Attachment 2
PRELIMINARY DRAFT
MPWMD ISSUE PAPER
2000-01
ASSESS POTENTIAL FEASIBILITY OF DREDGING EXISTING
RESERVOIRS ON THE CARMEL
RIVER
Prepared by Andrew Bell,
District Engineer; and Henrietta Stern, Project Manager
Monterey Peninsula
Water Management District
August 2000
This
preliminary draft is for discussion purposes only. This issue paper will be updated following
receipt of the Revised Draft EIR for the San Clemente Dam Seismic Retrofit
Project, which is being prepared by the California Department of Water
Resources.
ISSUE
The storage capacities of the two
existing reservoirs on the Carmel
River have been
significantly reduced due to sediment deposition over the past eight
decades. This paper reviews current
information to address the question, “Should the District, in cooperation with
California- American Water Company (Cal-Am), pursue sediment removal from the
two existing reservoirs to either: (a) maintain storage capacities at current
levels, or (b) increase storage capacities to all or part of the original
reservoir volumes?” This effort is
pursuant to Objective 8 of the Monterey Peninsula Water Management District
(MPWMD) Year 2000 Water augmentation Plan approved by the Board on April 17,
2000, which states:
Evaluate
feasibility of increased storage in existing reservoirs through sediment
removal (dredging).
OBJECTIVE
The objective of this document is
to summarize into a concise format:
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existing information on sediment volume and
management issues for each reservoir;
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potential benefits of sediment removal;
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potential problems in removing sediment from the
reservoirs; and
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additional information needs in order to guide decision-making
regarding sediment removal from San
Clemente and Los Padres Reservoirs.
SUMMARY
This paper reviews current
information about sedimentation trends at the two existing reservoirs -- San
Clemente Reservoir, completed in 1921, and Los Padres Reservoir, completed in
1948 -- as well as the potential feasibility of dredging and other sediment
removal methods. The paper concludes
that sediment management at San Clemente Dam and Reservoir, whether by dredging
or other means, has become increasingly important due to potential effects on
aquatic life and downstream flood elevations.
This paper finds that sediment removal from the reservoirs is
technically feasible, but may result in potential significant adverse impacts
to people, aquatic resources, and
wildlife. The high cost of dredging may
be partially offset by marketing the dredged materials. More information is needed on the
characteristics and volume of dredged materials at both reservoir sites, as
well as further information on the effects of existing regulations, such as the
Endangered Species Act, before a definitive determination can be made on the
feasibility of dredging in the Carmel
River.
BACKGROUND AND LOCAL SETTING
Sediment deposition has
significantly decreased the storage volumes of the two reservoirs. Storage capacities in acre-feet (AF) at the
respective spillway elevations (525 feet at San Clemente Dam and 1,040 feet at
Los Padres Dam) are as follows:
Year Original Current Decrease Avg. Rate
Reservoir Completed Capacity Capacity in
Volume of Decrease
San Clemente 1921 1,425 AF 149 AF 1,276 AF 16
AF/yr
Los Padres 1948 3,033 AF 1,569 AF 1,464 AF 28
AF/yr
These decreases in storage have prompted
investigations of the feasibility, costs, and impacts of sediment removal, both
to preserve the ability to operate and maintain the reservoirs, and as a
potential means of augmenting the Monterey
Peninsula’s water
supply. The District has included
dredging of the two reservoirs in its evaluation of water supply project
alternatives since the late 1980s.
Previous District evaluations have determined that dredging as a water
supply option is too costly when compared with other options, and that dredging
activities would result in significant potential environmental problems. Cal-Am has retained consultants to
investigate the potential for sediment removal from San Clemente Reservoir. The purpose of a 1996 study prepared for
Cal-Am (Moffatt & Nichol Engineers, 1996) was “...to evaluate several
dredging alternatives for removal of accumulated sediments, such that the water
storage capacity of San Clemente Reservoir is restored to, and maintained at,
functional levels.” Cal-Am is currently
considering sediment management options at San Clemente Reservoir as part of
its plans for strengthening San Clemente Dam.
These studies contemplate bypassing incoming sediment in order to
maintain storage capacity at or slightly above the current volume, but not restoring
the original capacity.
PREVIOUS FINDINGS
To date, information relating to
dredging San Clemente and Los Padres Reservoirs
has been obtained primarily from reservoir dredging experience by the City of Santa Barbara and from
studies commissioned by Cal-Am.
