Memo 3 Technical

Appendix C

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

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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)

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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.

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The potential increased entitlements assumed in this analysis are 750, 1000 and 1200 acre- feet/year. In addition, three different diversion scenarios were assumed. The first diversion scenario assumes the entitlements would be conveyed to the reservoir under the historical monthly average ratio of 18:25:21:16:11:8:1 percent from 1 April to 15 October each year. The second and third diversion scenarios assume the entitlements would be conveyed to the reservoir during the months of April, May and June at a ratio of 40:40:20 percent, respectively. The second and third diversion scenarios differ in the number of days per month diversions would occur. The second diversion scenario assumes water would be diverted every day during April, May and June, whereas the third diversion scenario assumes diversions would occur at increased flow rates for only 15 days of each month. Therefore the third diversion scenario (40:40:20 percent, April-June, 15-days/month) represents the maximum potential flow rate water would be diverted from the West Fork of the Carson River. These proposed scenarios were selected because peak flows in the West Fork of the Carson River have historically occurred in April, May and June.

The analysis that follows assumes that improved unlined open channels and pipelines should be sized to convey the maximum potential flow rate. The Snowshoe Thompson #1 and Upper Dressler Ditches should be sized to convey 1,200 acre-feet/year under the proposed 40:40:20 percent, April-June, 15-days/month diversion scenario. This entitlement and diversion scenario represents the maximum potential flow rates for all of the scenarios discussed above. Sizing improved channels and/or pipelines to these criteria will provide the District with the greatest operational flexibility.

It was also assumed that improved unlined open channels and pipelines proposed for the recycled water system should be sized to convey the maximum potential flow rate. An estimate of the maximum potential flow rate that may be required in the Diamond Ditch system was first calculated based on the current operational management and storage capacity of Harvey Place Reservoir and flow projections for the treatment plant at South Lake Tahoe. Under normal operations, Harvey Place Reservoir fills between 15 October and 1 April of the following year. Release of recycled water from the reservoir typically may start after 1 April and continues until 15 October when the reservoir is to be drained to its minimum pool.

Recycled water quantity projections indicate that the treatment plant will produce an estimated 5.8 MGD (approximately 6,498 AFY or 9.0 cfs) by 2028. To estimate the maximum potential reservoir drawdown rate, it was assumed that during a wet year, little or no recycled water would be required for irrigation until 1 June. If the reservoir were to remain full up to 1 June (3,800 acre-feet), the flow rate required to drain the reservoir to its minimum pool by 15 October would be approximately 24 cfs. This reservoir drawdown rate assumes a continuous inflow of new recycled water at 9.0 cfs from the District’s C-line, 800 acre-feet of flood storage from runoff and no outflows from evaporation or seepage. The analysis that follows indicates that existing Diamond Ditch system appears to have a sufficient capacity for this potential reservoir drawdown rate. However, a continuous inflow of new recycled water at 9.0 cfs between 15 October and 1 April will almost fill the reservoir to its current capacity. Therefore, by the year 2028, release of recycled water from the reservoir should start by April 1 at the latest.

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In order to provide the District with its greatest operational flexibility, it was assumed that proposed improvements to the Diamond Ditch system should be sized based on an existing limiting section. It was assumed that the existing inverted siphon under Indian Creek represents the limiting section of the Diamond Ditch system, because it cannot be easily improved or replaced. It would be more feasible to improve existing channel sections or replace them with a pipeline. The existing maximum capacity of the siphon under Indian Creek is approximately 40 cfs. This value is assumed to represent the maximum potential flow rate of the Diamond Ditch system.

As noted previously, the potential enlargement of existing unlined channel sections or the replacement with pipelines was evaluated. For this analysis, it was assumed that all improved channels and new pipelines would use the existing ditch alignments and slopes. However, the District has indicated an interest in potentially assessing new pipeline alignments because the natural contours of the land would not need to be followed to maintain pipe flow. The capacities of the proposed channels and pipelines were based on the maximum potential flow rates discussed above.

FRESH WATER SYSTEM – HYDROLOGY AND HYDRAULICS

As noted previously, diverted fresh water from the West Fork of the Carson River is conveyed to Indian Creek Reservoir via the Snowshoe Thompson #1 and Upper Dressler Ditches. Freshwater is also diverted from Indian Creek into the Upper Dressler Ditch. Existing fresh water facilities and gaging station data evaluated in this analysis include the following:

ƒ West Fork of the Carson River and USGS Gaging Station Number 10310000.

ƒ Snowshoe Thompson #1 Ditch, Federal Water Master Gaging Station (C76), culverts, and inverted siphon under the Upper Harvey Channel (Indian Creek).

ƒ Indian Creek diversion structure and the Upper Dressler Ditch.

ƒ Indian Creek Reservoir.

Figure 2 (at the end of this memorandum) presents a map of all of the components of the District’s fresh water system that are discussed in the following sections.

WEST FORK OF THE CARSON RIVER

Flow in the West Fork of the Carson River was analyzed based on existing stream gaging data from the USGS gage located at Woodfords, California (Station Number 10310000). This gaging station is located upstream of the Snowshoe Thompson #1 Ditch diversion structure and approximately 0.3 miles downstream from the bridge on State Highway 88-89 (Figure 2). The drainage area above the USGS gage is approximately 65 square miles (USGS, 2001). A continuous record of daily mean flows has been recorded at this site from 1 October 1938 to present. During this period of record, the maximum discharge recorded was 8,100 cfs measured

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on 1 January 1997, and the minimum daily discharge recorded was 5.3 cfs measured on 2 September 1977 (USGS, 2001).

An average annual hydrograph was developed for the West Fork of the Carson River at Woodfords based on the 61 years of data (1938 – 1999) from the USGS gage. Table 1 and Figure 3 present the average annual hydrograph, hydrographs for 60 and 30 percent of the average annual hydrograph, and the annual hydrograph for the driest year on record (1977). These hydrographs were developed to assess the potential average flow available for diversion per month and the feasibility of increasing the amount of water diverted to Indian Creek Reservoir (ICR) via the Snowshoe Thompson #1 Ditch.

