EXHIBIT 24-A
MONTEREY PENINSULA WATER
MANAGEMENT DISTRICT
MEMORANDUM
DATE: April
7, 2008
TO: Darby
Fuerst, General Manager
FROM: Andrew
M. Bell, District Engineer
SUBJECT: Report
on Factors to Consider in Determining the Feasibility of Raising Los Padres
Dam, or Otherwise Increasing the Storage Capacity of Los Padres Reservoir
BACKGROUND
At
the February 28, 2008 Board meeting, the Board approved a request by Director
Bob Brower to direct staff to prepare, for review at a future Board meeting, a
report regarding the feasibility of increasing the height of the existing Los
Padres Dam. The stated purpose of the
project is to create additional storage capacity in the reservoir so that
stored water could be released to increase streamflow in the
The
existing Los Padres Dam, constructed in 1948 and 1949 by California Water and
Telephone, predecessor of California American Water, is an embankment dam
(zoned earthfill) approximately 150 feet high.
It is located on the
DISCUSSION
Some
important questions to consider in determining a project description for
raising Los Padres Dam are as follows:
1.
How much additional storage is
desired (and therefore how much would the height of the dam need to be
increased)?
One
set of inputs to this decision is what benefits in terms of increased
streamflow are desired. This would
require a hydrologic analysis of streamflow, combined with biological input on
the needs for different animal and plant species that are intended to benefit
from the project.
2.
Can all or any of the existing
structures be kept as part of the ultimate project?
It
is most likely that the existing dam is not capable of being built upon, and
that in order to have a higher spillway at the same location would require
removing the existing dam, spillway, and outlet works and replacing them with
newly-engineered structures. See further
discussion below under “Determining Whether the Existing Dam Could Be Built
Upon” for additional information.
3.
When full, would the new reservoir
extend into the Ventana Wilderness?
If
so, a land exchange similar to that proposed for MPWMD’s New Los Padres Dam and
Reservoir Project and CAW’s Carmel River Dam and Reservoir Project, or other
arrangement acceptable to the U.S. Forest Service and other entities with
interests in wilderness areas, would be required. An increase in the elevation of the existing
spillway of five feet or more would cause the reservoir to extend into the Ventana
Wilderness along Danish Creek near its mouth.
Attachment 1 is a figure from the
1998 Draft Supplemental EIR for California American Water’s proposed Carmel
River Dam and Reservoir Project, which was a new, higher concrete gravity dam
to be located approximately one-half mile downstream of the existing dam and a
reservoir with a capacity of 24,000 acre-feet.
This figure shows the “Existing Wilderness Boundary” and the “Proposed
Los Padres National Forest Boundary Adjustment,” which would be an addition of
approximately 140 acres (Parcel B) to the Ventana Wilderness in exchange for
removing approximately 23 acres (Parcel A).
4.
Would the existing fish passage
facilities be adequate, or would improved fish passage be a requirement of the
project?
Steelhead
passage facilities at Los Padres Dam are considered by some to be
inadequate. Upstream migrants must find
a fish ladder that leads to a holding tank.
During the upstream migration season, the tank is checked daily by a
California American Water (CAW) employee, and if steelhead are in the tank, the
CAW employee transports them to the reservoir.
Downstream migration is achieved only via the spillway, resulting in
significant mortality to migrants. If
the dam were raised or replaced, California Department of Fish and Game and
National Marine Fisheries Service would likely call for significant
improvements to the existing methods of fish passage for both upstream and
downstream migrants. One potential
method of providing improved passage would be to install a fish ladder.
5.
How would access to upper Los
Padres Reservoir, the Ventana Wilderness, and private parcels upstream of the
dam be maintained, both during construction and once the new project facilities
are in place?
Existing
access is provided by a road across the dam face and extending approximately
one-half mile upstream along the west side of the reservoir. From there, trails suitable for hikers,
fishermen, backpackers, horses, and other pack animals extend southerly and
westerly into the Ventana Wilderness.
These access paths would need to be maintained during and after any
construction.
6.
What routes would be used for
construction access?
Existing
public road routes are (1) Carmel Valley Road to Cachagua Road, southerly and
southeasterly on Cachagua Road to Nason Road in Princes Camp, then southerly on
Nason Road to the existing dam; and (2) Carmel Valley Road to Tassajara Road in
Jamesburg, southwesterly on Tassajara Road to Cachagua Road, westerly on
Cachagua Road to Nason Road in Princes Camp, then southerly on Nason Road to
the existing dam.
