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 nondrought 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. A more exact relationship is presented in the report. It is interesting to note that ES-3
for some of these release scenarios the worst drought of record switched from the drought of 2002 to the drought that occurred in 1930. For a constant release requirement, the effect on safe yield is simply the magnitude of the release. That is, if the safe yield of the system with no stream conservation release is 16.0 MGD, imposing an 8.0 MGD constant release would reduce the safe yield of the system by 8.0 MGD. Mechums River Pumping Station. This study determined that operation of the proposed Mechums River Pumping Station would not increase the safe yield of the system. Reactivation of the pumping station may still be desirable for other reasons including: (1) transmission of storage releases from Lake Albermarle and/or Beaver Creek Reservoir to the Observatory Water Treatment Plant, and (2) if the storage of the Ragged Mountain Reservoirs was increased, it could be used to refill the reservoirs following a drought event. STUDY LIMITATIONS AND RECOMMENDATIONS This study was limited to evaluating the safe yield of the RWSA Urban Area source water system. Other RWSA raw water sources including, but not limited to, Beaver Creek Reservoir and Totier Creek Reservoir were not analyzed. Safe yield analyses performed for this study are dependent upon estimates of reservoir storage volume prepared by others. It was also assumed that there are no transmission capacity limitations between raw water sources and water treatment plants. Likewise, no evaluation of the capacities of the water treatment plants were performed, and it was assumed that the treatment capacity of the water treatment plants would not limit or negatively impact the safe yield of the system. It is suggested that the following recommendations be implemented to enable a comprehensive and focused evaluation of expansion alternatives currently under consideration. 1. Any future bathymetric surveys should include preparation of contour maps of the reservoirs. Such mapping would provide valuable information that can be used to determine the location and depth of sediment deposits, and prepare accurate elevationarea-storage relationships for future safe yield analyses. 2. Future water supply expansion alternatives need to carefully consider capacity limitations that may exist between raw water sources and water treatment plants, and the treatment capacity of individual water treatment plants, to make sure that the operation of the system is feasible and maximizes use of all source water facilities. Comparisons of existing safe yield and demands should be made to confirm existing and future projects against projected safe yields and localized demands. 3. If the planned operation of the system relies on transferring/releasing stored water from Sugar Hollow Reservoir to South Fork Rivanna Reservoir by in-stream gravity flow during drought periods, it is recommended that potential water losses from groundwater infiltration, transpiration, evaporation, etc. be evaluated and quantified. This can best be ES-4
accomplished by field tests during an extended dry period that would be expected to occur during the summer months. If it is determined that water losses during in-stream gravity flow transmission are significant, the impact these losses have on the safe yield of the system should be evaluated. 4. Perform coordination with the State Water Board to establish probable design criteria with respect to reservoir release rates and river intake pass-by flows at the Authority s facilities. Confirmation of reservoir release requirements is critical to establishing the viability of any future project. 5. Consider integrating Beaver Creek Reservoir or a portion of the existing reservoir storage volume into the RSWA Urban Area source water system as a means of increasing the safe yield of the system. Storage from this reservoir could be designated for use only as a last resort during extreme drought events. 6. Consider adding other available raw water resources to the RWSA Urban Area water system including Lake Albermarle and Chris Green Lake as a means of increasing the safe yield of the system. Like the use of Beaver Creek Reservoir, these resources could be designated for use only as a last resort during extreme drought events. ES-5