SNOWSHOE THOMPSON #1 DITCH

The existing Snowshoe Thompson #1 Ditch is an unlined open channel along its entire length and there are no gaging stations in the ditch from the diversion structure that splits flows into the Millich Ditch, to the outlet of the siphon under the Upper Harvey Channel (Figure 2). The District installed a 2-foot Parshall flume just downstream of the outlet of the siphon. The actual amount of fresh water that has been historically conveyed in the ditch must be estimated. As discussed previously, historical flow in the Snowshoe Thompson #1 Ditch was estimated based on data from the upstream U.S. District Court Federal Water Master gage C76 (Figure 2). The gaging station data from this site provides a record of the amount of fresh water that has been diverted from the West Fork of the Carson River for use in both the Millich Ditch the Snowshoe Thompson #1 Ditch from 1 April 1984 to 15 October 2000. Table 2 and Figure 4 present the average annual hydrograph for the gage C76 site. Average monthly percentages of the total water diverted during this period were calculated to estimate the historical diversion scenario. As discussed previously, the results of this analysis indicates that the historical monthly average 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.

District personnel have indicated that ditch losses in the Snowshoe Thompson #1/Upper Dressler Ditch system are high. The District currently has a water right entitlement at ICR for 555 acre- feet/year. The irrigation season, or the period of time when diversions from the West Fork of the Carson River are allowed to occur, is defined in the Alpine Decree as beginning 1 April and ending 15 October (USA vs. Alpine Land and Reservoir Company, et al., 1980). However, District personnel believe that they have rarely been able to convey the full entitlement of 555 acre-feet/year to the reservoir due to operational and capacity limitations in the Snowshoe Thompson #1/Upper Dressler Ditch system.

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Table 1

West Fork Carson River at Woodfords, California USGS station number: 10310000

Annual Ave. (1938- 60% of Average 30% of Average Driest Year (1977) 1999) cfs AF/day cfs AF/day cfs AF/day cfs AF/da y Jan 53 104 32 63 16 31 14 28 Feb 51 101 31 61 15 30 16 32 Mar 72 143 43 86 22 43 18 36 Apr 201 398 120 239 60 119 65 130 May 363 719 218 432 109 216 56 112 Jun 248 492 149 295 74 148 60 120 Jul 98 194 59 117 29 58 18 36 Aug 45 88 27 53 13 27 11 22 Sep 29 58 17 35 9 17 7 14 Oct 25 50 15 30 8 15 10 21 Nov 39 77 23 46 12 23 16 31 Dec 46 91 28 55 14 27 21 41

Figure 3 West Fork of the Carson River at Woodfords

400

350

300 Annual Ave. (1938-1999) 250

200 60% of cfs Average 150 30% of 100 Average

50 Driest Year 0 (1977) Jul Oct Apr Jun Jan Mar Feb Aug Sep Nov Dec May

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Table 2

Historical Average Flow at the Federal Water Master Gage C761

Historical Average Diversion Rates Percent of Total cfs AF/day AF/month Jan 0 0 0 0 Feb 0 0 0 0 Mar 0 0 0 0 Apr 12 24 731 18% May 16 32 1012 25% Jun 14 28 843 21% Jul 10 21 655 16% Aug 7 14 434 11% Sep 5 11 330 8% Oct 1 3 12 1% Nov 0 0 0 0 Dec 0 0 0 0 Total = 4016 AF/year 100%

NOTE:

1 Water diverted from the West Fork of the Carson River for use in both the Snowshoe Thompson # 1 Ditch and the Millich Ditch.

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Figure 4 Federal Water Master C76 Gage Average Annual Hydrograph 18 16 14 12 10

cfs 8 6 4 2 0 Jul Apr Oct Jun Jan Feb Mar Aug Sep Nov Dec May

Table 3 presents the average minimum daily flow rates in the Snowshoe Thompson #1 Ditch required to convey 555 acre-feet/year of water from the West Fork of the Carson River to ICR. These flow rates assume a 50 percent ditch loss over the entire length of the Snowshoe Thompson #1/Upper Dressler Ditch system. In addition, Table 3 assumes the same historical monthly average diversion scenario as calculated from the Federal Water Master gage C76.

Table 3

Average minimum daily flow rates (in cfs) in the Snowshoe Thompson #1 Ditch required to convey 555 acre-feet/year to Indian Creek Reservoir

April May June July August September October 18% 25% 21% 16% 11% 8% 1% 2.5 3.4 2.9 2.2 1.5 1.1 0.3

The existing capacity of the Snowshoe Thompson #1 Ditch system was evaluated to assess the ability of the ditch to convey existing and increased entitlements from the West Fork of the Carson River to ICR. Table 4 presents the results of a reconnaissance level survey of the Snowshoe Thompson #1 Ditch system. XC# denotes channel cross-section locations. The average flow rate (Ave Q) represents the flow rate in the channel defined by erosional features along the channel banks. The extent of the area free of vegetation in the channel was assumed to represent the elevation of the mean annual flow rate. Flow area at Max Q indicates the cross

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sectional area of flow at the maximum capacity of the channel. Figure 2 presents a map of the locations of the noted on Table 4.

Table 4

Snowshoe Thompson #1 Ditch – Existing Capacity Analysis

Approx. Location Slope Ave Q Max Flow area at Notes Station (ft) (cfs) Q Max Q (ft2) (cfs) 0+00 Diversion - 12.5 Sluice gate diversion Structure structure 0+12 to Culvert 1 -0.8% - 39 8.4 Approx 50” X 31” CMP 0+34 arch pipe with an adverse slope. 1+00 XC1 0.2% 5.3 7.5 5.5 Max Q assumes 1 ft of freeboard 3+70 XC2 0.2% - 7.6 5.4 Max Q assumes 1 ft of freeboard 15+33 XC3 0.2% 0.3 21 14.5 Ave Q from local spring seepage into the ditch, Max Q with 1 ft of freeboard 41+25 to Culvert2 0.2% - 49 11.4 Approx 58” X 36” CMP 41+45 arch pipe, slope assumed. 48+13 XC4 0.1% 1.0 18 16.1 Max Q assumes 1 ft of freeboard 49+13 to Culvert3 0.8% 17.5 64 15.0 5’ X 3’ RCB under Hwy 89, 50+58 Ave Q assumes 1 ft of sed. and 1 ft flow depth. 51+07 XC5 0.3% 2.5 49 23.6 Max Q assumes 1 ft of freeboard 61+62 to Culvert4 0.2% - 24.4 4.9 30” CMP, slope assumed 62+10 63+02 to Wood 0.2% - 18.6 5.0 Old wooden diversion 63+12 Diversion structure. Max Q before water spills over 2 ft. board 87+20 to Culvert5 0.2% - 25.3 4.9 30” CMP, slope assumed 87+30 106+58 XC6 0.4% - 10.2 7.4 Max Q assumes 1 ft of freeboard 109+58 to Inverted - - 43.8 7.1 Single barrel 36” RCP 112+58 Siphon under Upper Harvey