Santa
Barbara’s Experience: The City of Santa Barbara
removed approximately 700 acre-feet of “wet silt” from Gibraltar Lake
over a five-year period from 1984 through 1988.
The project was discontinued due to the high costs in terms of water
supply benefits and limited space available for storing the dredged
material. A report is available for the
first three years of the dredging operations (City of Santa Barbara, 1987). A copy of the executive summary from the
report is attached (Attachment 1).
Cal-Am Sediment Removal
Study: A 1996 report by a
consultant to Cal-Am provides information on the potential for removing
sediment from San Clemente Reservoir (Moffatt & Nichol Engineers,
1996). This report summarizes a
reconnaissance-level investigation of sediment characteristics in the
reservoir, alternative dredging, conveyance and disposal methods, required
facilities and equipment, and estimated costs, assuming removal of two volumes
of sediment: 1 million and 2 million
cubic yards (620 and 1,240 acre-feet, respectively). Estimated total costs to dredge and dispose
of 1 million cubic yards range from $8 million to $29 million. For removing 2 million cubic yards, estimated
total costs range from $25 million to $48 million. A summary of the alternatives evaluated,
including a brief description, estimated cost per cubic yard, and years of
duration of dredging operations, is provided in Attachment 2. These costs do not include mitigation
costs. On the other hand, the report recognized
but did not consider potential cost reductions that could be achieved if the
sediment were marketed as construction materials. The report summary contains the following
statement: “A potential market value for the excavated sediments exists, and should
be explored in more detail including partnering efforts with construction
companies and miners” (Moffatt &
Nichol Engineers, 1996, page 76-3). In
addition, the report did not recognize the potential cost savings that could
occur if selected portions of the dredged materials were placed downstream of
the dam to improve steelhead habitat, rather than trucked offsite.
The evaluation of San Clemente
Reservoir sediment characteristics in the Moffatt & Nichol Engineers study
is based on a reconnaissance-level field sampling and testing program. Over 80 sediment samples from two-thirds of
the inundation area at various depths were collected and analyzed, either
visually or in a laboratory. Sediment
grain sizes ranged from cobble and gravel to fine silts and clays, with some
organic material. Samples taken
consisted predominantly of sand and gravel-sized particles; an independent
review of the sampling program by Woodward-Clyde Consultants concluded that
more than 95% of the sediment samples is in the size range from fine sand to
gravel (greater than 0.075 millimeter).
However, the maximum depth at which sediments were sampled and tested
was 14 feet, and the majority of the samples were taken from depths of 5 feet or
less. The Woodward-Clyde Consultants
assessment stated that “[t]his estimate [95% sand and gravel] is based on
sampling of the upper few feet and deeper sampling will be required to verify
that no pervasive fine-grained layers exist.”
Additional sampling is important in order to anticipate quantities of
fines that could degrade water quality during dredging operations, as well as
quantities and gradation of sediment that could be processed and sold.
Seismic Retrofit Studies
for San Clemente Dam: Studies
have been performed regarding the potential for passing sediment through San
Clemente Reservoir as part of Cal-Am’s evaluation of options for strengthening
San Clemente Dam, which is required to meet California seismic safety
standards. The California Department of
Water Resources (DWR) is the lead agency for an Environmental Impact Report
(EIR) on the seismic retrofit project. Cal-Am has indicated that a Revised
Draft EIR, including information related to sediment removal, will be released
for public review in the August-September 2000 time frame. One of the principal concepts being addressed
by Cal-Am and its consultants is the construction of gated sluiceways through
the dam below the spillway to allow sediment in the reservoir to be sluiced
downstream during periods of high winter flows.
During the review of the December 1998 Draft EIR, the National Marine
Fisheries Service and other parties expressed a preference for removing San
Clemente Dam entirely. To the knowledge
of MPWMD staff, sediment issues related to dam removal have not been
addressed. When the Revised Draft EIR on
the San Clemente Dam Seismic Retrofit Project is released, additional
information relating to the potential for sediment removal should be available.
