Fire-related debris flows damage houses in Spring Lake and Santaquin, Utah County, September 2002
On the evening of September 12, 2002, intense thunderstorm rainfall on Dry Mountain, about 18 miles south of Provo, triggered fire-related debris flows that traveled down drainages and onto alluvial fans, damaging houses and property in Spring Lake and Santaquin east of Interstate 15. Fire-related debris flows are debris flows that start in areas burned by wildfires.
The debris flows started high in the drainages on the east side of Dry Mountain that burned in the 2001 Mollie wildfire, a human-caused fire that burned 8,000 acres between August 18 and September 1, 2001.
The most damaging debris flow traveled through a subdivision in Santaquin. This debris flow moved and partially buried several vehicles, broke through a house wall, and entered other houses through broken basement windows and doors. Debris-flow impacts also tore gas meters from their mounts, causing gas leaks and a small fire. Sediment flow and burial on lots also damaged landscaping and property outside the houses.
A debris flow in Spring Lake filled part of the High Line irrigation canal with sediment, causing flooding in addition to debris-flow damage.
For additional information read the Geologic Hazards Program Technical Report (pdf-4MB).
Massive Rock Fall Near Devil’s Slide, Morgan County, March 22, 2004
On the evening of March 22, 2004, a rock-fall event occurred in Morgan County, south of the Croydon exit along Interstate 84, about 8 miles east of the town of Morgan.
The rock fall, which is easily visible from the road, released from the face of an 80-foot-high cliff and measured roughly 26x20x35 feet.
What triggered the rock fall is not known; however, rock falls occur haphazardly and such events are not uncommon during seasonal snowmelt.
Repeated freeze-thaw cycles can cause water trapped in fractures within the rock to freeze and expand. This process possibly led to the weakening of the rock mass, and triggered the fall.
After dislodging, the large block slid down the cliff face, launched outward, then fell 25 feet before landing on the slope below.
The size and velocity of the block was sufficient to form a large impact crater.
Upon impact, the block broke apart sending several sizeable boulders downslope, some traveling nearly ½ mile, damaging large trees and vegetation along the way.
The largest of these boulders measures about 20x15x13 feet and weighs about 250 tons.
Fortunately, given the relative remoteness of the area, no property damage or injuries occurred.
April 6, 2004, Fire-related Flooding and Debris Flows in Farmington
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 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.
East Capitol Blvd.-City Creek Landslide May Continue To Move Until The End Of May
Measurements taken April 9, 2004 confirmed that the East Capitol Blvd.-City Creek (CBCC) landslide, which began its most recent period of movement in 1998, has started to move again, after stopping briefly in the early 2000s.
Based on observations and survey data, the entire landslide began moving, with the exception of the northernmost edge, in early March 2004.
Evidence includes numerous ground cracks and shears on the slide and offset of the main scarp, just east of East Capitol Blvd. About 7 inches of movement has occurred at the toe this year and the rate of movement has increased about 40 percent between late March and early April.
Although March was dry compared to the same time period in 1998, the movement pattern in previous years indicates the CBCC landslide may continue to move, possibly a foot or more, until the end of May. However, above normal precipitation could cause even more movement.
Continued movement of the landslide may block drainages flanking the slide, which could result in minor flooding.
Further movement could also affect the stability of the main scarp and lead to the enlargement of the slide and damage to the back lots and houses on East Capitol Blvd.
July 26th, 2004, fire-related debris flows near Spring Lake and Santaquin, Utah
By Christopher B. DuRoss
and Greg N. McDonald
Intense rainfall in the afternoon and evening of July 26th, 2004, triggered two fire-related debris flows south of Provo, near Spring Lake and Santaquin, Utah. Mud-, cobble-, and boulder-rich sediment discharged from two drainages on the northwest flank of Dry Mountain, and was deposited on alluvial fans north of the range-front. The debris originated from drainage areas burned by the human-caused 8,000-acre Mollie wildfire of 2001.
The fire-related debris flows were in the same general location as the two northernmost flows of September 12th, 2002; however, the July 2004 flows were smaller and did not travel as far north as the 2002 flows (see maps below). No damage to residential homes or property was reported. Conversely, the September 2002 flows resulted in significant damage to a subdivision due to a large volume of debris that originated from a drainage farther to the south. No debris flows were generated from that drainage in the July 2004 storm.
The larger debris flow is located southwest of Spring Lake and traveled to the northwest, filling part of the High Line irrigation canal with cobbles and silt. The debris was quickly excavated to prevent overflow and flooding of the canal.
The thickness of new material in the 2004 deposit could not be determined as an unknown amount of the September 2002 debris was incorporated into the flow.
About a mile east of Santaquin, a smaller debris flow traveled to the northwest into a storm-water catchment basin. The deposit is approximately 9 to 12 inches thick.
