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Safe Yield Study Safe Yield Study January 2004 Safe Yield Study Prepared For Prepared By January 2004 RIVANNA WATER AND SEWER AUTHORITY CHARLOTTESVILLE, VIRGINIA SAFE YIELD STUDY EXECUTIVE SUMMARY BACKGROUND Since 1973 the Rivanna Water and Sewer Authority (RWSA) has been responsible for providing a safe and dependable water supply to its customers in the City of Charlottesville and surrounding Albermarle County. The current RWSA Urban Area source water system evaluated in this study includes South Fork Rivanna Reservoir, Sugar Hollow Reservoir, the Ragged Mountain Reservoirs, and a river intake on the North Fork Rivanna River. Other source water facilities that are currently not a part of the Urban Area system but are owned and operated by the RWSA include Beaver Creek Reservoir and Totier Creek Reservoir. The safe yield available from the RWSA Urban Area source water system is diminishing with time from significant loss of storage capacity primarily due to sedimentation at South Fork Rivanna Reservoir. Loss of reservoir storage capacity has also occurred in Sugar Hollow Reservoir due to a landslide in 1995. At the same time, demand analyses by others have determined that the raw water demand by RWSA Urban Area customers began to exceed the available safe yield of the Urban Area system in 2000 (VHB, 2001). Because of growing demands for potable water, the loss of reservoir storage from sedimentation, the rapid development occurring in the region, and the occurrence of a potential drought of record in 2002, the RWSA Board of Directors commissioned Gannett Fleming, Inc. to perform a safe yield analysis of the RWSA Urban Area system. This analysis is intended to aid in re-evaluating the appropriateness of the current improvement program in the wake of the recent severe 2002 drought. In addition, Gannett Fleming, Inc. was requested to evaluate water supply expansion options currently under consideration including the reactivation of an existing pumping station on Mechums River, increasing the storage capacity of South Fork Rivanna Reservoir by raising the pool level with spillway crest gates and dredging sediment deposits from South Fork Rivanna Reservoir. Since the RWSA Urban Area water supply system is a complex system consisting of four reservoirs and one river intake, development of a unique model which simulates system operating rules was required in order to apply mass balance techniques to determine the safe yield. For this study the Virginia State Water Control Board’s definition of safe yield for a complex intake (impoundments in conjunction with streams) was adopted which states: "The safe yield is defined as the maximum withdrawal rate available to withstand the worst drought of record in Virginia since 1930. If actual gauge records are not available, correlation is to be made with a similar watershed and numbers synthesized in order to develop the report." ES-1 SCOPE OF STUDY The scope of this study included: (1) reviewing past safe yield investigations; (2) developing an accurate hydrologic database of daily flows into each RWSA reservoir and at each river intake for the 78-year period of record from 1925 to 2003; (3) programming a computer model to simulate the daily operation of the RWSA system, and (4) analyzing the RWSA Urban Area raw water system as a combined system using the computer model to determine the safe yield of the system and to evaluate the sensitivity of the safe yield to different analysis assumptions. The various assumptions included changes in reservoir storage and release requirements from South Fork Rivanna Reservoir. The severity and significance of the 2002 drought was also investigated in relation to the “worst drought of record”. The safe yield investigations performed in this study are based on United States Geological Survey (USGS) streamflow records and climatic data specific to the RWSA Urban Area system. Most of the streamflow data was obtained from seven gaging stations within the Rivanna River Watershed. Where streamflow data was missing, it was reconstituted using statistical correlation with other gages. Reservoir storage was based on the most recent bathymetric survey for each reservoir. STUDY FINDINGS For an established set of critical hydrometerologic conditions, the safe yield available from the RWSA Urban Area system is primarily dependent on two important variables including: (1) reservoir storage available for water supply, and (2) release requirement(s) from the reservoirs and flowby requirements at the river intakes. The reservoir storage available for water supply, normally referred to as useable storage, is that component of total storage volume at each reservoir exclusive of dead storage. This volume changes with time due to sedimentation. Dead storage corresponds to a minimum pool level below which no storage can be used for water supply, which is typically based on the lowest intake level, a projected level of long-term sediment accumulation or some other restriction such as a change in