
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).
10K GIF version of this diagram
Figure 28. Flow chart for evaluating potential for drift
delivery.
Direct Evidence from Observed Drift
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:
-
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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