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Table 4

Snowshoe Thompson #1 Ditch – Existing Capacity Analysis

Approx. Location Slope Ave Q Max Flow area at Notes Station (ft) (cfs) Q Max Q (ft2) (cfs) Channel 112+60 Parshall - - 43.9 6 2 ft Parshall Flume with 3.0 Flume ft of head assumed 117+00 Culvert6 0.2% - 93 12.6 48” CMP, slope assumed

It should be noted that several of the culverts listed on Table 4 currently have a significant amount of accumulated sediment that will reduce the capacities noted by up to 30 percent. This analysis appears to indicate that the existing capacity of the Snowshoe Thompson #1 Ditch is sufficient to convey the current water right entitlement 555 acre-feet/year to ICR under the historical monthly average diversion scenario. The uppermost reach of the ditch appears to represent the limiting capacity of the ditch. Overtopping of the ditch banks may start to occur at approximately 18 cfs. The capacity of the Snowshoe Thompson #1 Ditch as reported in the District’s Operation and Maintenance Manual is 15 cfs at the take out of the West Fork of the Carson River (CWC-HDR, 1988). Where the Millich Ditch takes off, the flow splits and the actual capacities of the Millich Ditch and Snowshoe Thompson #1 have not been formally established.

As noted previously, the District would like to evaluate the potential of increasing its entitlement at ICR. In addition, the District would like to evaluate different operational scenarios that would allow it to divert its entire entitlement. Administrative Provision X of the Alpine Decree states that “No user entitled to use of water under this Decree shall be allowed to divert more than 40% of his entitlement in any one calendar month” (USA vs. Alpine Land and Reservoir Company, et al., 1980). This monthly diversion amount can be spread out over the total number of days in the month or any lesser number of days. Therefore, 40 percent of the entitlement can be diverted in 31 days, 15 days or any other number of days equal to or less than the total number of days in any one calendar month.

An analysis of the potential flow rates in the Snowshoe Thompson #1 Ditch with entitlements of 555, 750, 1,000 and 1,200 acre-feet/year under three different operational scenarios was conducted. As discussed previously, the three operational scenarios are the historical monthly average diversion scenario (18:25:21:16:11:8:1 percent, 1 April to 15 October) and the proposed diversion scenarios, 40:40:20 percent, April, May and June, 30-31 days/month and 40:40:20 percent, April, May and June, 15 days/month. These analyses include the additional flow rate required to account for the assumed 50 percent ditch loss for unlined open channels and an assumed 10 percent for pipe losses.

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Table 5 presents the minimum improved unlined open channel dimensions and pipe sizes required to convey 1200 acre-feet/year under the proposed 40:40:20 percent, 15 days/month diversion scenario. This entitlement and diversion scenario represents the maximum potential flow rates for all of the scenarios discussed above. Sizing the channels and/or pipelines to these criteria will provide the District with the greatest operational flexibility. Unlined open channels are approximated by trapezoidal channels with 4.5 foot bottom widths, 1.5H:1V side slopes. This assumed channel geometry was chosen because it approximates the existing channel geometry and minimizes new excavation. The required channel depths noted on Table 5 also include a minimum of 1.0 foot of freeboard.

Table 5

Proposed Improvements to the Snowshoe Thompson #1 Ditch Open Channel and Pipeline sizes required to convey 1200 acre-feet/year to ICR under the proposed 40:40:20 percent, April-June, 15-days/month diversion scenario

Improved Unlined Open Channels Pipelines Max Existing Depth Flow V Max Existing Min Min Q Slope (ft) area (ft/sec) Q Slope RCP Dia HDPE (cfs) (ft2) (cfs) (in)1 Dia (in)1 24 0.1% 2.2 16.7 1.4 18 0.1% 36 36 24 0.2% 1.8 12.9 1.9 18 0.2% 30 30 24 0.3% 1.6 11.2 2.2 18 0.3% 30 30 24 0.4% 1.5 10.1 2.4 18 0.4% 30 28 24 0.5% 1.4 9.3 2.6 18 0.5% 30 28

NOTE:

1 Pipe sizes are rounded up to nearest commonly available pipe diameter; RCP sizes are I.D. whereas HDPE pipe sizes are O.D. and assume a 2-inch wall thickness.

It should be noted that additional components of spring flows, creek flows and storm runoff will be intercepted and conveyed by an improved open channel system. These components are currently intercepted and conveyed by the existing Snowshoe Thompson #1 Ditch. The potential additional quantity represented by these components and the potential impact on the capacity of the existing and improved open channel system is currently unknown. An estimate of this additional quantity is recommended if an improved open channel system is chosen as the preferred alternative. However, the District has indicated that the pipeline alternative would be preferred due to improved conveyance capabilities and reduced maintenance requirements. If a pipeline alternative were chosen, it would not be necessary to quantify the potential additional inflows from springs, creeks and storm runoff. The District has indicated that it would be

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desirable to maintain an open channel along the lower section of the ditch in order to continue capturing flows from Scott Creek.

An analysis of the potential flows at the Federal Water Master gage C76 site was conducted under the three diversion scenarios discussed above. The flow rates presented on these tables (Appendix) effectively represent the diversion rates required from the West Fork of the Carson River to meet the four entitlement scenarios at ICR and the historical average requirements of the Millich Ditch.