Need for Sediment Control: It is anticipated that the Revised Draft EIR
for the San Clemente Dam Seismic Retrofit Project will discuss the changing
sediment situation on the Carmel
River due to the filling
of San Clemente Reservoir with sediment in recent years. Since 1998, there have been signs of
uncontrolled releases of fine sediment carried in river waters passing over San
Clemente Dam. This situation will become
more pronounced in the future unless action is taken. Sediment deposition downstream of San
Clemente Dam is of concern because it has the potential to degrade steelhead
habitat and increase flood elevations in Carmel Valley. Preliminary computer modeling performed by
consultants to Cal-Am and the District indicates that more than two feet of
sediment could be deposited in river reaches downstream of San Clemente Dam
without sediment management (Balance Hydrologics, personal communications,
2000). Because of the serious nature of
this situation, it is District staff’s understanding that the San Clemente Dam
Seismic Retrofit Project now identifies sediment management as a basic project
purpose (Marc Lucca, Cal-Am Project Manager, personal communication, July
2000). District consultants have
indicated that increased capacity in existing reservoirs, via dredging or other
means, would enable the reservoirs to serve as a “sediment shock absorber” to
retain the sediment and keep it from depositing downstream in an uncontrolled
manner.
It is notable that rivers without
dams tend to be in a state of dynamic equilibrium with regard to sediment
transport. Because the dams have been in
place for many decades, the river channel downstream of both dams has been
deprived of sediment. This has resulted
in larger-sized sediment, as well as channel downcutting in reaches downstream
of the dams. The river channel has
largely adjusted to these new changes and is in a new, altered state of
equilibrium. Sediment deposition is not
desirable today due to the extensive floodplain development that has occurred
throughout the 20th Century, as it would result in a higher
potential for loss of property and threat to public safety in flooding
situations. Unless there are major
changes in public policy and federal regulations, such a situation is not
considered acceptable.
Los Padres Dam: No specific evaluation has been conducted
regarding sediment removal options from Los Padres Reservoir. Feasibility and other issues, including
benefits and negative impacts, are expected to be similar to those for San
Clemente Reservoir, with one notable exception.
Dredging Los Padres Reservoir would increase the volume of surface water
available for release and use during summer months. In contrast, dredging San Clemente Reservoir
would serve primarily to continue the 70-year process of trapping sediment in
the reservoir. Increased storage of
surface water in San Clemente Reservoir would not necessarily increase water
available for release or diversion because San Clemente Reservoir is kept
essentially full to maintain head on Cal-Am’s Filter Plant and to allow
migration of steelhead past the dam.
MPWMD Yield Analysis: In 1997, MPWMD staff performed an analysis of
the effect of reservoir dredging on the Peninsula’s
water supply (MPWMD, 1997). The analysis
showed that dredging one or both of the reservoirs would have a minor effect on
the number of months of mandatory 20% rationing, but would significantly reduce
the number of months of voluntary 10% water rationing over a 91-year period
(Water Years 1902 through 1992).
Moreover, the analysis concluded that legal diversions at San Clemente
Reservoir could be increased if reservoir capacity were increased by
dredging. Attachment 3 presents the results of this
analysis. It should be noted that
certain assumptions in the 1997 report are no longer valid due to subsequent
reductions in reservoir storage as well as regulatory changes.
River Temperature Increases
Affect Aquatic Life: Increased sedimentation in San Clemente Reservoir
has created a wide, flat channel exposed to solar radiation. In turn, this topography results in the
heating of river water to temperatures greater than those known to be suitable
for steelhead. When warm reservoir water
is discharged from San Clemente Dam to downstream waters in summer, steelhead
and other aquatic organisms are affected until the stream naturally cools
farther downstream. The MPWMD Sleepy
Hollow Steelhead Rearing Facility, located one mile downstream of San Clemente
Dam, has been affected by warm water
discharges in recent years. The District
rears steelhead rescued from drying pools in the lower river until conditions
warrant their release in the fall. A
cooling tower is being installed at a cost of $335,000 to help combat the
dangerously high influent temperatures at the facility so that reared fish are
not subject to increased disease and mortality.
Restoration of reservoir capacity at San Clemente and Los Padres Reservoirs by
dredging or other means would result in deeper, cooler reservoirs that would
help correct the existing situation.
Addressing temperature impacts of existing facilities will likely become
more important in the future given the federal Endangered Species Act, Section
4(d) Rules that require protection of steelhead and their habitat.
POTENTIAL BENEFITS OF SEDIMENT
REMOVAL
As noted in the above paragraphs,
the potential benefits of removing sediment from reservoirs include increased
municipal yield and system operational flexibility, increased instream flows
released from reservoir storage in summer months, and cooler reservoir and
river water temperatures. Increased
capacity in the existing reservoirs, especially San Clemente Reservoir, would
serve as a “sediment shock absorber” to avoid uncontrolled releases of fine
sediment downstream of San Clemente Dam.
