Geologic, Hydrologic, and Urban Hazards for Design in Desert Environments - Design with the Desert: Conservation and Sustainable Development - page 97

a different problem where desert cities abutt steep mountain ranges that accumulate
significant snowpack in winter.
Debris flow is a second type of flood hazard recognized by the National Flood Insurance
Program 6 that occurs in arid and semiarid areas in the southwestern United States
(Figure 5.4). This type of hazard typically is restricted to mountainous areas, bedrock can-
yons, and the heads of alluvial fans and typically does not occur on master streams that
pass through urban centers. Numerous historical debris flows have occurred in the arid
Southwest, but perhaps the most famous is the flood of September 14, 1974, in El Dorado
Canyon, an ephemeral stream draining an arid part of the Mojave Desert leading into Lake
Mohave on the Arizona-California border. This debris flow killed 9 people and destroyed
at least 5 homes, 38 vehicles, and most of a marina on Lake Mohave. 28
Debris flows are two-phase slurries of water and sediment with the consistency of wet
concrete—albeit concrete that contains extremely large boulders—that initiate during
intense rainfall on steep terrain. Grand Canyon, the epitome of steep terrain subjected to
intense rainfall, has one of the highest frequencies of debris flows in the region with five
occurring in an average year throughout the canyon. 29 On the Colorado Plateau, debris-
flow frequency is highest where fine-grained rock known as shales occurs in steep cliffs. 30
Although debris flows are known from a variety of geologic terranes in the Sonoran Desert,
they are most common from granitic mountains, such as the Santa Catalina Mountains
north of Tucson. 31 Debris flows can destroy roads, bridges, and houses, but their influence
on filling channels with coarse sediment, and thereby decreasing flow capacity for future
floods, is perhaps their most important hazard.
Growth of housing developments on the alluvial fans skirting the mountains of
the southwestern United States portend increased future risk from debris flows and
underscore the need for new tools for floodplain management. Along the northern edge
of Tucson (Figure 5.1), debris-flow deposition on the apices of alluvial fans primarily is
of Pleistocene age, but areas with significant recent or Holocene debris-flow deposits
have been identified.32 32 Modeling of debris-flows deposition, which would be needed
to quantitatively map hazard areas, is challenging and inaccurate over complex
topography. One of the better ways of predicting mobility and inundation potential of
debris flows is to use stochastic modeling. One such model, called LAHARZ, 33,34 requires
the assumption of a simplified debris-flow event consisting of initiation, transport, and
deposition zones for each event as well as a high-resolution digital-elevation model of
the deposition zone.
An example of debris-flow deposition mapping as estimated using LAHARZ appears
in Figure 5.5. Using a range of volumes of sediment mobilized during intense rainfall, a
range of depositional areas is estimated at the apex of an alluvial fan. While this technique
creates a map of debris-flow deposition, and therefore could possibly be used to regulate
floodplain development, it does not provide information on risk because no return-period
information is used. Debris-flow frequency little known and is not measured at gaging
stations, unlike flood frequency, and estimation of risk from debris flows requires more
information than is available at present. The central question for debris-flow hazard is
whether hazard avoidance consisting of restrictions on construction along channels with
a past history of debris-flow occurrence is preferable to a risk-based system where the
amount of risk is unknown or poorly known.
Debris flows pose another, perhaps greater, risk downslope from where the slurry
stops and streamflow drains the remainder of the runoff. As with flood hazards, channel
conveyance is assumed to be unchanging from its original design characteristics, and
debris-flow deposition can greatly decrease conveyance and increase overbank flood