RPVCCA_CC_SS_2014_02_18_02b_Altamira_Canyon_Drainage_Project
TO: HONORABLE MAYOR AND CITY COUNCIL MEMBERS
FROM: MAYOR PRO TEM JIM KNIGHT
DATE: FEBRUARY 18, 2014
SUBJECT: ALTAMIRA CANYON DRAINAGE PROJECT
Recommendation
Staff to review the Altamira Canyon Drainage (ACD) Project as outlined in this report.
Staff to come back to Council with their recommendations as to the feasibility of the
project, any modifications of the ACD project or if another type of Altamira Canyon
drainage project altogether would be preferable.
ALTAMIRA C ANYON DRAINAGE (ACD) PROJECT
Background
Over the course of several years I have had the opportunity to have many discussions
with Bob Douglas about the drainage dynamics of Altamira Canyon. These discussions
occurred both individually and in my capacity as Board member of the Abalone Cove
Abatement District. We both agree that infiltration of water into the Abalone Cove
landslide through the bottom of Altamira Canyon needs to be abated.
Based on geological investigations as well as data and observations collected over the
years by ACLAD and the City, it has long been known that the infiltration of storm water
discharge in Altamira Canyon contributes a substantial amount of water to the ground
water beneath the Abalone Cove Landslide. 60-70% of the discharge from individual
storms can be lost down major fracture zones which act as a major conduit into the
subsurface. 1
ACLAD dewatering wells (WW) tells us something about water infiltration rates.
Normally, after a rainy period, most well production will increase 1-3 months from the
rain events. But WW18, near Altamira Canyon, will have a spike of well production
almost immediately following a rain event. This tells us that water is infusing directly
into subsurface via the canyon floor fissures.
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Altamira Canyon Drainage Project
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In addition it appears that the subsurface ground water from the canyon drains into the
western portion of the Portuguese Bend landslide and contributes to the slipping of
Palos Verdes Drive South between Portuguese Point and Inspiration Point, sometimes
referred to as the “ski jump”. (As illustrated in Exhibit A)
Over the years, the City has studied potential projects to reduce storm water infiltration
in the canyon but none of them have been attempted. Part of the problem was the
previous projects attempted to tackle the entire canyon as one project which would have
cost millions of dollars to fund.
ACD Project description
This project divides the canyon mitigation into several prioritized phases making the
entire project more manageable for funding options. (Exhibit A)
Phases 1, 2 and 3 could be reviewed as one project for the purposes of CEQA and the
Army Corps. of Engineers Sec.404 permitting with a time allowance for implementation
of the individual phases. Phase 4 could be included in the overall project once a
decision is made as to which options described below will be used.
Funding options
By dividing the project into phases, funding will be more feasible. Of course, if the city
does find a funding source for the completion of all phases at one time, the entire
project could be completed at an earlier date maximizing the mitigation benefits.
Phase 1 - Lower Altamira Canyon
The first priority is to line the bottom of the lower portion of the canyon from about the
Strauss property to the CMP inlet at the Pinkham property. This is an area of particular
importance because the canyon crosses the fissures at the head of the Abalone Cove
landslide that allow a high volume of storm drain water to infiltrate directly into the
subsurface contributing to a build-up of groundwater in the slide.
The lining would be composed of large boulders embedded into a cement bottom with a
wall on each embankment. The boulders would be staggered acting as energy
dissipaters to reduce the destructive force of fast moving water. The smaller rocks
would be embedded between the larger boulders along the canyon bottom as well as
into the cement along the canyon wall lip on each embankment. The combination of
different sized rocks and a natural color to the cement would help blend the project into
the natural surroundings. (Fig.1)
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Altamira Canyon Drainage Project
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Figure 1
Phase 2 - Beachside of PVDS
The next important project would be to line the canyon starting from the CMP culvert
beachside at PVDS. A large diameter pipe 2 would be installed that has the strength to
withstand the storm water flow and the flexibility to follow canyon contours. Where the
pipe ends will be a lining designed to slow down the storm water and direct it to the
shoreline in a manner to minimize erosion at the shoreline.
Option 2b
The section of CMP under the road (between Phase 1 & 2) currently is in marginal
shape. It has a fractured cement lining that covers the rusted-out bottom of the CMP.
The beachside pipe could be extended up this section of CMP to connect with Phase 1.
Phase 3 – From northern end of Phase 1 to Narcissa and Sweetbay
This phase would be identical to, and blend into, Phase 1. The combination of Phase 1
and 3 would minimize the infiltration of storm drain along this portion of Altamira Canyon
which would help to divert water away from the subsurface of the slide area between
Portuguese Point and Inspiration Point reducing the need to repair this stretch of road
along PVDS.
Possible Phase 4 – Altamira Cyn. from Phase 3 to the head of the Canyon
The portion of Altamira Canyon above Narcissa near Sweetbay would have several
options and can, if needed, be broken down into multiple projects.
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Options could be:
A) lining as with Phase 1 &2;
B) filling the fissures in the canyon; or
C) a combination of A and B.
Possibly some sections of the canyon could be straightened with added reinforcement
to prevent excess debris from flowing down the canyon.
1”The Creepy (Slow Moving) Landslides of the Portuguese Bend Area”, Bob Douglas,
Special Publication No. 24, 2013
2Polyethylene pipe with steel reinforcement and pressure rated HDPE resin
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