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. 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 elevation- area-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

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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

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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

EXHIBITS Exhibit No. Title

Exhibit 1 Rivanna Water & Sewer Authority Source Water Supply System

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RIVANNA WATER AND SEWER AUTHORITY CHARLOTTESVILLE, VIRGINIA

SAFE YIELD STUDY

TABLE OF CONTENTS - CONTINUED

APPENDICES

Appendix Title

Appendix A Plots of 4-, 5-, 6-, 7-, 8-, and 9-Month Moving Average Monthly Precipitation near Charlottesville from 1836 to 2003

Appendix B Scatter Graphs of Concurrent Monthly Average Streamflows for Key USGS Stream Gages Located Within and Near the Rivanna River Watershed

Appendix C Flow Duration Curves for Inflows Into RWSA Reservoirs and At RWSA River Intakes

Appendix D 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

Appendix E 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 mi2 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 mi2 Dam Height 77 feet Normal Pool Elevation 975 feet Surface Area of Permanent Pool 51.2 acres Volume of Permanent Pool (1995 Survey) 360 Million Gallons 0.40 MGD whenever reservoir storage Existing Voluntary Conservation Release exceeds 80 percent of total Seepage at Dam 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 19th and early 20th 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

1600

1400

1200

1000

8 800

600

400

200 Total Storage In South Fork Rivanna Reservoir (Million Gallons) (Million Reservoir Rivanna Fork South In Storage Total

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 mi2 Dam Height 60 feet Normal Pool Elevation 382 feet Surface Area of Permanent Pool 366 acres Volume of Permanent Pool (2002 Survey) 1,155 Million Gallons The Lesser of 8.0 MGD or Existing Voluntary Conservation Release the Natural Inflow Seepage at Dam 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 mi2 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 14th to June 15th 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 mi2 2 MGD when >33 cfs & 4 MGD >66 cfs Conservation Release Requirement (Conditional February 14 – June 15) Pumping Capacity 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 mi2 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

Rainfall At Fort Monroe Safe Yield Analysis Based On (1835-1870) Streamflow Records from 1925 - 2004 8.0

7.0

6.0

5.0

13 4.0

3.0

2.0 5-Month Moving Average, Precipitation In Inches Month Per 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 4.2 USGS Stream Gaging Station Records. An extensive database of streamflow data was developed from which to reconstitute records representing natural streamflow into the RWSA reservoirs and at the river intakes. Eleven USGS gaging stations were identified as “primary” gaging stations because they are located within the watershed of the Rivanna River. A list of these gaging stations and their summary statistics are presented in Table 8 and their locations are shown on Exhibit 1. Of these 11 primary stream gaging stations only three are currently active. Most of the primary gaging stations have periods of record spanning less than 30 years and some are located downstream of RWSA reservoirs where the streamflow has been significantly modified by regulation. Fortunately, most of the discontinued stream gaging stations were located on streams within the Rivanna River Watershed that provide important short-term streamflow data that can be correlated and/or transposed with the other streamflow data from primary gaging stations in order to reconstitute missing streamflows and obtain a continuous record of daily flows from 1925 to 2003. The historical streamflow data from the primary short-term records were also correlated with concurrent long-term streamflow data of nearby gaging stations that are located outside of the Rivanna River Watershed. Twelve long-term nearby gaging station records were identified for this purpose and were evaluated by plotting scattergraphs of monthly concurrent streamflows and computing regression statistics. Seven of the long-term nearby gaging stations have periods of record that begin prior to 1930 and eight of these gages are still active. The average period of record of the long-term gaging stations is 66 years. A list of the long-term gaging stations and their summary statistics are presented in Table 8. It is interesting to note that the lowest recorded flow at 7 of 12 stream gaging stations in Table 8 that have records through 2003, occurred during the drought of 2002. A timeline of active USGS streamflow gaging stations located within and near the RWSA Urban Area water supply system is presented on Figure 3. In general, the first source of data used to develop streamflow data for each reservoir and river intake was the streamflow data from the primary gaging stations transposed by making the appropriate linear adjustment based on contributing drainage area. To develop missing streamflow data, the monthly averages were estimated based on correlation between short-term primary gaging stations and long-term gaging stations. For these periods of missing streamflow, daily streamflow records from nearby gages were then used to convert the estimated monthly average to a daily streamflow value. In this way a continuous daily record of streamflows was developed for each source for the period from 1925 through 2003. The streamflow data available for each reservoir and

17 Table 8 USGS Stream Gaging Station Data for RWSA Safe Yield Analysis

USGS Drainage Period Years Annual Min. Index Area of of Runoff Flow Number Station Name (Mi2) Record Record (CFSM) (CFS) Comments Primary Gaging Stations 1942-51 02031000 Mechums River Near White Hall 95.4 1979-03 45 1.11 0.00* 1952-63 02031500 N. F. Moormans River near White Hall 11.4 1982-84 14 1.51 0.00 02032000 Moormans River near White Hall 18 1943-46 3 1.08 0.00 02032250 Moormans River near Free Union 74.6 1979-97 18 1.43 0.58 Regulation by Sugar Hollow Dam 02032500 S. F. Rivanna River near Earlysville 216 1951-66 15 0.89 1.3 02032400 Buck Mountain Creek near Free Union 37.0 1979-97 24 1.23 0.35 02032515 S. F. Rivanna River near Charlottesville 260 1979-97 18 1.20 1.10 Below Sug. Hol. & S. F. Rivanna 02032640 N. F. Rivanna River near Earlysville 108 1993-03 10 1.26 0.25* 02032680 N. F. Rivanna River near Proffit 176 1970-92 22 1.33 1.8 Some diversion by RWSA 18 02033500 Rivanna River Near Charlottesville 507 1925-34 9 0.86 4.0 02034000 Rivanna River at Palmyra 664 1933-03 70 1.10 5.2 Regulation since 1966 Long-Term Nearby Gaging Stations 01625000 Middle River near Verona 375 1927-03 86 0.85 8.9* Regulation by WWTPs & mills 01626000 South River near Waynesboro 127 1952-03 51 1.16 7.0 Regulation by WTP & SCS dams 1925-51 01627500 South River at Harriston 212 1968-03 60 1.23 17 Regulation by WTPs & SCS dams 01665500 Rapidan River near Ruckersville 114 1942-03 61 1.35 0.21* 01671000 North Anna River near Doswell 441 1929-88 59 0.86 1.0 01671020 North Anna River at Hart Corner 463 1979-03 24 0.81 25* Flow regulated by Lake Anna 02021500 Maury River at Rockbridge Baths 329 1928-03 75 1.17 5.8 Regulation by Lake Meriweather 02022500 Kerrs Creek near Lexington 35 1926-03 77 1.03 0.90 02028500 Rockfish River Near Greenfield 94.6 1943-03 60 1.48 0.07* 02030000 Hardware River near Scottsville 116 1938-03 65 1.11 0.0* 02030500 Slate River near Arvonia 226 1926-95 77 1.00 2.0 02034500 Willis River at Lakeside Village 262 1927-86 59 0.94 1.5 * Instantaneous low flow occurred during the 2002 drought event. Gage Period of Record Number Station Name 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s Primary Gaging Stations 2031000 Mechums River Near White Hall 2031500 N. F. Moormans River near White Hall

2032000 Moormans River near White Hall Regulated by Sugar Hollow Dam 2032250 Moormans River near Free Union 2032500 S. F. Rivanna River near Earlysville 2032400 Buck Mountain Creek near Free Union

2032515 S. F. Rivanna River near Charlottesville Regulated by Sugar Hollow and S. F. Rivanna Reservoirs 2032640 N. F. Rivanna River near Earlysville 2032680 N. F. Rivanna River near Proffit

