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Re: Appropriate Seismic Risk for Earthen Dam

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Laurence B. Oeth [mailto:viacalx(--nospam--at)europa.com] wrote:
> Subject: Appropriate Seismic Risk for Earthen Dam

> Please consider the following: Earthen dam, circa 1920's and 1940's
> construction (phase 1, then heightened in phase 2), currently Zone 3.
Slope
> stability on downstream face was analyzed for 10%/50 year event and found
to
> be 1.0.
> 
> This dam impounds drinking water for a moderately sized community
(70,000+),
> with the treatment plant immediately below.  High population densities are
> somewhat removed, however.  It seems to me, given the potential results of
> failure that this structure should be evaluated more stringently than just
> any old office building...say 2%/50 year OR set the performance level to
> "Immediate Occupancy" at 10%/50 year.
> 
> Anyone out there have an enlightened opinion regarding input ground
motion?

Ah, this is a public policy question that should be answered by involving
local (e.g., city/county) representatives. In essence, you're asking "what
is an acceptable risk?" Engineers may have a different perception of
acceptable risk from those who would be affected by (1) failure of the dam
and/or (2) loss of its beneficial use.

Recently we analyzed the effectiveness of outreach efforts associated with
California's Seismic Hazards Mapping Act. The Act empowers cities and
counties to determine what level of risk is acceptable. The Act requires
engineers and engineering geologists to investigate seismic hazards and, if
problems are recognized, recommend mitigation that at least satisfies the
city/county that the level of risk is acceptable. Per our conclusions and
recommendations:

	"Ideally, each local [general plan] should... consider issues
related to "acceptable risk." Such an analysis should include a discussion
of potential losses, costs and feasibility of mitigation, survivability, and
post-disaster economic recovery. Armed with this knowledge, local decision
makers can formally adopt a finding regarding the level of acceptable risk
that the city or county deems acceptable. That finding can, in turn, be used
by local building officials and planning departments in the permit review
and approval process..." [Smith, T.C. and B. McKamey, 2000, Summary of
Outreach Activities for California's Seismic Hazards Mapping Program,
1996-1998: California Division of Mines and Geology Special Publication 121,
p. 36]

Even if a community doesn't have a general plan, it's probably a good idea
to involve them in the decision-making process. It could well be that the
perceived risk of failure is so great in comparison of the benefit that they
decide the dam should be removed. Or it might be that the benefit is so
great that they decide some or all of the population downstream should be
moved. More realistically, they probably will be able to tell you what level
of risk they feel is reasonable and you can use that to select your design
event.

  -- Ted

Theodore C. "Ted" Smith, R.G. 3445, C.E.G. 1029 <tsmith(--nospam--at)consrv.ca.gov>
Supervising Geologist, Technical Information & Support Program
California Department of Conservation, Division of Mines & Geology
801 K Street, MS 14-33, Sacramento, CA 95814; Phone: 916-445-0394