The NCHRP Research Report 853 describes scientifically supported methods for defining the design hydrology for stream restoration and channel stability at stream crossings with a set of decision support tools that are both science-based and practical in guiding users to an appropriate combination of design tools and depth of analysis for design hydrology in a given hydrologic and geomorphic setting. Specifically, the guidance and tools provide support in: (1) assessing the current conditions adjacent to a stream crossing and in the upstream watershed to determine design effort, (2) performing the appropriate hydrological and geomorphic analysis using a set of analytical and analog tools, and (3) designing the channel through stream crossing for stability and sediment balance. The hydrologic metrics and tools developed in this project provide a general framework and stronger physical basis for design hydrology at stream crossings, including locations where watershed land use is changing. The report will be of immediate interest to hydraulic engineers.
Significant resources are being applied by public and private highway and rail organizations to design and construct restored streams in disturbed watersheds as well as to provide for stable transportation crossings (bridges and culverts) of streams. Lacking in this effort is a scientifically supported method for (1) defining the design hydrology for such efforts and (2) understanding how that design hydrology might change with land use changes.
Much stream restoration and stream stability work is performed at sites where the upstream watershed is experiencing changes in land use and runoff characteristics. These changes affect not only peak discharges, but also flow duration relationships, total runoff volume, stream power, sediment supply, and sediment transport. Increases or decreases in net sediment transport potential imply changes in the character of channel-forming discharge, consequently affecting both the geometry and stability of existing stream channels.
Understanding how hydrology may vary over time or with changes in the watershed is a weak link in protecting highway infrastructure from the effects of stream instability. Research was needed to quantify the effect of these hydrologic changes on the channel-forming discharges and the resulting channel geometry that are important in designing culverts and bridges for long-term performance.
Research was performed under NCHRP Project 24-40 by Colorado State University to develop guidance based on a scientifically supported method for determining the design hydrology for stream restoration and channel stability at stream crossings and for understanding how that design hydrology might change over time. Several decision support/analysis tools were developed to improve and facilitate design hydrology analyses. The tools include: (1) a decision tree to be used with web-based hydrologic analysis tools (eRAMS) for generating design hydrology metrics under existing and future land use scenarios, (2) guidance on relating channel response potential to an appropriate level of design analysis, (3) guidance on selection of analog reaches and performing rapid geomorphic assessments of channel instability in the field, and (4) a spreadsheet-based Capacity Supply Ratio Tool (CSR Tool) for computing analytical channel designs that account for the full spectrum of sediment transporting events.
This research report is Appendix C of the research agency’s final report, which documents the entire research effort. The research agency’s final report including Appendices A, B, and D is available on the summary web-page for NCHRP Research Report 853. The three appendices are titled as follow:
· Appendix A—Site-specific Information for Study Sites
· Appendix B— Tutorials for the use of eRAMS (the environmental Resource Analysis and Management System)
· Appendix D— Reference Manual: CSR Tool
The CSR Tool software and two illustrative examples for different stream types (sand bed, and gravel/cobble bed) are also available on the summary web-page for NCHRP Research Report 853.