2033500 Rivanna River Near Charlottesville Regulated by S. F. Rivanna Reservoir 2034000 Rivanna River at Palmyra 19 Long-Term Nearby Gaging Stations 1625000 Middle River near Verona 1626000 South River near Waynesboro 1627500 South River at Harriston 1665500 Rapidan River near Ruckersville 1671000 North Anna River near Doswell 1671020 North Anna River at Hart Corner 2021500 Maury River at Rockbridge Baths 2022500 Kerrs Creek near Lexington 2028500 Rockfish River near Greenfield 2030000 Hardware River near Scottsville 2030500 Slate River near Arvonia 2034500 Willis River at Lakeside Village

Figure 3. Timeline of Active USGS Streamflow Gages Within and Near the RWSA Water Supply System river intake of the RWSA Urban Area system and the methodology used to develop the missing streamflow data are discussed in more detail below. All references to stream gaging stations in this report refer to United States Geological Survey (USGS) stream gaging stations. 4.3 Riverflow At North Fork Rivanna River Intake. Historical streamflow data for the North Fork Rivanna River are available at two gages; Gage #02032640 - North Fork Rivanna River near Earlysville, and Gage #02032680 - North Fork Rivanna River near Proffit. These two gages provide nearly continuous streamflow data for the period from 1970 to the present with the exception of a brief period in 1993. The gage near Earlysville provides the most representative data since it is located upstream of the RWSA river intake and records flow from 94 percent of the watershed above the intake. The primary source of data for developing the inflow record into Sugar Hollow Reservoir was the 33 years of streamflow data recorded on the North Fork Rivanna River at Earlysville and at Proffit. These flows were transposed by making a linear adjustment based on contributing drainage area to obtain the river flows at the intake for the concurrent period of record. For the period of missing streamflow data, the stream gaging data with the best correlation with the 33 years of streamflow data recorded on the North Fork Rivanna River were used to estimate the inflows using the best fit statistical relationship. The best correlation was with two stream gages located further downstream on the Rivanna River Near Charlottesville (Gage #2033500) and at Palmyra (Gage #2034000). Two other gages; Buck Mountain Creek near Free Union (Gage #2032400) and Slate River near Arvonia (Gage #2030500) were also used to fill in brief missing periods of streamflow in 1933 and 1993. A timeline of all USGS streamflow gages considered in the analyses is presented on Figure 4. The timeline segments shaded in yellow indicate the streamflow data used to develop the riverflow at the North Fork Rivanna Intake. Scattergraphs showing the graphical correlation between stream gages used to reconstitute missing streamflows are presented in Appendix B. A flow-duration relationship developed using the daily inflow record for the river flows at the intake for the period from 1925 to 2003 is presented in Appendix C. The daily estimates of riverflow were computed assuming the effective drainage area upstream of the intake is 115.0 square miles. 4.4 Riverflow Into South Fork Rivanna Reservoir. For this study, river flows into South Fork Rivanna Reservoir include the natural runoff from the upstream watershed minus the contributions from Sugar Hollow Reservoir and the natural flows at the Mechums River

20 Gage Period of Record Number Station Name 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s Primary Gaging Stations 2031000 Mechums River Near White Hall 2031500 N. F. Moormans River near White Hall

2032000 Moormans River near White Hall Regulated by Sugar Hollow Dam 2032250 Moormans River near Free Union 2032500 S. F. Rivanna River near Earlysville 2032400 Buck Mountain Creek near Free Union

2032515 S. F. Rivanna River near Charlottesville Regulated by Sugar Hollow and S. F. Rivanna Reservoirs 2032640 N. F. Rivanna River near Earlysville 2032680 N. F. Rivanna River near Proffit

2033500 Rivanna River Near Charlottesville Regulated by S. F. Rivanna Reservoir 2034000 Rivanna River at Palmyra 21 Long-Term Nearby Gaging Stations 1625000 Middle River near Verona 1626000 South River near Waynesboro 1627500 South River at Harriston 1665500 Rapidan River near Ruckersville 1671000 North Anna River near Doswell 1671020 North Anna River at Hart Corner 2021500 Maury River at Rockbridge Baths 2022500 Kerrs Creek near Lexington 2028500 Rockfish River near Greenfield 2030000 Hardware River near Scottsville 2030500 Slate River near Arvonia 2034500 Willis River at Lakeside Village

Figure 4. Timeline of Active USGS Streamflow Gages (Yellow Designates Data Used for North Fork Rivanna River Intake) Intake/Pumping Station. The natural flows into Sugar Hollow Reservoir and at the Mechums River Intake were determined separately. The discharges from these sources into the South Fork Rivanna Reservoir are computed by the model as either spillway or low-flow releases from the reservoir and flowby at the intake. The drainage area of South Fork Rivanna Reservoir was therefore assumed to be the total watershed drainage area (259.1 square miles) minus the drainage area of Sugar Hollow (17.5 square miles) and Mechums River Intake/Pumping Station (91.4 square miles) as well as the surface area of the reservoir (0.41 square miles) for a total effective area of 149.8 square miles. Short-term historical streamflow data upstream of South Fork Rivanna Reservoir is available from several stream gaging stations. The best source was a stream gaging station located near the upstream end of the reservoir that recorded streamflow for 15 years from 1951 to 1966 (Gage #2032500). This gage recorded the streamflow from 216 square miles of the 259 square mile watershed (83 percent). The gage was abandoned following filling of the reservoir. In 1979, stream gages were installed on the three largest tributaries that discharge into the reservoir including the Moormans River near Free Union (Gage #2032250), Mechums River near White Hall (Gage #2031000), and Buck Mountain Creek near Free Union (Gage #2032400). Combined, these three gages record the flow from 207 square miles or 80 percent of the watershed. All but the stream gage on Mechums River at White Hall were discontinued in 1997. The gage on the Mechums River continues to remain active and includes streamflow records for the 2002 drought. The primary sources of data for developing the inflow record into South Fork Rivanna Reservoir were the 15 years of record of the stream gage on the South Fork Rivanna River near Earlysville, followed by the 24 years of combined streamflow from the three major tributaries to the reservoir. For the period from 1942 to 1951 and from 1997 to 2003, the streamflows recorded at Mechums River near While Hall were the primary source of flow data. These flows were transposed by making a linear adjustment based on contributing drainage area to obtain the river flows into the reservoir for their period of record. Review of the daily streamflow records published by the USGS at Mechums River near White Hall (Gage #2031000) indicates that the instantaneous low flow at this gage for the period of record occurred during the 2002 drought event. During the periods of August 23-28 and September 3-26, 2002, no flow was recorded at this gage. According to Mr. Gene Powell, the Senior Environmental Engineer with the Virginia Department of Environmental Quality who is responsible for this gaging station, these flow records are accurate and have been verified. Mr. Powell confirmed

22 that there was a 22-day period in September 2002 of essentially no flow at this gage. He noted that the riverbed had standing water but that no moving water was observed. Mr. Powell also made a site visit to Moormans River where he noted that the riverbed appeared to be dry. He also visited Ivy Creek and observed a trickle of flow. No observations were made of Buck Mountain Creek. According to the USGS published streamflow data, the lowest flow on record for Buck Mountain Creek was 0.23 MGD which occurred on August 21, 1987. On this same date the flow at the Mechums River Gage and the Moormans River Gage were 7.1 MGD and 0.49 MGD, respectively. It therefore appears to be a reasonable assumption that whenever there was no flow in the Mechums River, there was probably no flow in the other tributaries that flow into the South Fork Rivanna River. For the period of missing streamflow data, the stream gaging data with the best correlation with the 15 years of streamflow data recorded on the South Fork Rivanna River at Earlysville were used to estimate the inflows using the best fit statistical relationship. The best correlation was with the stream gage on the North Fork Rivanna River near Proffit (Gage #2032680) and the two gages on the Rivanna River near Charlottesville (Gage #2033500) and at Palmyra (Gage #2034000). The stream gage on the Slate River near Arvonia (Gage #2030500) was also used to fill in a brief missing period of streamflow in 1933. A timeline of all USGS streamflow gages considered in the analyses is presented on Figure 5. The timeline segments shaded in yellow indicate the streamflow data used to develop the riverflow into the South Fork Rivanna Reservoir. Scattergraphs showing the graphical correlation between stream gages used to reconstitute missing streamflows are presented in Appendix B. A flow-duration relationship developed using the daily inflow record for the river flows at the intake for the period from 1925 to 2003 is presented in Appendix C. 4.5 Riverflow Into Sugar Hollow Reservoir. Short-term historical streamflow data for the Moormans River are available from three discontinued gaging stations. The most relevant was the gaging station located on the Moormans River near White Hall (Gage #2032000) that was active for approximately 3 years from 1943 to 1946. This gaging station was located just downstream of the Sugar Hollow Damsite. Monthly average streamflows recorded at this gage were plotted against concurrent monthly streamflows recorded at other primary gages and nearby long-term gaging stations. Inspection of the scattergraphs showed poor correlation, especially for low-flow conditions. Similar plots were prepared for the gage on the North Fork Moormans River near White Hall (Gage

