Design Hydrology for Stream Restoration and Channel Stability at Stream Crossings

Significant resources are being applied by public and private road 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 defining the design hydrology for such efforts along with an understanding of how that design hydrology might change with land use changes. For natural stream watercourses, the morphology of the principal channel has been believed to be related to a comparatively frequent flood discharge. Fundamentally, the product of net sediment transport at a given flow rate and the frequency of occurrence of that flow rate is not monotonic, but achieves an extremum for some relatively frequent value. That value is called "dominant discharge" or "effective discharge." Some authors draw subtle distinctions between these two terms, but both are founded on the assumption that the geomorphology of the stream is an artifact of the dominant sediment transport processes. This discharge is often considered to be approximately represented in hydraulic geometry by the bankfull stage. The bankfull stage is the depth that occurs in the main channel before spilling over into the floodplain. Current thinking is that the recurrence interval of this flow is thought to vary somewhat across the United States, but in many studies the discharge at bankfull stage has been considered to be approximately a 2-year event or more frequent based on an annual peak flow series. Wolman and Miller (1960) have shown that for a variety of American rivers located in different climatic and physiographic regions, 90% of the sediment load is carried by events with less than a 5-year return period. Regardless of nomenclature associated with particular authors, it is evident that channel forming discharges correspond to flood-frequency return periods of less than 5 years. However, the focus on defining a design discharge based on traditional return period analysis of annual peak discharges has not proven successful. There is a need to look at the flow-duration curve to characterize the range of flows experienced by a channel and to identify how the flow-duration curve or key points on that curve may be used to derive design hydrology. In addition, much stream restoration and stream stability work is performed at sites were the upstream watershed is experiencing land-use changes. These changes, in turn, not only affect peak discharges but also the flow-duration curves. Discharges with recurrence intervals of 5 years or less are typically most significantly affected by land-use change such as urbanization, surface mining, removal of trees, and varying agriculture practices. In addition to peak flow values, considerable changes to flow duration relationships and total runoff volume can occur with changes in watershed character. For example, conventional wisdom is that urbanization usually involves the clearing of natural vegetation and replacement with impervious cover such as rooftops, roadways, parking lots, and sidewalks and increases the volume of flood runoff. Furthermore, "improved" stream channels through straightening or lining, curb-and-gutter streets and storm drains or sewers are thought to cause more rapid and dramatic watershed response to particular storm characteristics. Sauer and others (1983) showed that impervious cover of 30% and moderate engineered alteration of stream channels would approximately double the magnitude of discharge associated with a 2-year recurrence interval. Such increases in flow rate and flow volume imply related increases in net sediment transport potential. Conversely, diminished magnitude of frequent flows may result from a number of anthropogenic activities. Agricultural water conservation activities such as contour plowing, terracing, pumping from valley alluvium, and structural control such as flood control and water supply reservoirs can all result in dramatic reduction in both peak flow rate and flow duration associated with frequent events. The net result of diminishing flow would obviously be a reduction in net sediment transport potential and sediment throughput. Either increases or decreases in net sediment transport potential imply some similar change in the character of channel-forming discharge, consequently affecting both geometry and stability of existing stream channels. Many watersheds are affected by the land-use change. Changes in the character of frequently occurring event hydrographs imply that changes will also occur in the net sediment transport rates out of a watershed and in available stream power. The effects of these changes are unknown, but are thought to imply potential changes in-state of sediment throughput relationships. Research is needed to quantify the effects of land-use change on the channel forming discharges and resulting channel geometry that are important in designing culverts and bridges for long-term performance. The evolution of the stream channel is important to the design of culverts and bridges. If the stage-discharge curve of a bridge or culvert rises significantly more rapidly than the adjacent stream reach, stream power is diminished and local aggradation of the stream often occurs, reducing the effectiveness of the stream and the structure. If the stage-discharge curve of the structure does not rise as rapidly as that of the stream, it may serve as an obstruction, resulting in increased channel velocities and stream power that may lead to local stream degradation and instability. As a result, many state departments of transportation (DOTs) are designing these hydraulic structures to be consistent with bankfull or effective discharges. To obtain permits for highway construction, highway agencies are being required, by environmental agencies, to design stream restoration projects. The long-term hydrology of these designs has not been investigated, resulting in a weak link in protecting highway infrastructure from the effects of stream instability. This is especially significant in designing for bridge scour, the leading cause of bridge failures around the nation. The objective of this research is to develop a scientifically supported method for defining the design hydrology for such efforts along with an understanding of how that design hydrology might change with land-use changes. Steps to produce that methodology include the following: (1) Investigate flow metrics other than peak annual flood frequency curves for more consistent correlation with channel forming processes (such as distribution of daily mean discharge, flow duration, key points on a flow-duration curve, etc); (2) Develop quantitative methods for estimating the impact of land-use change on the design metric that is appropriate for design; and (3) Investigate the connection between these changes and changes in channel-forming discharge, and consequently bankfull channel hydraulic geometry. These methods should include changes due to urbanization, surface mining, agriculture, and forestry practices.

Language

  • English

Project

  • Status: Active
  • Funding: $350000.00
  • Contract Numbers:

    Project 24-40

  • Sponsor Organizations:

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590

    American Association of State Highway & Transportation Officials (AASHTO)

    444 North Capitol Street, NW, Suite 225
    Washington, DC  United States  20001

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001
  • Project Managers:

    Reynaud, David

  • Performing Organizations:

    Colorado State University, Fort Collins

    Fort Collins, CO  United States  80523
  • Principal Investigators:

    Bledsoe, Brian

  • Start Date: 20130722
  • Expected Completion Date: 0
  • Actual Completion Date: 20160121
  • Source Data: RiP Project 38344

Subject/Index Terms

Filing Info

  • Accession Number: 01547644
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 24-40
  • Files: TRB, RiP
  • Created Date: Dec 12 2014 1:01AM