RIVANNA WATER AND SEWER AUTHORITY CHARLOTTESVILLE, VIRGINIA SAFE YIELD STUDY TABLE OF CONTENTS Page EXECUTIVE SUMMARY...ES-1 1. BACKGROUND AND SCOPE OF STUDY...1 1.1 Background...1 1.2 Scope of Study...2 1.3 Study Limitations...2 2. DESCRIPTION OF WATER SUPPLY SYSTEM...4 2.1 Rivanna Water and Sewer Authority Water Supply System...4 2.2 Ragged Mountain Reservoirs...4 2.3 Sugar Hollow Reservoir...5 2.4 South Fork Rivanna Reservoir...6 2.5 North Fork Rivanna River Intake...9 2.6 Mechums River Pumping Station...10 2.7 Beaver Creek Reservoir...11 3. CLIMATOLOGICAL DATA...12 3.1 General...12 3.2 Precipitation...12 3.3 Gross Shallow Lake Evaporation...14 3.4 Net Reservoir Evaporation...15 4. STREAMFLOW DATA...16 4.1 General...16 4.2 USGS Stream Gaging Station Records...17 4.3 Riverflow At North Fork Rivanna River Intake...20 4.4 Riverflow Into South Fork Rivanna Reservoir...20 4.5 Riverflow Into Sugar Hollow Reservoir...23 4.6 Riverflow At Mechums River Intake/Pumping Station...26 4.7 Riverflow Into Ragged Mountain Reservoirs...29 5. SYSTEM COMPUTER MODEL...30 5.1 Model Development...30 5.2 Model Structure...30 5.3 Verification of Computer Model...32 5.4 Input Data Sources...32 5.5 Output Data Files...32 i
RIVANNA WATER AND SEWER AUTHORITY CHARLOTTESVILLE, VIRGINIA SAFE YIELD STUDY TABLE OF CONTENTS - CONTINUED Page 6. SAFE YIELD ANALYSIS...33 6.1 Definition of Safe Yield...33 6.2 Previous Safe Yield Studies...33 6.3 Dead Storage...34 6.4 Existing Safe Yield of the RWSA Urban Area System...36 6.5 Probabilistic Analysis of System Safe Yield...38 6.6 Effect of Changes in Storage at South Fork Rivanna Reservoir...40 6.7 Effect of Release Requirements During Severe Drought Events...44 7. SUMMARY AND RECOMMENDATIONS...46 7.1 Summary...46 7.2 Recommendations...47 8. REFERENCES...49 TABLES Table No. Title Page Table 1 Statistics for Upper and Lower Ragged Mountain Dams...5 Table 2 Statistics for Sugar Hollow Dam...6 Table 3 Statistics for South Fork Rivanna Reservoir...9 Table 4 Statistics for North Fork Rivanna River Intake...10 Table 5 Statistics for Mechums River Intake/Pumping Station...11 Table 6 Statistics for Beaver Creek Reservoir...11 Table 7 Average Monthly Gross Shallow Lake Evaporation Rates for RWSA Reservoirs...15 Table 8 USGS Stream Gaging Station Data from RWSA Safe Yield Analysis...18 Table 9 Summary of Operating Rules for the RWSA Urban Area System...30 Table 10 Summary of Assumptions and Findings from Previous Safe Yield Studies...35 Table 11 Assumed Distribution of Storage in RWSA Urban Areas Reservoirs (2002 Conditions)...36 Table 12 Summary of Assumptions and Findings from Safe Yield Studies...37 ii
RIVANNA WATER AND SEWER AUTHORITY CHARLOTTESVILLE, VIRGINIA SAFE YIELD STUDY TABLE OF CONTENTS - CONTINUED FIGURES Figure No. Title Page Figure 1 Plot Showing Change in Total Storage Over Time Due to Sedimentation at South Fork Rivanna Reservoir...8 Figure 2 Plot of 5-Month Moving Average Monthly Precipitation near Charlottesville From 1836 to 2003...13 Figure 3 Timeline of Active USGS Streamflow Gages Within and Near the RWSA Water Supply System...19 Figure 4 Timeline of Active USGS Streamflow Gages (Yellow Designates Data Used for North Fork Rivanna River Intake)...21 Figure 5 Timeline of Active USGS Streamflow Gages (Yellow Designates Data Used for S.F. Rivanna Reservoir)...24 Figure 6 Timeline of USGS Streamflow Gages (Yellow Designates Data Used for Sugar Hollow & Ragged Mountain Reservoirs)...27 Figure 7 Timeline of USGS Streamflow Gages (Yellow Designates Data Used For Mechums Creek Pumping Station Watershed)...28 Figure 8 Schematic of RWSA Raw Water Supply System...31 Figure 9 Summary of Reservoir Storage from Simulation of RWSA System (1925 2003)...39 Figure 10 Safe Yield Probability Relationship for RWSA Source Water Supply System...41 Figure 11 Relationship Between Safe Yield of the RWSA System and the Usable Storage in South Fork Rivanna Reservoir...43 Figure 12 Relationship Between Safe Yield of the RWSA System and the Release Requirement at South Fork Rivanna Reservoir...45 Exhibit No. Exhibit 1 EXHIBITS Title Rivanna Water & Sewer Authority Source Water Supply System iii
RIVANNA WATER AND SEWER AUTHORITY CHARLOTTESVILLE, VIRGINIA SAFE YIELD STUDY TABLE OF CONTENTS - CONTINUED APPENDICES Appendix Appendix A Appendix B Appendix C Appendix D Appendix E Title Plots of 4-, 5-, 6-, 7-, 8-, and 9-Month Moving Average Monthly Precipitation near Charlottesville from 1836 to 2003 Scatter Graphs of Concurrent Monthly Average Streamflows for Key USGS Stream Gages Located Within and Near the Rivanna River Watershed Flow Duration Curves for Inflows Into RWSA Reservoirs and At RWSA River Intakes Sample Graphical Plots of Key Variables from Simulation of RWSA System for 2002 Storage Conditions with a Release Requirement at South Fork Rivanna Reservoir of 8 MGD or the Natural Inflow for the Period from 1925 to 2003 USGS Streamflow Data for 2002 Drought iv