23 Gage Period of Record Number Station Name 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s Primary Gaging Stations 2031000 Mechums River Near White Hall 2031500 N. F. Moormans River near White Hall

2032000 Moormans River near White Hall Regulated by Sugar Hollow Dam 2032250 Moormans River near Free Union 2032500 S. F. Rivanna River near Earlysville 2032400 Buck Mountain Creek near Free Union

2032515 S. F. Rivanna River near Charlottesville Regulated by Sugar Hollow and S. F. Rivanna Reservoirs 2032640 N. F. Rivanna River near Earlysville 2032680 N. F. Rivanna River near Proffit

2033500 Rivanna River Near Charlottesville Regulated by S. F. Rivanna Reservoir 2034000 Rivanna River at Palmyra 24 Long-Term Nearby Gaging Stations 1625000 Middle River near Verona 1626000 South River near Waynesboro 1627500 South River at Harriston 1665500 Rapidan River near Ruckersville 1671000 North Anna River near Doswell 1671020 North Anna River at Hart Corner 2021500 Maury River at Rockbridge Baths 2022500 Kerrs Creek near Lexington 2028500 Rockfish River near Greenfield 2030000 Hardware River near Scottsville 2030500 Slate River near Arvonia 2034500 Willis River at Lakeside Village

Figure 5. Timeline of Active USGS Streamflow Gages (Yellow Designates Data Used for S. F. Rivanna Reservoir) #2031500) that had 14 years of record and is located upstream of Sugar Hollow Reservoir with much better results. Discussions with USGS technical experts revealed that the streamflow records from the gage on the Moormans River near White Hall were affected by RWSA withdrawals at an upstream river intake. Monthly RWSA withdrawals from the Moormans river were obtained for the period of interest and adjustments were made to the monthly streamflow record to account for the water supply withdrawals. The corrected streamflow data was then plotted against concurrent monthly streamflow records at other primary and nearby long-term gaging stations with better results. The stream gage located on the Moormans River downstream of Sugar Hollow Dam near Free Union (Gage #2032250) that includes the period of record from 1979-1997 was not used for any analyses associated with the development of the inflow record into Sugar Hollow Reservoir as it was judged to be overly regulated by Sugar Hollow Dam and not truly representative of the watershed conditions upstream of the dam. As shown on Exhibit 1, the watershed upstream of Sugar Hollow consists entirely of very steep mountainsides and is unlike the terrain in the watershed area downstream of Sugar Hollow Dam which is much flatter. In 1997, the RWSA completed analyses of the historic streamflow records for North Fork Moormans River in order to evaluate minimum flow releases from Sugar Hollow Dam. The analyses showed that the streamflow from the North Fork of the Moormans River is flashy with unusually low flows during periods of low rainfall. It was suggested that there is little groundwater storage in the watershed which accounts for the relatively low baseflows observed from the watershed. For several drought periods, no streamflows were observed in the river for periods lasting between 25 and 36 days. The absence of observed baseflow may also be due to the presence of deep, porous deposits of alluvium and colluvium (cobbles, boulders, and other unconsolidated material) in the valley bottom. Here the streamflow, if any, would occur below the channel surface as subsurface flow. The primary source of data for developing the inflow record into Sugar Hollow Reservoir was the 14 years of streamflow data recorded at the North Fork Moormans River near White Hall. These flows were transposed by making a linear adjustment based on contributing drainage area to obtain the reservoir inflows for the concurrent period of record. A similar transposition was made for the 3-year period of streamflow record (1943-1946) at the Moormans River near White Hall that were corrected to account for RWSA water withdrawals. For the period of missing streamflow data, the stream gaging data with the best correlation with the 14 years of recorded at Moormans River at

25 White Hall was used to estimate the inflows using the best fit statistical relationship. It is interesting to note that the best correlation was with two stream gages not within the Rivanna River Watershed (Gage #2028500 - Rockfish River near Greenfield, and Gage #1627500 - South River at Harriston). Both of these gaging stations are located on rivers close to the Moormans River and have similar steep mountain terrain. A timeline of all USGS streamflow gages considered in the analyses is presented on Figure 6. The timeline segments shaded in yellow indicate the streamflow data used to develop the inflow data record for Sugar Hollow Reservoir. Scattergraphs showing the graphical correlation between stream gages used to reconstitute missing streamflows are presented in Appendix B. A flow-duration relationship developed using the daily inflow record for Sugar Hollow Reservoir for the period from 1925 to 2003 is presented in Appendix C. The daily estimates of reservoir inflow were computed assuming the effective drainage area upstream of Sugar Hollow Reservoir is 17.43 square miles. This area excludes the 51-acre surface area of the reservoir. 4.6 Riverflow At Mechums River Intake/Pumping Station. Short-term historical streamflow data at the Mechums River Intake/Pumping Station is available from several stream gaging stations. The best source was a stream gaging station located on the Mechums River near White Hall (Gage #2031000). This gage recorded streamflows for 45 years covering the period from 1942 to1951 and from 1973 to the present. For the 15-year period from 1951 to 1966, the stream gage on the South Fork of the Rivanna River near Earlysville (Gage #2032500) was used. The remaining missing periods of streamflow record were estimated using the best correlation relationship of the Mechums River Near White Hall (Gage #2031000) with the stream gage on the North Fork Rivanna River near Profit (Gage #2032680) and the two gages on the Rivanna River near Charlottesville (Gage #2033500) and at Palmyra (Gage #2034000). The stream gage on the Slate River near Arvonia (Gage #2030500) was also used to fill in a brief missing period of streamflow in 1933. A timeline of all USGS streamflow gages considered in the analyses is presented on Figure 7. The timeline segments shaded in yellow indicate the streamflow data used to develop the riverflow at the Mechums River Intake/Pumping Station. As noted in Paragraph 4.4, the daily streamflow records published by the USGS at Mechums River near White Hall (Gage #2031000) indicates that the instantaneous low flow at this gage for the period of record occurred during the 2002 drought event. During the periods of August 23-28 and September 3-26, 2002 no flow was recorded at this gage.

26 Gage Period of Record Number Station Name 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s Primary Gaging Stations 2031000 Mechums River Near White Hall 2031500 N. F. Moormans River near White Hall

2032000 Moormans River near White Hall Regulated by Sugar Hollow Dam 2032250 Moormans River near Free Union 2032500 S. F. Rivanna River near Earlysville 2032400 Buck Mountain Creek near Free Union

2032515 S. F. Rivanna River near Charlottesville Heavily Regulated by Sugar Hollow and S. F. Rivanna Reservoirs 2032640 N. F. Rivanna River near Earlysville 2032680 N. F. Rivanna River near Proffit

2033500 Rivanna River Near Charlottesville Regulated by S. F. Rivanna Reservoir 2034000 Rivanna River at Palmyra 27 Long-Term Nearby Gaging Stations 1625000 Middle River near Verona 1626000 South River near Waynesboro 1627500 South River at Harriston 1665500 Rapidan River near Ruckersville 1671000 North Anna River near Doswell 1671020 North Anna River at Hart Corner 2021500 Maury River at Rockbridge Baths 2022500 Kerrs Creek near Lexington 2028500 Rockfish River near Greenfield 2030000 Hardware River near Scottsville 2030500 Slate River near Arvonia 2034500 Willis River at Lakeside Village

