Description:
Flooding issues and drainage problems associated with historical development on four active alluvial fan
study sites in central Arizona were examined to document the effectiveness of engineered flood protection
measures and floodplain management policies. The study sites are located in the metropolitan Phoenix
area and include (1) Ahwatukee-City of Phoenix, (2) Pima Canyon-City of Phoenix/Guadalupe, (3) Reata
Pass-Scottsdale, and (4) Lost Dog-Scottsdale. The four study sites have experienced different types of
urbanization, including master-planned communities, single lot residential development, public
transportation and utility networks, and major engineered drainage structures such as channels, detention
basins, culverts, and dams. The engineered drainage systems at the four historical alluvial fan study sites
have performed adequately during the 30-year period of record, at least with respect to controlling the
flow path uncertainty and sedimentation normally associated with active alluvial fans. Significantly, no
homes have been damaged by alluvial fan flooding at any of the study sites, and no avulsions have
occurred in the developed portions of the alluvial fans. Two floods exceeding the 100-year design storm
occurred on two of the fans, but many of the flood control measures on the other fan sites remain untested
by large floods. The absence of flood damages is likely due to lack of debris flow potential at any of the
sites, low rates of sediment yield at the fan sites, channelization and encroachment that increase sediment
transport off the fan surface, and to some degree, the relatively short period of record since development
first occurred.