RIVANNA WATER AND SEWER AUTHORITY CHARLOTTESVILLE, VIRGINIA SAFE YIELD STUDY 1. BACKGROUND AND SCOPE OF STUDY 1.1 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 an 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. Acting as wholesale distributor, the RWSA sells finished water to two customers: the Albermarle County Service Authority (ACSA) and the City of Charlottesville Public Works Department. Since potable water is a primary human need, it is a paramount responsibility of the RWSA to take necessary steps to protect the integrity and adequacy of its potable water supply. This responsibility is recognized in the Commonwealth of Virginia's laws and regulations governing water supplies, which require water purveyors to evaluate future supply needs when consumption reaches 80 percent of system capacity. The safe yield available from the RWSA Urban Area source water system is diminishing with time due to the significant loss of storage capacity from its primary source; South Fork Rivanna Reservoir. Since the South Fork Rivanna Reservoir was constructed in 1966 approximately 40 percent of the total reservoir storage has been lost due to sedimentation. Continuing loss of storage capacity at the present rate would result in less than 15 percent of the original useable storage remaining in 2050. 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 storage volume from reservoir 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 for existing and future conditions. In addition, Gannett Fleming, Inc. was requested to evaluate water supply expansion options currently 1
under consideration including the construction of a 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. 1.2 Scope of Study. The investigations performed in this study included the following components: (1) Review past RWSA Urban Area safe yield reports. (2) Develop hydrologic database consisting of a continuous record of daily riverflow into each reservoir and at each river intake from 1925 to 2003. (3) Develop climatological database consisting of a continuous record of precipitation and shallow lake evaporation in order to compute daily net evaporation from the surface of each reservoir from 1925 to 2003. (4) Program a custom computer model that simulates the daily operation of the RWSA Urban Area source water system. (5) Analyze/simulate the daily operation of the RWSA Urban Area system using the computer model to determine the safe yield for the worst drought of record. (6) Evaluate the safe yield for changes in storage at South Fork Rivanna Reservoir due to losses from sedimentation and gains from raising the normal pool using spillway crest gates. (7) Evaluate the effect on safe yield of various release requirements from South Fork Rivanna Reservoir during severe drought events. (8) Evaluate the 2002 drought event in context with past drought events. (9) Prepare an engineering report that summarizes and documents the methodology and findings of the safe yield investigations. Include charts and other graphic exhibits, augmented with commentary, to assist in interpreting the results of the study. 1.3 Study Limitations. The scope of this study was limited to evaluating the availability of source water from existing RWSA Urban Area sources of supply. Increasing the storage capacity of the Ragged Mountain Reservoirs, incorporating the use of Beaver Creek Reservoir, Lake Albermarle or Chris Green Lake, and new reservoir sites or pumped storage reservoir sites were not evaluated. 2
Variations in monthly demand were assumed to be relatively minor and were not included in the analyses. Seepage from the RWSA Urban Area reservoirs is unknown and was assumed to be insignificant. The scope of the study was limited to consider the availability of source water exclusive of existing physical and regulatory constraints for the water system. Important water system constraints not analyzed in this study include: (1) possible complex source water allocation permit restrictions that may be imposed on the water supply expansion option involving increasing the storage of South Fork Rivanna Reservoir using spillway crest gates; (2) transmission capacity limitations between raw water sources and water treatment plants; (3) treatment plant capacity limitations; and (4) potential water losses from groundwater infiltration, evaporation, etc., during in-stream gravity flow transmission. 3
2. DESCRIPTION OF WATER SUPPLY SYSTEM 2.1 Rivanna Water and Sewer Authority Water Supply System. The RWSA Urban Service Area is supplied by finished water from the following three water treatment plants (WTP): (1) South Rivanna WTP, (2) Observatory WTP, and (3) North Fork Rivanna WTP. These water treatment plants receive raw water from four reservoirs and one river intake. The South Rivanna WTP is served by the South Fork Rivanna Reservoir. Water from Sugar Hollow Reservoir can be released into the South Fork Rivanna Reservoir via the Moormans River, a tributary to the South Fork Rivanna River. The Observatory WTP is supplied by water from the Upper and Lower Ragged Mountain Reservoirs via an 18-inch pipeline and from Sugar Hollow Reservoir via another 18-inch diameter pipeline interconnected with the Ragged Mountain pipeline. Excess water from Sugar Hollow Reservoir can also be transferred to the Ragged Mountain Reservoirs. The North Fork Rivanna WTP treats water pumped from an intake on the North Fork Rivanna River. A map of the RWSA source water system is presented in Exhibit 1 at the end of this report. As part of the RWSA multi-step integrated water supply strategy for increasing the safe yield of the system, the RWSA is also exploring the feasibility of reactivating the Mechums River Pumping Station and incorporating Beaver Creek Reservoir as a supplemental source of supply. Each of the aforementioned source water facilities are described in the paragraphs that follow. 