By Patrick Dent, CAP Water Control Manager and Aaron Ashcroft, CAP Senior Civil Engineer
Earth fissures in Arizona aren’t new, but recently a two-mile long fissure in Pinal County made the news, bringing to light one of the consequences of extensive groundwater pumping.
Fissures like this can pose a risk to homes, buildings, roads and even the Central Arizona Project (CAP) system. Fortunately, CAP has never had service interrupted by fissures or subsidence, and takes many steps to ensure that continues.
A report entitled Land Subsidence and Earth Fissures in Arizona by the Arizona Land Subsidence Group describes subsidence and fissures as follows:
Land subsidence and earth fissure formation in Arizona are the result of substantial groundwater withdrawal from aquifers in sedimentary basins. Subsidence frequently results in bowl-shaped depressions, with loss of elevation greatest in the center and decreasing towards the perimeter. Earth fissures, the most spectacular manifestation of subsidence-related phenomena, occur about the margins of alluvial basins, near exposed or shallow buried bedrock, or over zones of changing alluvial characteristics in basins where differential land subsidence has occurred.
During the planning phase of CAP, a study of land subsidence and fissures was conducted to identify locations of subsidence and fissures along the CAP alignment. In areas with subsidence, the CAP Canal was designed with a greater depth of lining to allow for settling over time. In areas with fissures, the CAP Canal right-of-way was lined with steel sheet piles driven to adequate depths in the ground to prevent the fissures from advancing through the CAP Canal.
For more than a decade, CAP has participated with the Arizona Department of Water Resources and Arizona Geological Survey (AZGS) on subsidence and fissure mapping efforts in Central Arizona. CAP shares information, which is significant given the depth of detailed Information that was collected during the design phase of the project. CAP has provided and continues to provide funding for satellite imagery that has the amazing ability to identify small land surface elevation changes over time and identify subsidence areas over extremely large geographical areas. And finally, CAP monitors certain survey locations for vertical change and has many decades of historical data for a set of canal locations.
CAP routinely reviews this data to proactively identify potential areas of concern, avoiding unexpected impacts to the system and deliveries. When areas of concern are identified, CAP’s Reliability Engineering Department takes the lead.
Engineers identify the risk of a particular fissure, which depends on a number of factors associated with both the fissure itself and the geological and structural attributes of the canal. In situations where a fissure proposes a risk to the system, a geologic investigation is done and proposed engineering alternatives are developed. Actions may include ongoing monitoring of the fissure, remediation of local drainage or embankment stabilization to prevent water from entering the fissure, or in the most severe cases, an invasive canal structural fortification effort.
Anecdotally, Central Arizona’s best defense against these hazards is CAP! Renewable CAP water offsets groundwater pumping for cities, farms, and tribes and this reduction in groundwater overdraft has a direct positive impact on the continued expanse of fissures and subsidence.