water quality. Flowby and release requirements are important in defining the quantity of water that is lost from the system. Existing Safe Yield (2002 Reservoir Storage Conditions). For this study, the total usable storage for water supply as of 2002 is estimated to be 1,586 million gallons. This value is based on an evaluation of storage conditions at each reservoir. Although the RWSA has voluntarily committed to make conservation releases from Sugar Hollow and South Fork Rivanna Reservoirs during non- drought conditions, investigations by others (VHB, 2001) have not identified any regulatory release requirement. Therefore, during very infrequent drought events, when it is determined that the integrity and reliability of the RWSA Urban Area system is at risk, the RWSA can cease stream conservation releases in order to conserve as much water as possible for supply purposes. The safe yield of the RWSA Urban Area system, assuming 2002 reservoir storage conditions of 1,586 million gallons of usable storage with no stream conservation releases from their reservoirs, is 16.0 MGD. The worst drought of record corresponding to this safe yield value is the drought that occurred in 2002. It should be emphasized that operating the system with no conservation releases implies that no releases are made from South Fork Rivanna Reservoir during periods when the ES-2 reservoir falls below normal pool conditions. Any deviations from this operating assumption would result in less safe yield from the system. Future Safe Yield and Safe Yield for Expansion Alternatives Under Consideration. The future safe yield of the existing system is dependent on the rate of loss of usable storage within each reservoir due to sedimentation. Analysis of bathymetric surveys at South Fork Rivanna Reservoir indicates that reservoir storage has been lost at a relatively uniform rate of approximately 15.1 million gallons per year. This loss rate, which is higher than average for similar size reservoirs in the region is expected to continue, but can change as a result of land use changes within the watershed and changes in the sediment trap efficiency of the reservoir. Based on analyses using the computer model of the RWSA Urban Area source water system, an approximate relationship was developed between reservoir storage and system safe yield. The relationship indicates that the safe yield of the RWSA Urban Area system decreases by 1.0 MGD for every 190 million gallons of lost reservoir storage. If it is assumed that reservoir storage continues to be lost at a rate of 15.1 million gallons per year and no stream conservation releases are made from the reservoirs, by 2050, the safe yield of the system would gradually decrease at a rate of 0.079 MGD per year from 16.0 MGD in 2002 to 12.2 MGD in 2050. Likewise, the safe yield of the system can be increased by approximately 1.0 MGD for every 190 million gallons of storage added to the system through storage expansion options such as reservoir dredging, installation of spillway crest gates on South Fork Rivanna Dam, raising Ragged Mountain Dams, etc. For example, increasing the reservoir storage of South Fork Rivanna Reservoir by 550 million gallons by installing 4-foot-high gates on the crest of the spillway would result in an immediate increase in the safe yield of the RWSA Urban Area system by approximately 2.9 MGD. This increase assumes that there is no change in the stream conservation release requirement from the reservoir. It is important to note that the additional storage provided by installing spillway crest gates at South Fork Rivanna Dam will diminish with time due to reservoir sedimentation as will the additional yield afforded by this modification. Effect of Release Requirements on Safe Yield. Securing regulatory agency permits for the current Capital Improvement Program will likely require adopting new reservoir release requirements. Since there are currently no mandatory release requirements from any of the RWSA sources of supply, any newly established release requirement would need to be analyzed in order to determine its impact on the overall net safe yield benefits afforded by the respective improvement project being developed. Since the greatest impact on the RWSA Urban Area system safe yield would result as a consequence of a new mandatory release requirement from South Fork Rivanna Reservoir, the computer model of the system was used to evaluate various conservation release scenarios from this reservoir. The release scenarios evaluated included releasing the lesser of a specified flow (ranging between 0 and 24 MGD) or the natural flow. As an approximate rule of thumb, for every 1.0 MGD of specified flow release or natural inflow ranging between 0 and 15 MGD, the safe yield is reduced by approximately 0.34 MGD. For example, adopting a release requirement equal to the lesser of 8.0 MGD or the natural inflow would reduce the safe yield of the system by approximately 2.7 MGD as compared to operating the system with no releases.
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