To design economic in-stream flow control structures with confidence, hydraulic engineers need sound engineering design, installation, and maintenance criteria. These criteria must be supported by quantitative optimization of parameters, such as the amount of construction materials, life-cycle cost, size, spacing, and foundation depth, and their influence on the stream habitat, scour depth, sediment transport, and long-term structure and channel stability. The objective of NCHRP Project 24-33 was to develop quantitative engineering guidelines, design methods, and recommended specifications for in-stream, low-flow structures that address (1) erosion protection, channel stability, sediment transport, and scour stability of the stream; (2) cost-effectiveness, as well as long-term performance in terms of the low-flow structure stability, durability, and survivability; (3) recommended installation practices; and (4) maintenance requirements. To develop these guidelines, researchers at the University of Minnesota’s St. Anthony Falls Laboratory and Virginia Tech coupled an in-depth review of the current use of in-stream structures with a comprehensive study of five of the most commonly used in-stream structures using physical and numerical experiments. They conducted physical experiments at two scales: a laboratory-scale straight-channel flume and an outdoor field-scale meandering experimental channel. After extensive validation using the physical experimental results, a state-of-the-art coupled hydrodynamic and bed morphodynamic model was used to investigate the performance of various structure configurations under different geomorphic settings. This model, dubbed the Virtual Stream Lab (VSL3D), is capable of simulating the complex three-dimensional flows around in-stream structures and their interaction with the bed. This report presents design guidelines for in-stream flow control structures that are often used to limit lateral migration and reduce bank erosion. The guidelines include a description of conditions under which in-stream flow control structures are either successful or not effective in providing protection against erosion and scour and in performing applicable habitat restoration functions. Unlike rip-rap, which strengthens the bank to withstand the applied hydrodynamic forces, in-stream flow control structures alter the stream-flow patterns to shift the high-velocity thread away from the bank. (Author/publisher)
Abstract