HEADCUT EROSION MECHANICS IN GULLY SYSTEMS

Gullies are arguably the primary geomorphic pathway for soil erosion in many regions. The majority of erosion is often focused at gully headcuts, thus our research is focused on understanding the key elements needed to propagate a headcut in a gully, and the overall effect of a headcut on gully form. We use field observations to quantify the erosional mechanics at a headcut. From these observations, we build a reduced-complexity 1D numerical model of gully evolution.

At our study site in eastern Colorado, we observe gullies with vegetated valleys upstream of each headcut, a zone of bare soil below the headcut, and a grassy valley floor downstream. Aerial photos from 1937 to present show that headcuts retreat at average rates of 0.5 m/yr, and that these rates are dependent on upstream drainage area. We quantify the volume and pattern of erosion at a headcut with repeat terrestrial LiDAR. LiDAR measurements support the aerial photo erosion rate of 0.5 m/yr, but reveal that headcut erosion is primarily driven by block failure. Time-lapse photography and hydrologic measurements suggest that erosion at the headcut face is controlled by block fall due to freeze-thaw erosion, fluvial erosion, and desiccation.

Using our field observations, we develop a 1D numerical model to investigate the necessary and sufficient conditions to explain headcut shape and the downstream valley morphology. We prescribe a headcut retreat rate, which encompasses all headcut erosional processes into a single rate per year. Sediment generated by headcut retreat is deposited downstream according to an exponential decay rule. We apply rainfall to the gully and generate overland flow with an infiltration-excess overland flow rule. Water flow drives erosion and deposition of sediment based on soil type, the presence or absence of vegetation, and the incoming sediment concentration.

Our 1D model abstracts the specific mechanisms of the headcut but highlights key factors needed to propagate gully headcuts. Vegetation is needed upstream of a headcut to resist lip erosion. A bare soil zone downstream of the headcut is necessary to prevent headcut burial. Headcut retreat rate can be influenced by both upstream drainage area and headcut height. The overall gully slope downstream of a headcut is controlled by the headcut retreat rate and rainfall rate.