Geologic, Hydrologic, and Urban Hazards for Design in Desert Environments - Design with the Desert: Conservation and Sustainable Development - page 90

effect is that considerable regulatory authority is in place to manage and/or guide urban
design in this region.
All hazard design ultimately is based on the concept of risk, or, more explicitly, accept-
able risk. Clearly, we do not know the size of the absolute maximum flood that may
impact a river basin or can we predict the largest possible earthquake that might occur
in a given place; furthermore, elimination of risk from a given hazard would be prohibi-
tively expensive in design and construction. Specific hazards are regulated under the
concept of acceptable risk; for example, the acceptable risk for flood hazard is greater
than the 100-year flood for floodplain construction or infrastructure, or, for critical
infrastructure, greater than the 500- or 1000-year flood.1 1 Understanding the level of risk
in regard to geologic and hydrologic hazards is critical to design and sustainability of
urban landscapes.
This chapter uses examples of hazards from the southwestern United States with a
focus on the authors' experiences in Arizona to illustrate the array of hazards that may
be present and affect urban and landscape design. Here, we discuss the regional risk for
a variety of geologic, hydrologic, and urban hazards, focusing on the ones with greatest
regional impact, and we refer readers to other publications by urban designers for unique
ways to mitigate against these hazards.* We emphasize that while there may be common-
ality in hazards that affect floodplain management, urban design, and hazard mitigation,
in most cases, local approaches based on scientific or engineering analyses of the hazard
threat may be more important regionally than the typical “one-size-fits-all” approach
used in the past.
5.2 Flood Hazards
Flooding on major rivers is the most common hydrologic hazard in arid and semiarid
regions of the southwestern United States, and societal measures to protect against flood
damage are readily apparent in the banks of most rivers. Nationwide, flood damage
increased through the twentieth century (Figure 5.1) † and is statistically related to increases
in multiday precipitation. 3 The reason for increasing amounts of flood damage involves a
complex interaction among societal pressures to develop river floodplains, climatic varia-
tion, or change that influences storm intensities and our ability to estimate flood hazards
and regulate flood-plain development.
All of the rivers in the southwestern United States have annual floods with discharges
significantly higher than typical flows, which is by definition zero for most of the year in
ephemeral channels (see Chapter 4). Although floods generally are considered as single-
phase flow (water), desert rivers are decidedly two-phase flow with significant entrained
sediment. Rivers with high-elevation headwaters, such as those crossing the Colorado
Plateau, the Humboldt and Truckee Rivers in Nevada, the Rio Grande in New Mexico,
and the Salt and Gila Rivers in Arizona, have annual flood peaks in response to snow melt
or rarely as rain falling on existing snow packs. Lower-elevation rivers, such as the Santa
* Numerous publications discuss current and future designs for hazard mitigation, including Burby 2 ; additional
planning documents are available from the National Association of Floodplain Managers, http://www.floods.
org/inex.asp?menuID=298&irstlevelmenuID=188&siteID=1 (accessed June 17, 2010).
† http://www.nws.noaa.gov/oh/hic/flood_stats/Flood_loss_time_series.shtml (accessed June 24, 2009).