Figure 6. Timeline of USGS Streamflow Gages (Yellow Designates Data Used for Sugar Hollow & Ragged Mountain Reservoirs) Gage Period of Record Number Station Name 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s Primary Gaging Stations 2031000 Mechums River Near White Hall 2031500 N. F. Moormans River near White Hall

2032000 Moormans River near White Hall Regulated by Sugar Hollow Dam 2032250 Moormans River near Free Union 2032500 S. F. Rivanna River near Earlysville 2032400 Buck Mountain Creek near Free Union

2032515 S. F. Rivanna River near Charlottesville Regulated by Sugar Hollow and S. F. Rivanna Reservoirs 2032640 N. F. Rivanna River near Earlysville 2032680 N. F. Rivanna River near Proffit

2033500 Rivanna River Near Charlottesville Regulated by S. F. Rivanna Reservoir 2034000 Rivanna River at Palmyra 28 Long-Term Nearby Gaging Stations 1625000 Middle River near Verona 1626000 South River near Waynesboro 1627500 South River at Harriston 1665500 Rapidan River near Ruckersville 1671000 North Anna River near Doswell 1671020 North Anna River at Hart Corner 2021500 Maury River at Rockbridge Baths 2022500 Kerrs Creek near Lexington 2028500 Rockfish River near Greenfield 2030000 Hardware River near Scottsville 2030500 Slate River near Arvonia 2034500 Willis River at Lakeside Village

Figure 7. Timeline of USGS Stream Gages (Yellow Designates Data Used for Mechums Creek Pumping Station Watershed) Scattergraphs showing the graphical correlation between stream gages used to reconstitute missing streamflows are presented in Appendix B. A flow-duration relationship developed using the daily inflow record for the river flows at the intake for the period from 1925 to 2003 is presented in Appendix C. 4.7 Riverflow Into Ragged Mountain Reservoirs. The Ragged Mountain watershed is ungaged. Since the Sugar Hollow watershed is the most representative of the Ragged Mountain watershed, its reservoir inflows were transposed for Ragged Mountain using a linear adjustment based on contributing drainage area. The effective drainage area of the Ragged Mountain Reservoirs is 1.70 square miles which excludes the 70.4-acre surface area of the reservoirs. Since the drainage area of the Ragged Mountain Reservoirs is relatively small, the error associated with transposing the flows as described above will have minimal impact on the safe yield computed for the entire RWSA Urban Area system. The most important feature of the Ragged Mountain Reservoirs is the additional storage capacity they contribute to the system.

29 5. SYSTEM COMPUTER MODEL 5.1 Model Development. A computer model was developed to simulate the daily operation of the RWSA Urban Area source water system. The purpose of the model is to simulate the operation of each reservoir and river intake as a composite system for an extended period of record to determine the safe yield. The influence of different operating practices, conservation and drought contingency measures, regulatory releases, and expansion alternatives can also be evaluated using the model. 5.2 Model Structure. For each day in the period of simulation, the model computes the natural inflows to each pump station river intake and reservoir. Then, based on a set of assumed operating rules, the model computes the release, flowby, water supply draft, spillway flow, net evaporation, pumpage and the changes in reservoir storage as appropriate. A schematic depicting the overall structure of the RWSA Urban Area system is presented in Figure 8. Based on conversations with RWSA staff, the operating rules were chosen to determine when to use each source for water supply draft. The rules were based on percent of remaining storage in South Fork Rivanna Reservoir. Table 9 describes the operating rules for this study. The operating rules were chosen to approximate the way in which RWSA would operate the system and to maximize the safe yield. Sensitivity analyses revealed that the safe yield of the system was not overly sensitive to the operating rules. Table 9 Summary of Operating Rules for the RWSA Urban Area System

Percent N. Fork Storage in S. Rivanna Ragged S. Fork Fork Rivanna River Sugar Hollow Mountain Rivanna Reservoir Intake Reservoir Reservoir Reservoir 100% 99.5 – 100 ------Demand Maximum Balance of 80 – 99.49 Draft ------Demand Maximum Lesser of 50% Remaining Balance of 50 – 79.99 Draft Demand or Max. Available --- Demand Maximum Balance of 40 – 49.99 Draft Maximum Draft --- Demand Maximum Maximum Balance of < 40 Draft Maximum Draft Draft Demand

30 Watershed of Unreg. Watershed of Mechums Pump S. Fork Rivanna Station Reservoir

Watershed of Sugar Hollow Reservoir

Watershed of Watershed of Ragged Mountain N.Fork Rivanna Reservoirs River Intake Mechums Pump Station Net Evaporation Sugar Hollow Reservoir

Net Evaporation Net Evaporation N. Fork Ragged Mountain South Fork Rivanna Rivanna Reservoirs Reservoir River Intake

Minimum Release & Minimum Release & Minimum Flowby & Spillway Flows Spillway Flows Flows Past Intake

Treatment and Distribution

Legend

Occasional Pumpage Watershed Pipeline Transmission Reservoir Streamflow Pump Station or Intake Water Lost from System

Figure 8. Schematic of RWSA Raw Water Supply System

31 Pumpage from the Mechums River Pump Station was based on storage in Ragged Mountain Reservoirs and on streamflow at the pump intake in accordance with the regulatory permit requirements. 2 MGD or 4 MGD was pumped into Ragged Mountain Reservoirs whenever the streamflow was greater than 33 or 66 cfs, respectively and the reservoirs were less than 97 percent full. It was assumed that the reactivated pumping station would include a fish passage facility and that no pumping restrictions would be imposed between February 14th and June 15th. 5.3 Verification of Computer Model. Before performing the analyses, the computer model was thoroughly tested to ensure that all computations and internal logic were being performed correctly. The daily computations of spillway flow, reservoir evaporation, and change in reservoir storage were checked. In addition, an overall check was made by performing a mass balance test for the entire system for the 78-year period of simulation. The mass balance test involved computing the total inflows, withdrawals, losses, and the net change in reservoir storage for the simulation period, and verifying that there were no net gains or losses of water computed by the model. 5.4 Input Data Sources. The data needed to run the model are contained in two sources: the input database and the hydrologic data file. The input database is a Microsoft Access Database containing all of the data that is used to define the operation of the system. It includes the following user-supplied information: 1. Drainage area of upstream watersheds; 2. Minimum release/flowby requirement(s); 3. Spillway elevations; 4. Dead storage levels; 5. Reservoir stage-storage and stage-area relationships. 6. Triggers for operating rules The hydrologic data file contains daily river flows, reservoir inflows and daily net evaporation rates. 5.5 Output Data Files. After the input files were prepared and the program executed, several options are available to review, print, or plot the model output. The program output was stored in a file containing the daily computations for the entire simulation period. Sample printouts of the output of key statistics plotted in 10-year increments on summary graphs for each component of the RWSA Urban Area source water system are presented in Appendix D. Providing a complete sample printout in numeric tables of the data presented graphically in Appendix D would require over 1,500 pages of output and therefore was not included in this report.

