Monitoring channel head erosion processes in response to an artificially induced abrupt base level change using time-lapse photography 2301

Authors: Nichols, Mary; Nearing, Mark; HERNANDEZ, M. - University Of Arizona; POLYAKOV, V. - University Of Arizona

Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2016
Publication Date: 5/7/2016
Citation: Nichols, M.H., Nearing, M.A., Hernandez, M., Polyakov, V. 2016. Monitoring channel head erosion processes in response to an artificially induced abrupt base level change using time-lapse photography. Geomorphology. 265:107-116.

Interpretive Summary: Channels are an important source of sediment in semiarid areas and erosion of the channel headwall can be a major contributor to sediment production. A study was conducted from 2004 to 2014 on the Walnut Gulch Experimental Watershed (WGEW) in southeastern Arizona, USA to determine how the headwall of a small channel would respond to an abrupt lowering of the channel bed. Field observations and time-lapse photography were coupled with hydrologic measurements and models to identify the dominant erosion process and to quantify the range of conditions under which erosion resulted in headcut propagation. The most frequent erosion processes observed through time-lapse photography were plunge pool erosion and mass wasting through sidewall or channel headwall slumping that occurred during summer months in response to soil moisture fluxes that appear to be an important controlling factor in channel erosion dynamics. No runoff occurred during winter months, however, minor dry ravel and individual grain movement was observed. A large erosion event occurred in August 2014 that advanced the channel head 7.4 m (51% of the overall advance) and removed 11.3 m3 of sediment. High temporal resolution time-lapse photography was critical for identifying the opening of a vertical pipe that diverted surface runoff underground and led to subsurface erosion which weakened the surrounding soil in advance of the large erosion event. In the absence of time-lapse images the occurrence of subsurface erosion would not have been known. This study has identified the previously unobserved process of piping as an important erosion process on the WGEW and points to the need for much longer periods of observation and data collection to support development of statistical and process-based models of event scale headcut advance.

Technical Abstract: Headcut and channel extension in response to an abrupt base level change in 2004 of approximately 1m was studied in a 1.29 ha semiarid headwater drainage on the Walnut Gulch Experimental Watershed (WGEW) in southeastern Arizona, USA. Field observations and time-lapse photography were coupled with hydrologic measurements and simulations to identify the dominant erosion process and to quantify the range of conditions under which erosion resulted in headcut propagation. During the 10 year period from 2004 to 2014 the headcut migrated upchannel a total of 14.5 m reducing the contributing area at the headwall by 21.5%. Beginning in July 2012, time lapse photography was employed to observe event scale channel evolution dynamics. The most frequent erosion processes observed during the 3 seasons of time-lapse photography were plunge pool erosion and mass wasting through sidewall or channel headwall slumping that occurred during summer months in response to soil moisture fluxes that appear to be an important controlling factor in channel erosion dynamics. No runoff occurred during winter months, however, minor dry ravel and individual grain movement was observed. Geomorphic change during the 10 year period was dominated by a single piping event in August 2014 that advanced the channel head 7.4 m (51% of the overall advance) and removed 11.3 m3 of sediment. High temporal resolution time-lapse photography was critical for identifying the erosion processes, in the absence of time-lapse images piping would not have been identified as a dominant erosion mechanism at this site. This single geomorphically significant event dominated the landscape change during the study period within which the overall headcut advance was 14.5 m. As a result, there is no basis for developing statistical relationships between hydrologic drivers (precipitation, runoff volume, and peak runoff rate) and headcut advance; however, the results provide insight into channel head erosion processes that will inform testable hypotheses as part of ongoing field research. This study has identified the previously unobserved process of piping as an important erosion process on the WGEW and points to the need for much longer periods of observation and data collection to support development of statistical and process-based models of event scale headcut advance.