Spillway near 14-Mile Tunnel to be abandoned

By From page A8 | April 18, 2012

The spillway nearest the entrance to 14-Mile Tunnel will be abandoned and the next one east of that will be replaced and automated.

The 14-Mile Tunnel is nearly the last leg before a short section of canal empties into Forebay Reservoir in Pollock Pines.

The mitigated negative declaration — 3/4-inch thick —won unanimous backing from the El Dorado Irrigation District Board of Directors April 9.

The spillway closest to the tunnel is being abandoned because any use of it results in substantial erosion, because the area terrain below it is only loose volcanic soil and no rocks. The spillway itself is a funky-looking wooden chute.

Trying to improve that spillway would cost $3.4 million, according to the engineer’s estimate, while abandoning it and improving the tunnel headworks and Spillway 46 farther east will cost $1.7 million.

Spillways are placed along the 22-mile conveyance system of canals, flumes and tunnels in case of a blockage backing up water in the canal. The spillway prevents the canal or flumes from spilling uncontrollably.

The 22 miles of flume, canal and tunnels bring water down from four alpine reservoirs in three counties. The system is called Project 184 by the Federal Energy Regulatory System, which licenses it because EID operates a 22-megawatt powerhouse. Most of the water runs through the powerhouse, but 15,080 acre-feet of water is used for household tap water. That water right predates 1914, which puts it out of reach from the state. An additional 17,000 acre-feet of consumptive water comes from Project 184.

As a consequence of receiving a new operating permit from FERC in 2006 EID is required to make a number of improvements, this being one of them.

Last June the board approve nearly $325,000 for design costs for this project, which included $189,700 for Carlton Engineering.

This fall contractors who win the bid will do the work to upgrade the tunnel entrance and trash gate, upgrade and electrify Spillway 46. Part of the project includes acquiring additional property near the tunnel entrance to provide a staging area for district crews and equipment to clean out the tunnel and to improve the security fencing. An access road will be graded and gravelled as well.

An additional factor leading to the $1.7 million estimated cost of the project is work to stabilize the uphill area along this section of the canal and remove  and replace 250 feet of canal lining and rock line 80 feet of Randolph Creek near the Spillway 46 outfall.

District crews would remove Spillway 47, which is the one being abandoned. That would be done in the fall of 2013. Included in that removal is a spillway control shed.

Michael Raffety

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