MODELING POST-FIRE STORM FLOWS WITHIN THE JASPER FIRE

The Jasper Fire started August 24, 2000, devouring over 83,500 acres within the Black Hills National Forest, making it the largest fire in the recorded history of the Black Hills. The fire occurred between Custer, SD and New Castle, WY, in an area known locally as the Limestone Plateau. Due to the highly permeable nature of the underlying geology, stream flow prior to the fire was almost nonexistent. Drainage channels are predominantly ephemeral or intermittent and often vegetated with mature ponderosa pine growing within the channel. There are short, isolated sections of spring-fed streams, but generally the area is considered to be a ground water recharge zone. Precipitation averages approximately 20 inches per year, most of which falls as rain from high-intensity thunderstorms in spring and early summer.

To characterize the post-fire watershed response, four thunderstorm events were modeled in HEC-HMS using a distributed geo-spatial approach. Burn severity was developed to define post-fire watershed conditions for input into the HEC-HMS model. GIS storm grids were generated from total storm precipitation depths that were distributed based on 50% and 20% exceedence probabilities for 1st quartile storms. ArcView GeoHEC-HMS was used to derive watershed characteristics. Channel cross-section data was collected and input into WinXSPro to provide calibration flows for the HEC-HMS model.

Results show that the highest intensity rain event occurred on Saturday, July 7, 2001. An average of approximately 2.5 inches fell over the entire fire, with an overall range of 1.0 to 4.5 inches. The next evening on Sunday, July 8, major flash flooding and channel scour occurred, despite it being a less severe storm. The third storm on Monday, July 9, also produced minor flash flooding originating from the northeast portion of the fire, which the two previous storms had not concentrated over. Hillslope runoff from these storms mobilized sediments and large debris within the channels, resulting in debris flows from watersheds as small as 1/10 acre. The runoff from these storms increased 200 to 1000% from pre-burn flows in Redbird, Gillette, and Hell Canyons.