John J. Braswell
Years participated in RESESS:
Rainfall, overland flow, and gully erosion at Bijou Creek, Colorado
Academic Affiliation: Graduate, University of Michigan, Geology/Earth Science
Science Research Mentors: Greg Tucker and Ph.D. student Francis Rengers, University of Colorado at Boulder
John J. Braswell is a returning intern who grew up in Ann Arbor, Michigan. He recently graduated from the University of Michigan, is currently working for a small oil company in Texas, and hopes to attend graduate school in planetary geology next year.
Understanding how hydrology shapes geomorphologic processes such as gully erosion will help us monitor landscape evolution in response to changes in climate. Gully erosion is a process by which water flows in narrow channels during or after heavy rains, causing widening, deepening, and headcutting of small channels and waterways through erosion. In this study, we measured rainfall, overland flow, and gully retreat at Bijou Creek, Colorado, from 2007 onwards.
For rainfall, we focused on storm events and used rainfall amounts during the 60-minute period of greatest intensity (the 60-minute peak rainfall) and storm duration. For overland flow, we used a stream gauge to measure water surface height in a typically gully channel and marked overland flow events when the water height exceeded 5 cm. Plotting the 60-minute peak rainfall against the duration of the storm and identifying which events triggered overland flow, we see that storm duration does not influence the likelihood of overland flow. On the other hand, rainfall intensities above 10 mm/hr do appear to trigger overland flow, indicating that this is the rainfall intensity threshold for overland flow. Recent LiDAR (Light Detection and Ranging) measurements of the gully headwall position indicate rapid retreat in the spring of 2010 and a more steady, slower-paced retreat in the previous two years. We are correlating these retreat rates with the number of overland flow events over each period to determine whether there is a statistically significant relationship. If there is, then we will be able to forecast the rate of gully erosion in Bijou Creek from observations of rainfall events and expect to be able to extend these results to other areas of the Colorado Front Range where rainfall data are available.