Topaz Lake with the West Walker River Diversion to and from the Lake. Photo courtesy of Robert Johnson.
(Click on the photo to enlarge - note that the enlarged photo will open in a new window. Note that the enlarged photo is a very large file and may take time to load.)
Walker River Irrigation District - Topaz Reservoir Improvement Project
Topaz Reservoir (formerly know as Alkali Lake) is located on the Nevada-California border, approximately 26 miles south of Lake Tahoe in Mono County, California. Most of Topaz Reservoir, its tunnel outlet and outlet canal is located in Nevada; however, a large portion of the Reservoir, its entrance canal, levee (existing dam) along the entrance canal and diversion structure are in California. Topaz Reservoir is filled by regulation of a diversion structure on the West Walker River in California, approximately one mile east of Highway 395 and two miles south of the California-Nevada border. A 1.5-mile canal transports water from the diversion structure to Topaz Reservoir. The Walker River Irrigation District (WRID) owns and operates all structures and canals and utilizes the water.
Water was first diverted into the off-stream storage reservoir in 1921. A gated tunnel was constructed through a low topographic saddle in the northeast rim of the basin and provides the only outlet from the reservoir. An open channel leading from the terminus of the tunnel returns water released through the tunnel back to the river. In 1937, an earth fill, rock-faced levee (existing dam) was constructed along a portion of the 1.5-mile canal. In 1997, the California Division of Safety of Dams determined that this levee/dam is subject to their jurisdiction. Dead storage below the level of the outlet tunnel amounts to approximately 65,000 acre feet.
The existing Inlet Channel Diversion Structure (Diversion Structure) which was constructed in 1921 is located on the West Walker River about 5,400 feet south of the most southern shore of the lake at maximum reservoir level. The structure consists of two concrete buttress/flashboard diversion dams (one on the West Walker River Channel and the other on the Inlet Channel) are original to the facility and allows limited regulation of the flow of the river between the two channels.
The channel capacity of the river below the diversion is choked with sediment and growth. During peak flows, water backs up and flows down the canal to the lake. This potentially causes lakeside flooding and overtopping the man-made barrier. This over filling occurred during late December 1996 and early January 1997, escalating to an emergency situation. The solution to the problem includes rebuilding the diversion structure.
Since the floods of 1986, WRID has become increasingly concerned about the adequacy of the diversion structure to reliably regulate the flows of the West Walker River within the limits of the District's water rights and within the limitation of the outlet tunnel to safely pass uncontrolled flood lows accruing in the reservoir without damage to the control gates, tunnel and downstream channel. During the floods of 1996 and 1997, the diversion structure could not operate with the high flows and was severely damaged. In addition, it allowed uncontrolled flood flows and large amounts of sediment to enter the reservoir. It has become increasingly apparent that the District's inability to control the inflow to the reservoir during extreme flood events poses a serious risk to downstream life and property. Further, installation or removal of flashboards during high flows is difficult if not impossible.
On February 11, 1997, the California Department of Water Resources, Division of Safety of Dams (DSOD) inspected the earthen levee/dam feature of the Topaz Reservoir, and based on their findings during the inspection, determined that the levee/dam was in fact subject to State jurisdiction (i.e., the dam was higher than 6 feet from the lowest point at the downstream toe of the embankment to the permanent storage level and the storage impounded by the dam was in excess of 50 acre-feet). The District was advised of this finding by letter dated March 11, 1997 and was instructed to:
(1) File an "Application for Approval of Plans and Specifications for Construction or Enlargement of the Dam," or
(2) File and "Application for Approval of Plans and Specifications for the Repair or Alteration of a Dam of Reservoir" and lower the dam to less than jurisdictional size, or
(3) Remove the dam completely to eliminate the additional storage that was constructed in 1937, or
(4) Construct a levee on the northerly side of the river below the diversion to force flows over range land on the opposite side, or
(5) Increasing the channel capacity.
At its meeting of May 7, 1997, the District's Board of Directors adopted a motion regarding option (1), above. That is, to file an "Application for Approval of Plans and Specification for Construction or Enlargement of the Dam."
Topaz Lake with the West Walker River Diversion to the Lake. Photo courtesy of Robert Johnson.
(Click on the photo to enlarge - note that the enlarged photo will open in a new window.)
Inlet Channel Control Structure (Diversion Structure) and Control System
A major component of the project is the replacement of the existing flashboard diversion structure on the West Walker River and the entrance to the 1.5-mile inlet canal. The new control structure will consist of 10-foot high drop leaf control gates at the entrance to the inlet channel and 6-foot high drop leaf control gates on the river channel. Eventually, bridges will be incorporated into the design to provide vehicle access from the west side of the inlet canal to both the inlet canal levee and the east side of the river channel. The existing flashboard structure will be completely removed.
The controllable gates on the inlet canal and the river channel will allow great operational flexibility. The new gate system will allow for the following:
1. Real time regulation of reservoir inflow to maximize efficient diversion to storage.
2. Reservoir operations that maximize the routing of West Walker River flood flows, reducing downstream peak flows.
3. Ability to induce scouring flows in both the inlet canal and the river channel, potentially reducing the accumulation of sediment.
