HomeMy WebLinkAbout12/20/1988, 4 - WATER STATUS PROJECTIONS ITEM #4 WATER STATUS PROJECTIONS
REPORT TO BE MADE AVAILABLE UNDER SEPARATE COVER.
This item was brought up under Communications on 12/6 with
special request to add to the 12/20 meeting.
Icity of Sa►n LUib OBISPO Mtn DATETR
Dec. 20, 1988
COUNCIL AGENDA REPORT ffE "
From: William T. Hetland, Director of Utilities
Subject: Salinas Reservoir Safe Annual Yield
RECEIVED
Recommendation: Receive and file report. Direct staff as necessary. O 16
ft
r,1T►CLERK
SANIUSCOM CA
io:X 0 AW
Summary
Over the years a number of analyses have been done on the safe annual yield to adjust and
verify for changed conditions. Council has expressed some concern about our current safe
annual yield figures and requested staff to submit a status report. A specific concern is
how releases required to maintain the live stream are impacting the safe annual yield.
As part of the City's Salinas Dam Expansion project a task was included to review the safe
annual yield for the Salinas Reservoir. In addition to the live stream a number of other
factors are being evaluated. Though the report is not yet complete and it is only
preliminary, it appears that a reduction of the safe annual yield of 300 to 400 acre
feet/year (this represents 4% to 5% of our total yield) may be warranted. Most of the
reduction is attributed to an increase in siltation at the reservoir and not the live
stream releases.
The reduction in safe annual yield may be offset by additional groundwater development and
by an increase in the cooperative use of the Salinas and Whale Rock Reservoirs. At this
time it is felt to be premature to take any specific policy action. Staff therefore
recommends Council direct staff to receive the consultant's final report and then return
to Council with alternative options as soon as possible.
Issue
Because of the City's current water situation, questions have been raised on the
reliability of the safe annual yield calculation for the Salinas Reservoir. Though the
live stream factor has raised the most concern, this status report will try to identify
the other key factors which may impact the safe annual yield and provide an explanation of
their impact.
Concerns
The safe annual yield is defined as the maximium amount of water (measured in acre
feet/year) a reservoir can provide year after year without being drawn down to an
unacceptable low level, considering the period of lowest rainfall. For the City's
reservoirs it is based on the driest six year period (1945-1951) in the last 115 years of
record. A number of factors are taken into account in determining the safe annual yield.
It is changes in these factors that have raised concerns on the reliability of the safe
annual yield calculation. These concerns are summarized below:
city of sz luis OBispo
AGO COUNCIL AGENDA REPORT
Salinas Reservoir
Page 2
A. Live Stream
Incresased water consumption in the north county may be resulting in additional
withdrawals from the Salinas River downstream of the dam. This impacts the live
stream which must be established before the City may store water in the reservoir.
Was this adequately taken into account when the safe annual yield was calculated?
B. Water Quality
At the low reservoir levels our ability to treat the water to an acceptable level both
from a health and an aesthetic standpoint is an issue. Is there a minimum pool of
water that is not treatable and how would that minimum pool effect the safe annual
yield?
C. Minimum Pool
Most reservoirs have a minimum pool to be left in the reservoir for the benefit of
fish and wildlife. What is Salinas reservoirs minimum pool and was it taken into
acocunt when determining the safe annual yield?
D. Siltation
Over the life of a reservoir the storage capacity will be diminished due to siltation
and this will reduce the safe annual yield. Periodically the safe annual yield needs
to be adjusted to account for this change in storage capacity.
These concerns will be addressed in this report.
History of Salinas Safe Annual Yield
The Salinas Reservoir was constructed by the United States Government in 1941. At the
time of construction the safe annual yield was determined to be 5600 acre feet/year. In
1972 the State Water Resources Control Board issued an order which requires the Salinas
Reservoir Project to satisfy the downstream vested water rights, but only to the extent
they could be satisfied in the absence of the project. The order states:
"Permittee shall release water into the Salinas River channel from Salinas Reservoir
in such amounts and at such times and rates as will be sufficient, together with
inflow from downstream tributary sources, to supply downstream diversions of any
surface flow and ground-water extractions under prior vested rights. Until further
order of this Board, it will be conclusively presumed that prior vested downstream
rights will be met if at all times either a visible surface flow exists in the Salinas
River between the Salinas Reservoir and the confluence of the Nacimiento River or the
total inflow to the Salinas Reservoir is released therefrom into the channel of the
Salinas River below the Salinas Dam. The foregoing is not intended to preclude other
operational criteria that will fully satisfy prior vested downstream rights."
