HomeMy WebLinkAboutBLD1990-00361 Geo Tech Anderson & Albrecht / Sec. 33, West of Port Townsend
away). Occasionally, a failure may cut even deeper into the
edge. At a frequency of perhaps once in 20-60 years a large
failure may result in bluff-edge retreat of 10-35 feet.
Setback Revision
Based on long-term average bluff retreat rates and the sizes
of failures seen at this site and also based on those previously
seen along bluffs similar to this one (5-35 feet in a single
failure), I believe the setback can be reduced to about 85 feet
without a great increase in risk. However, this change, of
necessity, does increase the risk slightly, simply because there is
somewhat less distance to the current bluff edge.
In considering the overall risk (regardless of whether the
85-foot or 100-foot setback is used), I might again emphasize two
points covered in my original report:
1) Because the timing and exact size of failures is impossible
to predict, the more generous the setback, the less
uncertainty there will be in the long-term adequacy of the
setback; and
2) My estimates of an adequate setback try to anticipate
bluff-edge losses during events such as earthquakes.
However, because of many unknown factors regarding the
size and effects on bluffs of very large earthquakes in
the past (and for that matter, the size of some
earthquakes prior to the relatively brief written history
of the Northwest), it is impossible to guarantee that a
given site would be totally safe from massive bluff-edge
retreat under all conceivable circumstances.
If I can answer any questions about my observations or
conclusions, please contact me.
Sincerely,
Ralph F. Keuler, M.S.
Consulting Geologist
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