4. Provide freeboard sufficient to pass peak flow without over-topping of adjacent embankments.
5. Provide automated gate controls and remote control/monitoring capabilities.
6. Provide control gates that can pass large debris.
An overflow spillway structure (spillway), 600 feet in length, will be constructed along the existing dam.
Inlet Canal Levee
During major flood events, flood flows in the West Walker River downstream from the diversion structure overtop the westerly (left) bank of the river and flow back into the inlet canal between the diversion structure and the lake causing damage to the structure and the easterly (right) bank of the inlet canal.
In order to limit the adverse impacts resulting from these occurrences the project includes a compacted earthen levee section which will be constructed along the easterly side of the inlet canal beginning at the southerly end of the existing dam and runs southerly some 5,300 feet to the junction of the two diversion dams.
The existing dam is a compacted, homogeneous, earth embankment approximately 5,700 feet in length which extends in a southeasterly direction from the southern edge of the mountainous terrain which forms the eastern rim of the lake ("Gray Hills") to high ground near the mouth of the Inlet Canal. The dam has a maximum height (downstream toe to dam crest) of about 11 feet and has a crest elevation of about 5,008 feet MSL based on the National Geodetic Vertical Datum of 1929. The dam has about 3.0 feet of freeboard above the maximum lake elevation of 5,005 feet MSL. The existing dam will be raised 2 feet as required by the California Division of Safety of Dams for flood control purposes.
An engineering analysis of the structural integrity of the concrete outlet tunnel is necessary because this tunnel was constructed in 1921. This analysis could include a survey of the location and flow rate of each leakage site in the tunnel and assessment of seepage out of the bank near the tunnel. The analysis must characterize potential weak areas and possible failure points and evaluate remedial actions to strengthen weak areas to prevent a failure.
The Walker River Irrigation District applied for a grant under the AB 237 program to assist with a capital improvement project for the Topaz Reservoir. The grant was approved by the Board for Financing Water Projects on March 14, 2002, for $4,262,750 (85% of the total eligible project cost of $5,015,000). The scope of this project is to 1) reconstruct the Inlet Channel Diversion Structure, and install a remote control system and demolish the existing structure; 2) construct a new overflow spillway; 3) increase the inlet canal levee height; 4) construct improvements to the existing dam; 5) construct a gated outlet through the existing dam; and 6) perform an engineering evaluation of the outlet tunnel.
The following images include photos of the Walker River Irrigation diversion site in December 2006.
(Place your cursor over the photo for a brief summary of what your are seeing. Click on the photo to enlarge - note that the enlarged photo will open in a new window.)
Project designs were submitted to the California DSOD in the fall of 2004. The plans and specification were finally approved at the end of 2005. Design and permitting took 3 years. Permitting is complete from all regulatory authorities. The District has pre-purchased the mechanical parts for this project.
WRID provided an update to the Board on the project's progress at the November 9, 2006, Board meeting.
The project was bid three times. The first bids were opened February 2006. Three bids were received with two of the bids more than twice the engineering estimate. Pazargad, with a bid nearly half the amount of the other two bidders, was the successful bidder; however, they were denied bonding. The project was rebid in October 2006 as two schedules: concrete and earthwork. Four bids were received with the high bidders being Pazargad (concrete) and Burdick Excavating (earthwork). These bids were still well above the money that WRID has for construction of this project and what WRID felt was a fair amount for the job (e.g., mobilization costs were excessive). A third round of bids was received in December 2006 with WRID assuming more of the workload. WRID received more reasonable bids the third time; however, the new project cost is estimated at $7,864,897.87.
Construction costs over the five years, since the original grant was approved by the Board, have increased significantly. WRID has attempted to take on elements of the project within their purview in order to provide the lowest possible construction costs. Engineering costs have already exceeded the original grant amount due to changes in project agreement/scope required to complete the design and permitting. The additional project bids as well as the upcoming construction oversight and field testing requires additional engineering funding.
At the special Board meeting of January 22, 2007, WRID requested additional grant funding totaling $2,422,413.19 for a total grant amount of $6,685,163.19 (85% of $7,864,897.87). WRID also requested an extension of a year to the total project time (March 2008). WRID requested that the original amount allocated in the grant for contingency remain the same due to the possibility of additional requirements of the California DSOD and unforeseen conditions that may be encountered in the field. No increase in funding for the tunnel evaluation was sought in this request. The Board approved both the additional funding and time.
The WRID Board awarded the construction constract to MKD Construction. Project construction began in July 2007 with work on both the diversion structure and levee/spillway structures running concurrently. A final walk-through of the levee and spillway was completed on February 11, 2008. Both the CA-DSOD and the State of Nevada, Division of Water Resources, Dam Safety Branch were at the final walk-through. The diversion gates on the Walker River and Topaz Reservoir were also complete at that time; however, the generator, compressor and operating appurtenances still needed to be installed in the operations building.
Go to project photo page --> click here