For the purpose of a comparative analysis, it is assumed that the current entitlement of 555 acre- feet/year can be conveyed in the Snowshoe Thompson #1 Ditch under the historical monthly average diversion rates. Therefore the relative increase in diversion rates for entitlements of 750, 1,000 and 1,200 acre-feet/year at ICR are compared to the historical monthly average diversion rates. This analysis was conducted to estimate the potential impact of increased diversions to downstream users on the West Fork of the Carson River. Therefore the tables in this Appendix indicate the potential percentage increase over the historical monthly average diversion rates. Estimated percentage increases are noted for unlined open channels and pipelines and include estimated conveyance losses.

Figures 5 and 6 present the estimated flow rates for entitlements of 750, 1,000 and 1,200 acre- feet/year under the proposed 40:40:20 percent, 15 days/month diversion scenario compared to the historical monthly average diversion rates and the average monthly flow rates in the West Fork of the Carson River that were observed during 1977, the driest year on record. Figure 5 presents this hypothetical scenario for a proposed improved open channel with estimated 50 percent conveyance losses. Figure 6 presents this hypothetical scenario for a pipeline with estimated 10 percent conveyance losses. These figures represent the worst-case scenario were maximum diversion rates coincide with minimum flows in the river. These figures indicate that if this scenario where to occur, it would have a significant impact on river flows downstream of the diversion. Therefore, during a relatively dry year, it may not be feasible to divert water from the river under the maximum diversion scenario without impact to downstream users.

Upper Dressler Ditch

The District’s Operation and Maintenance Manual indicates that one of the purposes of the Upper Dressler Ditch is to divert runoff from the local Harvey Place Reservoir watershed into Indian Creek Reservoir. In addition, the Ditch diverts flushing water from Indian Creek and makeup water from the West Fork of the Carson River via the Snowshoe Thompson #1 Ditch. The ditch runs along the 5,640 to 5,620 contour lines with a bed slope of 0.0015 ft/ft and is approximately 7,170 feet long. Several side channel spillways are located on the channel bank with the crests of the spillways parallel to the channel alignment. The upper portion of the ditch above the flood control structure is both lined and unlined and has a capacity of 30 cfs. The lower portion of the ditch below the flood control structure is lined and has a capacity of 70 cfs. The ditch drains into a 60-inch RCP that conveys water to ICR.

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Figure 5 Potential Flowrates at the Federal Water Master Gauge C76 Using an Open Channel and a 40:40:20%, 15 days/month Diversion Scenario 70 West Fork Carson River at Woodfords Driest Year (1977) 60 1200 AF/year

50 1000 AF/year 750 AF/year 40 Historical Ave. cfs 30

20

10

0 Jul Oct Apr Jan Jun Feb Mar Aug Sep Dec Nov May

Figure 6 Potential Flowrates at the Federal Water Master Gauge C76 Using a Pipeline and a 40:40:20%, 15 days/month Diversion Scenario 70 West Fork Carson River at Woodfords Driest Year (1977) 60 1200 AF/year 1000 AF/year 50 750 AF/year 40 Historical Ave.

cfs 30

20

10

0 Jul Apr Oct Jan Jun Mar Feb Aug Sep Nov Dec May The diversion structure in Indian Creek intercepts and diverts all but flood flows into the Upper Dressler Ditch. This diversion structure is referred to in the District’s Operation and Maintenance Manual as the Harvey Diversion Structure (CWC-HDR, 1988). The diversion structure consists of a concrete weir and a 36-inch RCP placed at right angles to the stream

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flow. All flows in excess of 30 cfs pass over the weir and into the Upper Harvey Channel (CWC-HDR, 1988). The District installed a 2 foot Parshall flume just downstream of the outlet of the diversion structure and a 5 foot Parshall flume near the terminus of the ditch. In addition, a Federal Water Master stream gage exists in the 5 fit flume.

The existing capacity of the Upper Dressler Ditch was evaluated to assess the ability of the ditch to convey existing flows from Indian Creek and the Snowshoe Thompson #1 Ditch and increased flows from the Snowshoe Thompson #1 Ditch. Table 6 presents the results of a reconnaissance level survey of the Upper Dressler Ditch system. XC# denotes channel cross-section locations. An estimation of the average flow rate was not available in this ditch system because there are no distinct flowline indicators on the concrete lined sections and maintenance was recently conducted on the unlined sections. Flow area at Max Q indicates the cross sectional area of flow at the maximum capacity of the channel. The locations noted on Table 6 are presented on the map on Figure 2.

Table 6

Upper Dressler Ditch – Existing Capacity Analysis

Flow Area Location Slope Max Q (cfs) at Max Q Notes (ft2) Diversion - 30 12.5 36 “ RCP from weir diversion on Indian Creek Flume - 30 4.7 2 ft Parshall Flume with 2.35 ft of head XC1 0.15% 42 10.0 Concrete lined trapezoidal channel, 9 ft top width, 3 ft bottom width, 3 ft deep. Max Q assumes 1 ft of freeboard Culvert1 0.15% 41.7 12.6 48” RCP under access road, slope assumed XC2 0.15% 34 17.3 Unlined channel, recently cleaned, just upstream of old FWM stream gage. Max Q assumes 1 ft of freeboard Culvert2 0.15% 40.9 12.6 48” CMP just upstream of flood control structure. Inlet - 45 10.8 Trapezoidal concrete weir at Weir entrance to flood control structure, 2 ft flow depth assumed Outlet - 28.5 8.0 Rectangular concrete weir at outlet Weir to flood control structure

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Table 6

Upper Dressler Ditch – Existing Capacity Analysis

Flow Area Location Slope Max Q (cfs) at Max Q Notes (ft2) XC3 0.15% 59 13.3 Concrete lined trapezoidal channel, 12 ft to width, 4 ft bottom width, and 3 ft deep. Max Q assumes 1 ft of freeboard Flume - 85.6 12.5 5 ft Parshall Flume with 2.5 ft of head assumed

As noted previously, the capacity of the Upper Dressler Ditch as reported in the District’s Operation and Maintenance Manual is 30 cfs above, and 70 cfs below the flood control structure (CWC-HDR, 1988). The analysis conducted on Table 6 indicates that the capacity of the existing channels and culverts upstream of the flood control structure is at least 30 cfs, and the capacity of the 2-foot wide rectangular outlet weir in the flood control structure has now been improved to convey at least 30 cfs.

