Geologic, Hydrologic, and Urban Hazards for Design in Desert Environments - Design with the Desert: Conservation and Sustainable Development - page 103

hazard. These maps clearly show that seismic hazard in the arid and semiarid areas of the
western United States is highly localized, and some areas (i.e., west of Phoenix, Figure 5.8)
have essentially no seismic hazard, while others (e.g., Yuma, Arizona) have a high seismic
hazard. As previously noted, the details of soils, geology, and substrate determine local
responses to seismic shaking, and these factors generally are available to designers through
soil engineering reports that may be required in the zoning process.
5.6 Other Geologic Hazards
5.6.1 Radon
Radon is an urban hazards issue because this noble gas, which can cause lung cancer,
can accumulate in poorly ventilated houses and buildings, particularly in basements.*
Radon is one of the daughter products of the radioactive decay of uranium, and its dis-
tribution in Arizona, as one example, is related to certain specific lithologies that underly
urban areas. † This noble gas enters buildings through cracks in floors and foundations or
through release from water, and its presence can be mitigated by proper ventilation or pre-
vented by eliminating cracks in foundations or other points where gas may seep from soils
or rock into occupied areas. In Arizona, as in other desert regions, the problem of radon
may be relatively low 44 owing to a climate that encourages higher ventilation of structures.
5.6.2 Expansive Soils
Expansive soils are a problem throughout the United States, and some assessments assert
that damages from expansive soils exceed all other geologic hazards combined. ‡ Although
these soils are common in arid regions, the problem is not as extensive as in other regions.
Expansive soils typically result from soil-forming processes that create expansive clays, §
although some of the larger problems in Arizona result from shales or marine origin that
contain multilayer clays, most commonly smectites. Shrinking and swelling of soils in
response to rainfall, irrigation, or water leaks cause buckling of roadways, sidewalks, and
other infrastructure and are the most common cause of cracked floors, foundations, and
walls in arid regions not subject to subsidence or frequent earthquakes.
5.7 Urban and Man-Made Hazards
Until this point, we have discussed the hazards occurring from forces of nature to the
environment as a result of an imbalance in a natural system or state. In many cases, human
activities can cause hazards of nature by disrupting the natural hydrologic, geologic, or
climatic processes. For example, the removal of upland vegetation by fire, construction, or
other human activities can lead to downstream flooding because runoff, unimpeded by
* http://www.epa.gov/radon/ (accessed June 23, 2010).
† http://www.azgs.az.gov/HomeOwners-OCR/radongasinarizona.pdf (accessed June 23, 2010). 44
‡ http://geology.com/articles/expansive-soil.shtml (accessed June 23, 2010).
§ http://www.azgs.az.gov/hazards_problemsoils.shtml (accessed June 23, 2010).