2.2 Ragged Mountain Reservoirs. The RWSA owns and operates two dams located in the Ragged Mountain region which is immediately west of the City of Charlottesville. The two dams, Upper Ragged Mountain Dam and Lower Ragged Mountain Dam, are in series on an unnamed tributary to Moores Creek, and together form the Ragged Mountain Reservoir system. Upper Ragged Mountain Dam was constructed around 1885 and originally had a normal pool at Elevation 653.0 feet (El. 654.7 ft., GF 2002 datum). A 10-inch outlet pipe and transmission line passes through the embankment and follows the original streambed downstream to the intake tower of Lower Ragged Mountain Dam. Reportedly, a break exists along the 10-inch transmission line within the Lower Ragged Mountain Reservoir such that the pool levels for the Upper and Lower reservoirs equalize during normal low-flow conditions. During these conditions, the upper reservoir pool is at Elevation 640.5 feet (El. 641.0 ft., GF 2002 datum), or approximately 12.5 feet (13.7 feet, GF 2002 datum) lower than the original reservoir level. The Lower Ragged Mountain Dam was constructed in 1908. The drainage area upstream of Lower Ragged Mountain Dam is 1.81 square miles. 4
For the purposes of this study, Upper and Lower Ragged Mountain Reservoirs are assumed to act as one system. The current combined storage capacity of both reservoirs with normal pools at Elevation 640.5 feet (El. 641.0 ft., GF 2002 datum) is 513.6 million gallons. Prior to the inoperability of the 10-inch transmission line, the combined reservoir storage was 611 million gallons. The aforementioned reservoir storage volumes were taken from Figure 5 of a report prepared in 1959 by Polglaze & Basenberg Engineers titled Report on Water Works System, Charlottesville, Virginia. No known bathymetric surveys have been performed on the reservoirs since their construction. Reservoir sedimentation does not appear to be an issue at these reservoirs due to the relatively small size and undisturbed condition of the watershed. Water from Sugar Hollow Reservoir can be transferred to the Ragged Mountain Reservoir system via an 18-inch transmission line. There is no regulatory minimum release requirement from the Ragged Mountain Reservoir system and seepage from the Lower Ragged Mountain dam appears to be insignificant. Although the watershed is ungaged, evaluation of nearby streamflow data indicates that there is no natural inflow into the reservoirs during drought events. Important project statistics are summarized in Table 1. Table 1 Statistics for Upper and Lower Ragged Mountain Dams Feature/Parameter Value Drainage Area 1.81 mi 2 Dam Height (Lower Dam) 67 feet Normal Pool Elevation 640.5 feet (El. 641.0 ft., GF 2002 datum) Surface Area of Permanent Pool 70.4 acres Volume of Permanent Pool (Current) 513.6 Million Gallons Existing Conservation Release Requirement 0.0 MGD Seepage at Dam (Lower Dam) Unknown 2.3 Sugar Hollow Reservoir. Sugar Hollow Dam was constructed in 1947 on the Moormans River to expand the public water supply system. An inflatable crest gate was added to the spillway crest in 1999 to increase spillway capacity while maintaining original storage capacity. The drainage area upstream of the dam is 17.5 square miles. The normal pool with the inflatable crest gates in the 5
raised position is at Elevation 975 feet. At this normal pool condition, the existing reservoir storage is approximately 360 million gallons (Waterway Surveys and Engineering 1995). This storage volume is based on a bathymetric survey of the reservoir performed in September 1995 following a landslide in the reservoir that occurred in late June 1995 as a result of a severe rainfall event. The original storage capacity of the reservoir (prior to the landslide) at this same normal pool elevation was 430 million gallons. For this study the 2002 storage condition of Sugar Hollow Reservoir was assumed to be 360 million gallons. Although there is no regulatory minimum release requirement from the reservoir, the RWSA has made a voluntary commitment to release 400,000 gallons per day to the Moormans River at all times unless total available reservoir storage falls below 80 percent. Current analysis of streamflow records indicate that the natural flows at the dam are less than 400,000 gallons per day approximately 5 percent of the time and during extreme drought events