The District has indicated that they would like to evaluate the alternative of improving existing unlined open channel sections in the Upper Dressler Ditch or replacing them with pipelines. Therefore, Table 7 presents the minimum improved unlined open channel dimensions and pipe sizes required to convey the combined maximum flow from Indian Creek (30 cfs) with the maximum proposed flow from the Snowshoe Thompson #1 Ditch (1,200 acre-feet/year under the proposed 40:40:20 percent, 15 days/month diversion scenario). Sizing the channels and/or pipelines to these criteria will provide the District with the greatest operational flexibility.

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Table 7

Proposed Improvements to the Upper Dressler Ditch Open Channel and Pipeline sizes required to convey 1200 acre-feet/year to ICR under the proposed 40:40:20 percent, April-June, 15-days/month diversion scenario

Improved Unlined Open Channels Pipelines Max Existing Depth Flow V Max Q Existing Min Min Q Slope (ft) area (ft/sec) (cfs) Slope RCP Dia HDPE (cfs) (ft2) (in)1 Dia (in)1 49 0.1% 3.0 28.3 1.7 47 0.1% 54 48 49 0.2% 2.5 21.9 2.2 47 0.2% 48 48 49 0.3% 2.3 18.9 2.6 47 0.3% 42 42 49 0.4% 2.1 17.0 2.9 47 0.4% 42 42 49 0.5% 2.0 15.7 3.1 47 0.5% 36 36

NOTE:

1 Pipe sizes are rounded up to nearest commonly available pipe diameter; RCP sizes are I.D. whereas HDPE pipe sizes are O.D. and assume a 2-inch wall thickness.

A comparison of the above analysis with the existing capacity analysis presented on Table 6 appears to indicate that the capacity of the existing Upper Dressler Ditch, upstream of the flood control structure, is inadequate to convey the maximum proposed flow. Whereas the capacity of the existing ditch downstream of the flood control structure appears to be sufficient to convey the proposed increased flow rate. It should be noted that an additional component of storm runoff will be intercepted and conveyed by an improved open channel system. Storm runoff is currently intercepted and conveyed by the existing Upper Dressler Ditch. The majority of the runoff produced by the watershed area above the ditch is concentrated in an unnamed ephemeral stream channel that crosses the ditch approximately midway. A concrete flood control structure has been constructed at this location, which allows flood flows above a specific flow rate to bypass the ditch and flow towards Harvey Place Reservoir. However, all runoff and flood flows below this specific flow rate are diverted into the ditch and then into ICR. The District’s Operation and Maintenance Manual indicates the 100-year storm will produce a peak flow of 100 cfs and the Upper Dressler Ditch below the flood control structure has a capacity of 70 cfs (CWC-HDR, 1988). The potential additional quantity of storm runoff intercepted by the existing ditch or a proposed unlined open channel system is currently unknown. An estimate of the total quantity of runoff produced by the watershed area above the ditch is recommended if either an unlined open channel system or a pipeline is installed. If a pipeline replaces the existing Upper Dressler Ditch, storm runoff to Harvey Place Reservoir will increase and may potentially reduce the storage capacity for recycled water.

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The Upper Dressler Ditch discharges to a 60-inch RCP pipeline that in turn discharges to ICR. It is assumed that the capacity of this pipeline is sufficient to convey existing and proposed flows from the ditch into the reservoir.

Indian Creek Reservoir

Indian Creek Reservoir (ICR), constructed in 1967 was initially used for storage of tertiary treated wastewater. However with the construction of the larger Harvey Place Reservoir in 1989, it was converted to a freshwater reservoir and is still an integral part of the District’s reuse system (CWC-HDR, 1988). ICR provides flood control for Harvey Place Reservoir and the District also has a contractual obligation to maintain the reservoir for fishing and recreation.

Flushing flows obtained from Fred Dressler’s (now Bentley and the Carson Water Subconservancy District’s) Indian Creek winter water flows are “wheeled” through the reservoir at approximately the same rate as it is diverted from Indian Creek between 1 October through 31 March. Indian Creek flows received between 1 April and 30 September are available for storage as part of the District’s water rights that were acquired with the purchase of lands (CWC-HDR, 1988).

Figure 7 presents a bathymetric map of Indian Creek Reservoir with a table of elevations, corresponding surface areas and volumes. The minimum operational pool elevation is approximately 5,588 feet, which corresponds to approximately 1,515 acre-feet of storage in the reservoir. The maximum pool elevation is approximately 5,600 feet, which corresponds to approximately 3,100 acre-feet of storage in the reservoir. 5,600 feet is equivalent to the spillway crest elevation.

The District’s Operation and Maintenance Manual indicates that the maximum reservoir fill rate should be limited to 2 ft/day (CWC-HDR, 1988). If the maximum proposed flow rate from the Upper Dressler Ditch (46 cfs) were to continuously enter the reservoir over a 24-hour period, this would be equivalent to approximately 91 acre-feet/day. Figure 5 indicates that a 2 foot rise in water surface elevation above the minimum pool elevation (5,588 feet to 5,590 feet) would correspond to approximately 175 acre-feet of water. Therefore, if the maximum proposed flow rate from the Upper Dressler Ditch were to correspond to the minimum pool elevation, the reservoir fill rate would be well below 2 ft/day. If the same 46 cfs fill rate were to be maintained, the rate of water level rise would continue to decrease as the surface area of the lake increases.

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Recycled Water System – Hydrology and Hydraulics

Existing recycled water facilities and gaging station data evaluated in this analysis include the following:

ƒ Harvey Place Reservoir and the outlet structure.

ƒ Diamond Ditch system including the inverted siphon under Indian Creek, the Snowshoe Thompson # 2 Ditch diversion structure, the Upper Celio-Dressler Ditch diversion structure and the Diamond Ditch Pipeline along Diamond Valley Road.

Figure 8 (at the end of this memorandum) presents a map of all of the components of the District’s recycled water system that are discussed in the following sections.

