RECONSTRUCTION OF A GEOMORPHICALLY-EFFECTIVE FLOOD FOLLOWING THE 2012 HIGH PARK FIRE

In June 2012, the High Park Fire burned over 353 km² near Fort Collins, Colorado, making it the third largest fire in Colorado’s recorded history. A storm event that occurred on 6 July 2012, a few days after the fire ended, provides an opportunity to examine how the spatial and temporal dynamics of convective summer thunderstorms interact with watershed topography to produce persistent geomorphic changes in burned areas. This event affected Skin Gulch, a 15 km² watershed that had 62% of its area burned at moderate or high severity, and it produced tremendous channel erosion and localized deposition of over 0.5 m in the middle and lower portions of the watershed along with the transport of boulders up to 1 m in diameter. The objectives of this work are to quantify the spatial pattern and intensity of the rainfall that produced this flood, and to characterize the resulting peak discharge and spatial patterns of boundary shear stress and relate these to the observed geomorphic changes. Because this event occurred shortly after the fire when the watershed was not instrumented, it is challenging to quantify its hydrologic and hydraulic characteristics, so we turn to remote sensing data and modeling to reconstruct this event. Rainfall is characterized with hydro-NEXRAD data from the Cheyenne NWS Doppler radar as well as data from the CSU-CHILL radar. Airborne and terrestrial LiDAR combined with surveyed channel topography and grain size measurements are used to generate the computational mesh for hydraulic analyses. Both 1-D (HEC-RAS) and 2-D (FaSTMECH) hydrodynamic models are being used to estimate peak discharges and calculate the local sediment transport capacity as well as locations where boulder deposition is expected. Model verification is being provided by the surveyed high water marks and the calculated shear stress needed for incipient motion of the boulders that were moved during the event. Preliminary results suggest that short duration (<1 h) instantaneous rainfall rates of 60-100 mm/h localized to a small portion of the watershed were responsible for most of the erosion. This analysis sheds light on the potential for relatively small post-fire storms to produce extraordinary geomorphic response comparable to those resulting from much larger storms in unburned conditions, such as the 1976 Big Thompson flood event.