there are brief periods of no riverflow. Important project statistics are summarized in Table 2. Seepage from the dam is unknown. It is important to note that releases and seepage from Sugar Hollow Dam are inflows into South Fork Rivanna Reservoir. Table 2 Statistics for Sugar Hollow Dam Feature/Parameter Value Drainage Area 17.51 mi 2 Dam Height Normal Pool Elevation Surface Area of Permanent Pool Volume of Permanent Pool (1995 Survey) Existing Voluntary Conservation Release Seepage at Dam 77 feet 975 feet 51.2 acres 360 Million Gallons 0.40 MGD whenever reservoir storage exceeds 80 percent of total Unknown 2.4 South Fork Rivanna Reservoir. South Fork Rivanna Reservoir is located on the South Fork Rivanna River and was constructed in 1966 to increase the safe yield of the RWSA Urban Area source water system and currently serves as the primary source of raw water. The drainage area upstream of the dam is 259.1 square miles. Other significant impoundments within the watershed 6
include Sugar Hollow Reservoir, Beaver Creek Reservoir (a flood control and water supply reservoir), and Lake Albemarle (a recreation reservoir). The Mechums River Pumping Station built in 1965, is also located upstream of South Fork Rivanna Reservoir. The original storage capacity of South Fork Rivanna Reservoir with a normal pool at Elevation 382.0 feet is approximately 1,700 million gallons. The lowest water supply intake level was set at Elevation 367 feet. The storage volume below this elevation (dead storage) immediately following construction was 492 million gallons. The original designers of the dam determined that reservoir sedimentation would be significant and predicted an average loss of reservoir storage volume of 19.6 million gallons per year (Bowler, 2002). Since the construction of the dam in 1966 the reservoir volume was surveyed six times. The most recent bathymetric survey conducted in March 2002 determined that the reservoir storage at that time was 1,155 million gallons. The corresponding dead storage below Elevation 367 feet was 355 million gallons. This equates to an average annual loss of reservoir storage of 15.1 million gallons per year. In other words, over the last 38 years, approximately 1 percent of the original reservoir volume was lost each year due to sedimentation. A plot showing the change in total storage due to sedimentation at South Fork Rivanna Reservoir is presented on Figure 1. As shown in Figure 1, the rate of loss of reservoir storage is relatively uniform, and if it continues, by the year 2050 less than 200 million gallons of useable storage will remain. As part of an overall watershed management plan, the RWSA investigated the potential sources of sediment and were unable to identify the dominant process as either landscape erosion or streambank erosion. One hypothesis is that near complete deforestation of the watershed in the 19 th and early 20 th centuries led to extreme landscape erosion. The landscape erosion sediment loads may have been so great that the streams lacked the energy to transport all the sediment and as a result the sediment was deposited in the stream floodplains. Subsequent reforestation may have prevented additional landscape erosion; however, it may be that the sediment deposits in the streambanks of the floodplain are now being eroded and transported downstream. The RWSA is currently investigating practical efforts to reduce sediment inflow into the South Fork Rivanna Reservoir in order to prolong the useful life of the reservoir and obviate future dredging. Although there is no regulatory minimum release requirement, the RWSA has made a voluntary commitment to release a minimum flow of 8.0 MGD from South Fork Rivanna Reservoir 7
1800 8 Total Storage In South Fork Rivanna Reservoir (Million Gallons) 1600 1400 1200 1000 800 600 400 200 0 1960 1970 1980 1990 2000 2010 2020 2030 2040 2050 Time Figure 1. Plot Showing Change In Total Storage Over Time Due to Sedimentation at South Fork Rivanna Reservoir
except during severe drought conditions when such a release would threaten the ability to meet public water supply needs. When the reservoir inflow is less than 8.0 MGD, release from the reservoir would equal the natural inflow to the reservoir. In other words, the streamflow downstream of the dam would be at its natural level rather than being artificially augmented by reservoir storage. Streamflow records indicate that the natural flows into the reservoir are less than 8.0 MGD approximately 3 percent of the time, and that extended periods of no flow occurred during the 2002 drought. According to the 1978 Phase I Inspection Report for the dam, no seepage was observed. Important project statistics are summarized in Table 3. Table 3 Statistics for South Fork Rivanna Reservoir Feature/Parameter Value Drainage Area 259.1 mi 2 Dam Height Normal Pool Elevation Surface Area of Permanent Pool Volume of Permanent