Determining Whether the
Existing Dam Could Be Built Upon
Primary
in determining the feasibility of “raising” the existing dam is to determine
the condition of the foundation and abutment formation materials in the area of
the damsite. Some of this type of
exploration work was done prior to building the existing dam, but it would be
necessary to do additional testing, including
foundation coring and possibly exploration of nearby seismic
faults. Deciding factors in determining
whether all or part of the existing dam could be preserved include whether
proposed structures would be capable of withstanding seismic and flooding
standards of the California Department of Water Resources, Division of Safety
of Dams (DSOD). In order to learn
whether the new structures would be approved by DSOD typically requires
conducting geotechnical and engineering field studies and analyses, and
presenting the results of those studies along with a proposed design as part of
a permit application to DSOD.
It
would be necessary to determine what types of construction (e.g., embankment,
concrete gravity, concrete arch) would be appropriate. In the event DSOD would allow adding to the
height of the existing dam, the appropriate method of new construction may be
earthfill, the same category as the existing structure. However, DSOD no longer allows spillways to
be incorporated in large embankment dams on streams with the potential for
significant streamflow. Alternative
spillway types include a tunnel separated from the embankment. This means that either a different type of
spillway than the current concrete-lined channel built into the embankment
would be required, or else the existing dam would need to be removed and
replaced with a different type of dam construction.
Flood and Seismic Safety
Current
DSOD standards require a dam to safely resist forces that would occur during
significant flood and seismic events. The basic requirements are to safely pass the
peak streamflow during what is termed the Probable Maximum Flood, and to safely
resist the Maximum Credible Earthquake.
Hydrologic, seismologic, and structural analyses must be conducted to
determine to the satisfaction of DSOD that a proposed structure will meet these
requirements.
Environmental Review and
Permitting Requirements
Environmental
review would likely require completion of state (CEQA) and federal (NEPA)
environmental review processes. Permits
for a major revision or reconstruction
of Los Padres Dam would at minimum require the following permits:
California
Department of Fish and Game Stream Alteration Agreement
It
may also be necessary to obtain a change to CAW’s water rights license for Los
Padres Dam and Reservoir. In conducting
the studies needed to bring the New Los Padres Dam and Reservoir Project to a
vote in 1995, the District learned that there are a host of other requirements,
including mitigations for potential impacts to steelhead,
Rights of Way
Los
Padres Dam and Reservoir and much of the surrounding land belong to CAW. It would be necessary to obtain either
ownership of some or all of the facilities and land, or agreement by CAW to
allow any proposed modifications to their property. In addition, if the spillway elevation were
raised five feet or more, the new reservoir would extend into the Ventana
Wilderness along Danish Creek near its mouth.
In this case, some property arrangement acceptable to the U.S. Forest
Service and other entities with interests in the Ventana Wilderness, and
wilderness areas in general, would be required.
ALTERNATIVES THAT COULD BE
CONSIDERED
Staff
is aware of two alternatives to raising the existing dam that would increase
the storage capacity of the reservoir.
These are described below.
Remove Accumulated Sediment
from the Existing Reservoir
A
significant amount of sediment has become deposited in Los Padres Reservoir
since it was created by the construction of Los Padres Dam in 1948 and
1949. MPWMD has periodically looked at
the potential for removing the sediment.
The most recent MPWMD report on this topic, dated July 28, 2000 and
titled “Preliminary Draft, MPWMD Issue Paper 2000-001, Assess Potential
Feasibility of Dredging Existing Reservoirs on the Carmel River,” is attached
as Attachment
2. In 1996, CAW consultant
Moffatt & Nichol issued a report titled “San Clemente Reservoir Dredging
Feasibility Study,
Install an Inflatable Dam
in Los Padres Spillway
An
inflatable dam could be installed in the spillway to raise the reservoir
elevation during periods when it is considered safe to do so without adversely
affecting the spillway’s ability to safely pass flows. To determine this would require confirming
with DSOD the dates when the reservoir level could be raised.
CONCLUSION
Increasing
the height of Los Padres Dam, or otherwise increasing the storage capacity of
Los Padres Reservoir, would require extensive engineering, geotechnical,
environmental review, and project planning efforts. In order to evaluate the technical
feasibility of this project, the District would need to retain expertise in dam
construction and rehabilitation, in environmental impacts of such a project,
and most likely in fish passage biology and engineering. MPWMD staff members are not qualified to make
such determinations. The timelines and
costs for such studies would require staff to developing scopes of work for the
desired studies, obtain proposals from consultants, and analyze the proposals.
Attachments
1. Figure
2-2. Location Map - Proposed
2. Preliminary
Draft, MPWMD Issue Paper 2000-001, Assess Potential Feasibility of Dredging
Existing Reservoirs on the Carmel River, July 28, 2000, prepared by Andrew
Bell, District Engineer, and Henrietta Stern, Project Manager
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