q i City of san Im. JBispo
MINGS COUNCIL AGENDA REPORT
Salinas Reservoir
Page 3
This concept is commonly referred to as the "live stream". It essentially means that the
water flowing into the reservoir is released until a live stream is developed. Once the
live stream is developed water may be stored in the reservoir. The stored water does not
have to be released in the future. As a result of the Board order, the Army Corps of
Engineers performed a new yield study that took into account the live stream. The study
made some calculations on what the downstream usage was from the river and its underflow
and some assumptions on what the future usages would be. The study finally determined
that the safe annual yield of the reservoir should be reduced to 5,000 acre feet/year.
In 1977 the U.S. Geological Survey analyzed the siltation in the reservoir. The study
surveyed the reservoir and determined it's storage capacity. The long term sediment yield
for the Salinas River Basin upstream of the lake was estimated. A model correlating the
annual sediment deposition to the annual water discharge was developed. As a result of
this study, the safe annual yield was reduced to the current 4800 acre feet/year.
A key element of the Salinas Dam expansion study currently underway is a reservoir
operations analysis. The purpose of this analysis is to review the previous safe annual
yield work and to check its validity. This was included in the study because staff felt
that this was an opportune time to review past work and because of some concerns that
certain events warranted it. The attached chart shows a schematic representation of the
model for determining the yield and the key considerations which are being evaluated, each
of which can have an impact on the final safe annual yield number.
Tentative Study Results
Leedshill-Herkenhoff is performing the yield studies for the Salinas Dam Expansion
Project. The computer model has been developed and model runs are being performed. Staff
has not received a written report on this analysis and the information being presented
here is very preliminary and will have to be verified.
Initial discussions with the consultant have indicated that the new safe annual yield is
likely to be less that the 4800 acre feet/year that the City has used in the past. The
preliminary numbers indicate a safe annual yield in the range of 4400 to 4500 acre
feet/year. The consultant is still working on the computer model and verifying the
statistical accuracy of the results. The final determination will be a fixed safe annual
yield number. If the preliminary analysis is confirmed and the safe annual yield is
indeed 4400 to 4500 acre feet/year, the City's overall safe annual yield would be between
4% to 5.4% less than our current estimate.
A. Live Stream
The impact of the live stream on the safe annual yield did not seem to have a significant
effect. The consultant did not compare his analysis with the previous work done and
therefore cannot make an accurate determination on why the results seem to be similar.
I
��v
a city of sa*uis owpo
COUNCIL AGENDA REPORT
Salinas Reservoir
Page 4
But it looks like the initial work done and the estimates that were used have continued to
be valid.
Staff has put together a list of all water rights users on the Salinas River from the dam
to the County line. There are three major users and seven minor users which have water
rights other than the City and Corps. We are also trying to verify the usage and
determine if any water rights violations have occurred.
B. Water Quality
A� the water levels become lower in the reservoir it sometimes becomes more difficult for
us to treat the water to a level acceptable to our customers. Also at certian times of
the year the reservoir will "turn over . This occurs when the water at the surface cools
and becomes more dense and sinks to the bottom. This results in mixing of the poorer
quality of water from the bottom of the reservoir which also causes difficulity in
treating the water.
These water quality concerns can be addressed by increases in our treatment process
capabilities and by developing more flexibility in our outlet works at the reservoir. The
issue of the treatment processes is being addressed in our plant upgrade which will begin
design after the first of the year. The county is currently modifying the intake tower
which will provide us more flexibility. The new intake tower will allow us to withdraw
water from different levels in the reservoir. The operators would then be able to
determine and withdraw from the level which would provide the best quality of water for
treating. Additional analysis by staff needs to be done to determine a fixed minimum pool i
based on water quality concerns. This may result in a further adjustment in the safe
annual yield.
C. Minimum Pool
The operations manual for the Salinas Reservoir indicates that a minimum pool of 5000 acre
feet should be maintained if it does not impact the live stream or in case of an
emergency. It is believed that this operations criteria was established by the County
Flood Control Staff based on input for the Department of Fish and Game. The requirement
is not in the City's water rights permits as is the case with the Whale Rock Reservoir
minimum pool and therefor may not be as leagally binding. The consultant assumed that an
extended drought would be classed as an emergency. He therefore used the dead pool (the
amount of water below the lowest withdrawal point) of 400 acre feet as the minimum pool in
the safe annual yield calculation. This is a reasonable assumption providing water
quality concerns can be handled.
