Technical 3 Memo C Appendix Appendix C Technical Memo 3 Hydrology and Hydraulics Review TECHNICAL MEMORANDUM – HYDROLOGY AND HYDRAULICS REVIEW September, 2008 TECHNICAL MEMORANDUM NO. 3 INTRODUCTION AND PURPOSE The purpose of this technical memorandum is to evaluate the fresh water and recycled water facilities owned and operated by the South Tahoe Public Utility District (District) in Alpine County, California. The hydraulic capacity and operational management of the existing conveyance and storage structures will be evaluated, as well as the capacity required for potential new operational scenarios. In addition, the hydrology of the watershed areas that affect the District’s facilities will be reviewed. The components of the District’s fresh water system that were reviewed in this analysis include the West Fork of the Carson River, the Snowshoe Thompson #1 and #2 Ditches, Indian Creek, the Upper Dressler Ditch, Millich Ditch, the Upper and Lower Harvey Channels and Indian Creek Reservoir. The components of the District’s recycled water system reviewed in this analysis include the Harvey Place Reservoir and the Diamond Ditch system to the Upper Celio Dressler Ditch Diversion. SITE DESCRIPTION The District’s fresh water and recycled water facilities are located in northeastern Alpine County on the eastern slope of the Sierra Nevada. The facilities analyzed in this memorandum extend from approximately Woodfords, California and Indian Creek Reservoir to the California/Nevada State line (Figure 1). The fresh water system begins with a diversion from the West Fork of the Carson River near Woodfords at an elevation of approximately 5,750 feet above sea level. Fresh water diverted from the West Fork of the Carson River is conveyed to Indian Creek Reservoir via the Snowshoe Thompson #1 and Upper Dressler Ditches (reference Figure 1). Freshwater is diverted from Indian Creek into the Upper Dressler Ditch. The minimum and maximum pool elevations for Indian Creek Reservoir are approximately 5,588 feet and 5,600 feet above sea level, respectively. The recycled water system begins at the wastewater treatment plant in South Lake Tahoe. Secondary treated wastewater (recycled water) is pumped out of the Lake Tahoe basin over Luther Pass and into Alpine County. Recycled water is conveyed in an open channel from the outlet of the District’s C-Line pipeline to Harvey Place Reservoir. The elevation of the primary spillway Harvey Place Dam is approximately 5,563 feet above sea level. Recycled water from the reservoir is conveyed in the Diamond Ditch (Figure 1, at the end of this memorandum) through Diamond Valley and applied for agricultural reuse to several properties located between the reservoir and the State line. Recycled water reuse facilities are located in Wade Valley and the Fredericksburg area. The recycled water irrigation system was developed to provide Technical Memorandum 3 1 of 28 Recycled Water Facilities Master Plan Hydrology and Hydraulics Review Appendix C TECHNICAL MEMORANDUM – HYDROLOGY AND HYDRAULICS REVIEW additional treatment of wastewater through land application and to augment the supply of irrigation water in a water-short area (CWC-HDR, 1988). Two major drainage areas contribute stream flow and runoff to the District’s fresh water and recycled water facilities in Alpine County, the West Fork of the Carson River and Indian Creek watersheds. The West Fork of the Carson River watershed above the Snowshoe Thompson #1 Ditch diversion is approximately 66 square miles. The Indian Creek watershed above Harvey Place and Indian Creek Reservoirs is approximately 10.4 square miles (Parsons et. al., 1982). The majority of the precipitation that falls on these watersheds occurs as snowfall from November through March. The annual average precipitation for Woodfords, California is approximately 21 inches (WRCC, 2001). The depth of snowfall varies significantly throughout the area, with the higher elevations (above 9,000 feet) receiving up to 50 percent more snowfall than the lower elevations. The District’s facilities reviewed in this analysis range in elevation from approximately 5,680 feet at the Snowshoe Thompson #1 Ditch diversion to approximately 5,245 feet at the Upper Celio Dressler Ditch Diversion. The elevation of Woodfords is approximately 5,640 feet. Therefore, precipitation totals for Woodfords should be relatively representative of the majority of the project area. Monthly and annual averages of temperature, precipitation and snowfall for the National Weather Service meteorological station at Woodfords, California are included in the Appendix. Stream flows in the West Fork of the Carson River and Indian Creek depend on the snowpack at the higher elevations of the watersheds. A deep snowpack and a cool spring characterize a wet year. This scenario results in relatively high stream flows continuing into June or July. A normal year is characterized by a moderate snowpack with a warm spring and high stream flows continuing until early June and dropping off throughout the remainder of the summer. A dry year is characterized by a poor snowpack with a warm spring