BACKGROUND
Natural resource agencies have encouraged many state DOTs to use “natural” (context-sensitive) stream stability and restoration measures in lieu of traditional engineering responses to stabilize rivers and streams against erosion and scour. These measures include the construction of shallow, in-stream, low-flow structures across all or part of a stream channel. These measures have gained wide acceptance with national and state regulatory agencies responsible for protecting natural habitats because the structures often enhance stream habitat. Structure types include cross vanes, j-hooks, w-weirs, constructed riffles, and stream barbs usually constructed of rock riprap. Proponents of these structures have claimed that they can be durable and cost-effective and provide scour stability, but the necessary case studies have not been documented to verify these claims. Comprehensive engineering and financial criteria do not exist for evaluating, designing, installing, and maintaining these structures.
In-stream, low-flow structures sometimes require less rock than traditional engineered streambank stabilization, promising greater economy, especially where material sources are scarce. The state of the art in design and installation does not include proven engineering design criteria, thus current efforts to apply these structures often result in frequent instances of failure and associated costs for repair. Validated engineering criteria promise reduced risk of failure and increased cost-effectiveness of installations.
Quantitative optimization of parameters such as lifecycle cost, size, spacing, and foundation depth and their influence on scour depth, sediment transport, and long-term structure and channel stability will support the development of engineering design, installation, and maintenance criteria necessary for hydraulic engineers to design economic in-stream, low-flow structures with confidence.
OBJECTIVE
The objective of this project is 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, long-term performance in terms of the low-flow structure stability, durability, and survivability; (3) recommended installation practices; and (4) maintenance requirements.
The guidelines should include a description of conditions under which in-stream, low-flow structures are either successful or not effective in providing protection against erosion and scour and in performing applicable habitat restoration functions.
Accomplishment of the project objective will require at least the following tasks.
TASKS
PHASE I
(1.) Perform a comprehensive review of relevant literature. The literature review should identify research in progress as well as completed work. (2.) Conduct a survey of all state DOTs and state and federal natural resource agencies to describe and evaluate existing low-flow structure applications including effective uses, limitations, material specifications, installation guidelines, possible failure modes, and design methods. Provide the survey questionnaire and contact list to the project panel for approval prior to performing the survey. From the survey responses and follow-up discussions, identify potential sites for use in calibrating the Task 4 modeling efforts. (3.) Prepare an interim report that includes the results from the Task 1 literature search and the Task 2 survey. The interim report should also describe the prevalent low-flow structures and their current installation guidelines and practices; identify exemplary practices and limitations of existing design guidance; include a list of different types of structures in a matrix format that (a) classifies according to hydraulic type and common characteristics, (b) identifies the top six (plus or minus) most common and successful types, and (c) identifies modes of failure and possible failure reasons, including extreme events, design and installation problems; and provide an updated work plan for Phase II.
PHASE II
(4.) Evaluate the stability of the individual low-flow structures. Develop physical and numerical models (two- and/or three-dimensional) to analyze the forces on the stream structures (general and detailed, based on hydraulics and sediment transport) and to understand the in-stream structure mechanics for all selected types of structures for applicable design flow regimes. (5.) Using appropriate models, evaluate and compare the effectiveness of selected low-flow structure types to protect surface transportation infrastructures. Develop new structure types to improve and optimize their effectiveness. (6.) Develop methods for predicting the performance of in-stream, low-flow structures based on the results of Tasks 4 and 5. Prepare design and installation guidelines. Classify structures according to uses (when and where each structure can be installed) and types of flow regimes and stream conditions. (7.) Verify the predictive methodology and design guidelines by using the physical and numerical models of existing installations identified in Task 2. (8.) Prepare guidelines describing recommended structure types, design principles, and proper installation methods. Provide a final report summarizing the total research effort (including but not limited to physical and numerical modeling analyses, predictive methodology rationale, and an explanation of how adopted design and installation hwere developed).
A link to NCHRP Report 795: Design Methods for In-Stream Flow Control Structures presents design guidelines for in-stream flow control structures used to limit lateral migration and reduce bank erosion is here. Appendices A through E and Appendix G are available here.
