Work to start soon on Anderson Dam Tunnel

Costing Santa Clara county valuable drinking water resources, earthquake retrofitting is expected to improve reliability and endow the reservoir with its design storage capacity. Current water storage is restricted to a level around 17m below the dam’s crest. This protects the public in the event of structural damage and consequent uncontrolled water release that could result from a major earthquake.

The project forms part of the larger Anderson Dam Seismic Retrofit Project (ADSRP) and comes under a federal emergency order that will see a replacement dam built to address seismic safety concerns.

In preparation for the US$630m project, the Valley Water authority started lowering the reservoir water level in October 2020. Now at just 3% of reservoir capacity, the water level is at the lowest that can be reached through the existing outlet tunnel which is deemed undersized at its current capacity of 500ft3/sec (14.1m3/sec).

At around 548m long and up to 7.3m in diameter, the proposed Anderson Dam Tunnel will allow Valley Water a five-fold increase in water release rate from the reservoir in an emergency. It will eventually be used to achieve full reservoir drawdown and to bypass flows during the removal and replacement of the existing dam.

The tunnel will comprise three sections:

(1) A 122m-long, 2.4m-diameter ‘lake tap’ pipe upstream of the dam, connected to
(2) A 30.5m microtunnel boring-machine launch chamber, connected to a 282m-long, 5.8m-diameter reinforced concrete-lined pipe within a 7.3m-diameter tunnel; connected to
(3) A 114m-long, 4m-diameter steel pipeline within a 5.6m-diameter horseshoe tunnel.

Construction on the tunnel is expected to start in June 2021 and is expected to last for between 24-36 months.