and high stream flows only through early May (CWC-HDR, 1988). METHODOLOGY The methods used to evaluate the conveyance and storage capacity of the District’s existing facilities were based on existing data, information provided in the District’s Operation and Maintenance Manual (CWC-HDR, 1988), calculations and computer analysis of reconnaissance level survey data, personal communications with District staff and several assumptions. The hydraulic capacity of the existing conveyance system was evaluated, as well as the capacity required for potential new operational scenarios with increased flows. The maximum capacity of all existing and proposed channels was assumed to include 1 foot of freeboard. Unless otherwise noted, concrete lined and unlined channels were assigned, assumed Manning’s coefficients of 0.015 and 0.040, respectively. The relatively high Mannings coefficient for unlined open channels reflects the fact that existing channel surfaces are rough with numerous loose boulders and cobbles. Existing channel side slopes with woody vegetation were assigned an assumed Mannings coefficient of 0.060. Existing and proposed High Density Polyethylene (HDPE) Technical Memorandum 3 2 of 28 Recycled Water Facilities Master Plan Hydrology and Hydraulics Review Appendix C TECHNICAL MEMORANDUM – HYDROLOGY AND HYDRAULICS REVIEW pipelines were assigned standard Mannings coefficients of 0.010. It was assumed that all proposed unlined open channels should be designed for subcritical flow. The capacity of existing and proposed channels and pipelines was estimated using reconnaissance level survey data and the computer program FlowMaster (Haestad Methods, 1995). The maximum capacity of all existing culverts was calculated by assuming maximum allowable headwater elevations equal to the difference in elevation between the invert and the top of the bank in the upstream channels. Tailwater depths were assumed to equal the normal depth in the downstream channels. Therefore inlet control conditions were assumed. Corrugated Metal Pipe (CMP) culverts were assigned a standard Mannings coefficient of 0.024. Reinforced Concrete Pipe (RCP) and Reinforced Concrete Box (RCB) culverts were assigned a standard Mannings coefficient of 0.013. The capacity of the existing culverts was estimated using reconnaissance level survey data and the computer program CulvertMaster (Haestad Methods, 1994). The maximum capacity of the existing siphons under Indian Creek was also evaluated. The capacity of the existing siphons was estimated based on assumptions of pipe length, geometry and head losses. Fresh water is diverted by the Snowshoe Thompson #1 Ditch/Upper Dressler Ditch system (Figure 1) and conveyed to Indian Creek Reservoir from the West Fork of the Carson River and Indian Creek. There is a gauging station at the three-foot flume at the take out from the West Fork for flow that goes to both Snowshoe Thompson #1 and the Millich Ditch. The U.S. District Court Federal Water Master gage C76 (Figure 1) monitors the fresh water that is diverted from the West Fork of the Carson River for use in both the Millich Ditch the Snowshoe Thompson #1 Ditch. The gaging station is located at the three-foot flume at the take- out from the West Fork of the Carson River for flow that goes to both Snowshoe Thompson #1 and the Millich Ditch. Historical records of flow have been collected at gage C76 from 1 April 1984 to 15 October 2000. Average monthly percentages of the total water diverted during this period were calculated to estimate the historical monthly average diversion scenario. This analysis indicates that the historical average monthly diversion scenario at gage C76 has been approximately 18, 25, 21, 16, 11, 8 and 1 percent during the months of April, May, June, July, August, September and October, respectively. As noted above, this is the average monthly diversion percentages of fresh water from the West Fork of the Carson River that have been historically supplied to both the Millich Ditch and the Snowshoe Thompson #1 Ditch. The District has indicated they would like to evaluate the potential of increasing its entitlement at Indian Creek Reservoir. In addition, they would like to evaluate different operational scenarios that would allow it to divert its entire entitlement when water is available in the West Fork of the Carson River. The District believes ditch losses in the current system are as high as 50 percent. The District would like to evaluate the alternative of enlarging existing unlined channel sections or replacing them with pipelines. Existing lined channel sections will continue to be used until they fail, at which time they will most likely be replaced with pipeline. The District has indicated they would eventually like to replace the majority of the open channel system with pipelines, which require much less maintenance. Technical Memorandum 3 3 of 28 Recycled Water Facilities Master Plan Hydrology and Hydraulics Review Appendix C TECHNICAL MEMORANDUM – HYDROLOGY AND HYDRAULICS REVIEW The potential increased entitlements assumed in this analysis are 750, 1000 and 1200 acre- feet/year.