Pool (2002 Survey) Existing Voluntary Conservation Release Seepage at Dam 60 feet 382 feet 366 acres 1,155 Million Gallons The Lesser of 8.0 MGD or the Natural Inflow Unknown 2.5 North Fork Rivanna River Intake. The RWSA has a river intake and pumping station on the North Fork Rivanna River just downstream of the confluence of Jacobs Run. The drainage area of the watershed upstream of the river intake is 115.0 square miles. The only impoundment of significance upstream of the river intake is Chris Green Lake, a recreation reservoir constructed on Jacobs Run. Releases from Chris Green Lake to supplement extreme low-flow conditions at the river intake were not evaluated as part of this study. The effective pumping capacity of the river intake is 2.0 MGD. There is currently no flowby requirement at the river intake. Important project statistics are summarized in Table 4. 9
Table 4 Statistics for North Fork Rivanna River Intake Feature/Parameter Value Drainage Area 115.0 mi 2 Existing Conservation Flowby 0.0 MGD Pumping Capacity 2.0 MGD 2.6 Mechums River Pumping Station. A river intake and pumping station was built in 1965 on the Mechums River as an interim measure until the South Fork Rivanna Dam, Reservoir, and Water Treatment Plant were constructed. The purpose of the Mechums River Pumping Station was to provide additional source water by transferring water from the Mechums River to the Ragged Mountain Reservoirs or directly to the Observatory Water Treatment Plant. The existing pumping station and intake includes two 2.0 MGD pumps, a stoplog dam and an intake channel. The pump station is interconnected with an existing 18-inch transmission main used to deliver water from Sugar Hollow Reservoir to the Ragged Mountain Reservoir/Observatory Water Treatment Plant. The drainage area of the watershed upstream of the pumping station is 91.4 square miles. Both Beaver Creek Reservoir and Lake Albermarle are located on upstream tributaries to the Mechums River and their locations are shown on Exhibit 1. The benefits of storage releases from Beaver Creek Reservoir and Lake Albermarle were not analyzed as part of this study. Since early 2002 the RWSA has been evaluating the benefits and cost of rehabilitating the Mehcums River Pumping Station. Previous safe yield investigations by others (RWSA 2002) concluded that operation of the Mechums River Pumping Station would not increase the safe yield of the system. Reactivation of the pumping station has still been viewed as desirable for the following three reasons: (1) it could be used to transmit storage releases from Lake Albermarle and/or Beaver Creek Reservoir to the Observatory Water Treatment Plant, and (2) if the storage capacity of the Ragged Mountain Reservoirs was increased, it could be used to refill the reservoirs following a drought event. Regulatory permit requirements for reactivating the Mechums River Pumping Station stipulate that the RWSA can begin pumping 2.0 MGD whenever the riverflow is greater than 33 cfs and can pump up to 4 MGD when the riverflow exceeds 66 cfs. To prevent the blockage of fish passage of resident species, the stoplogs at the pump station impoundment must be removed from 10
February 14 th to June 15 th of each year; or, in consultation with the Virginia Department of Game and Inland Fishery, the RWSA must construct acceptable fish passage at the impoundment prior to activating the pumps. Important project statistics are summarized in Table 5. Table 5 Statistics for Mechums River Intake/Pumping Station Feature/Parameter Value Drainage Area 91.4 mi 2 Conservation Release Requirement Pumping Capacity 2 MGD when >33 cfs & 4 MGD >66 cfs (Conditional February 14 June 15) 2.0 MGD & 4.0 MGD 2.7 Beaver Creek Reservoir. The RWSA manages Beaver Creek Reservoir as a source of water supply for the Town of Crozet. The water from Beaver Creek Reservoir is treated at a nearby 1.0 MGD water treatment plant. The Town of Crozet s average water demand is approximately 0.6 MGD. The drainage area of the watershed upstream of the dam is 9.55 square miles and the total storage in Beaver Creek Reservoir is 520 million gallons. Beaver Creek Reservoir is currently not used to supply source water to the RWSA Urban Area system. Important project statistics are summarized in Table 6. Table 6 Statistics for Beaver Creek Reservoir Feature/Parameter Value Drainage Area 9.55 mi 2 Volume of Permanent Pool 520 Million Gallons Existing Voluntary Conservation Release 0.0 MGD Seepage at Dam Unknown 11
