Arizona Geological Survey OPEN-FILE REPORT OFR 08-06

Arizona Geological Survey
The term “mud flow” is sometimes used synonymously with “debris flow” (e.g. Federal Emergency
Management Agency, 2008), or to describe a flow with 0‐20% coarse clasts and a soft, remoldable clay
matrix with high plasticity and liquid limit values (Hungr, 2005). However, the term “mud” has geologic
and compositional implications, which is why Pierson and Costa (1987) recommend disregarding the
term. Current research describes debris flows as two‐phased, non‐Newtonian flows dominated by
coarse‐particle interactions (Iverson, 1997). Thus, use of the term “mud flow” is confusing and
unnecessary.
Debris flows can be triggered by different types and sizes of slope failures, often referred to as
landslides. Some researchers use the term landslide to describe large, deep‐seated rotational or
translational mass movements (Glenn and others, 2006), while other researchers apply no scale to the
term and use it for all sizes and types of slope failures (Terranova and others, 2007). Shallow landslides
are the most common trigger of debris flows (Iverson and others, 1997), especially in areas that have
not been subject to recent fire. Generally, the term landslide‐induced debris flow is used to describe a
shallow translational failure of thin soil over bedrock that liquefies and transforms into a debris flow
(Iverson and others, 1997; Cannon and others, 2002; Santi and others, 2007). This description applies to
the 2006 debris flows in the Santa Catalina Mountains.
Debris flows have three zones ‐ initiation, transport and deposition ‐ where different processes occur
(Hungr, 2005) (Figure 2). Initiation zones are where slope failures occur, usually identified by distinct
head scarps, and debris flows are generated. Once initiated, debris flows travel downslope through
existing channels. The character of a debris flow changes in time and space as it travels downslope.
Debris flows commonly move in surges led by a coarse‐boulder front (head), followed by a liquefied
slurry (body), and a more watery tail which is often a hyperconcentrated flow (Hungr, 2005). Levees,
which confine the flow, may be deposited anywhere along the channel due to longitudinal sorting
(Hungr, 2005), but are most obvious in areas with less lateral topographic confinement. Debris‐flow
volumes can change significantly during downslope movement as scouring or deposition occurs (Iverson
and Vallance, 2001). Debris‐flow deposition occurs in areas where lateral confinement decreases and/or
channel slope decreases. Depositional areas are often alluvial fans located at the mouths of drainages.
Initiation
Hydrological conditions that can trigger debris flows include prolonged or intense convective rainfall
(Webb and others, 2005), rain‐on‐snow events (Meyer and others, 2001; Lenzi, 2006), or rainfall on
recently burned areas (Cannon and Gartner, 2005). In Arizona, debris flows have been generated by
dissipating tropical storms, prolonged winter storms, intense convective summer storms, and postwildfire
summer storms. Debris flows usually initiate on steep slopes between 20 o and 45 o (Hungr, 2005)
but have initiated on slopes as low as 11° (Lenzi, 2006). Nearly all of the 2006 debris flows in the Santa
Catalina Mountains initiated on steep slopes (Webb and others, 2008).
Debris flows in undisturbed (unburned) areas are triggered when soils become saturated and
destabilize. Soil pore pressure in saturated soils increases and shear strength decreases to a critical point
at which failure occurs, rapidly mobilizing the soil mass into a viscous slurry through liquefaction or
dilatancy (Costa, 1984). As described above, the initial failure can be rotational or translational (Costa,
1984). Debris flows can also form in channels when sediments are mobilized by runoff (Costa, 1984).
Debris flows formed by mobilized channel sediment, called progressive sediment bulking, have most
often been noted following wildfires (Wohl and Pearthree, 1991; Cannon, 2001; Santi and others, 2008).
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