Reconstructing Peak Discharge in a Colorado Front Range Headwater Stream During the September 2013 Storm

Monday, 15 December 2014
Amanda Elizabeth Brenner1,2, Robert S Anderson3, Suzanne P Anderson4, Eric W. Winchell5, David Schellhase2,4 and Jorgie Anthony Marquez1,6, (1)Institute of Arctic and Alpine Research, Boulder, CO, United States, (2)Colorado State University, Ecosystem Science & Sustainability, Fort Collins, CO, United States, (3)INSTAAR and Department of Geological Sciences, Boulder, CO, United States, (4)University of Colorado at Boulder, INSTAAR, Boulder, CO, United States, (5)University of Colorado at Boulder, Boulder, CO, United States, (6)Arapahoe Community College, Littleton, United States
A storm with a recurrence interval characterized as “millennial” inundated the Colorado Front Range in September 2013. Boulder’s watershed is heavily studied and the effects of this storm on its catchments were severe. Gordon Gulch is a small part of the watershed affected during this event. Unfortunately the discharge measurements from this area are missing due to equipment failure. In order to recover this data, and estimate the peak discharge during the storms, two methods were implemented: (1) an estimation of Paleodischarge from cross-sections and Manning's Equation and (2) a critical shear stress analysis of flow depth. The purpose of this paper was to look critically at the data collected throughout our research and assessing whether or not our methods support one another. The information gained through these methods allowed us to approximate that discharge was between 2 m3/s and 5 m3/s. Cross sectional analysis yielded stream flow heights of 0.7 and 0.8 m. We were also able to infer that the flow depths in Lower Gordon Gulch would have had to be between 0.09 and 0.46 m high in order to move the pebbles measured within the stream at Lower Gordon Gulch. With flow heights of 0.7 and 0.8 m high, we concluded our estimated paleodischarge is well within reason based on the results of our data. Although our methods allowed some room for error, we were able to draw reasonable conclusions that the peak discharge in Lower Gordon Gulch was 10 - 30 times greater than peak discharges during annual spring runoff floods. A shift in discharge this great in such a small stream could have a dramatic impact on the geomorphology of the area.