Harvey Place Reservoir

Harvey Place Reservoir, constructed in 1989, has a maximum capacity of approximately 3,800 acre-feet. There is also approximately 250 acre-feet of dead storage below the outlet of the reservoir (CWC-HDR, 1988). The elevation of the primary spillway on Harvey Place Dam is approximately 5,563 feet above sea level, which corresponds to the maximum elevation water will rise to during normal operation. The District’s normal operation is to fill the reservoir with recycled water from 15 October to 1 April, without any discharge. After 1 April, the District drains the reservoir to minimum pool before 15 October to allow for it to be filled again during the winter (CWC-HDR, 1988).

The Harvey Place Reservoir has a watershed area of approximately 1.9 square miles. As noted previously, ICR and the Upper Dressler Ditch effectively capture the majority of runoff from the watershed area that would normally drain to Harvey Place Reservoir. The 100-year storm from the 1.9 square mile watershed is estimated to produce 420 acre-feet of runoff into the reservoir (CWC-HDR, 1988). The primary and emergency spillways are designed to pass the probable maximum flood (PMF) if the dam for ICR failed (CWC-HDR, 1988).

Based on recycled water flow projections, the treatment plant at South Lake Tahoe will produce an estimated 5.8 MGD by 2028 (Kennedy/Jenks, 2001). This is equivalent to a flow rate into the reservoir of approximately 9.0 cfs. Assuming normal operations with the reservoir beginning to fill on 15 October and 3,000 acre-feet of recycled water storage capacity, at an inflow of 9.0 cfs, the reservoir will fill to its capacity just after April 12 of the following year. This projected fill date assumes the additional 800 acre-feet required for possible floodwater storage is not available for additional recycled water storage. In addition, no evaporation or seepage losses are assumed. If the reservoir fills with 800 acre-feet of runoff and 3,000 acre-feet of recycled water and begins discharging on 1 April, the minimum flow rate required to drain the reservoir by 15 October is approximately 16.6 cfs. This reservoir drawdown rate includes a continuous 9.0 cfs

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inflow of new recycled water into the reservoir from the District’s “C” line in the year 2028. In addition, no additional inflows from precipitation and runoff or outflows from evaporation or seepage losses are assumed. Therefore, 16.6 cfs represents the minimum drawdown rate required to drain the reservoir by 15 October in the year 2028.

Figure 9 presents idealized fill and drawdown curves for Harvey Place Reservoir for 2000 and projected for 2028. These curves assume normal operations with filling occurring from 15 October to 31 March and drawdown from 1 April to 15 October. The 2000 curve assumes a constant recycled water inflow of 7.1 cfs and the 2028 curve assumes a constant projected inflow of 9.0 cfs. No additional inflows or outflows are assumed. The 2028 curve indicates that the reservoir will potentially be 98 percent full by 1 April 2028. This projected scenario indicates the District will have very little operational flexibility by 2028, unless alternative uses of additional storage of recycled water is identified.

Figure 9 Harvey Place Reservoir Operational Management Curves

4000 Maximum Storage Capacity w ith Flood Storage - 3,800 AF 3500

Recycled Water Storage Capacity - 3,000 AF 3000 2028 (projected) 2500

2000 2000

1500

1000 Reservoir Storage (Acre-Feet)

500

0 15-Oct 15-Nov 15-Dec 15-Jan 15-Feb 15-Mar 15-Apr 15-May 15-Jun 15-Jul 15-Aug 15-Sep 15-Oct

Diamond Ditch System

The Diamond Ditch system begins at the outlet works of the Harvey Place Reservoir (Figure 8). The ditch is a concrete lined trapezoidal channel that runs parallel to Diamond Valley Road then crosses under the road and Indian Creek in a double barrel inverted siphon. The siphon is located in Diamond Valley just upstream of a short narrow canyon that connects Diamond Valley to the head of Dutch Valley. A concrete lined trapezoidal channel continues from the outlet of the siphon southwest towards Wade Valley. As the channel crosses a saddle and enters Wade Valley it flows in a short rip-rap lined channel section and then into a steep unlined section. A large headcut was observed in this steep unlined channel section. A potential opportunity exists to construct energy dissipation structures in this section to prevent further

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erosion and also increase treatment of the recycled water through aeration and volatilization. At the base of the steep unlined channel, the gradient flattens and recycled water is routed to a 36- inch HDPE pipeline approximately 1,200 feet long. The pipeline discharges to another concrete lined trapezoidal channel in Wade Valley. From the end of the concrete lined channel, flow is routed in an unlined channel where it eventually intersects the Snowshoe Thompson #2 Ditch diversion structure. This complex structure presents a constriction in the ditch system and has had several flooding and erosion problems in the past. The District has indicated that this structure needs replacement. From the Snowshoe Thompson #2 Ditch diversion structure, the gradient increases and the ditch winds downhill to the Upper Celio-Dressler Ditch diversion structure. At this diversion structure flow can either be routed northward towards Diamond Valley Road in the unlined Diamond Ditch North (Figure 8) or into a 30-inch RCP that conveys flows into the Upper Celio-Dressler Ditch system. Numerous irrigation turnout structures and side channel spillways exist along the bank of the Diamond Ditch North in Wade Valley.

Recycled water flowing in the Diamond Ditch North from the diversion structure crosses under a South Tahoe Public Utility District posted fence then runs parallel to Diamond Valley Road. There is no fence between the road and the ditch at this location. A potential public exposure hazard exists. From the roadway, the ditch continues down a steep embankment before entering into a pipeline that crosses over the West Fork of the Carson River (Figure 8). The District is concerned about the potential of the entrance to the pipeline clogging with debris and recycled water spilling into the West Fork of the Carson River at this location. The District has identified replacement of this open channel section with a pipeline up to the Upper Celio Dressler Ditch diversion as a potential priority.

The existing capacity of the Diamond Ditch system was evaluated to assess existing and proposed increased discharge rates from Harvey Place Reservoir. Table 8 presents the results of a reconnaissance level survey of the existing Diamond Ditch system. The average flow rate (Ave Q) was assumed by the height of the stain line along the channel banks. Recycled water historically flowing in the ditch has stained the concrete lining a dark blackish brown. Therefore the average (or maximum) historical depth of flow in the ditch is clearly visible. Flow area at Max Q indicates the cross sectional area of flow at the maximum capacity of the channel. The locations noted on Table 8 are presented on the map on Figure 8.

