BRIEFING PAPER - SOURGRASS DEBRIS FLOW

 

A series of storms in the latter part of December 1996 and the first week in January 1997 caused flooding and erosion in the Sierra Nevada and adjacent Central Valley.  One of the larger landslides triggered by this climatic event was the Sourgrass debris flow.  The debris flow occurred at about 6:30 pm on January 1, 1997.  The U.S. Geological Survey noted this slope movement during an aerial reconnaissance of the central Sierra Nevada after the New Years storm.  Their report refers to the feature as the Dorrington debris flow.

 

 

Triggering Mechanism

 

+       Debris flow was triggered by precipitation during major storm event.  A total of over 10 inches of precipitation fell at nearby Calaveras Big Trees State Park between December 31, 1996 and January 2, 1997.

 

+       By December 27, 1996, rainfall stations at 5600-foot elevation within the vicinity of Sourgrass debris flow were at 200% of average precipitation.  It is known snow covered the area where the debris flow was initiated by January 1, 1997 making this a rain-on-snow event.

 

+       U.S. Army Corps of Engineers estimates the storm event has a 100 year return frequency (1% probability of occurrence).  Unimpaired peak flow at New Melones dam was 125,000 cfs.  The significant storm of February 1996 generated a flow at New Melones of 80,000 cfs.  For comparison, the historic 1955 storm event caused a peak flow of 102,000 cfs.

 

 

Initial Movement

 

+       Initial movement was a debris slide on a defined slip surface on a 40 percent slope at the head of an unnamed tributary to the North Fork of the Stanislaus River.  The initial movement involved an estimated 96,300 cubic yards.  The slide mass is defined by a headscarp and well-defined lateral shear zones.  The headscarp is at an elevation of about 5960 feet.

 

+       The slip surface "daylighted" from the slope about 12 to 15 feet above the surface of a timber access road which contoured across the slope below it.  The top of the cutslope is 5 feet above the road surface and indicates slope movement was unlikely to be a result of removal of material to construct the road.

 

+       The initial slide was a block of Tahoe-age (?) till nearly 50 feet thick overlying volcanic mudflow breccia of the Merhten Formation.  The slip surface generally coincides with the upper part of the volcanic unit.

 

+        The debris slide almost immediately began to disaggregate into a debris flow mass.  A remnant of the initial block which includes several 6 to 10-foot diameter granitic boulders remain within the slide scar.  About 64,500 cubic yards of material is the estimated initial volume of the debris flow mass which moved downslope.

 

 

Main Movement

 

+       From upslope of the uppermost timber access road to the North Fork of the Stanislaus River, the slope movement was a debris flow.  Total distance from the headscarp to the river is 2.4 miles.

 

+       Evidence of debris flow movement includes: superelevation of the movement as it passed through major bends in the channel, formation of levees at the margins of the flowpath, matrix-support of large rock fragments and debris in levees and deposit, 'smoothed' passage over unconfined areas of path with scour to bedrock within confined path, and impact damage to vegetation but no significant scour damage.

 

+       Debris flow passage from the initial movement to several thousand feet downslope of Highway 4 was at a speed of 2-3 miles per hour (based on field measurements).  Only a few feet of material was eroded and added to the flow volume along this part of the path.

 

+       Once the debris flow entered the steeper, more defined channel downslope from Highway 4 area, it increased in speed and scoured many feet of material from the channel which increased the volume reaching the river.  At the North Fork of the Stanislaus River, the debris flow was traveling over 12 miles per hour.  This nearly equals the average peak velocity of past debris flows in the Sierra Nevada.

 

+       The volume of the debris flow increased due to erosion of material along the flowpath.  By the time it reached the river, it had increased in volume to an estimated 191,170 cubic yards which is about 3 times the initial volume.

 

+       The debris flow deposit partially dammed the river.  Rising water behind this impoundment floated a 40-foot equipment van left in the Sourgrass campground by a contractor.  The van was found tipped on its side and snagged on vegetation downstream from its original location where it was apparently carried by water once unimpeded flow resumed.

 

+       The flood flow in the North Fork of the Stanislaus River mobilized the debris downstream.  It took about an hour for unimpeded flow to resume.  Little debris flow deposit remains.

 

 

Damage Effects

 

+       The debris flow removed trees along the 2.4 mile long, 100-500 foot width of its flow path beginning at 5860 feet elevation and ending at 3960 feet elevation.  Many trees along the margins of the flowpath were damaged by impact from trees and large rocks carried by the debris flow.

 

+       From the point of initial movement to downslope from Highway 4, the damage was to land owned by Georgia Pacific.  From downslope of Highway 4 to the river, the damage was to national forest system land administered by the Stanislaus National Forest.

 

+       No significant damage to the two timber access roads upslope from Highway 4 or Highway 4 from passage of the debris flow.  Post-movement damage occurred to the lower access road from water flow and water-flow damage occurred at Highway 4 from debris blocking the culvert.

 

+       In the same area above Highway 4, a Pacific Gas and Electric Company 21,000 volt distribution line was broken when one pole was toppled and 1300 feet of line was pulled down.  A Pacific Bell Corporation fiber optic lines buried in a 3-foot deep trench was severed, too.

 

+       At the North Fork of the Stanislaus River, the two-lane paved Forest Service road was severely damaged on the north side of the river.  The bridge across the river was destroyed.  Additional damage was incurred at the Sourgrass campground.  A rubber-tired skidder parked near the campground was carried down river and destroyed.

 

+       McKays Reservoir lost 200 acre-feet of storage capacity.  This represents 10 percent of its storage capacity.  The Sourgrass debris flow likely accounts for slightly more than 100-acre-feet of this lost capacity.

 

JEROME V. DEGRAFF

Forest Geologist

Jan. 22, 1997