Modelling of gully widening, a review. Implications for research on gully evolution and restoration

Abstract

Gully erosion is recognised as a major soil degradation process, with gully growth being a complex phenomenon that is controlled by different processes: incision, gully headcut retreat and gully widening, coupling at different spatial and temporal scales. While gully modelling research has paid considerable attention to headcut retreat processes and incision, our review of the available literature in Web of Science database shows that less attention has been paid to the modelling of gully widening processes. This might hamper progress on gully evolution studies or on gully control and restoration programmes. Our review has found seven models described in the indexed literature: three of them empirical, three based on processes, and a seventh one that can be classified as hybrid. The latter, despite being based on process-based equations, is simplified and requires empirical calibration for implementation. Our review has also noted a limited number of publications using these models, rather for validation or for case studies, with very few examples of models used outside the team that developed it. Of the four process-based models, two of them are the most comprehensive - BSTEM and GULTEM - both incorporating a combination of processes to predict bank failure based on soil, climate and vegetation parameters, coupled with others describing the effect of concentrated channel flow. A third one, the Bank Failure Model (hereafter BFM), is also embedded in the well know landscape evolution model CHILD. Of the seven models identified, only two of them, BSTEM and BFM through CHILD, are directly available to potential users with access to executable versions and instructions, while the others need to be requested from the authors or built from the published equations. The models cover a wide range of spatial and temporal scales, from event to centuries, and from gully stretch to regional scale. Our review suggests that process-based models are the ones providing the most comprehensive approach, especially those including geotechnical approaches, although they would benefit from further development to include wider spatial and temporal scales. This research suggests that more studies and field data are required for calibration, understanding lateral flows, wetting and drying cycles, tension crack prediction, and integrating widening with other gully erosion processes. They should also include further testing and validation of available models beyond the teams that developed them, ideally in model intercomparison exercises.

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