San Bernardino Report
Debris Flows at Forest Falls, San Bernardino County, CA
July 11, 1999
By Douglas M. Morton
Update based on email dated December 22, 1999
San Bernardino County Fire Department
Forest Falls Fire Protection District
The following information has been added to this report:
- The area in question is Snow Creek, flowing out of Snow Canyon (USGS Topo Map incorrect). It is correct that the street affected is Spring Dr.
- Creek affected, from East to West were: Rattlesnake Creek, Snow Creek and Slide Creek
- Final count was 1 fatality with reports of any missing persons being unfounded.
High-intensity short-duration rainfall produced debris flows in the late afternoon of July 11 in the community of Forest Falls. Forest Falls is located along the south side of a major canyon, Mill Creek Canyon, in the southeastern part of the San Bernardino Mountains. Mill Creek Canyon is a very steep-walled canyon developed along the Mission Creek strand ("North Branch") of the San Andreas Fault zone. Erosion of the highly fractured rock along the fault zone in this rapidly uplifting part of the San Bernardino Mountains has produced an unstable, steep-sided, deep canyon. The canyon wall on the south side of Forest Falls rises abruptly about 3,000 feet to elevations of nearly 9,000 feet at the crest of Yucaipa ridge that is underlain by a variety of gneissic rocks. Numerous avalanche chutes scar the canyon wall. Most of Forest Falls is located on bouldery debris flow deposits that form a relatively narrow shelf sloping from the base of Yucaipa ridge northward to Mill Creek. Mill Creek (Figure 1) is frequently subject to flood conditions, and has eroded the distal parts of the debris flow deposits on which Forest Falls is located. Pervasive physiographic features that typify debris flows cover the surface on which Forest Falls is located. The most evident features are widespread active debris flow channels and levees, remnants of debris flow levees.
Forest Falls has frequently been the site of debris flows produced primarily during periods of high-intensity short-duration rainfall during the summer monsoon season. Based on the past 45 year period of time, the recurrence interval for significant debris flows in the Forest Falls area is about 4 years. The most recent damaging debris flows prior to the July 11, 1999 debris flows was on September 4,1997. Other relatively recent significant debris flows have occurred in 1955, 1958, 1961, 1963, 1965(two debris flow events on two consecutive days), 1984 (two debris flow events on two consecutive days), and 1989. In the late afternoon of July 11, 1999, about 1.5" of rain fell on Yucaipa ridge south of the eastern part of Forest Falls. This intense rainfall produced boulder debris flows particularly in the small drainages of Spring Creek, Rock Creek, and Slide Creek. In these drainages the debris flows over-topped their natural channels spreading boulder debris over extensive areas. Debris flows deposited wide stretches of boulder-laden debris to depths up to about 6 feet on the only access road to Forest Falls, isolating the community until the afternoon of the 12th (Figure 2). Over 30 homes and numerous automobiles were damaged or destroyed. One confirmed fatality occurred at Spring Creek and on July 12 there were still several persons reported missing.
At Spring Creek the existing debris flow channel, estimated about 10 feet in depth, was topped where the channel has a 400 bend to the right (down stream). A very large, high-velocity debris flow, containing boulders of six feet or more in length, overtopped the debris flow channel at the 400 right bend in the channel (Figure 3). The flow crossed over a prominent, several-foot-high, debris flow levee, where it scarred trees at heights of about 8 feet above the levee crest (Figure 4). The vertical distance from the base of the debris flow channel to the top of the debris flow crossing the crest of the levee is estimated at over 20 feet. Still rapidly moving down-slope after topping the levee, the debris flow spread-out damaging and destroying homes in its wake (Figure 5, Figure 6, and Figure 7). It was in this area most of the damage and the death occurred. Mud-laden water followed the debris flow(s). It appears most of the debris flow activity transpired over a period of a relatively few minutes.
It is not known if there was some blockage in the Spring Creek debris flow channel at the bend that diverted or helped to divert the debris flow, or whether the inertia of the high-velocity flow carried the debris up and over the levee. On July 12 there was no blockage in the channel, suggesting the over-topping was primarily due to the inertia of the high-velocity debris flow continuing in a straight course, up and over the levee.
Where debris flows came to rest on undeveloped parts of the old debris flow surface they formed characteristic boulder laden snouts at their down-slope end (Figure 8).
On the afternoon of July 14, 1999, the weather conditions of the 11th were repeated with about 2" of rain falling in a 30-minute period. This rainfall produced numerous debris flows that once again coursed through Forest Falls damaging several homes.
Figure 1 Mill Creek channel at Forest Falls. View looking downstream (west).
Figure 2 Recently cleared access road into Forest Falls. Photo taken about 3PM, July 12, 1999. Debris flows covering the road came down Slide Creek to the left of the road. View looking west.
Figure 3 View looking south up Spring Creek debris flow channel at the 400 bend in the channel. Fragments of trees in the foreground are located where the debris flow left (topped) the debris flow levee. Water following the debris flow(s) has modified the channel bottom. Yucaipa ridge in the background.
Figure 4 Debris flow-scarred yellow pine tree near crest of the debris flow levee where the debris flow(s) left the channel. Height of scarring is about 8 feet above debris flow surface; the elevation of the debris flow surface prior to July 11 was lower than the present surface.
Figure 5 Destroyed home and buried car (partly visible in central part of left side of photo); location just below position of Figure 4.
Figure 6 Destroyed homes in area where debris flow(s) spread laterally, unconfined by channel walls.
Figure 7 Destroyed cars in the medial part of the debris flow.
Figure 8 Snout of stabilized boulder debris flow on undeveloped part of the old debris flow surface. This snout is a characteristic feature of the distal end of debris flows.
|Douglas M. Morton
U.S. Geological Survey
UC - Riverside
Riverside, CA 92521
Department of Earth Science, Room
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