Factor influencing headcut migration within a cohesive Texas Black-Land gully.

Gully erosion degrades the Black-Land Prairies (BLP) (Texas), affecting agricultural productivity. It is initiated by rill and inter-rill erosive processes and exacerbated by storm frequency and duration. Few studies have assessed the factors influencing headcut development in cohesive gullies, mainly due to the limitations posed by field instrumentation and the high shear strength of clay soils. Cohesive gullies, e.g. those found in Vertisols may be regarded as an end member gully types in erosion studies. As such, critical assessments of the evolution and failure mechanisms of these headcuts are necessary for channel evolution modeling and determining the potential contribution of these channels to sediment budgets. The El Niño season provided ideal atmospheric perturbations to foster such a study. For the first time, this paper attempts to demonstrate relationships between aerial and subaerial factors influencing gully growth, using new remote sensing methods, hydrology, and in-situ soil properties. As such, the objective is to elucidate the environmental influences leading to erosion in a permanent cohesive gully, by systematically investigating the headcut through space-time and time-lapse observations. Geomorphological processes were validated through offsite observations. Precipitation, migration, discharge and volumetric soil moisture was measured to investigate relationships impacting retreat. Drying days was found to be the most critical factor, with an R2 of 0.95. The total rainfall between migration periods, and discharge at the over-fall also critically informed migration, with R2 values of 0.89, and 0.78, respectively. A vertical zonation of the headcut was found based on soil geotechnical properties that produced a distinct failure mechanism. Tridimensional photo reconstruction (3D PR) (Structure-From-Motion) techniques were used for topographic analysis.