PLT11 - EAP Activation to Validate TTX and Guide Dam Modification Design

On May 6, 2022, during a routine inspection of Christmas Tree Dam, White Mountain Apache Tribe (WMAT) Land Operations team members noticed unusual discharge out of the inoperable outlet works conduit. The Emergency Action Plan (EAP) was activated to Level 1, and WMAT and GEI personnel were mobilized to the site with a Remotely Operated Vehicle (ROV) camera inspection crew and a dive crew to inspect the outlet works structure and determine the cause of the unusual increase in seepage.

The results of the ROV inspection determined aquatic vegetation was collecting at the upstream side of the inoperable slide gate. A dive team was mobilized to the site to inspect the gate for damage and attempt to repair the structure. From the time the WMAT and GEI decided a dive team was required and their arrival onsite, the unusual discharge stopped, and the outlet works pipe did not flow.

On May 17, 2022, two dives were performed at Christmas Tree Dam to investigate the condition of the outlet works which verified there was no damage to the intake structure or slide gate, the structure was mildly silted in, there was no aquatic vegetation located near the structure, and there was no undocumented secondary structure in the reservoir. Upon returning from the second dive, WMAT Land Operations staff noticed the vent pipe was corroded and severed, and plugged with vegetation. Divers removed the vegetation plug and the unusual discharge started again. The air vent pipe was plugged with oakum and hydrophobic grout to prevent future drainage.

The EAP was deactivated on May 18, 2022 after the cause of the unusual seepage was determined to be a corroded and severed air vent pipe. The process of activating the EAP, notifying the public, and deactivating the EAP was practiced during a Tabletop Exercise (TTX) in October 2019, and the emergency preparedness was helpful during this event. Signs, fliers, and tribal meetings were used to notify the public, and the dam was monitored around the clock during the EAP activation.

The WMAT is using results from this incident to guide retrofit designs to Christmas Tree Dam. The modifications will accommodate information collected during the EAP activation and dam site reconnaissance.