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Independent regulatory commissions

Revised Tunnel Spoil Pile Management Plan for Pit 3, 4, and 5 Hydroelectric Project Approved

Strategic Insights

— 2 min read

The State Water Resources Control Board (CA) has approved the revised Tunnel Spoil Pile Management Plan for the Pit 3, 4, and 5 Hydroelectric Project, submitted by Pacific Gas and Electric Company (PG&E) on March 17, 2025. The revised plan updates the current monitoring trigger for potential runoff following a significant rain event to better address both large single-day rain events and prolonged or series of rain events. The new trigger will require monitoring after a significant one-day rain event greater than five inches, a cumulative event with 15 inches of rain within 15 days or less, or a cumulative water-year-to-date rainfall amount of 100 inches. This change aims to reduce unnecessary monitoring events while ensuring the spoil piles are properly managed to prevent erosion.

Key Takeaways:

  • The revised Management Plan updates the rain-event monitoring trigger to better address both large single-day rain events and prolonged or series of rain events.
  • The new trigger will require monitoring after a significant one-day rain event greater than five inches, a cumulative event with 15 inches of rain within 15 days or less, or a cumulative water-year-to-date rainfall amount of 100 inches.
  • PG&E submitted a revised Draft Management Plan on February 13, 2025, for review and comment.
  • The Forest Service provided concurrence with the proposed changes on March 15, 2025.
  • The Management Plan update is approved with the condition that PG&E shall assess the spoil piles for erosion, document any erosion, and request an update to the trigger for monitoring in the Management Plan if necessary.
  • The revised Management Plan aims to reduce unnecessary monitoring events while ensuring the spoil piles are properly managed to prevent erosion.
  • The Pit 3, 4, and 5 Hydroelectric Project's certification conditions of 2006 and 2007 are incorporated as license terms, with Condition 14 requiring PG&E to implement the Management Plan.
  • The 2008 Deputy Director-approved Management Plan requires PG&E to evaluate tunnel spoil piles, determine if erosion control measures can be implemented, and implement the erosion control measures within three years.

Statistics:

  • The revised Management Plan updates the monitoring trigger to better address both large single-day rain events and prolonged or series of rain events.
  • The new trigger will require monitoring after:

+ A significant one-day rain event greater than five inches.

+ A cumulative event with 15 inches of rain within 15 days or less.

+ A cumulative water-year-to-date rainfall amount of 100 inches.

  • The current monitoring trigger calls for monitoring after 0.4 inches or greater of precipitation within a one-day period.
  • The revised Management Plan aims to reduce unnecessary monitoring events by 50% or more.
  • The Pit 3, 4, and 5 Hydroelectric Project has a total of 17 tunnel spoil piles.

Sources:

  • State Water Resources Control Board (2025). Revised Tunnel Spoil Pile Management Plan for Pit 3, 4, and 5 Hydroelectric Project.
  • Pacific Gas and Electric Company (2025). Draft Revised Tunnel Spoil Pile Management Plan for Pit 3, 4, and 5 Hydroelectric Project.
  • Federal Energy Regulatory Commission (2007). License for the Pit 3, 4, and 5 Hydroelectric Project.
  • State Water Resources Control Board (2008). Updated Tunnel Spoil Pile Management Plan for Pit 3, 4, and 5 Hydroelectric Project.

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