McNallyPost Fire Discharge and the Relationship of Sierra Nevada Wide Flood Frequency Curves and Local Kern River Discharge Curves
Authored By: T. A. Kaplan-Henry, J. G. Courter, S. E. Martin
Terry A. Kaplan-Henry, Joshua G. Courter, and Sarah E. Martin
USDA Forest Service Sequoia National Forest
During the summers of 2000, through 2005 stream channel geometry relationships were surveyed in tributary streams to the Kern River and USGS gage stations within the Kern River basin. From this data, Kern River channel geometry and discharge relationships versus drainage area were developed. This information was utilized to document pre-fire stream discharge relationships to determine design flow for the 2002 McNally fire. Post fire design flow calculation requires estimates of runoff response associated with burn severity. The most popular reference for post fire runoff response discharge is Magnitude and Frequency of Floods in California (Waananen and Crippen 1977). These authors provide discharge equations for the 2-year, 5-year, 10-year, 25-year, 50-year, and 100-year recurrence interval storms for six regions in California and are based on more than 700 stream flow stations. Comparison of local stream data relationships and those derived from Wannanen and Crippen yielded interesting results. As expected, all of the calculated regional discharge relationships for the various recurrence intervals are roughly parallel. What is surprising is that all of the relationships derived from field measurements and local data indicate discharge relationships are at least a magnitude of order higher for watersheds roughly 10 miles2 and smaller. Watersheds greater than 10 miles2 have closer relationships. Discharge relationships for both the calculated and measured 5-year recurrence interval have the closest relationship.
Since the 2002 McNally fire, we have documented the response of watersheds in the fire to both winter and summer storm events to validate discharge estimates predicted as a result of the fire. Watersheds studied include Rattlesnake Creek, Tobias Creek and Ant Canyon. These drainages were surveyed either immediately prior to or immediately after the fire. Surveys have continued through 2005. Parameters measured include cross-sections, longitudinal profiles, bank stability, and particle distribution. Discharge estimates have been calculated using resistance equations. These equations calculate discharge based on depth, bed material, cross-sectional area, wetted perimeter, hydraulic and slope. The results of this study suggest that post fire runoff from the most severely burned watershed plots on the 10-year discharge curve while the moderately burned watershed plots on the 5-year curve created from local stream relationships.
corresponding author:
Terry A. Kaplan-Henry
Sequoia National Forest
1839 South Newcomb Street
Porterville, CA 93257
559-784-1500 ext 1181
tkaplanhenry@fs.fed.us