32 6. SAFE YIELD ANALYSIS 6.1 Definition of Safe Yield. The criteria used to determine safe yield is found in Chapter 590 Paragraph 12VAC5-590-830 of the Virginia Department of Health's Waterworks Regulations, August 2002, and is also stated in the Commonwealth of Virginia, Virginia State Water Control Board Planning Bulletin #334 titled: Safe Yield of Municipal Surface Water Supply Systems in Virginia. According to these sources the safe yield of a source should be determined as follows: Simple Intake (free-flowing stream). “The safe yield is defined as the maximum withdrawal rate available during a day and recurring every 30 years (30-year-1 day low flow). To generate the report for this, data is to be used to illustrate the worst drought of record in Virginia since 1930. If actual gauge records are not available for this, gauges are to be correlated from similar watersheds and numbers are to be synthesized.” Complex Intake (impoundments in conjunction with streams). "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." Complex systems are systems that are made up of more than one source. For these systems, the Virginia State Water Control Board specifies that a unique model be developed that defines system boundaries, identifies system constraints, and simulates a set of operating rules. Mass balance simulation modeling techniques are then applied to determine the safe yield. Since the RWSA Urban Area system is a complex system, the safe yield was computed using State’s procedures for a complex intake. The safe yield contribution from the RWSA river intake on the North Fork Rivanna River and on Mechums River were determined using the system model rather than the State’s definition of a “simple intake”. The safe yield is based on a constant demand or draft rate. 6.2 Previous Safe Yield Studies. An accurate determination of the safe yield is vital to the successful evaluation of expansion alternatives and determination of the most appropriate course of action. The purpose of this section is to present a summary of previous estimates of safe yield for the RWSA Urban Area system. The evaluation is based on a review of five existing reports prepared by

33 four different investigators between 1959 and 2002. The safe yield of the RWSA Urban Area system reported in these studies ranges between 19.2 MGD and 4.5 MGD. A summary of the data sources, study methodology, and key assumptions used in each safe yield study is presented in Table 10. Additional safe yield analyses of the RWSA Urban Area system were performed by O’Brien & Gere in 2001/2002 to evaluate source water expansion alternatives. The results of these investigations are not summarized in Table 10. The safe yield date specified in each column of Table 10 represents the year for which the safe yield was computed. For example, the 1997 O’Brien and Gere study determined the safe yield of the RWSA Urban Area system for the year 1997 and 2050. Table 10 shows that there are significant differences in the assumptions made in each study. To simplify the analyses, rather than developing statistical relationships between the gages, all of the investigators transposed the streamflows from different stream gaging stations making a linear adjustment based on contributing drainage area. It is interesting to note that the worst drought of record used for the safe yield analyses was not the same for the four studies. Other differences between the previous safe yield studies include the method of accounting for evaporation losses, the assumed release requirement, and most importantly the assumed available storage for water supply. The relative importance and consequences of the assumptions related to release requirements and useable storage is discussed in more detail later in this report. 6.3 Dead Storage. Dead storage accounts for the volume of water in a reservoir that cannot be used to satisfy water demands due to reservoir siltation, water quality, or hydraulic limitations of the system. The dead storage of each reservoir source was assessed independently. At the South Fork Rivanna Reservoir, the dead storage was considered to be the volume of storage below Elevation 367 feet, the elevation of the lowest water intake. For Sugar Hollow and Ragged Mountain Reservoirs, since no known site specific information is available the dead storage was assumed to be 10 percent of the total storage. Table 12 tabulates the storage volumes assumed at each reservoir. Beaver Creek Reservoir was not considered to be part of the RWSA Urban Area water supply system for this study.

34

Table 10 Summary of Assumptions and Findings from Previous Safe Yield Studies

Previous Studies Polglaze & Virginia Water Black & Basenburg Control Board Veach O’Brien & Gere O’Brien & Gere Pertinent Data (1959) (1985) (1994-1995) (1997) (1997) Safe Yield of Urban Raw Reference Title Report on Water Municipal Water Water Mangmt. Raw Water Supply Raw Water Supply Works System Supply Systems Plan Study Study Safe Yield Date 1990 1985 1994 1997 2050 Effective Drainage Area S. Fork Rivanna 258.5 mi2 231.6 mi2 - 243.0 mi2 243.0 mi2 Sugar Hollow 17.8 mi2 18.0 mi2 18.0 mi2 18.0 mi2 18.0 mi2 U&L Ragged Mountain 1.9 mi2 1.8 mi2 1.8 mi2 1.8 mi2 1.8 mi2 N. F. Rivanna R. Intake - 109.0 mi2 - 121.0 mi2 121.0 mi2 Total Storage: S. Fork Rivanna 1,760 MG 1,624 MG 1,333 MG 1,294 MG 604 MG Sugar Hollow 430 MG 430 MG 430 MG 356 MG 280 MG U&L Ragged Mountain 611 MG 513.6 MG 513.6 MG 514 MG 514 MG Total 2,801 MG 2,567.6 MG 2,567.6 MG 2,203 MG 1,398 MG Useable Storage: S. Fork Rivanna 1,172 MG 1,242 MG 880 MG 841 MG 112 MG Sugar Hollow 430 MG 430 MG 430 MG 356 MG 280 MG U&L Ragged Mountain 611 MG 513.6 MG 513.6 MG 514 MG 514 MG Total 2,213 MG 2,185.6 MG 1,823.6 MG 1,711 MG 906 MG Dead Storage: S. Fork Rivanna 588 MG (Silt) 382 MG 492 MG 453 MG 453 MG Sugar Hollow 0 MG 0 MG 0 MG 0 MG 0 MG U&L Ragged Mountain 0 MG 0 MG 0 MG 0 MG 0 MG Total 588 MG 382 MG 453 MG 453 MG 453 MG ~7” 44”/Year 44”/Year Evaporation Losses 44”/Year - (0.94 mgd) (1.88 mgd) (1.88 mgd) Streamflow Source USGS Gages USGS Gages USGS Gages USGS Gages USGS Gages S. Fork Rivanna 3350 & 3250 3250 6655 3350 & 3050 3350 & 3050 Sugar Hollow Oprtg. Records 3150 3150 3350 & 3050 3350 & 3050 U&L Ragged Mountain Oprtg. Records 3150 3150 3350 & 3050 3350 & 3050 N. F. Rivanna R. Intake - 3268 3268 3350 3350 Critical Drought S. Fork Rivanna 1930 1953-54 - 1930 1930 Sugar Hollow 1930 1953-54 1953-54 1930 1930 U&L Ragged Mountain 1930 1963 1963 1930 1930 N. F. Rivanna R. Intake - 1971-82 1971-82 1930 1930 Mass Balance Mass Balance Computer Model Spreadsheet Spreadsheet Computational Method Separately Equation - Separately Separately Separately Separately Gross Safe Yield (No Rel.) S. Fork Rivanna 19.65 mgd 18.4 mgd 19.8 mgd 15.2 mgd <8.0 mgd Sugar Hollow 5.2 mgd* 5.4 mgd 5.4 mgd 4.1 – 4.8 mgd 3.9 – 4.2 mgd U&L Ragged Mountain (included above) (included above) (included above) (included above) (included above) N. F. Rivanna R. Intake - 1.0 mgd 2.0 mgd** 0.6 mgd 0.6 mgd Total Gross Safe Yield 24.85 MGD 24.8 MGD 27.2 MGD 19.9-20.6 MGD <12.8 MGD Release Requirement S. Fork Rivanna 5.85 mgd 8.0 mgd 8.0 mgd 8.0 mgd 8.0 mgd Sugar Hollow 0.0 mgd 0.0 mgd 0.0 mgd 0.0 mgd 0.0 mgd U&L Ragged Mountain 0.0 mgd 0.0 mgd 0.0 mgd 0.0 mgd 0.0 mgd N. F. Rivanna R. Intake - 0.0 mgd 0.0 mgd 0.0 mgd 0.0 mgd Total Safe Yield 19.0 MGD 16.8 MGD 19.2 MGD 11.9-12.6 MGD 4.5-4.8 MGD * Does not include intake on Mechums River ** Includes supplemental flow from Chris Greene Lake

35 Table 11 Assumed Distribution of Storage in RWSA Urban Area Reservoirs (2002 Conditions)

Storage at Dead Useable Normal Pool Storage Storage Description Reservoir (MG) (MG) (MG) of Dead Storage South Fork Rivanna Reservoir 1,155 355 800 Volume below El. 367 Sugar Hollow Reservoir 360 36 324 10% of usable storage Ragged Mountain Reservoirs 514 51 463 10% of usable storage Beaver Creek Reservoir 520 ------Not modeled

