California planning US$1.8 million Hell Hole Dam core raise project

Placer County Water Agency

The Placer County Water Agency (PCWA) is in the process of planning the Hell Hole Dam core raise project at the 725-kW Hell Hole hydroelectric scheme near the town of Foresthill in Placer County, Calif.

PCWA estimates the project will cost US$1.8 million and has said to safely control seepage that could occur during probable maximum flood (PMF) loading, it must increase the dam’s core height.

The contract to complete the project has not yet been awarded, but June 13 marks the end of a 30-day public review period of the project’s Mitigated Negative Declaration.

The Hell Hole facility is one of five that PCWA operates as part of its 224-MW Middle Fork American River hydropower project (MFP). The four additional MFP hydro facilities include: 15.3-MW French Meadows; 122.4-MW Middle Fork; 79.2-MW Ralston; and 6.128-MW Oxbow.

Hell Hole Dam was completed in 1966. It is a zoned rockfill and earthfill embankment dam that has a maximum height of about 410 ft and a cambered crest, 1,550-ft-long by 35-ft-wide. Camber is the extra height added to the crest of embankment dams to ensure the difference in elevation between the maximum water surface in the reservoir and the dam crest will not be diminished by foundation settlement or embankment consolidation.

The dam impounds the Rubicon River and Five Lakes Creek to form Hell Hole Reservoir, which has a storage capacity of 207,590 acre-feet.

The crest of the dam is reported to have settled about 2.6 ft since its construction, resulting in a reduction of the 10-ft camber included in the original design. The current crest elevation varies from 4,650 to 4,657.4 ft.

PCWA, in its contract addendum to the project bid released May 26, indicated the Hell Hole Dam core must be increased about 2.5 ft, in part, to withstand PMF conditions “based on the proposed addition of spillway gates.”

In preparation of relicensing from the Federal Energy Regulatory Commission (FERC), according to a PCWA executive summary that included “Project Betterments and Improvements,” part of the organization’s consideration to modernize the dam includes the installation of eight, 10-ft-high crest gates on the existing Hell Hole Dam spillway.

According to PCWA, addition of spillway gates could provide an additional 9,750 to 12,000 acre-feet of increased seasonal storage in Hell Hole Reservoir.

In 2013, Black & Veatch Co. engineers revised the PMF water surface elevation for Hell Hole Dam to 4648.58 ft, based on the proposed addition of spillway gates. The PMF elevation is higher than portions of the dam core.

As a condition for the California Division of Safety of Dams (DSOD) to approve the project and meet guidelines for FERC licensing, DSOD said PCWA must raise the dam core elevation to 4,651 ft, which is about 2.5 ft above the PMF flood level.

The 2013 investigation determined that without raising the core, overtopping would occur for a distance of about 700 ft adjacent to the left abutment and 700 ft adjacent to the right abutment.

In published news reports, PCWA spokesman Ross Branch said FERC licensing procedure is targeted for completion in 2019.

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