HomeMy WebLinkAbout0552.091-
December 12. 1991
THE HONORABLE CITY COUNCIL
Palo Alto, California
Members of the Council:
:Report in Brief
This report presents the results of a study made on the condition
of Arastraderc Lake Dam. No Council action is required.
Bagkground
As part of the 1990-95 Capital Improvement Program, Council
approveQ $30,000 (CIP 19102) for testing and design develop~ent
services to analyze the condition and adequacy of the dam to
withstand major storms an1!jor earthquakes. On April 18, 1991
Council approved an agreement with Geomatrix Consultants, Inc. to
provide these testing and design development services (CMR 225:91j.
The consultant has recently completed an evaluation of the dam and
has found that the dam spill'io.'ay is under!:; ized and too poorly
constructed to handle the volume of ¥ater generated from a 1D year
Qr greater storm. As a result. the lake ~ould fill and overtop the
crest of the dam causing additional erosion on the downstre~m face.
Previous e~osion on the face due to p~st storms over the last 35
years has created large gullies which have eroded the 55-foot crest
width to 35 teet.. continued erosion due to major storms could
result in a poE-~ble breach of the dam.
This erosion also has the potential to weaken the dam's resistance
to seismic deformation~ The darn \IIould likely aeform downwards
approximately foi.u· feet in the. event of a major earthquake,
resulting in a possible breach (approximately +8 on the Richter
Scale on the San Andreas fault) ..
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CMR,552:91
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Staff surveyed the houses downstream ~hich could b~ i~pacted in the
event of a breach in the dam. There are seven houses cDnstructed
adjacent to the <!;rcek ",hich leao3 from the daM. Of tnese, three
houses are located close enough to the cre~'k and 10\01 enough in
elevation to experience some risk of flooding above their finished
fleors. Ho'Wever, it is unlikely that these buildings .,.:auld pe
structurally damaged.
Discussion
Based on the results of the consultant stud~~, staff has examined
the follo'Wing options~
• Do Nothing.. Under this option, no improvements "Would be Ir.ade
to the dam. Although it is u;11ikcly th"t one large storm
would cause an immediate breach of the darn. a series of starros
or a significant earthquake could cause dam failure, resulti~g
in flood da~age to dc~nstream houses~ Fo~ this reason staff
does not recommend this option.
II DecomI!'tission th-E.......ne..m. TJnder this option, the lake .'ould be
permanently drained. A small pond \-,.ould remain to mitigate
State Fish and Game ~etlands reguir~~ent5. Several trees and
shrubs would be lost and thereiore need to be mitigated. A
culvert pipe through the dam .... ould be installed to handle
storm runoff ~ The lakebed .... ould be graded to provide
drainage, and the existing spill-....:ay .. -auld be filled .'ith seil.
This option would cost approximately $170,000. Because the
cost of decommissioning exceeds that of imprOVing the dam,
staff does not recommend this option.
• Improve the o.~_m. This option .. QuId provide protection from
both major storms and earthquakes. Itt\prQve~\ent.5 -.. 'Quld consist.
of constructing a concrete spill+ ... 'ay, clearing most of the
trees and vegetation from the do· ... '['stream and upstream face of
the dam, filling all holes an\:! gUllies +with engineered fill
(soil) ~aterial, a~o hydroseeding. No .. etlands but some tree
and shrub mitigation .. ould be. required. Staff recoTI\l!1ends this
option, which would cost approximately $150 /000.
Conclusion
Staff believes that improving the darn is the pre.ferable course of
action~ This option is less cost ly than deco:-:'.miss ion ing the dam
and results. in fewer environment.al irr.pacts. It 'Will provide
protection against dam failure and resulting property dam3ge.
Staff will include a project for inproving the dam in the F'f
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CMR:5S2:91
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1992-94 CIP program. Proposed timing and phasing ""ill be presented
during the FY 1992-94 budget process.
Respectfully submitted,
~~A~
Assistant Director of ~~blic Works
DAVID G. ADAMS
Director of Public Works
~, 2-
WI LL! AM ZANER
City Manager
Related Ct'!R:
CMR:552:91
225:91