What type of gully is that? The need for a classification of gullies

Abstract Despite over a century of investigations into gullies and gully erosion, the characterization and categorization of gullies and the varied definitions, nomenclatures and terminology used has caused some confusion in understanding and communicating the relationships of gully forms and processes around the world. We firstly review the gully literature and highlight how a lack of consistency in gully definition and characterization prevents unifying theory from being developed within this important field of research, since it is often unclear whether different landscape features being discussed are comparable. We propose that conventionally employed qualitative planform and cross‐sectional characteristics of gullies alone are inadequate to define gully types, yet both these features remain central to most modern gully descriptions. We discuss the need to revise and augment these basic characteristics with clearly defined morphogenetic attributes such as landscape context, soil material characteristics, erosion processes, hydrological integrity, modes of development, and head/side‐wall morphology for an effective, practicable, generic gully classification scheme. Central to a gully classification scheme is the need for a clear definition of what a gully is – and is not – for which geomorphological criteria are proposed to differentiate a ‘gully’ from other ‘incisional land surface forms’. This gully definition hinges largely on the identification of a retreating head scarp and the internal erosion by mass‐movement and other sidewall slope erosion processes, coupled with the transport of the soil materials from the gully void. By defining a gully and synthesizing descriptions of gully ‘types’ from the literature and our own experience, we propose key morphogenetic attributes of gullies necessary to form a framework for a systematic gully classification scheme. An initial, eclectic classification framework is presented as both a summation and a synthesis of the literature review, and as a progenitor to a dynamic generic classification scheme that is proposed in a follow‐up article.