Dry Dams in the Desert: Chasing Land Subsidence and Earth Fissures with Geophysics

Abstract

Homogenous embankment dam flood retention structures (FRSs) were built throughout central Arizona desert basins in the mid-twentieth century to protect mostly agricultural lands and military installations from rare flood events. Many of these FRSs eventually suffered transverse cracking due to desiccation of the embankment soils in the hot, dry climate. While this dam safety hazard has been recognized and mitigated, another hazard was developing under several dams. Differential land subsidence was gradually tilting some FRSs by several feet; ground cracks known as earth fissures have developed near two FRSs. These earth fissures are large (typically) tension cracks in basin alluvium with lengths of hundreds of feet to several miles and depths up to perhaps hundreds of feet. They can become conduits for uncontrolled erosion and earthen structure failure. Compounding the impacts of these geohazards, continuous metropolitan growth was changing the land use that these FRSs protected so the consequences of dam failure were increasing dramatically.