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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-05-072
Date: July 2006
Assessing Stream Channel Stability At Bridges in Physiographic Regions
This chapter provides two examples for using the stream stability assessment method based on photos shown in the appendix A. The first example is Jayne Avenue over Jacalitos Creek near Coalinga, CA. Using http://terraserver-usa.com and the coordinates in table 5, a larger view of the bridge-stream intersection can be seen. The stream is seen to be mobile laterally, as evidenced by the large scars, deposits, and remnant channels. The photos of Jacalitos Creek in appendix A show views of the channel upstream and downstream of the bridge. Using the modified Thorne reconnaissance sheets given in figures 18-20, data collection begins by recording the map location, the GPS location, and the date. Next, characteristics of the watershed and flood plain are recorded. From the aerial photo on http://terraserver-usa.com, it is clear that the watershed use is primarily agricultural, while at least a portion of the flood plain is natural to allow for lateral movement of the stream. A visit to the site showed that cattle grazing is also a large part of the land use. The channel is braided to meandering with a riparian buffer of trees and shrubs upstream, but minimal buffer downstream. The channel was classified according to the Montgomery-Buffington and USACE methods. This yields a dune-ripple bed that is meandering to braided. The width and depth measurements are taken upstream and downstream of the bridge at approximate bankfull elevation. (Note that these measurements are not needed for the assessment method, but provide a record from which to compare the channel over many years.) The number of measurements needed depends on the variability of the channel dimensions. For this channel, three measurements upstream and downstream of the bridge were adequate to describe an average width-to-depth ratio. The measurements should be taken out of the influence of the bridge so that contraction scour does not influence the recorded dimensions. The general rule of thumb for reach length is 20 channel widths. In this case, where the channel is fairly uniform, 10 widths upstream and 10 downstream are adequate. The remaining data are recorded for the channel sediment, obstructions, and bank characteristics. The primary purpose of recording these data on the modified Thorne sheets is to help familiarize the user with the channel, focus the user's attention on various aspects of the channel, and provide a record of conditions that can be compared in subsequent years. While still onsite and after the reconnaissance sheets are filled out, the stability assessment sheet in table 8 should be used to determine the ratings for each of the 13 indicators. For Jacalitos Creek, the ratings and the total sum are given in table 9. Since this channel is in the dune-ripple category, table 10 is used to determine the overall rating.
The second example is S.R. 445 over Roaring Run in the Valley and Ridge Province of central Pennsylvania. Photos for this channel are provided in appendix A. Using http://terraserver-usa.com and the coordinates in table 5 to obtain aerial photos provides a larger view of the channel, the bridge-channel intersection, and watershed characteristics. Land use is primarily natural forest. The channel classifies as a step-pool channel according to the Montgomery-Buffington method and a mountain torrent, according to the USACE method. The modified Thorne sheets are completed to familiarize and focus the user on the channel, flood plain, and watershed. In this channel, the dimensions and other characteristics are relatively constant, so 10 channel widths upstream and downstream were adequate for estimating average dimensions and other observations. Cross-sectional width and depth were measured at the approximate bankfull elevation. Since the channel is very uniform both upstream and downstream, only one to two cross sections needed to be measured. The detail of the cross-sectional measurement depends on the user and the need to have detailed cross sections to compare in the future. For the purposes of the assessment method developed here, no cross-sectional data are used; however, as stated previously, it may be desirable to measure several cross sections in greater detail for future use. For the Thorne reconnaissance method, only an average width and depth are recorded. Although it is unnecessary for an initial or near-bridge channel stability assessment, it should be noted that walking much longer lengths of the channel can reveal disturbances that could eventually affect the bridge-channel intersection of interest. After the modified Thorne sheets are completed and the user is familiar with the stream, the stability assessment ratings can be determined. In this example, all indicators except for #13 show that the channel is very stable. The alignment of the channel and the bridge, however, cause #13 to be much higher than the others. The tight bend on which the bridge sits could clearly migrate in the future and cause problems at the bridge. Otherwise, the channel stability is rated as excellent, according to table 11.
If a stability rating is determined to be fair or poor, it might be desirable to return to the site for more detailed channel measurements by survey. Lateral movement and bed degradation can be measured over time by using such detailed, repeated measurements. However, to conduct rapid, preliminary assessments, as provided in this report, such detailed measurements are not necessary. If a channel is deemed to be stable (good to excellent), additional detailed measurements likely will not be needed. Thus, this method can be used as a decisionmaking tool regarding the need for more detailed and costly assessments.