D. Siltation
The consultant has indicated that most of the reduction of the yield is due to an increase
in the siltation of the reservoir and a diminishing reservoir capacity. The U.S.
Geological Survey model was used to calculate new sedimentation rates taking into account
the specific storms experienced since the model was originally developed. From this it is
estimated that the capacity of the reservoir has been reduced by 1500 to 2000 acre feet.
�#
Grp city of san tui,. jBispo
COUNCIL AGENDA REPORT
Salinas Reservoir
Page 5
As you may recall the Water Management Plan identified siltation as a component impacting
the safe annual yield and recommended that additional water resources be developed to
compensate based on an average yearly loss of yield. So we have been taking into account
siltation in our water resource studies. Staff is also negotiating with the consultant to
perform a new aerial photography survey of the reservoir and to recalculate the capacity.
This would verify the sedimentation calculations and increase our confident level in these
values.
Coopeative Use
The cooperative use of the Whale Rock and Salinas Reservoir is an operational procedure
which maximizes the individual reservoirs yields. As part of the Water Management Plan an
analysis was performed which indicated that 500 acre feet/year in increased safe annual
yield could be developed with cooperative use. An amendment to the Leedshill-herkenhoff
contract is being processed which will use the new yield work done on the Whale Rock and
Salinas Reservoirs and review the yield from cooperative use. Initial discussions with
the consultant are favorable that a larger yield number may result with this study.
Safe Annual Yield, Annual Water Operational Plan Water Allocation Ordinance
The safe annual yield is a calculated number based on historical information and educated
assumptions. It is important to remember it is a theoretical calculation and that the
exact same conditions may or may not occur again. Though there is a probable degree of
error in the calculation, the assumptions are on the conservative side to help further
increase the reliability of the calculation.
In addition the City has developed two items which monitor and regulate our water
operation on an annual basis. One is the Annual Water Operational Plan which yearly looks
at our water use and the storage in our reservoirs. This helps to provide additional
assurances that if any negative factors arise the City's use and development of its water
resources can be adjusted to accomodate them.
The City's Water Allocation Ordinance clearly regulates the use of water as its relates to
growth. Like the Annual Water Operational Plan it provides a means for the City to be
aware of and compensate for any difficulties with it's water resources.
Conclusions
The City's safe annual yield is composed of the following components:
Salinas Reservoir 4800 acre feet/year
Whale Rock Reservoir 2057 acre feet/year
Cooperative Use 500 acre feet/year
Groundwater M acre feet/year
Total 7857 acre feet/year
I
V
city of salluis oBispo
COUNCIL AGENDA REPORT
Salinas Reservoir
Page 6
If the consultants analysis is correct and the Salinas yield is reduced 300 to 400 acre
feet/year, it would represent a decrease in the City's total safe annual yield of 4% to
5.4%. The analysis seems to show that this reduction is due to siltation and not an
increase in reservoir releases. The amount of siltation can be determined by surveying
the reservoir and thereby verifying the reduction in yield. Staff is negotiating a
contract to perform this work. Additional staff work is also underway to verify the
minimum pool assumptions based on water quality criteria.
Additional resource development through groundwater and cooperative use seem to be
favorable options in minimizing the impact of this potential reduction in safe annual
yield.
Though this reduction may not normally be major in the total picture of things it is
significant when you are operating on the edge both in allocation for future development
and meeting daily water demand. The City's Water Allocation Ordinance and the Annual
Water Operational Plan are two existing means which can address this reduction.
This is a status report of an ongoing study and therefore it would be imprudent to take
any policy action at this time. Once the final study is completed staff will return to
Council with final recommedations for further action.
say1220/n
I
8719129P/PRP-9 • • II
SALINAS RESERVOIR EXPANSION
tRESERVOIR OPERATIONS STUDIES
OBJECTIVE: TO DEFINE PROJECT YIELD BASED ON ANALYSIS OF HISTORICAL
HYDROLOGIC PERIOD
SCHEMATIC REPRESENTATION OF MODEL
RAINFALL EVAPORATION
SPILLS
SALINAS
SEE~ PAGE RESERVOIR
DOWNSTREAM
S
WITHDRAWALS ' RELEASES
KEY CONSIDERATIONS
• DEFINITION OF YIELD • SILTATION
• LIVE STREAM REQUIREMENT • MINIMUM POOL
• DIVERSION CAPACITY • FLOOD CONTROL
0 WATER DEMAND • COORDINATED OPERATION