Flash Flood — Tuolumne, California

2018-03-22 · near Moccasin, Tuolumne, California

Event narrative

A period of heavy rain upstream from Moccasin Dam, in which about three inches fell during four hours led to significant erosion and at least one landslide. It caused a massive amount of water and debris to run downhill into the Moccasin Reservoir. The water level rose to 3 times the normal reservoir capacity and the emergency spillway was utilized. There was severe erosion of the emergency spillway and potential for the dam to fail. The Power house and staff at the Moccasin Fish Hatchery were evacuated due to potential failure. All the fish in the Moccasin Creek Hatchery were killed when it flooded from the water coming over the emergency spillway. The National Weather Service issued a flash flood warning associated with the probable dam failure, and the sheriff advised residents to avoid the area as a matter of public safety. The warning was reduced to a watch until the reservoir was drained and then the watch was canceled once the reservoir was drained.

Cleanup and repair efforts associated with a major storm that damaged Tuolumne County's Moccasin Creek Dam in March is expected to cost roughly $43 million, according to an estimate by the San Francisco Public Utilities Commission, which owns and operates the dam. That total includes $3.2 million in damage to the state fish hatchery at Moccasin Creek, which lies downstream.

Wider weather episode

Torrential rain from an atmospheric river with locally heavy convective showers brought flash flooding to portions of the northern Sierra and Motherlode foothills. The heaviest flooding was in Groveland, where 4-5 of rain fell, combining with 8-9 of rain in higher elevations upstream. There was also widespread damage in Tuolumne County to roads from debris flows, soil erosion, erosion of water conveyance systems, sewer systems that were inundated with excess water and debris. Roads damaged inluded state Highways 49 and 132. Moccasin Dam was damaged and came close to failure, with road closures. A tornado struck an amphitheater in Wheatland and caused minor damage. Heavy snow caused travel problems over Sierra highways.

View location on OpenStreetMap → (37.8155, -120.3134)

Source: NOAA Storm Events Database, event_id 747046. Narrative written by the NWS forecast office that issued the report.