February 20, 2005 425 East South Weber Drive Landslide South Weber, Davis County
By Richard E. Giraud and Gary E. Christenson
On the evening of February 20, 2005, a landslide in South Weber flowed across South Weber Drive (State Route 60) at 425 East, blocking State Route 60. The highway was subsequently closed and cleaned up by the Utah Department of Transportation. The landslide destroyed a barn south of the highway, and flowed into a field north of the highway. The landslide was about 480 feet long and 80 feet wide.
The landslide is just below the Davis-Weber Canal and likely started moving as a rotational slide in the canal embankment, but quickly transformed into a rapid earthflow about midway downslope and ran out 150 feet beyond the toe of the slope. The landslide occurred in one of the steeper parts of a north-facing slope underlain by a prehistoric landslide that has been locally historically active.
The geologic material in the slope under the canal embankment is sand, silt, and clay of the Pleistocene Lake Bonneville Weber River delta. The slope formed as the Weber River cut down into its former delta as Lake Bonneville receded 16,000 years ago and the shoreline retreated to the present level of Great Salt Lake. Shallow ground water and weak soil materials characterize the slope.
The main scarp of the landslide is in the outer canal and canal road embankment. The upper part of the main scarp is composed of canal embankment fill, and about 20-25 feet of the embankment and canal road remains between the crown of the landslide and the canal. Water is not presently flowing in the canal, and it was undamaged, but the canal is threatened by eventual retreat of the main scarp.
Fresh scarps in the hillside in and just below the embankment are found both east and west of the landslide, indicating that adjacent slopes remain marginally stable.
At the time of the UGS investigation on the morning of February 21, water was flowing from the landslide scar below the main scarp about midway down the slide. The vegetation types indicate that shallow ground water was present in the hillside adjacent to the landslide, particularly to the east.
Records from the National Weather Service stations indicate the Layton-South Weber-Ogden area received 148% of normal precipitation for the period since September 1, 2004 prior to the landslide. In addition, the area received greater than normal precipitation last year, and an additional 0.72 inches of rain fell on the day of the landslide.
Shallow ground-water conditions, along with the steepness of the slope (up to 65% locally), presence of the canal embankment fill, and underlying weak geologic materials in the slope probably all contributed to the landslide.
This report is preliminary and subject to revision; it is presented here to facilitate a timely release of pertinent information.
Sage Vista Lane Landslide, Cedar Hills, Utah County
By Francis Ashland and Greg McDonald
On the afternoon of April 28, 2005, a landslide that had moved in 1983 reactivated above a Cedar Hills subdivision and slid against the lower portion of the back wall of a four-unit townhouse.
Residents of the impacted four-unit townhouse evacuated and a neighboring family in a separate unit across the street voluntarily moved out of their home temporarily.
The landslide toe has crushed vinyl fencing, air conditioners, and deck supports at the back of three of the units, but according to a Cedar Hills building inspector, had not as of Friday (April 29) afternoon caused any structural damage to the townhouse.
Given the current position of the landslide toe, however, continued movement could result in damage to the building.
The landslide is on a southwest-facing slope and is a reactivation of a 1983 landslide that is part of a larger, prehistoric landslide complex associated with the Manning Canyon Shale. The active landslide is approximately 375 feet long and 150 feet wide at its toe.
Landslide debris consisted of cobbles and boulders in a olive-green to brownish-gray clay matrix. Locally, weathered fragments of black Manning Canyon Shale were observed in the debris.
The upper part of the landslide consists of a narrow main scarp zone composed of several scarps less than 2 feet high and a relatively shallow area of translational sliding. The landslide widens and deepens downslope, giving it a teardrop shape.
Residents indicated that the toe initially emerged partway up a cut slope behind the townhouse and debris subsequently flowed down against the back wall of the building. The main scarp of the landslide is approximately 25 to 30 feet high; the upper 20 to 25 feet represents the scarp from the 1983 event.
Ground deformation in the slide varies from top to bottom. Fresh, polished slickensides were observed in the upper part of the slide. Near the toe of the landslide, the debris is disrupted, but relatively intact blocks of soil exist in the middle of the slide.
Measurements by the UGS on Friday, April 29 indicated a rate of movement of about 0.3 foot per hour in the morning and that the movement rate had slowed to about 0.1 foot per hour in the afternoon.
Utah County surveyors, using high-resolution GPS techniques, measured points both on and off the landslide over the weekend (April 30-May 1).
Their data indicate a stake in the uppermost part of the landslide moved about 19 inches between Friday (4/29) afternoon and Sunday (5/1) morning and suggests the rate of movement at the head is slowing. Plots of stakes lower in the slide show no significant movement, but suggest movement at a rate below the resolution of GPS surveying.
Two other small landslides were observed above the main slide, also in the prehistoric landslide, further suggesting the marginal stability of the hillslope. Measurements indicated no movement of these slides during the afternoon of Friday, April 29. A reconnaissance of the remainder of the prehistoric landslide indicated no other areas of active landsliding.