3. CLIMATOLOGICAL DATA 3.1 General. The determination of safe yield requires review and analysis of historic climatological data to identify critical drought periods and to estimate gains and losses of source water from the surfaces of reservoirs due to precipitation and evaporation. Climatological data is normally available for a longer period of record than that of streamflow data and is useful in evaluating the relative severity of drought events that occurred outside of the period of streamflow record. 3.2 Precipitation Data. Precipitation data is available from a climatological station located in Charlottesville that is maintained by the National Oceanic and Atmospheric Administration - National Weather Service (NOAA-NWS). The Charlottesville precipitation station began operation in 1869 and has remained active ever since with only minor gaps in the data. Daily precipitation records were obtained from 1930 through 2003. For brief periods where daily data were missing, they were estimated using data from the next closest precipitation station. For the period from 1925 (the beginning of the period of record of streamflow data) to 1930, monthly precipitation totals from Charlottesville were used. Even though precipitation data from one climatological station cannot be directly used to accurately determine the safe yield of the RWSA Urban Area system, it does provide an indication of the relative severity of past droughts. An analysis was performed consisting of plotting the 4-, 5-, 6-, 7-, 8-, and 9-month moving average of monthly precipitation for the period of record from 1836 to 2003. This precipitation data is of particular interest since it covers a period of 167 years and is a good indication of the severity of droughts which occurred prior to the 78-year period of streamflow data (1925-2003) used to determine the safe yield of the RWSA Urban Area water supply system. The closest climatological station with precipitation data prior to 1868 is the station at Fortress Monroe (now Langley Air Force Base). Precipitation data was recorded at Fortress Monroe from 1836 to 1890. A complete record of monthly precipitation from 1836 to 1890 was obtained by piecing together the precipitation records of Fortress Monroe with those recorded at Charlottesville. Plots of monthly moving averages of durations between 4 and 9 months were plotted in order to evaluate the sensitivity of various drought durations. Based on the daily simulation of the system, which is discussed in detail later in this report, it was determined that the two worst droughts of record for the RWSA Urban Area system occurred in 2002 and 1930 and lasted between 145 and 187 days (5-6 months). A plot of the 5-month moving average monthly precipitation is presented 12
9.0 8.0 Rainfall At Fort Monroe (1835-1870) Safe Yield Analysis Based On Streamflow Records from 1925-2004 13 5-Month Moving Average, Precipitation In Inches Per Month 7.0 6.0 5.0 4.0 3.0 2.0 1.0 Average Monthly Precipitation = 3.63 Inches 0.0 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 Year Figure 2. Plot ot 5-Month Moving Average Monthly Precipitation near Charlottesville from 1836 to 2003
in Figure 2. Plots of the 4-, 5-, 6-, 7-, 8-, and 9-month moving average monthly precipitation for the period of record from 1836 to 2003 are presented in Appendix A. Figure 2 shows that during the past 167 years there were extended periods of low average rainfall with the potential of being more severe than the 1930 and 2002 drought events; two of which fall within the 1868-1925 period of record at Charlottesville, and two more which fall within the 1836-1868 period of record at Fortress Monroe. It should be noted that the relationship between rainfall and runoff is very complex. Some of the factors affecting this relationship include rainfall intensity, spatial and temporal distribution of storm events, antecedent soil conditions, seasonal variation in evaporation and moisture demand by vegetation, land use and watershed topography. Further, the precipitation data used represents rainfall measured at one point. Spatial variability of precipitation can be considerable. Streamflow, however, represents an integration of all of the hydrologic process within a drainage area. Nevertheless, there appears to be a remarkable correlation between the worst drought periods as determined by simulating the operation of the system and the lowest 5-month moving average monthly rainfall. More importantly, the precipitation data shows that a more severe drought has occurred prior to 1925 which is outside of the period of streamflow record used to determine the safe yield of the system. This important finding should be considered when making decisions based on the safe yield computed using streamflow data from 1925 through 2003. 3.3 Gross Shallow Lake Evaporation. Reservoir evaporation losses can be substantial, especially in shallow reservoirs storing more than a year s water supply. Most of the reservoirs of the RWSA are not particularly sensitive to evaporation as they are relatively deep and the duration of the worst droughts for the system is only several months. Evaporation losses are still significant and need to be estimated as accurately as possible. Monthly average shallow lake evaporation rates are available from published sources. Table 1 lists the monthly average evaporation rates used to compute net evaporation from the RWSA Urban Area reservoirs. At normal pool, the combined surface area of South Fork Rivanna Reservoir, Sugar Hollow Reservoir and the Ragged Mountain Reservoirs is approximately 488 acres. Annual gross shallow lake evaporation in the Charlottesville area is approximately 36 inches or 3 feet per year. During summer months, shallow lake evaporation can be as high as 0.24 inch per day which is equivalent to a loss of 3.2 MGD assuming the RWSA reservoirs are at normal pool. 14