6.4 Existing Safe Yield of the RWSA Urban Area System. Although the RWSA is committed to being environmentally responsible and has made voluntary commitments to specific conservation releases at Sugar Hollow and South Fork Rivanna Reservoir during non-drought conditions, investigations by others (VHB, 2001) has 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 releasing water for stream conservation purposes to conserve as much water as possible for public water supply purposes. The safe yield of the RWSA Urban Area system assuming 2002 reservoir storage conditions and 1,586 million gallons of usable storage with no stream conservation releases from the reservoirs is 16.0 MGD. The drought of record occurred in 2002 and the second-worst drought occurred in 1930. Other less severe droughts occurred in 1954, 1963, 1966, and 1977. For this simulation, during the 2002 drought, reservoir levels fell below normal pool on May 20, 2002 and just emptied all usable storage in every reservoir on October 16, 2002. The reservoirs did not refill to normal pool volumes again until February 24, 2003, which is about 280 days after the date when the reservoir levels fell below normal pool. It should be emphasized that operating the system assuming no stream conservation releases implies that no releases are made from South Fork Rivanna Reservoir during periods when the reservoir falls below normal pool conditions. Any deviations from this operating assumption would result in less safe yield from the system. Table 11 summarizes the assumptions and results of this analysis. The safe yield of the RWSA Urban Area system was also assessed assuming the release from South Fork Rivanna Reservoir is the lesser of 8 MGD or the inflow to the reservoir and the release

36 Table 12 Summary of Assumptions and Findings from Safe Yield Studies

2002 1930 1930 2002 4-Foot Drought Drought Drought Drought Crest Gate Pertinent Data No Rel. No Rel. With Rel. With Rel. With Rel. Safe Yield Date 2002 2002 2002 2002 2002 Effective Drainage Area S. Fork Rivanna 241.20 mi2 241.20 mi2 241.20 mi2 241.20 mi2 241.20 mi2 Sugar Hollow 17.43 mi2 17.43 mi2 17.43 mi2 17.43 mi2 17.43 mi2 U&L Ragged Mountain 1.70 mi2 1.70 mi2 1.70 mi2 1.70 mi2 1.70 mi2 N. F. Rivanna R. Intake 115.01 mi2 115.01 mi2 115.01 mi2 115.01 mi2 115.01 mi2 Total Storage: S. Fork Rivanna 1,155 MG 1,155 MG 1,155 MG 1,155 MG 1,707 MG Sugar Hollow 360 MG 360 MG 360 MG 360 MG 360 MG U&L Ragged Mountain 514 MG 514 MG 514 MG 514 MG 514 MG Total 2,029 MG 2,029 MG 2,029 MG 2,029 MG 2,580 MG Useable Storage: S. Fork Rivanna 800 MG 800 MG 800 MG 800 MG 1,351 MG Sugar Hollow 324 MG 324 MG 324 MG 324 MG 324 MG U&L Ragged Mountain 462 MG 462 MG 462 MG 462 MG 462 MG Total 1,586 MG 1,586 MG 1,586 MG 1,586 MG 2,137 MG Dead Storage: S. Fork Rivanna 355 MG 355 MG 355 MG 355 MG 355 MG Sugar Hollow 36 MG 36 MG 36 MG 36 MG 36 MG U&L Ragged Mountain 51 MG 51 MG 51 MG 51 MG 51 MG Total 443 MG 443 MG 443 MG 443 MG 443 MG Shallow Shallow Shallow Shallow Shallow Evaporation Losses Lake Lake Lake Lake Lake Release Requirement S. Fork Rivanna 0.0 mgd 0.0 mgd 8 mgd/Inflow 8 mgd/Inflow 8 mgd/Inflow Sugar Hollow 0.0 mgd 0.0 mgd 0.4 mgd/80% 0.4 mgd/80% 0.4 mgd/80% U&L Ragged Mountain 0.0 mgd 0.0 mgd 0.0 mgd 0.0 mgd 0.0 mgd N. F. Rivanna R. Intake 0.0 mgd 0.0 mgd 0.0 mgd 0.0 mgd 0.0 mgd Streamflow Source USGS USGS USGS USGS USGS S. Fork Rivanna Gages Gages Gages Gages Gages Sugar Hollow - - - - - U&L Ragged Mountain - - - - - N. F. Rivanna R. Intake - - - - - Critical Drought S. Fork Rivanna 2002 1930* 1930 2002* 1930 Sugar Hollow 2002 1930* 1930 2002* 1930 U&L Ragged Mountain 2002 1930* 1930 2002* 1930 N. F. Rivanna R. Intake 2002 1930* 1930 2002* 1930 Conjunctive Conjunctive Conjunctive Conjunctive Conjunctive Computational Method Use Model Use Model Use Model Use Model Use Model Average Yield S. Fork Rivanna 9.59 MGD 14.03 MGD Sugar Hollow 2.20 MGD 1.88 MGD U&L Ragged Mountain 2.99 MGD 2.34 MGD N. F. Rivanna R. Intake 1.22 MGD 1.75 MGD Total Safe Yield 16.0 MGD 20.0 MGD 13.3 MGD 13.4 MGD 16.2 MGD

* Second worst drought of record for given assumptions

Note: The safe yield of the RWSA Urban Area raw water system for future time periods can be estimated by adjusting the safe yield values in Table 12 using relationships presented in Sections 6.6 and 6.7 of this report for anticipated reservoir storage conditions and negotiated reservoir release requirements.

37 from Sugar Hollow Reservoir is 400,000 gallons per day unless the available reservoir storage falls below 80 percent. The safe yield of the system with these assumptions was estimated to be 13.3 MGD. With these assumptions, the drought of record occurred in 1930 and the second-worst drought occurred in 2002. Other less severe droughts occurred in 1954, 1963, 1966, and 1977. Based on the computer simulation, during the 1930 drought, the reservoirs would have been completely full on June 19, 1930 and just emptied the usable storage in each reservoir on December 26, 1930. The reservoirs would not have completely refilled until August 24, 1931, over 400 days later. Table 12 summarizes the results of this analysis. Simulating the daily operation of the system produces more than 28,000 lines of output per simulation. Rather than analyze volumes of output, the results of the simulations are summarized and presented graphically. For example, numerous plots were generated from the simulation of the system using the previously stated release requirements with a demand of 13.3 MGD (the safe yield) and they are contained in Appendix D. The sets of graphs developed illustrate plots of key variables from the simulation of the system for the period 1925 through 2003. Each plot presents results for a 10-year period resulting from a constant system demand. The total usable storage for water supply as of 2002 was assumed to be 1,586 million gallons. Figure 9 is a summary of reservoir storage conditions in each of the reservoirs for the entire period of record. It is interesting to note that the drought of record based on simulation of the system with no release requirements is different than the drought of record based on simulation of the system with release requirements from South Fork Rivanna and Sugar Hollow Reservoirs. Table 12 compares the safe yield estimates for the worst and second-worst droughts for both simulation scenarios. From these analyses, it can be inferred that both the 1930 and 2002 droughts are both extreme droughts. The relative magnitude of these droughts is discussed in Section 6.5. Section 6.7 examines the effect of various release requirements at South Fork Rivanna Reservoir on the safe yield of the system. 6.5 Probabilistic Analysis of System Safe Yield. In order to evaluate the severity of the worst droughts of record for this system (the 1930 and 2002 droughts), a probabilistic analysis was completed using the computer model and the hydrologic database of reservoir inflows and flows at the river intakes from 1925 through 2003. The hydrologic database was divided into 78 1-year periods. Each 1-year period was assumed to begin in May (the beginning of the normal dry period) and run through the following April. The computer model of the RWSA Urban Area system was then simulated for each 1-year period in order to compute the safe yield for each year.

