Debris Flow Triggered on July 28, 1999 – Interstate I-70 Corridor
Debris flows triggered by the rainstorm of July 28, 1999, Interstate-70 corridor, Georgetown to the Eisenhower Tunnel, central Colorado
Warning—Provisional report, subject to revision
On July 28, 1999, about 80 debris flows were triggered by an afternoon rainstorm along Interstate 70 (I-70) between Georgetown and the Eisenhower Tunnel in central Colorado (fig. 1).
The rainstorm was embedded in a period of prolonged flow of monsoonal moisture which lasted from about July 26 - August 2, 1999. Early indications are that in excess of 3 inches of rain fell during the storm. The debris flows, therefore, were triggered by intense rainfall on saturated colluvium and talus that overlies bedrock on steep mountain slopes.
Debris flows occurred in two primary areas along this portion of the I-70 corridor: 1) just north of Georgetown along the lower portion of the east-facing flank of Columbia and Democrat Mountains, and 2) in the vicinity of Loveland Pass along and near the Continental Divide. The Georgetown debris flows occurred directly above the highway on a hillslope that has had multiple debris flow events in the last 20 years. Debris flows occurred in about 20 channels on this hillslope during the July 28 storm. The volume of debris from these flows was relatively small (generally less than 100 cubic meters). About five of these flows deposited debris on I-70.
Debris flows in the vicinity of Loveland Pass occurred in a north-south trending area about 17 km long and 6 km wide. The area extends from the Arapahoe Basin ski area (fig. 2), south of I-70, to the north flank of Woods Mountain (fig. 3), just north of I-70. All debris flows in this area were initiated on steep slopes (30-40 degrees) at elevations above 3,350 meters (11,000 feet). The largest of the debris flows occurred in Watrous Gulch on the south flank of Mount Parnassus elevation 4,137m, 13,574 feet (fig. 4).
This debris flow deposited an estimated 100,000 cubic meters of debris, up to about 7 m deep and with boulders up to 2 m in diameter, on and near I-70, closing the interstate for about 24 hours.
On July 29 and August 4, USGS scientists Al Chleborad, Jeff Coe, Jonathan Godt, Ed Harp, Bill Savage, and Pierre Tachker inspected debris flows in the Georgetown and Loveland Pass areas. Particular attention was given to the Watrous Gulch debris flow because of its size and location above I-70.
The Watrous Gulch Debris Flow
The Watrous Gulch debris flow coalesced in a single channel from smaller debris flows that began in, or at the heads of, about 6 channels incised in talus at about 3,960 m (13,000 ft). These 6 channels are best described as large rills, or erosional gullies. There were no discrete, large landslides at the heads of any of the channels. Material at the heads of the channels is angular and ranges from sand to small boulder size (fig. 5).
There is essentially no fine-grained material (clay or silt) at the heads of the channels. Some of the channels were newly created by the storm and others appear to have existed prior to the storm those that were newly created were located beneath two large snow fields located on an east-facing aspect of the south-facing slope (fig. 6).
These channels were incised less than 1 meter and were probably established by rainfall, snowmelt, and runoff from the snowfields.
Previously existing channels, which were widened, deepened, and lengthened by the storm, were located on south- and west-facing aspects of the slope and did not have snowfields above them. The maximum dimensions of the largest of these channels were about 4.9 meters (16 feet) deep and 9.8 meters (32 feet) wide (fig. 7).
The lack of large landslides at the heads of any of the channels indicates that the debris flows were mobilized by the progressive entrainment of material by running water along the bottoms of the channels. Matrix supported levees and debris lobes exist along the unincised flanks of these channels (fig. 8 & fig. 9) from about 3,930 meters (12,900 feet) down to the fan at I-70 at about 3,090 meters (10,150 feet).
