Improving understanding of river restoration design and techniques
“I thought how lovely and how strange a river is. A river is a river, always there, and yet the water flowing through it is never the same water and is never still. It’s always changing and is always on the move. And over time the river itself changes too. It widens and deepens as it rubs and scours, gnaws and kneads, eats and bores its way through the land.”
―Aidan Chambers, "This is All: The Pillow Book of Cordelia Kenn"
Rivers and streams are dynamic systems—often reworking their beds and banks as sediment is picked up, carried along and deposited downstream. Annually, billions of dollars are spent on critical stream and river bank stabilization and restoration projects to slow or halt eroding stream banks and protect our nation’s built environment, and transportation and natural resource infrastructure alongside and across these waterways.
A common approach to bank stabilization involves armoring stream banks with concrete or riprap. However, this approach is costly and eliminates habitat along the margins of streams and rivers (riparian zone) and aquatic in-stream habitat. As a result, natural resource agencies now promote the use of in-stream, low-profile, flow control structures as a stabilization technique that is softer and more responsive to nature. These structures use less material, are less costly and can promote diverse aquatic habitat by allowing pools to form in streams and rivers.
Performance issues
Many in-stream structures fail in what they were designed to accomplish, primarily due to the difficulty in predicting the complex three-dimensional flow patterns in the vicinity of structures and the interaction of these flow fields with the streambed and banks.
Site-specific conditions such as stream gradient, bed and bank material, and meander bend characteristics can exert significant influence on the performance of in-stream structures.
Current design methodology for in-stream structures is often based on prior experiences or one-dimensional models that do not account for the complex physics governing the flow of water and sediment in unique meandering streams. In addition, monitoring efforts to evaluate the performance of in-stream structures and identify signs of failure are underutilized. The result—numerous stream stabilization and restoration efforts fail to protect the stream channel, yet the techniques used are replicated for the next projects because the causes of failures are unknown.
Common in-stream structures:
In-stream structures support stream bank stabilization by channeling the flow of water away from banks to reduce erosion and scour by slowing or redirecting the flow of water.
To address this critical need for physics-based research to improve in-stream structure design, a team led by the St. Anthony Falls Laboratory (SAFL) developed a new approach to identify and evaluate designs for in-stream structures for successful stream and river bank stabilization. This novel methodology is the result of a $600,000 project funded by the National Cooperative Highway Research Program (project NCHRP 24-33) to enhance the work of transportation and natural resources practitioners across the nation. This project was also supported by the National Center for Earth-surface Dynamics (NCED), a National Science Foundation Science and Technology Center.
The project team was led by SAFL Director Fotis Sotiropoulos and includes Professor Panos Diplas, Lehigh University (formerly Virginia Tech), Dr. Ali Khosronejad and Dr. Jessica Kozarek (both SAFL research associates). Other team members included Craig Hill, Seokkoo Kang, Anne Lightbody, Ryan Radspinner, Read Plott, Rajan Jha, and numerous undergraduate research assistants.
Building on current practice
Before launching in-depth analysis on in-stream structures, researchers surveyed transportation and natural resource agencies across the nation to gather key information on current practice, such as preferred structures, design methods and effectiveness for stream bank stabilization. Use, long-term durability and maintenance of a variety of in-stream structures surfaced in this data gathering phase, allowing researchers to select the most widely used techniques (see sidebar) on which to conduct extensive laboratory, field and numerical analyses.
Research was conducted using a multi-pronged approach spanning a range of spatial scales from a small laboratory flume to a field-scale meandering channel to large virtual river channels. Laboratory analysis in SAFL’s Indoor StreamLab (ISL) utilized the three-foot-wide tilting bed flume to study the effects of structure configuration on stream flow and bed erosion.
The most promising structures were tested in SAFL’s unique Outdoor StreamLab (OSL) facility, producing field-scale studies under laboratory control. In both cases, SAFL-designed data acquisition systems measured the complex flow fields and scour around the structures and stream banks.
Data from the laboratory and field analyses fed into the SAFL Virtual StreamLab (VSL3D), an advanced three-dimensional numerical model capable of predicting both flow and sediment processes in real-world complex streams. Using the collected data, the VSL3D model was validated and tested to ensure it can simulate true-to-form flow fields and scour patterns.
The team then utilized the VSL3D in conjunction with SAFL’s parallel supercomputers to optimize the number of structures and structure angle, spacing, rock size and installation depth, among other variables in two virtual river channels.
As a result of the comprehensive VSL3D modeling effort, the research team produced a novel simulation-based approach to select an appropriate structure and layout given a set of stream characteristics.
Next steps
The result of the study is a report that includes engineering guidelines and design methods, as well as recommended specifications for installation, monitoring and maintenance of in-stream structures in meandering channels. The guidelines will be published as a report from NCHRP available for distribution across its network of transportation and natural resource practitioners. This research will support a wide group of transportation and natural resource practitioners to establish effective and long-lasting stream bank stabilization solutions in streams and rivers across the nation by incorporating a physics-based approach to in-stream structure design.
Recent related publications
Kang, S., A. F. Lightbody, C. Hill & F. Sotiropoulos (2011). “High-resolution numerical simulation of turbulence in natural waterways.” Advances in Water Resources, 34, 98-113.
Kang, S. & F. Sotiropoulos (2011). Flow phenomena and mechanisms in a field-scale experimental meandering channel with a pool-riffle sequence: Insights gained via numerical simulation. Journal of Geophysical Research-Earth Surface, 116.
Kang, S., Khosronejad, A., and Sotiropoulos, F. (2012). Numerical simulation of turbulent flow and sediment transport processes in arbitrarily complex waterways. Environmental Fluid Mechanics (Eds: Rodi, W. and Uhlmann, M.), IAHR Monograph, CRC Press.
Kang, S., & F. Sotiropoulos (2012). “Numerical modeling of 3D turbulent free surface flow in natural waterways.” Advances in Water Resources, 40, 23–36.
Kang, S., & F. Sotiropoulos (2012). “Assessing the predictive capabilities of isotropic, eddy viscosity Reynolds-averaged turbulence models in a natural-like meandering channel.” Water Resources Research, 48(6), W06505.
Khosronejad, A., S. Kang, I. Borazjani, and F. Sotiropoulos, (2011). Curvilinear immersed boundary method for simulating coupled flow and bed morphodynamic interactions due to sediment transport phenomena, Advances in Water Resources, 34(7), 829-843.
Khosronejad, A., S. Kang, and F. Sotiropoulos, (2012). Experimental and computational investigation of local scour around bridge piers, Advances in Water Resources, 37, 73-85.
Khosronejad, A., C. Hill, S. Kang, and F. Sotiropoulos, (2013). Computational and experimental investigation of scour past laboratory models of stream restoration rock structures, Advances in Water Resources, 54, 191-207.
Plott, J.R., P. Diplas, J.L. Kozarek, C.L. Dancey, C. Hill, and F. Sotiropoulos (2013). A generalized log law formulation for a wide range of boundary roughness typically encountered in natural streams. Journal of Geophysical Research: Earth Surface. 118: 1419-1431.
Sotiropoulos, F., Diplas, P., Khosronejad, A. (2012). Scour around hydraulic structures. Handbook of Environmental Fluid Dynamics (Edited by H. J. S. Fernando), Taylor and Francis. CRC Press.