineering Department and Extension Specialist for NC Sea Grant based at North Carolina State University located in Raleigh, North Carolina. Barbara holds a Ph.D. in Biological and Agricultural Engineering and is a licensed professional engineer. Barbara leads the Stream Restoration Program at NC State University, which carries out research, trains students, conducts numerous training workshops for professionals and organizes EcoStream, the Southeast Regional Stream Restoration Conference. Doll is the principal investigator for more than $1 million in grant funds and leads a team of engineers and students that conduct outreach and research projects focused on evaluating the performance of stream restoration efforts, developing new techniques for ecological restoration and assisting communities with flooding and water quality challenges.
Doll’s engineering team conducts geospatial analysis, and hydrology and hydraulic modeling to assist communities with identifying flood mitigation opportunities. The team has evaluated flood mitigation opportunities for the Cashie River in Windsor, Meadow Branch in Lumberton and two creeks with flooding problems in Goldsboro. With grant funds from NC DOT, Doll’s team evaluated flood mitigation efforts for the Neuse River Basin, including identifying ways to improve early warning systems for transportation-related infrastructure and evaluating future storm severity. Doll recently completed a 16-month multi-disciplinary inter-institutional effort to evaluate the flood reduction potential of nature-based solutions (e.g. wetlands, water farming, and reforestation) focused in the Neuse Basin. Results of the modeling and economic analyses were shared with the NC General Assembly.