PRELIMINARY EVALUATION OF THE FIRE-RELATED DEBRIS FLOWS ON STORM KING MOUNTAIN, GLENWOOD SPRINGS, COLORADO
REMAINING HAZARDS AND STEPS NECESSARY TO EVALUATE THEIR IMPACT
A number of different scenarios exist for the hazards posed by the current conditions at Storm King Mountain.
These are discussed below in order of perceived probability of occurrence. In addition, steps to evaluate the potential
impact of the first, and most likely, hazard (debris flows from the mobilization of channel alluvium and hillside
materials), are described. Further, steps to assess the potential for the occurrence of the following two, less
likely, hazards (debris flows from the mobilization of the colluvial wedge and reactivation or headward erosion
of the old landslide deposits), are outlined.
Debris Flows from Mobilization of Channel Alluvium and Hillside Materials
Although the net result of the September rainstorm in the burned drainages on Storm King Mountain was to remove
the most easily eroded material from the channels and hillsides, a considerable hazard from such intense rainfall
events still remains. In the months following the September events, considerable incision into the channel alluvium
was observed, in some places up to 8 ft. Incision and entrainment of channel alluvium has been cited by several
investigators as a debris-flow sediment source (e.g., Beaty, 1963; Scott, 1971), and abundant evidence for bulking
of flood discharges with channel alluvium and subsequent transformation to debris-flow conditions was observed
in lahar events following the 1980 Mount St. Helens eruptions (Scott, 1988). More specifically, work by Florsheim
et al. (1991) in a southern California watershed that experienced a wildfire shows that the initial sedimentation
events involving the dry ravel material in the channels was followed the next year by debris flows containing material
that was eroded from the channel itself. In addition, there is still abundant loose material left on the hillsides
that, given another intense storm, could be delivered into the channels and then mobilize into debris or hyperconcentrated
flows. Considerable rainfall and resulting surface flow would be necessary to mobilize hyperconcentrated or debris
flows from the existing channel alluvium, and the volume of material that would be involved in such an event, or
that would reach the canyon mouths in any given rainfall event, is unknown at this time. Further, the volume of
material that would be eroded from the hillsides and transferred to the canyon mouths by debris flow in any given
storm is also unknown.The following steps are recommended to fully assess the impact of this hazard:
- Determine the volume of material available for mobilization in the channels and hillsides.
- Characterize the erosion susceptibility of the materials in the channels and on the hillsides relative to rainfall
events of varying intensities and durations. Determine where within the topography failure is likely to occur and
at what rates.
- Develop a model for debris-flow processes that can be used to simulate debris-flow travel through a digital
elevation model of the area. Start with existing sediment routing models (e.g. McEwen, 1989; O'Brien, 1985), and
modify to reflect the factors evaluated above. Such a model could potentially predict paths and flow rates within
channels in the watershed.
Debris Flows from Mobilization of Colluvial Wedge
A further hazard exists from incision into channel alluvium in the form of the destabilization of the colluvial
wedges located in drainage B. If sufficient channel downcutting occurs, and the entire mass fails catastrophically
into the channel, approximately 403,000 yds³ of material would be available to be mobilized from drainage
A and 323,000 yds³ of material from drainage B. Again, at this time it is not known how much of the wedges
would fail, or how much of the material would be mobilized by high surface flows, how much of this material would
then reach the canyon mouth, or under what rainfall conditions this would occur. The following steps are recommended
to evaluate the potential for the occurrence and the impact of this hazard:
- Determine rates of channel incision under different rainfall and surface flow conditions. If incision is proceeding
at unprecedented rates, then the following procedures are recommended in order to asses the impact of this hazard.
- Determine both the failure susceptibility of the colluvial wedges and the erosion susceptibility of the materials
once deposited in the channel, again relative to rainfall events of varying intensities and durations. This information
could then be incorporated into the model for debris flow described above to determine flow rates and volumes within
the channel and at the canyon mouths.
Reactivation or Headward Erosion into Old Landslide Deposits
Some possibility exists that the removal of the vegetation by the fire may result in the destabilization of the
old landslide deposits. Destabilization could occur in response to two processes: the headward erosion of the canyons
at the base of the landslide, and dissection of the surface of the landslide deposits by deepening gullies. Examination
of the aerial photographs indicates that a few steep drainages are superimposed on the lower reaches of the landslide
deposits. Some drainage continues from the landslide deposits down into the heads of the canyons. If accelerated
erosion in a very intense and long duration storm or storms should occur, the headwalls of these canyons could
continue to retreat upslope, leading to the destabilization of the landslide mass by removal of downslope support.
In addition, if the conditions of increased surface flow, brought about by the removal of vegetation, were to result
in the formation of numerous very deep gullies into the surface of the landslide deposits, the breakup of the landslide
mass and its eventual destabilization by removal of lateral support might occur. It is unlikely that the entire
mass of the landslide deposits would be destabilized at once, and the time frame and rainfall conditions under
which this process would occur are unknown. It is further not known if such a destabilization would result in the
mobilization of the material into debris flows, or as translational failure of the smaller, broken up blocks. And
again, it is not known how much material from such a failure would reach the canyon mouths. The following steps
are recommended to evaluate the potential for the occurrence of this hazard and to preliminarily appraise its impact:
- Determine both the rates of headward erosion of the canyons into the foot of the old landslide deposits and
rates of gully incision into the surface under varying rainfall conditions. If either of these two processes appear
to be proceeding at unprecedented rates, the following steps are recommended:
- Carefully map and determine the complete aerial extent of the landslide deposits and the depth to the failure
surface.
- Determine the location and volume of unstable blocks through careful and detailed evaluation of movement kinematics.
Asses the stability of these blocks in terms of both failure and mobilization mechanisms.
- If analyses indicate the unstable blocks may mobilize into debris flows, evaluate flow behavior and travel-distance
potential using the model for debris-flow processes described above. Note that careful and detailed field mapping
on accurate large-scale maps is essential to all of the above objectives.
SUGGESTIONS FOR IMMEDIATE RESPONSE
We suggest the following actions as an immediate response to the potential hazards described above. Note that these
actions do not mitigate the hazards, and the steps outlined above are necessary to properly evaluate their potential
impact. Monitor conditions at Storm King Mountain throughout the spring snowmelt and thunderstorm season for a
number of years. Repeated field examination is necessary to determine if headward erosion of steep canyons into
the old landslide deposits is occurring, if gullies are forming and deepening on the surface of the landslide deposits
at unprecedented rates, and if the colluvial deposits are being undercut. Install an automated weather station
on Storm King Mountain to track approaching storms. The station needs to be able to detect storms of the appropriate
scale. It will also be necessary to define those weather conditions that present a hazard in order for a warning
to be issued. Weather information for the September event was insufficient to draw any conclusions from, but ongoing
observations of the occurrence of sedimentation events following storms could eventually lead to the qualitative
definition of a rainfall threshold for hazardous conditions. Consider installing an early-warning device across
susceptible canyons. The motion of a large flow could trigger the device, allowing either warning signs to be activated,
or the interstate to be closed.