There is a good possibility that a portion of dredged materials could be
marketed to the construction industry, which could help offset dredging
costs. Selected portions could also be
placed downstream of the dam to improve steelhead habitat.
POTENTIAL CONSTRAINTS TO
SEDIMENT REMOVAL
A key concern in the removal of
sediment from reservoirs is the potential adverse impacts to the fishery and
aquatic habitat downstream of dredging operations. Releases of fine sediment during dredging or
de-watering operations can negatively impact both water quality (e.g.,
turbidity, anaerobic or release of compounds toxic to aquatic life) and the
substrate (e.g., fine sediments in the channel bottom could degrade steelhead
habitat and food sources). If the
dredged materials were trucked to an offsite location for storage or use,
traffic impacts would be significant.
For example, 1 million cubic yards (CY) of sediment, which is equivalent
to 620 AF, would result in the need for
50,000 to 125,000 truckloads, depending of the loading rate of the truck
(CY/truck). Using an alternative slurry
pipeline method of transport would require a significant amount of water (in
the range of 850-1,200 AF/year) to keep the sediment suspended in the
pipeline, thereby reducing streamflow
available for downstream aquatic life, or water stored in a reservoir, or
both. Other potential problems include
limited disposal sites without adverse environmental effects and a relatively
high cost per acre-foot of yield.
These potential problems are
further described in Appendix A, Evaluation of Water Supply Alternatives, to
the Draft Supplemental EIR for the Carmel River Dam and Reservoir Project
(MPWMD, 1998). The portions of Appendix
A addressing dredging of San Clemente
and Los Padres Reservoirs (pages A-25 through A-28 and Table A-8) are included
as Attachment 4.
ADDITIONAL INFORMATION NEEDED
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Further information is needed regarding sediment
characteristics in both reservoirs. This
information is especially important to accurately assess the type and volume of
dredged materials that could be marketed or released below the dam for habitat
enhancement. (For San Clemente
Reservoir, sediment samples extended to a maximum depth of 14 feet, and the majority
were to 5 feet or less. No analysis of
sediment characteristics has been conducted at Los Padres Reservoir.)
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The 1996 Moffatt & Nichol Engineers study,
the most comprehensive local study available, addressed sediment removal only
at San Clemente Reservoir. Similar
analysis is required to assess potential sediment removal, de-watering, and
transport methods and costs for Los Padres Reservoir.
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Critical review of the 1996 Moffatt & Nichol
Engineers report is needed. Information
in the report such as transport routes, potential sediment storage sites,
environmental impacts, and estimated costs should be reviewed for accuracy and
completeness.
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Further information is required regarding
existing regulations regarding reservoir dredging and required permits.
FINDINGS AND CONCLUSIONS
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Removal of sediment from San Clemente and Los Padres Reservoirs is
technically feasible.
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The high costs of removing, transporting, and
disposing of sediment reported in the 1996 Moffatt & Nichol Engineers
report could be partially offset by the sale of the portion of dredged material
that is usable.
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In addition to high costs, key concerns are
traffic impacts associated with trucking operations, the need for large
quantities of water if a slurry pipeline were used, and instream impacts to
water quality and substrate conditions.
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Sediment management at San Clemente Dam and
Reservoir has become increasingly important due to warm water releases that
affect downstream aquatic life, and the potential for increased flood
elevations in lower Carmel
Valley.
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For San Clemente Reservoir, ongoing Cal-Am
consideration of methods for retrofitting San Clemente Dam includes sediment
management options. The final outcome of
these efforts will determine how sediment in San Clemente Reservoir is managed.
REFERENCES CITED
City of Santa
Barbara, May 1987, Gibraltar
Lake Restoration Project,
Final Report
Moffatt & Nichol Engineers,
September 10, 1996, San Clemente Reservoir Dredging Feasibility Study, Carmel Valley, California
(prepared for California-American Water Company, Monterey Division)
Monterey Peninsula Water
Management District, September 29, 1997, Preliminary Water Supply Analysis of
Reservoir Dredging Alternatives (memo from Darby Fuerst to PUC Workshop
Participants)
Monterey Peninsula Water
Management District, November 13, 1998, Draft Supplemental Environmental Impact
Report for the Carmel River Dam and Reservoir Project, Appendix A. Evaluation of Water Supply Alternatives
PERSONAL COMMUNICATIONS
Balance Hydrologics, Inc. (Barry
Hecht and Mark Woyshner). Telephone and
in-person discussions in January-June 2000.
Marc Lucca, Cal-Am Project Manager. Discussions in July 2000.
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