This report is preliminary (5/1/2005) and subject to revision; it is presented here to facilitate a timely release of pertinent information.
Rock Fall in Provo, May 12, 2005
By Richard Giraud
At around 5:00 p.m. on May 12, 2005, a rock fall impacted and destroyed a guest house at 1468 North 1550 East in Provo. Luckily, no one was home at the time.
The rock impacted the southwest corner of the guest house and came to rest against a conifer tree in the front yard. The structure is likely a total loss.
The rock measures approximately 7×5.1×4.5 feet and weighs approximately 13 tons. Many other fresh rocks from the same fall were also present in the area, indicating that either multiple rocks were involved in the rock fall or the main rock broke up during its fall down the slope.
The rock fall occurred shortly after a significant storm on May 10-12 that dropped a total of 3.7 inches of mixed rain and snow at the Cascade Mountain Snotel site, which is 3 miles southeast of the starting zone. It was raining lightly at the time of the rock fall.
Impact craters (bounce marks) were evident on the 20 degree slope above the house. The source was a cliff band in the Mississippian Deseret Limestone on “Y” Mountain about 2,600 feet above the house.
The rock(s) traveled a total slope distance of over a mile (about 5,500 feet) and probably achieved high velocity and bounce height as it traveled down the slope.
The rock was bouncing just above the guest house and had almost stopped when it rolled into the house foundation wall.
The average slope from the rock-fall source to the rock’s resting place is about 28.5 degrees. The lower slope is mostly colluvium (material that fell or was washed onto the slope) and the upper slope below the cliff band is talus (rock debris derived from former rock falls).
The source area for this rock fall is one of the upper cliffs, but plentiful source-area cliffs extend throughout the area to the top of the ridge above.
Abundant rock-fall litter among the homes in the area indicates a relatively high rock-fall hazard.
Although the occurrence of this rock fall does not necessarily indicate a heightened rock-fall hazard above what normally exits under present conditions, rock falls are possible in this area at any time and typically occur with no warning, often during and following storms, periods of snowmelt, and earthquakes.
This report is preliminary (05/17/05) and subject to revision; it is presented here to facilitate a timely release of pertinent information.
April 9, 2006, 1650 East South Weber Landslide, Davis County
By Richard Giraud, Greg McDonald, and Gary Christenson
Around 9:30 p.m. on Sunday evening, April 9, 2006, a hillside in South Weber above a house at 7687 S. 1650 E. failed, flowed over the recently covered Davis-Weber Canal at the base of the slope, and impacted the back of the house below the canal.
The failure originated at the top of the hill and flowed rapidly down the slope, crossing a dirt road, the canal, and a rock wall at the back of the lot before hitting the house, causing significant damage and injuring a child inside. Because water has not yet been turned into the canal for the irrigation season, and it has recently been covered through this area, obstruction to flow in the canal by the landslide was not an issue.
The slope that failed above South Weber has experienced other recent historical failures. The slope was formed as Lake Bonneville receded and the Weber River cut down through the old Weber River delta that had built into Lake Bonneville 16,000-18,000 years ago. The relatively fine-grained material in the delta is prone to landsliding, and slopes in this area on both sides of the Weber River Valley in South Weber and Washington Terrace have failed repeatedly in the past.
The relatively long and rapid runout of the April 9 landslide, and last year’s February 20, 2005, 435 East South Weber landslide just 2 miles to the west, indicate the risks involved in building close to the base of these slopes.
The upper part of the slope has been modified over the years by gravel-pit operations on the bluff top, about 200 vertical feet above the subdivision, and loose material had been pushed onto the slope and an embankment built to prevent water from flowing over the crest and down the slope.
The landslide was a rapid earth flow that occurred in a steep (more than 50%) part of the slope, and much of the material in the initial failure was human-placed fill. The landslide is about 80 feet wide and 600 feet long.
The head of the slide is about 75 feet from a pond in gravel pits on the bluff top. Adjacent to the landslide near its head we found “piping” holes that indicate active underground seepage. These piping holes indicate that saturation and seepage were occurring through the embankment and underlying soils, mostly fill, near the head of the slide, probably causing local instability and ultimately the landslide.
The pond is being drained to lower the ground-water table at the top of the slope to prevent saturation of the slope crest. However, vegetation types at the bluff top, particularly large, old cottonwood trees, indicate that shallow ground water is a common occurrence in much of the area. Test pits excavated on April 10 at the top of the slope encountered shallow ground water, in places less than 10 feet deep, perched on clay layers beneath the gravel beds capping the bluff.
Much of northern Utah is experiencing a second consecutive wet year, following last year’s very active spring when the UGS recorded over 100 landslides, including the425 East South Weber landslide on February 20, 2005.
The UGS monitors ground-water levels in other landslides in northern Davis County, and we’ve noted a significant rise in ground-water levels between mid-March and April 10, probably in part a result of the significant snow and rain that fell on April 6.
This report is preliminary and subject to revision.
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