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Potential Drift Accumulation at Bridges
Information that does not bear directly on the presence or absence of drift may be useful in assessing potential for drift delivery to a site, but always requires a large element of subjective judgment as part of its application. Such information can bear on drift generation, transport, or both. If the potential for generation can be determined to be low, then the potential for drift delivery is also low. If abundant drift is generated, the ability of the stream to transport drift and the maximum dimensions of drift that can be transported determine the potential for drift delivery.
Trees introduced into the channel by bank erosion are the dominant type of large drift at most sites. Consequently, most indirect evidence regarding drift generation is evidence of existing or potential bank erosion. Basins containing bridges with chronic drift accumulation generate drift continuously or during annual high-water periods. Most basins have a high potential to generate drift in infrequent, catastrophic events such as large floods, ice storms, and intense wind storms.
Indirect evidence for abundant drift generation includes the following observations:
Indirect evidence for low potential for drift generation includes the following observations:
If indirect evidence suggests that the rate of drift generation is high, or could become high, the potential delivery of drift will be controlled by the ability of the stream to transport it. Most streams are capable of transporting some drift. Assume that a given stream can transport drift unless evidence shows that drift accumulates where it is generated, rather than being transported downstream. Forested channels (where trees are numerous and dense across the channel bottom) transport little drift, and can be assumed to have low potential for delivery of drift as long as the forest is not cleared.
Channel dimensions upstream from the site, particularly channel width, affect the size of drift that can be transported, and thereby influence the potential size of accumulations. The width and depth of the channel upstream from the site may increase over the life of the bridge. Indicators of potential increases should be taken into account when estimating future dimensions. Such indicators include: a history of channel migration, widening, or down-cutting; existing or planned dams; and history or prospect of human alteration of the channel network. Erosion-resistant bed and bank material may limit future channel evolution, and effective channel stabilization may prevent future widening and deepening.
The length of transported drift exceeds the channel width in some situations. Where deep water flows unimpeded by forest over the width of a valley, the width and depth of the valley-wide flow control transported-drift dimensions. Deep valley-wide flow is most common in relatively steep, narrow valleys subject to infrequent, large floods. In V-shaped valleys with slopes of 3 percent or more, debris torrents are capable of moving full-sized logs, uprooting or shearing off mature trees, and incorporating them as additional drift.
Estimate the maximum design log length on the basis of channel width upstream from the site. Channel width is the perpendicular distance between banks or lines of permanent vegetation, and it should be measured at inflection points between bends (Lagasse and others, 1991). The longest logs in wide channels reach a maximum length of about 24 m (80 ft) in much of the United States. Channels less than 12 m (40 ft) wide transport logs equal in length to the upstream channel width. In channels in the Eastern United States from 12 m (40 ft) to 60 m (200 ft) wide, the estimated design log length is 9 m (30 ft) plus one quarter of the channel width (figure 24).
A relatively shallow channel may transport logs of a shorter maximum length than would be estimated on the basis of channel width. The depth sufficient to float logs is the diameter of the butt plus the distance the root mass extends below the butt, or roughly 3 to 5 percent of estimated log length. Therefore, the length of transported logs with attached root masses rarely exceeds about 30 times the maximum flow depth; larger logs accumulate in the channel. If flow deep enough to float large logs off the bed can occur, however infrequently, large logs stored in the channel may eventually be transported.
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