Geologic, Hydrologic, and Urban Hazards for Design in Desert Environments - Design with the Desert: Conservation and Sustainable Development - page 95

in the late 1970s through the mid-1990s, these floods did not occur in southern Utah,
southern and central Nevada, or the Mojave Desert.
The primary lesson from the regional flooding of the1970s and 1980s in central and south-
ern Arizona is that lateral channel change is the dominant flood hazard, 25 not overbank
inundation as is regulated under the National Flood Insurance Program. To minimize lat-
eral channel change, several types of bank protection were installed through metropolitan
or other areas where lateral channel shifts would cause significant damage to infrastruc-
ture (such as at bridge crossings). For example, in 1982, soil cement—a weak form of con-
crete created by mixing a small amount of cement with ambient channel sediments—was
installed on both channel banks along the Santa Cruz River through Tucson. Combined
with grade-control structures to minimize the potential for channel downcutting, these
flood-control measures effectively minimized damages in treated reaches during large
floods in 1983 and 1993 except at the beginning and end points of bank protection. Similar
types of continuous bank protection are used throughout the Phoenix metropolitan area
and other urban areas in the region.
Nonstationarity in flood frequency can have severe economic impacts. In the case of the
Santa Cruz River, bank protection was found to be necessary through much of the Tucson
metropolitan area in response to the large floods from 1977 through 1993. If design of that
bank protection was completed in 1970, when the 100 year flood was 20,870 ft 3 /s, the floods
of 1983 and 1993 (peak discharges of 52,700 and 37,400 ft 3 /s, respectively) likely would not
have been contained within the engineered channel. Conversely, if the high flood peaks of
1977-1993 do not continue (Figure 5.2B), the flood protection along the channel may have
been too expensive for the level of protection required under the National Flood Insurance
Program. The central question of flood-hazard mitigation is whether it is a risk-based sys-
tem (i.e., floodplain regulation and design based on a 100 year flood or other return period)
or whether it is a hazard-avoidance system with minimal risk (i.e., overdesigned flood-
plai n struct ures).
Installation of bank protection along an engineered channel is by no means the end of
flood-hazard mitigation, as illustrated by the Santa Cruz River at Tucson. On August 23,
2005, thunderstorms in the early morning hours caused a substantial rise in the normally dry
channel. Based on water height within the engineered and soil-cemented channel, observ-
ers believed that the flood had a discharge of greater than 40,000 ft 3 /s, which would make
it the second largest flood in recorded history, and emergency responders began to close
bridges and issue alerts. Careful measurements established the peak discharge at 16,300
ft 3 is which is high but not extraordinary for this river. The combination of sedimentation
and establishment of riparian vegetation in the engineered channel had reduced flood con-
veyance, thereby increasing the stage for a given discharge and increasing flood hazard.
One of the key challenges to design of channels is the trade-off of flood-hazard miti-
gation in a watercourse that is mostly dry with the public expectation and environmen-
tal value of riparian corridors through urban environments. If the objective solely is
hazard mitigation, then engineered channels with stabilized banks and periodic main-
tenance to remove vegetation and sediment buildup is appropriate, but these chan-
nels are viewed as ugly and maintenance of them is considered to be environmental
degradation. However, if expectations of environmental quality in river channels are
desirable, then engineering design must account for the changing amounts of riparian
vegetation and its influence on sediment aggradation between the stabilized banks.
One way that this is accomplished in Arizona is creating an abnormally wide chan-
nel with low-flow controls on lateral channel change and high-flow areas that sustain
riparian vegetation.