Table 7 Average Monthly Gross Shallow Lake Evaporation Rates for RWSA Reservoirs Month Evaporation (inches) Month Evaporation (inches) January 1.05 July 5.30 February 1.50 August 4.90 March 1.75 September 4.00 April 3.05 October 2.75 May 3.40 November 2.00 June 4.50 December 1.20 3.4 Net Reservoir Evaporation. Net evaporation must be accounted for to accurately simulate the performance of the water system. Net evaporation is gross evaporation minus rainfall. On average the RWSA reservoirs receive approximately 46 inches of direct rainfall annually. Gross shallow lake evaporation from the reservoirs averages about 35.4 inches per year; therefore, net evaporation is about -10.6 inches per year. In other words, precipitation exceeds gross evaporation most of the time. During extreme drought events, however, gross evaporation significantly exceeds precipitation. For the simulation of the RWSA Urban Area system, daily net evaporation rates from each of the RWSA reservoirs were computed by subtracting the daily precipitation recorded by the National Climatic Data Center (NCDC) at Charlottesville (if any) from the estimated gross shallow lake evaporation. The daily total evaporation from each reservoir was then computed by multiplying the net evaporation rate by the exposed surface area of the reservoir. The exposed surface area of each reservoir was computed by the computer model based on changes in storage volume. As discussed in Section 3.2, prior to 1930, only monthly totals of precipitation were available. These totals were distributed evenly over each day of the month, and net evaporation was computed. 15
4. STREAMFLOW DATA 4.1 General. Determining safe yield and other system performance statistics requires extensive streamflow data to identify critical drought periods and to determine the availability of water at each source. Developing a reliable hydrologic database is therefore critical. The accuracy of the analysis will only be as good as the data used in the analysis. The primary objective of analyzing streamflow records is to develop a continuous daily streamflow record into each reservoir and at each river intake from the present to a time prior to the 1930 drought, often considered the worst drought of record in Virginia. Based on the Commonwealth of Virginia s definition of safe yield, the safe yield must be determined from the worst drought of record since 1930. Of equal interest for this study is the recent severe drought which occurred in 2002. The streamflow from a drainage basin depends upon the climate and the physical characteristics of the basin. In areas of the Rivanna Watershed below the Blue Ridge Mountains the topography and climate is relatively uniform. Farther northwest within the Blue Ridge Mountains, the topography is very steep and the climate can vary because of the positions of the mountain ranges and the different altitude of the basins. The principal drainage basin characteristics that affect the amount and distribution of runoff are location, size, shape, physiography, geology, soils, vegetative cover, and man-made developments. Evaluating the extent to which one of these basin characteristics affects the streamflow and relating the streamflow of one basin to that of another by analyzing the differences in their basin characteristics would be an enormous endeavor. Such an endeavor is not needed for a safe yield analysis because the complex interrelationships of climate and drainage basin characteristics are integrated in the flow of the stream, and their aggregate effect is measured directly at the stream gaging station. The measured runoff from drainage basins therefore furnishes the best basis for comparing their runoff characteristics. Considerable work has been done by the Hydrologic Engineering Center of the U.S. Army Corps of Engineers, the United States Geological Survey, and others to develop procedures for reconstituting missing streamflows. Use of these procedures involves numerical and graphical correlation of short-term records at a site with a long-term record. Reconstruction of missing streamflow data for this study was performed using correlation methods with both graphical and numerical techniques. Relationships between the stream flows at stream gaging stations were based on concurrent monthly streamflows (by calendar month). 16