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Table 8

Diamond Ditch – Existing Capacity Analysis

Location Slope Ave Q Max Q Flow Area at Notes (cfs) (cfs) Max Q (ft2) Culvert at Old Airport 0.15% - 42 12.6 48” RCP, slope and Road length assumed. XC1 - Lined 0.15% 28 59 13.3 12 ft top width, 4 ft Trapezoidal Channel bottom width, and 3 ft Upstream of Siphon deep. Max Q assumes 1 ft of freeboard Inverted Siphon under - 28 40 3.1, 4.9 Double Barrel Indian Creek Inverted Siphon. West - 24” steel pipe, East – 30” RCP XC2 - Lined 0.11% 28 38 10.8 12 ft top width, 4 ft Trapezoidal Channel bottom width, and 2.7 Downstream of Siphon ft deep. Max Q assumes 1 ft of freeboard Steep Unlined Channel 0.5% to 28 80 to 22.5 Large Headcut (~15 ft 4.0% 210 deep) erosive velocities. HDPE Pipeline 0.2% 26 44 7.1 Existing 1200 ft pipeline, 36” slope assumed XC3 - Lined 0.11% 26 48 12.7 11 ft top width, 4 ft Trapezoidal Channel bottom width, and 3 ft Downstream of deep. Max Q assumes Pipeline 1 ft of freeboard XC4 - Unlined Channel 0.11% - 27 14.0 12 ft top width, 3 ft upstream of Snowshoe deep, recently cleaned, Thompson #2 slope assumed. Max Diversion Q assumes 1 ft of freeboard XC5 - North Ditch – 0.5% to - 74 to 22.5 Unlined channel, Unlined 3.0 % 180 recently cleaned, slope varies, erosive velocities. Diamond/Upper Celio - - 56 14.1 2 weir openings, 3.2’ Ditch Diversion X 2.2’ Structure Technical Memorandum 3 23 of 28 Recycled Water Facilities Master Plan Hydrology and Hydraulics Review Appendix C

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Table 8

Diamond Ditch – Existing Capacity Analysis

Location Slope Ave Q Max Q Flow Area at Notes (cfs) (cfs) Max Q (ft2) Pipeline to Upper Celio 3.3% - 27 4.9 30” RCP at diversion Ditch structure, estimated slope XC6 - North Ditch – 0.2% - 47 22.5 Unlined channel, north Unlined of diversion, recently cleaned

The capacity of the Diamond Ditch and Inverted Siphon under Indian Creek, as reported in the District’s Operation and Maintenance Manual, is 40 cfs (CWC-HDR, 1988). The analysis conducted above indicates that the existing ditch system has sufficient capacity to convey up to 40 cfs, with the exception of two choke points that limit flow to about 20 cfs: at the Snowshoe Thompson #2 Ditch/Diamond Ditch diversion structure (Bar Screen No. 3), and at Bar Screen No. 5 where the Diamond Ditch flow enters a pipeline that leads to the Paynesville Bridge. Therefore, it is recommended that these structures be replaced and the open channel section above the Snowshoe Thompson #2 Ditch diversion structure be enlarged.

By contract with the Diamond Ditch users, a minimum of 2,000 acre-feet/year of recycled water is provided for irrigation (CWC-HDR, 1988). As discussed previously, flow projections for the treatment plant at South Lake Tahoe indicate outflow from the District’s “C” line will be 9.0 cfs by 2028. To estimate the maximum flow rate that may be required in the Diamond Ditch system by 2028 it can be assumed that during a wet year little or no recycled water is required for irrigation until 1 June. If the reservoir remains full up to 1 June (3,800 acre-feet), the flow rate required to drain the reservoir to 800 acre-feet by 15 October would be approximately 20 cfs. This reservoir drawdown rate assumes a continuous inflow of new recycled water at 9.0 cfs from the District’s “C” line and no additional inflows from runoff or outflows from evaporation or seepage. The existing ditch system appears to have a sufficient capacity for this potential reservoir drawdown scenario.

Sizing the Diamond Ditch system to its limiting section will provide the District with its greatest operational flexibility. Since the inverted siphon under Indian Creek is not easily improved or replaced, it is assumed that it represents the limiting section at 40 cfs. Table 9 presents the minimum improved unlined open channel dimensions and pipe sizes required to convey 40 cfs in the Diamond Ditch system. Trapezoidal channels with 5 ft bottom widths with 1.5H:1V side slopes approximate proposed unlined channels. This assumed channel geometry was chosen because it approximates the existing channel geometry and minimizes new excavation. The required channel depths noted on Table 9 also include a minimum of 1.0 foot of freeboard.

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Table 9

Proposed Improvements to the Diamond Ditch System Unlined Open Channels and Pipelines required to convey 40 cfs

Improved Unlined Open Channels Pipelines Max Existing Depth Flow V Max Q Existing Min Min Q Slope (ft) Area (ft2) (ft/sec) (cfs) Slope RCP Dia HDPE (cfs) (in)1 Dia (in)1 40 0.11% 3.6 23.5 1.7 40 0.11% 48 48 40 0.2% 3.3 18.9 2.2 40 0.2% 42 42 40 0.4% 2.9 14.7 2.5 40 0.4% 36 36 40 0.6% 2.7 12.7 2.8 40 0.6% 36 36 40 0.8% 2.6 11.5 3.0 40 0.8% 36 32 40 1.0% 2.5 10.6 3.82 40 1.0% 30 32 40 1.4% 2.4 9.4 4.32 40 1.4% 30 30 40 2.0% 2.2 8.3 4.82 40 2.0% 30 28 40 3.0% 2.1 7.2 5.62 40 3.0% 24 26 40 4.0% 2.0 6.5 6.22 40 4.0% 24 24 40 5.0% 1.9 6.0 6.72 40 5.0% 24 24

NOTES:

1 Pipe sizes are rounded up to nearest commonly available pipe diameter; RCP sizes are I.D. whereas HDPE pipe sizes are O.D. and assume a 2-inch wall thickness.