38 Summary of Reservoir Storage 1920-2010 Constant Demand: 13.28 MGD Release Req'mts: S. Fork Rivanna: 8 MGD/Inflow N.F. Rivanna R. Intake: none Sugar Hollow: 0.4 MGD/80% Mechums P.S.: 33 cfs/66 cfs Ragged Mountain: none 2500

2000

1500 Total Storage, 2,029 MG

(MG) 1000 Dead Storage, 443 MG 500 Total Reservoir Storage Reservoir Total 0 1200

1000 Total Storage, 1,155 MG

800

600

(MG) Dead Storage, 355 MG 400

200 South Fork Rivanna Reservoir

0 400

Total Storage, 360 MG 200 Dead Storage, 36 MG Suger Hollow

Reservoir (MG) Reservoir 0 600

400 Total Storage, 514 MG

(MG) Dead Storage, 51 MG 200 Reservoir Ragged Mountain 0 600

400 Total Storage, 520 MG (MG) 200 Note: Storage in Beaver Creek Reservoir not included in Analysis Reservoir Beaver Creek

0 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 Figure 9. Summary of Reservoir Storage from Simulation of RWSA System (1925 - 2003)

39 In the driest year (1930), the safe yield of the system assuming 2002 reservoir storage conditions and a release requirement at South Fork Rivanna Reservoir equal to the lesser of 8 MGD or the natural inflow was computed to be 13.3 MGD. In the second driest year (2002), the safe yield of the system was computed to be 13.4 MGD. For other scenarios where there is no release requirement from South Fork Rivanna Reservoir, the 2002 drought is the worst drought of record. In the wettest year (1975), the safe yield of the RWSA Urban Area system was computed to be 167 MGD. The 78 safe yield estimates were ranked and assigned a plotting position and return period based on the Weibull Formula: T = (n + 1)/m Where; T = approximate return period in years, n = number of years of record, and m = rank or order. Figure 10 shows these data plotted on probability paper. The small degree of scatter and strong linear tendencies indicate that the data match the Weibull distribution and the estimates of return period appear valid. The best-fit line was fit by the method of least squares. As shown on Figure 10, the 2002 and 1930 droughts were extreme events. Using the best-fit relationship, it can be estimated that the corresponding return period for a safe yield of 13.3 MGD is 94 years. This analysis indicates that the 2002 and 1930 events are very rare. According to the best-fit relationship presented on Figure 10, the 100-year safe yield for conditions modeled is approximately 13 MGD. The safe yield of the system for other return periods can also be estimated from Figure 10. 6.6 Effect of Changes in Storage at South Fork Rivanna Reservoir. The future safe yield of the entire system is highly dependent on the rate of loss of usable storage due to sedimentation at the South Fork Rivanna Reservoir. Analysis of bathymetric surveys at South Fork Rivanna Reservoir indicate that reservoir storage has been lost at a relatively uniform rate of approximately 15.1 million gallons per year. This loss rate is expected to continue, but can change as a result of land use changes within the watershed and changes in the sediment trapping efficiency of the reservoir, and future frequency of severe storm events that typically cause the majority of sediment laden flows. As discussed in Section 2.4, the total storage in South Fork Rivanna Reservoir has been decreasing due to reservoir sedimentation. Any loss in usable storage would cause the safe yield of the RWSA Urban Area system to decrease. Conversely, any increase in usable reservoir storage would increase the safe yield of the system.

40 1,000

1975 167 MGD

Best-Fit Line

100 41

1930 Drought 13.3 MGD Simulation Assumptions: 1. Minimum Release Requirements: - S. F. Rivanna Reservoir = Lesser of Inflow or 8 MGD 10 - Sugar Hollow Reservoir = 0.4 MGD (When > 80%) - Ragged Mountain Reservoirs = 0 MGD - No Flowby Required from N. F. Rivanna River Intake 2. Reservoir Storage

Safe Yield of RWSA Raw Water Supply System (MGD) System Supply Water Raw of RWSA Yield Safe - S. F. Rivanna Reservoir: Dead Storage = 355 MG; Total Storage = 1,155 MG - Sugar Hollow Reservoir: Dead Storage = 36 MG; Total Storage = 360 MG 2002 Drought - Ragged Mountain Reservoirs: Dead Storage = 51.4 MG; Total Storage = 514 MG 13.4 MGD

1 1000 500 200 100 50 2010 51.221.5 1.1 1.05 1.02 1.01 1.005 1.001 Recurrence Interval (Years) Figure 10. Safe Yield Probability Relationship for RWSA Source Water Supply System In order to investigate the effect of changes in storage at South Fork Rivanna Reservoir, an approximate relationship was developed between reservoir storage and system safe yield. The RWSA Urban Area system was simulated with release requirements of 8 MGD or the natural inflow at South Fork Rivanna Reservoir and 400,000 gallons per day at Sugar Hollow Reservoir unless the available reservoir storage falls below 80 percent. The usable storage in Sugar Hollow and Ragged Mountain Reservoirs was assumed to be 324 million gallons and 462 million gallons, respectively. The usable storage in South Fork Rivanna Reservoir was varied from about 200 million gallons to about 2,000 million gallons. Figure 11 depicts the relationship between safe yield of the RWSA Urban Area system with the assumptions stated and the usable storage in South Fork Rivanna Reservoir. The usable storage in South Fork Rivanna Reservoir has also been translated into normal pool elevations (based on 2002 storage conditions) on the secondary Y-axis. Decreases in usable storage can occur due to sedimentation. These cases are represented by usable storage below 800 million gallons or below Elevation 382 feet. Increases in usable storage can be achieved by modifying the dam to raise the normal pool level or by dredging the reservoir to remove that quantity of sediment which has accumulated above the dead storage level of Elevation 367 feet. These cases are represented by usable storage above 800 million gallons or above Elevation 382 feet. For instance, installing 4-foot crest gates on South Fork Rivanna Reservoir would increase the normal pool elevation to Elevation 386 feet and increase the usable storage to 1,351 million gallons. Using Figure 11, it can be determined that the safe yield of the RWSA Urban Area raw water supply system would increase to 16.2 MGD, an increase of 2.9 MGD assuming that there is no change in the release requirement from the reservoir. This case is also summarized in Table 9. Figure 11 can also be used to investigate other scenarios involving increases or decreases in usable storage from the entire system not just from South Fork Rivanna Reservoir. In general, the relationship indicates that the safe yield of the RWSA Urban Area system decreases by 1.0 MGD for every 190 million gallons of reservoir storage volume reduction. 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 reservoir dredging, installation of spillway crest gates on South Fork Rivanna Reservoir, raising Ragged Mountain Dams, etc. For example, if it is assumed that reservoir storage continues to be lost at a rate of 15.1 million gallons per year, by

42 2500

Assumptions: 1. Minimum release from S. F. Rivanna Reservoir = lesser of 8 MGD or natural inflow 2. Useable storage In Sugar Hollow Reservoir = 324 MG 390 2000 3. Useable storage In Ragged Mountain Reservoirs = 462 MG 4. No release requirement from Ragged Mountain Reservoir 5. Minimum release from Sugar Hollow Reservoir = 0.4 MGD whenever reservoir storage exceeds 80 percent of total 6. Pumping capacity at N. F. Rivanna River Inake = 2 MGD with no flowby requirement 1500

386 Existing Pool of South Fork Rivanna Reservoir

43 El. 382 feet 800 MG Usable Storage (2002 Conditions) k Reservoir Elevation RWSA System Safe Yield = 13.3 MGD 1000

382

500 Usable Storage in South Fork Rivanna Reservoir (MG) 377

372 Normal Pool Elevation at South For at South Elevation Normal Pool

0 0 5 10 15 20 25 RWSA Urban Service Area Raw Water Safe Yield (MGD)

Figure 11. Relationship Between Safe Yield of the RWSA System and the Usable Storage in South Fork Rivanna Reservoir 2050, the safe yield of the system would gradually decrease by approximately 0.079 MGD per year from 16.0 MGD in 2002 to 12.2 MGD, assuming no release requirements from the reservoirs. 6.7 Effect of Release Requirements During Severe Drought Events. Securing regulatory agency permits for some of the water supply expansion alternatives under consideration may require adopting new release requirements. Since there are currently no mandatory release requirements from any of the RWSA Urban Area sources of supply, any newly established release requirement(s) would need to be analyzed in order to determine impacts on the overall net safe yield benefits afforded by the respective alternatives under consideration. Since the greatest impact on the RWSA Urban Area system safe yield would result as a consequence of a new release requirement from South Fork Rivanna Reservoir, the computer model of the system was used to evaluate various conservation release scenarios. The release scenarios evaluated included releasing the lesser of a specified flow (ranging between 0 and 24 MGD) or the natural inflow. Figure 12 depicts the results of this analysis. It is interesting to note that for some of these release scenarios, the worst drought of record switched from the drought of 2002 to the drought that occurred in 1930. As a general rule of thumb, for every 1.0 MGD of specified flow release or natural inflow ranging from 0 to 15 MGD, the safe yield is reduced by approximately 0.34 MGD. For example, adopting a release requirement equal to the lesser of 8 MGD or the natural inflow would reduce the safe yield of the system by approximately 2.7 MGD to about 13.3 MGD. It should be noted that the relationship presented in Figure 12 is not truly linear. For accurate results, Figure 12 should be used rather than using the rule of thumb described above. Further, to estimate the effects of more complex release requirements such as those that vary seasonally or with storage conditions, additional model simulation should be completed.