The presence of these levees and lobes suggest that debris flows were initiated at or near the heads of the channels and traveled about 2.6 km (1.6 miles) to the fan at I-70. The position of the levees and lobes well above the incised base of the channels (fig. 8 & fig. 9) suggests that the debris flows occurred early in the cycle of channel incision and were either responsible for, or were followed by significant downcutting. It is unclear whether downcutting was accomplished by the debris flows themselves, or by water-dominated flow after the debris flows had passed. Video footage of moving debris at the fan at the base of Watrous Gulch (Rob McClure, News4, Denver) suggests that the later scenario is most feasible. The video shows a tumbling mass of mud and boulders being pushed by a torrent of muddy water.
The material deposited on the fan at I-70 is composed of small-to-large boulders in a silty-sand matrix (fig. 10).
This material is derived exclusively from channel incision along the entire length of the flow. Previously existing deposits at the head of the fan have been incised to a depth of about 6 meters by the channel (fig. 11).
Channel incision in the middle and lower portions of Watrous Gulch averaged about 4 meters and was generally down to the bedrock surface (fig. 12).
The materials incised in the middle and lower portions of the flow were of glacial and debris flow origin. Some of these deposits were fine-grained (fig. 13).
Boulders and cobbles in the deposits generally became more rounded with decreasing elevation. The debris flow deposit at the fan and I-70 therefore, had a finer-grained matrix and more rounded clasts than did the debris flow levees along the upper channels.
Because of the significant incision, channel walls along the debris flow path are oversteepened. Landslides along the channel walls will occur in the near future. These landslides will supply material to the bottom of the channel. This material could subsequently mobilize as a debris flow(s) given adequate precipitation and runoff.
Figure 1
Figure 1 Map showing the location of July 28 debris flows.
Figure 2
Figure 2. Debris flows in and above the Arapahoe Basin Ski area. Photo taken 7/29/99 by E. Harp.
Figure 3
Figure 3 Debris flow deposit at the base of the northern flank of Woods Mountain. See geologist for scale. Photo taken 8/3/99 by J.A. Coe.
Figure 4
Figure 4 Debris flow tracks on the southern flank of Mount Parnassus. Photo taken 7/29/99 by J.A. Coe.
Figure 5
Figure 5 Talus at the head of the largest incised channel at about 13,000 ft. on the south flank of Mount Parnassus. See gallon sized sample bag for scale. Photo taken 8/4/99 by J.A. Coe.
Figure 6
Figure 6 Snowfield and channels on southeast-facing aspect of the south flank of Mount Parnassus. Snowfield is about 40m wide. Photo taken 8/4/99 by J.A. Coe.
Figure 7
Figure 7a Incised channels (rills) in the debris flow source area on the south flank of Mount Parnassus. Relief shown is about 300m. Photo taken 8/4/99 by J.A. Coe.
Figure 7b Nearly vertical face of largest incised channel in source area. See geologist for scale. Interstate 70 in background. Photo taken 8/4/99 by J.A. Coe.
Figure 8
Figure 8 View looking up (northeast) from the bottom of the largest channel on the south flank of Mount Parnassus. See geologist for scale. Photo taken 8/4/99 by J.A. Coe.
Figure 9
Figure 9 Hanging channel of Watrous Gulch at intersection with unnamed tributary in which the debris flow occurred. View is to the northwest. Note matrix supported levees and about 2 m of incision below base of Watrous Gulch channel. Watrous Gulch above this point did not contribute material to the debris flow. Photo taken 8/4/99 by J.A. Coe.
Figure 10
Figure 10 Fresh debris fan deposit just above (north of) Interstate 70. Photo taken 7/29/99 by J.A. Coe.
Figure 11
Figure 11 Incision of old deposits at the head of the fan. Photo taken 7/29/99 by J.A. Coe.
Figure 12
Figure 12 Channel incision in lower portion of Watrous Gulch. Note size of boulders in deposits and that channel is incised to bedrock. Photo taken 7/29/99 by J.A. Coe.
Figure 13
Figure 13 Fine-grained sediments in channel exposure in middle portion of Watrous Gulch. Photo taken 7/29/99 by J.A. Coe.
Contact Information
| Jeff Coe 303-273-8606 U.S. Geological Survey Golden, Colorado |
