SEDIMENTATION RATES AND ALLUVIAL PROCESSES DURING EXTREME PRECIPITATION EVENTS IN SEMIARID ENVIRONMENTS: DRAMATIC GROWTH OF AN ALLUVIAL FAN AT THE URBAN–WILDLAND INTERFACE, COLORADO, SEPTEMBER 2013

The Colorado Front Range is a semiarid region along the urban–wildland interface characterized by summertime monsoon circulation. Understanding erosion resulting from extreme precipitation and flooding events, such as those experienced by Colorado during September 2013, is vital for mitigating consequences of more frequent extreme precipitation events expected with anthropogenic climate change. Located in Colorado Springs near the mouth of Cheyenne Canyon, the study area is a small, <1 km², alluvial basin composed of decomposed granite situated beneath several paved and unpaved switchback roads.

We collected differential GPS data of an alluvial fan surface before and after flooding. Using spatial modeling software to compare the pre vs. post flood fan surfaces we calculated the change in alluvial fan volume. USGS stream gauging data was used to constrain the timing of deposition. From September 10, 2013 to September 17, 2013 the USGS Evans Street gauging station on Cheyenne Creek, ~1 km NE of the study area, recorded a 7 day accumulation of 13.59” of precipitation. 11.59” fell during a high intensity period lasting ~60 hrs. During this interval, the fan surface area expanded ~200%. The wetted fan surface aggraded >2 m and deposited >1000 m³ of sediment over an area of ~400 m² on the toe of the alluvial fan. Depositional timing can be conservatively constrained by the onset and end of precipitation. A more plausible constraint is that sedimentation only occurred above a precipitation intensity threshold at which we observed alluvial processes to initiate. In either scenario, results of the rapid sedimentation are striking.

Precipitation events associated with monsoonal atmospheric circulation like the Colorado event are projected to be more extreme in nature with climate change. Coupled with periods of extended aridity, precipitation events will have compounded consequences for downstream communities as large and more frequent floods redistribute sediment over wide areas. The rates and magnitude of sedimentation from this study yield an analog for possible impacts expected during future extreme precipitation events of this nature.