44 17

Assumptions: 16 1. Minimum release from S. F. Rivanna Reservoir = lesser of given value or natural inflow 2. Useable storage In System = 1,586 MG 3. No release requirement from Ragged Mountain Reservoir 15 4. Minimum release from Sugar Hollow Reservoir = 0.4 MGD whenever reservoir storage exceeds 80 percent of total 5. Pumping capacity at N. F. Rivanna River Inake = 2 MGD with no flowby requirement 14

13 Release at S.F. Rivanna Reservoir = lesser of 8 MGD or natural inflow RWSA System Safe Yield = 13.3 MGD 45 12 Safe Yield (MGD) Yield Safe

11

10

9

8 0 5 10 15 20 25 Release from South Fork Rivanna Reservoir (MGD)

Figure 12. RelationshipBetween Safe Yield of the RWSA System and the Release Requirement at South Fork Rivanna Reservoir 7. SUMMARY AND RECOMMENDATIONS 7.1 Summary. The safe yield investigations performed in this study are based on a 78-year period of record from 1925 to 2003 using streamflow and climatic data specific to the site. The method of analysis involved programming a custom computer model to simulate the daily operation of the RWSA Urban Area system and includes the North Fork Rivanna River Intake, Mechums River Pumping Station, South Fork Rivanna Reservoir, Sugar Hollow Reservoir, and Ragged Mountain Reservoirs. Past safe yield investigations were critically evaluated. For the worst drought of record, the safe yield of the RWSA Urban Area system with no release requirements was computed to be 16.0 MGD. With release requirements of the lesser of 8 MGD or the natural inflow at South Fork Rivanna Reservoir and 400,000 gallons per day at Sugar Hollow Reservoir when the storage remaining is greater than 80 percent of the total, the safe yield of the RWSA Urban Area system was computed to be 13.3 MGD. For an established set of hydro-meteorological conditions, the safe yield available from the RWSA Urban Area system is primarily dependent upon two important assumptions including (1) the reservoir storage available for water supply, and (2) the release requirement(s) from the reservoirs and flowby requirements at the river intakes. As part of this study, a relationship expressing safe yield as a function of usable storage in South Fork Rivanna Reservoir was established. This relationship can also be used to estimate the effect on safe yield of any increase in usable storage for the entire system. In general, the relationship indicates that the safe yield of the RWSA Urban Area system increases/decreases by 1.0 MGD for every 190 million gallons of gained/lost reservoir storage. If it is assumed that reservoir storage continues to be lost at a rate of 15.1 million gallons per year, the safe yield of the system would gradually decrease by approximately 0.079 MGD per year from 16.0 MGD in 2002 to 12.2 MGD in 2050, assuming no release requirements. A relationship between the release requirement at South Fork Rivanna Reservoir and the safe yield of the system was also generated. In general, for every 1.0 MGD of specified flow release or natural inflow (ranging from 0 to 15 MGD), on average the safe yield is reduced by approximately 0.34 MGD. This relationship is not linear and Figure 12 should be referenced for accuracy. Statistical analyses were also performed to evaluate the severity and return period of past drought events. Both the 1930 drought and the 2002 drought were severe and rare drought events of similar magnitude.

46 7.2 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. The analyses 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 elevation-area-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 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

47 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.

48 8. REFERENCES 1. Beard, L.R. 1970, Estimating Monthly Streamflows Within a Region, Technical Paper No. 18, U.S. Army, Corps, of Engineers, Hydrologic Engineering Center, Davis, California, 15 p. 2. Black & Veatch, Interim Memorandum for the Feasibility of Flashboards on the South Rivanna Dam, prepared for Rivanna Water & Sewer Authority, November 1994 3. Black & Veatch, Urban Raw Water Management Plan Summary Report, prepared for Rivanna Water & Sewer Authority, August 1995, 24 p. 4. Bowler, Stephen P., South Fork Rivanna Reservoir, Reflecting on 35 Years, Anticipating 50 Years, Draft Report, 2002, 83 p. 5. Bowler, Stephen P., South Fork Rivanna Reservoir and Watershed, Reflecting on 36 Years, Anticipating 50 Years, Expanded Summary, Spring 2003, 11 p. 6. Earthinfo, Inc., 2000. NCDC Summary of the Day 2000, CD contains precipitation data. 7. Meyer, A.F., 1942. Evaporation from Lakes and Reservoirs, Minnesota Resources Commission, St. Paul, Minnesota, 111 p. 8. NCDC Website, National Climatic Data Center, http://www.ncdc.noaa.gov/oa/ncdc.html 9. O’Brien & Gere Engineers, Inc., Vanasse Hangen Brustlin, Inc., Water Supply Analysis, Raw Water Supply Facility Permitting, Supply Analysis, Albemarle County and City of Charlottesville, October 1997, 20 p. 10. Polglaze & Basenberg, Engineers, Report on Water Works System, Charlottesville, Virginia, December 1959, 101 p. 11. Rivanna Water & Sewer Authority Revised Staff Recommendation, A Multi-Step, Integrated Water Supply Strategy, Urban Water Service Area, Revised October 28, 2002, 13 p. 12. Schwinn, Michael A., Western Virginia Regulatory Section, Department of the Army, Written communication to Lawrence Tropea, Rivanna Water & Sewer Authority, January 7, 2003. 13. Searcy, J.K., 1960, Graphical Correlation of Gaging-Station Records, Manual of Hydrology: Part 1, General Surface Water Techniques, Geological Survey Water-Supply Paper 1541-C, U.S. Department of the Interior, Washington D.C., 100 p. 14. USGS Website, Daily Streamflow for the Nation, http://waterdata.usgs.giv/nwis/discharge

49 15. Vanasse Hangen Brustlin, Inc., Handouts from Public Meeting for Water Supply Project Preliminary Alternatives, April 20, 1999 16. Vanasse Hangen Brustlin, Inc., O’Brien & Gere Engineers, Inc., Ellis & Thorp, LLC, Revised Draft of the Rivanna Water and Sewer Authority Water Supply Project Recommended Alternatives, May 2001, 41 p. 17. Vanasse Hangen Brustlin, Inc., O’Brien & Gere Engineers, Inc., Ellis & Thorp, LLC, Revised Draft of the Rivanna Water and Sewer Authority Water Supply Project, Summary of Recommended Alternatives, May 2001, 41 p. 18. Vanasse Hangen Brustlin, Inc., O’Brien & Gere Engineers, Inc., Raw Water Supply Facility Permitting Demand Analysis, Albemarle Count and City of Charlottesville, October 1997, 34 p. 19. Virginia State Water Control Board, Commonwealth of Virginia, Safe Yield of Municipal Surface Water Supply Systems in Virginia, Planning Bulletin #335, March 1985. 20. Waterway Surveys & Engineering, Hydrographic Survey of Sugar Hollow Reservoir, Albemarle County,, VA, October 5, 1995. 21. Waterway Surveys & Engineering, Volume Calculations at the South Fork Rivanna River Reservoir, Charlottesville, VA, May 31, 2001. 22. Waterway Surveys & Engineering, Volume Calculations at the South Fork Rivanna River Reservoir, Charlottesville, VA, April 19 2002.

50