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.
Direct evidence for high delivery potential includes the following observations:
- Multiple cases of drift accumulation at bridges.
- Chronic drift accumulation at one or more sites.
- Drift accumulation at sites where potential for drift accumulation would be low if drift were not abundant.
- Abundant drift in the channel.
- Past need for drift removal in the channel system.
Direct evidence of currently low drift delivery, suggesting possible low potential for drift delivery, includes the following observations:
- Negligible drift after floods at sites with high potential to trap drift.
- Negligible drift delivered in large floods, ice storms, and wind storms.
- Drift absent after floods in typical drift-accumulation sites other than bridges.
- All drift accumulates in forested channel upstream.
- Drift in the channel is stationary during floods because of low flow velocity.
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