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Potential Drift Accumulation at Bridges
Potential for a River to Deliver Drift
The potential for a river to deliver drift combines the potential for wood to be introduced into the channel and the potential for drift to be transported downstream to the bridge site. Information gathered to aid in the assessment of drift may bear on potential for drift generation, drift transport, or both. Some information is direct information about large woody debris and drift. Other information about the stream and its basin has implications for drift generation and transport. Direct evidence should be evaluated first and given greater weight than indirect evidence (figure 28).
Figure 28. Flow chart for evaluating potential for drift delivery.
Observations of drift provide the most direct evidence for assessing potential for drift delivery to a site. However, a lack of drift at the site does not indicate a low potential for drift delivery (Pangallo and others, 1992). Observations of drift may come from bridges or from other sites of accumulation, and from the channel system upstream from the site or from channel systems in similar basins. Even if drift is currently sparse, infrequent catastrophic events or changes in the basin may provide abundant drift in the future.
Direct evidence for high delivery potential includes the following observations:
Direct evidence of currently low drift delivery, suggesting possible low potential for drift delivery, includes the following observations:
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