New Mexico Geological Society Annual Spring Meeting — Abstracts

Debris flows, sometimes incorrectly described as mudflows, are a common and. in one case, fatal geologic process in the mountainous area surrounding Red River and Questa, New Mexico . Previous studies have shown that debris flow source areas are hydrothermally altered, pyrite-rich (< 3%), igneous rocks commonly underlying south-facing slopes steeper than 20°. Acidic pH values have been measured in seeps from debris flow channels, presumably as a consequence of the high pyrite content. It has been postulated that the debris tlows are mobilized from landslides; however. large rotational landslides that have not mobilized into debris flows are also common throughout the area.

A series of aerial photographs shows signs of continuing activity in the Bitter Creek drainage from the mid 1930s through the early 1990s, as well as vegetated and healed debris source areas and fans where activity has occurred in the past The most active debris flow complex in the Bitter Creek drainage consists of a cirque-like. heavily weathered debris source area, an incised debris channel. and a broad depositional debris fan Debris channel terraces marked by tree populations of different heights suggests a complicated recent history of alluvial and debris now processes, involving both incision and sedimentation. A longitudinal profile down the active channel shows no significant gradient change between the channel and the fan : thus, the channel probably serves to maintain the thickness and fluidity of debris flows and allows material to be transported a kilometer or more from the source area. Models of debris flow behavior based upon Bingham rheology suggest that once confinement is lost (or is never attained). the flows will spread laterally and basal shear stress will decrease to the point where the debris is transformed from a tluid into a rigid plastic layer. In the Bitter Creek drainage, channelization
appears as though it may be a necessary condition for moblization of landslides into debris flows because landslides and sediment sources areas that are not associated with constrictions do not seem to produce significant debris flow activity.

The practical hazard assessment question is, which areas will be susceptible to future debris now hazards? Areas characterized by soil protile development and the presence of mature forests can be considered relatively stable at the present time. although future debris flow activity cannot be ruled out unless the debris source area has been exhausted For example, debris flows have inundated mature forests on apparently stable portions of the currently active Bitter Creek fan. Anecdotal accounts also suggest that flows are most common during heavy summer rains Historical precipitation records show that summer rainfall is much less variable than total annual precipitation, so the debris flow hazard is likely to remain constant regardless of annual precipitation trends. Any systematic assessment of debris flow hazards in the area should take into account the mobilization potential of potential sources (contractive soils thicker than the critical Bingham thickess), the volume of potential sources. the channelization potential. and signs of recent activity and/or landscape stability.