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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