2 Exceeds the maximum permissible mean channel velocity for erosive soils (Fortier and Scobey, 1926)

As noted previously, a large headcut was observed in the steep unlined channel section. In addition, the soil types observed in the majority of all of the channel sections in the freshwater and recycled water systems were erosive soils consisting of loose silty sands with some boulders and cobbles. The maximum permissible mean channel velocity for erosive soils is 3.0 ft/sec (Fortier and Scobey, 1926). The analysis above indicates that mean channel velocities will approach 3.0 ft/sec at slopes of 0.8 percent. Therefore, it is not recommended that unlined open channels be constructed in areas where existing slopes exceed 0.8 percent. As noted on Table 8, several sections of the existing Diamond Ditch North system have slopes exceeding this value.

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SUMMARY AND RECOMMENDATIONS

ƒ The existing capacity of the Snowshoe Thompson #1/Upper Dressler Ditch system was evaluated. The results indicate the Snowshoe Thompson #1 Ditch has a limiting capacity that has not been formally established.

ƒ The capacity of Harvey Place Reservoir is currently adequate. Additional storage capacity will be required after the year 2028 based upon a 5.8 MGD projected inflow to HPR. Efforts should be made to reduce the storage needs over the 20-year planning period or to increase the available storage of Harvey Place Reservoir.

ƒ The historical monthly average freshwater diversion scenario, as observed at the Federal Water Master gage C76 (1984 – 2000), has been approximately 18:25:21:16:11:8:1 percent from 1 April to 15 October. The District’s current water rights entitlement at Indian Creek Reservoir is 555 acre-feet/year. However, the District has rarely been able to convey their full entitlement to the reservoir due to operational and capacity limitations in the existing Snowshoe Thompson #1/Upper Dressler Ditch system.

ƒ A proposed diversion scenario of 40:40:20 percent during April, May and June was selected for consideration corresponding to historic peak flows in the West Fork of the Carson River. The District would have a better potential of conveying their full existing entitlement or proposed increased entitlements to the reservoir with this diversion scenario.

ƒ Potential increased entitlements of 750, 1,000 and 1,200 acre-feet/year at ICR were assumed. Proposed enlarged unlined open channels and pipelines for the fresh water system were sized to convey an assumed maximum potential flow rate. This flow rate would occur under a proposed 1,200 acre-feet/year entitlement, diverted at 40:40:20 percent diversion scenario, from April-June for 15 days/month.

ƒ Unlined open channels and pipelines were assumed to have 50 and 10 percent conveyance losses, respectively. The maximum potential flow rates required to convey the maximum entitlement to ICR in the Snowshoe Thompson #1 Ditch would be 24 cfs for unlined ditches and 18 cfs for pipelines.

ƒ The results of the existing capacity analysis indicate that in order to convey the maximum potential fresh water flow rate, several sections of the Snowshoe Thompson #1 Ditch will require enlargement or replacement with a pipeline. In addition, the existing channel upstream of the flood control structure in the Upper Dressler Ditch will also require enlargement or replacement with a pipeline. It is recommended that the outlet to the flood control structure be replaced or the opening enlarged. The existing Upper Dressler Ditch downstream of the flood control structure appears to have a sufficient capacity to convey the maximum potential flow rate.

ƒ The steep channel section in the Diamond Ditch system has experienced excessive erosion. It is recommended that energy dissipation and erosion control measures be implemented at

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this location. Properly designed structures may also be able to increase treatment of the recycled water through aeration and volatilization.

ƒ The Diamond Ditch at bar screen number three near the Snowshoe Thompson #2 diversion structure has been identified as a choke point in the Diamond Ditch system. Flooding and erosion problems have occurred in the past at this location. It is recommended that a more detailed analysis of this location be conducted in order to develop a more effective alternative design.

ƒ Several existing unlined channel sections in the Diamond Ditch North system have erosive soils and slopes exceeding 0.8 percent. The maximum permissible mean channel velocity for erosive soils is 3.0 ft/sec. Channels with velocities exceeding this value will potentially experience excessive erosion. This analysis indicates that velocities of 3.0 ft/sec or more will occur in unlined channels with slopes exceeding 0.8 percent. It is recommended that no new unlined open channels be constructed with slopes of 0.8 percent or greater. Furthermore, it is recommended that the District consider replacing all existing unlined channel sections with slopes of 0.8 percent or greater with pipelines.

ƒ This analysis assumed the existing ditch alignments and slopes would be utilized for new pipelines. However, the District has indicated an interest in assessing potential new pipeline alignments.

ƒ With the fresh water system, it is desirable to minimize conveyance losses.

The original draft of this technical memorandum was prepared by Kennedy/Jenks Consultants in 2003 and updated by Stantec Consulting in 2008.

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REFERENCES AND WEBSITES

CWC-HDR, Inc. 1988. South Tahoe Public Utilities District, Operation and Maintenance Manual, Volume III, Harvey Place Dam and Irrigation Works.

Fortier S. and F.C. Scobey, 1926. Permissible Canal Velocities, American Society of Civil Engineers, Volume 89, pp 940-956.

Haestad Methods, Inc., 1994. FlowMaster v 6.1, Computer Program for the Hydraulic Design and Analysis of Open Channels, Ditches and Pipelines.

Haestad Methods, Inc., 1995. CulvertMaster v 1.0, Computer Program for the Hydraulic Design and Analysis of Culverts.

Kennedy/Jenks, 2001. Recycled Water Quality and Quantity Projection to 2020, Technical Memorandum No. 2. Prepared for the South Tahoe Public Utility District.

Parsons, Brinckerhoff, Quade & Douglas, 1982. Hydrology Report, Harvey Place Dam and Reservoir. Prepared for J.H. Kleinfelder and Associates.

Stone, G. 2001. Personal communication. Federal Water Master, United States District Court for the District of Nevada.

USGS, 2001. United States Geological Survey, Nevada, NWIS (Historic water data) website http://waterdata.usgs.gov/nwis-w/NV/

United States of America vs. Alpine Land and Reservoir Company, et al., 1980. Civil No. D-183 BRT, Final Decree, in the United States District Court for the District of Nevada.

WRCC, 2001. Western Regional Climate Center website http://www.wrcc.dri.edu/

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