Phoenix debris-flow hazard assessment: House location matters

Three catchments that debouch into Phoenix housing developments from the Ma Ha Tuak Range, South Mountain show a consistent pattern of declines in the magnitude of debris flows over time. We used a mix of field and geospatial methods to estimate volume, and the varnish microlamination technique to estimate minimum ages of debris deposits. Estimates of debris-flow volumes show a drop of more than two orders of magnitude from latest Pleistocene to 20th century or Little Ice Age. Debris-flow run-out lengths also shortened tremendously, with the most recent events stopping inside incised channels. In contrast, a catchment above houses in the Gila Range, South Mountain, where houses were built very close to the debris-flow source area, reveals an increase in debris-flow magnitude over time. These contrasting findings emphasize the importance of the location of home sites with respect to the debris-flow system in the type of small desert mountain catchments that interface with the sprawling urbanism found in Southwestern USA deserts.