Areas of floods and debris flows from the April 6, 2004 storm. Map also shows areas of potential flooding (alluvial fans and small drainages) from fire-related floods and debris flows (except where protected by debris basins) from the burn area of the 2003 Farmington fire.
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by Richard E. Giraud and Greg N. McDonald

Intense thunderstorms on the evening of April 6, 2004, caused flooding and debris flows in areas along the east bench of Farmington between Farmington and Shepard Canyons. The sources of the floodwaters were mainly small, range-front drainages burned in the July 2003 Farmington fire.

Most damage occurred in subdivisions on small alluvial fans below two unnamed drainages south of Shepard Creek (see adjacent map).

Floodwaters and sediment deposition were mostly restricted to streets and yards, but damage also occurred to several vehicles, garages, and homes (see photos below).

At one locality, erosion by floodwaters threatened a section of a Weber Basin Water Conservancy District aqueduct running along the mountain front (see photo below).

The lower slopes of the Wasatch Range above Farmington were burned in the July 2003 Farmington fire. Following the fire, the Utah Geological Survey (UGS) assessed the heightened fire-related debris-flow hazard and produced a map showing debris basins and areas of possible flooding and debris flows. Letter Report to Wasatch-Cache National Forest (pdf).

A debris flow and flood across the Weber Basin aqueduct resulted in partial failure of the embankment. Crews are in the process of repairing the embankment.
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A U.S. Forest Service (USFS) Burned Area Emergency Rehabilitation (BAER) team assessed the severity of the burn and resulting increased flood hazard.

Following the BAER team assessment, the USFS and U.S. Natural Resources Conservation Service undertook various watershed-protection measures to reduce flooding and debris-flow hazards. These were completed in late fall 2003.

Factors probably contributing to the flooding in addition to heavy rainfall and burned hillsides include steep slopes, ample supplies of sediment, and increased runoff caused by already wet soil conditions from recent snowmelt.

Davis County and Farmington have a long history of flooding and debris flows, and as a result many of the larger drainages such as Farmington and Rudd Canyons are protected by debris basins. However, most small drainages are not.

Although emergency watershed protection measures, such as were completed last fall following the fire, are designed to reduce flooding and debris-flow hazards, they do not eliminate hazards and are not permanent. Therefore, the heightened flooding and debris-flow hazard from the fire will exist for several more years while the watershed recovers to preburn conditions.

A field survey on April 7, 2004, by UGS geologists found that drainages in the flood area contain ample sediment for future debris flows.

	A debris flow crossed North Compton Road and buried the driveway, pushing the trucks and breaking the lower garage door panel. Water and sediment flowed into the garage, damaging the contents inside. The sediment burial depths are 1 to 3 feet. Click here for larger image.
	After crossing North Compton Road, the debris flow deposited 3 to 6 inches of sediment on the front lawn of this lot. Click here for larger image.
	Looking downslope at the three lots damaged by the April 6, 2004 debris flows. Crews are removing sediment from the lots. The debris flow exceeded the channel capacity of the small alluvial-fan channel in the foreground and flowed onto the fan surface. The alluvial-fan channel gradient decreases from 9 to 7 degrees downfan and average channel depth is 2.5 to 3 feet. Click here for larger image.
	Looking upslope into areas burned by the July 2003 Farmington fire. Thunderstorm rainfall rapidly converged into small channels, eroding sediment and forming debris flows. Here the runoff is eroding a coarse sand and fine gravel Lake Bonneville beach face just downslope of the Bonneville shoreline. From the inception of channel erosion to the damaged lots, the debris flows traveled approximately 3,000 feet and dropped 1,200 feet in elevation. Click here for larger image.