Environmental Engineering Reference
In-Depth Information
city as a dry riverbed
(Figure 8.15),
except for short periods when it carries runoff from
flash floods, and the basin is semidesert with sparse vegetation.
Extraction of large quantities of groundwater from the basin for farming, the mining
industry, and the growing population has caused the water table to drop by as much as
130 ft (40 m) since 1947 alone. Human activities have disturbed nature's balance and cre-
ated desert conditions and other associated problems, including increased erosion, flood-
ing, land subsidence, and surface faulting.
Study Purpose and Scope
The Pima County Association of Governments recognized the need to establish guidelines
for water conservation and flood protection in the valley, and for the planning and devel-
opment of the essentially virgin area of 200 mi
2
known as Canada del Oro, adjacent to the
city on the lower slopes of the mountains (see high-altitude stereo photos,
Figure 2.11).
The association engaged a consultant to perform an environmental protection study.
The study scope included evaluations of the following aspects:
Groundwater and surface water resources, the principal groundwater recharge
areas, and their maintenance and protection
●
Minimization of flooding and erosion
●
Suitability of the soil-water regime for liquid and solid waste disposal
●
Potential for severe areal subsidence from increasing groundwater withdrawal
●
Maintenance of the vegetation and wildlife regimes
●
Foundation conditions for structures as affected by subsidence faulting
●
Extraction of construction materials and their consequences, especially sand and
gravel operations in the washes, which were increasing the erosion hazard along
the banks and bridge foundations
●
Master Planning Concepts Developed for Canada del Oro
A geologic constraint map, given in
Figure 8.58,
was prepared of the Canada del Oro area.
The map was based on information obtained from the interpretation of stereo-pairs of both
high- and low-altitude aerial photos, ground reconnaissance, and a review of the existing
literature, including the soil association map for the area prepared by the Soil
Conservation Service. With the map as the database, a number of master planning con-
cepts were developed.
For the Canada del Oro area it was recommended that the major washes and floodplains
be zoned to prohibit any development, except that accessory to recreational uses. In addi-
tion to being flood-prone areas, they are the major source of recharge for much of the
basin. (In some semiarid regions, recharge is being accomplished by deep-well injection of
water stored temporarily in flood-holding reservoirs to avoid high losses to evaporation;
ENR, 1980).
Water conservation requires a reduction in ostentatious uses, such as watering lawns
and swimming pools by discouraging their use with substantial tax premiums.
Landscaping with desert vegetation, for example, will help to conserve water.
Residential development should be located to avoid blocking the larger of the natural
drainage ways that traverse the area as sheet wash gullies (see
Figure 2.11)
as shown on the
tentative land-use map given
Figure 8.59.
Roadways should be planned to parallel the nat-
ural drainage ways with a minimum of crossovers
(
Figure 8.16)
or blocking. Housing con-
structed on a cluster or high-density basis will minimize site grading, roadways, and other
disturbances to the surface, and reduce the costs of waste disposal. Extensive site grading
