ESPL State Of Science Draft What Type of Gully is That? I. The Need for a Classification of Erosion Gullies R. N. Thwaites, A.P. Brooks, T.J. Pietsch, J. Spencer Centre for Coastal Management, Griffith University, Gold Coast, QLD 4111, Australia. [email protected] +61 408986957 Keywords: Gully classification, erosion forms, soil erosion, gully mapping, gully typology Abstract Despite over a century of investigations into gullies and gully erosion, the characterisation and categorisation 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 characterisation 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 erosion 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 and soil material characteristics, erosion processes and 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 an erosion gully is – and is not – for which geomorphological criteria are proposed to differentiate a ‘gully’ from a ‘stream channel’.DRAFT This gully definition hinges largely on the identification of a retreating headcut and the internal erosion and deposition of soil materials to form the gully void. By defining a gully and synthesising 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 inceptive classification framework is presented as both a summation and a synthesis of the literature review, and as a progenitor to a dynamic generic 1 THWAITES ET AL. classification scheme that is proposed in Part II of this article (see Thwaites et al., this volume). 1. Introduction 1.1 A landform of many guises In addition to the term ‘gully’, the literature contains innumerable references to gully-like features, and concepts, using a diversity of local (specific or general) terms, for example arroyos, barrancos, torrentera, calanchi, ovrag, dongas, ravines, wadis, gulches, draws, trenches, vocaroca (vossoroca), bocoroca, ramps, the lavaka of Madagascar (Haigh, 1984; Radoane et al., 1995; Wells, 2004) and others listed by Zachar (1982). We also have and amalgam of generic terms and concepts for types of gullies, such as the ‘classical’, or ‘permanent’, and ‘ephemeral’ distinction of gully forms (Foster, 1986; Poesen, 1993). Many authors have proposed morphology- and process-based groupings, such as valley-side and valley-bottom gullies (Brice, 1966); dendritic, linear, and bulbous gullies (Ireland et al., 1939); bank gullies (Poesen, 1993) and upland gullies (Higgins, 1990); continuous and discontinuous gullies (Leopold and Miller, 1956). Feature-based categories, modifying gully types, such as head wall morphology (Deitrich and Dunne, 1993) and side-wall morphology and process (Crouch and Blong, 1989) amongst others have also added to the mix. Incisional and erosional land surface features that have commonly been described in different global locations such as arroyos (Cooke and Reeves, 1976), ravines (Tejwani, et al., 1975; Haigh, 1984; Dagar, 2018), wadis (Goudie, 2004; Wells, 2004a) may not actually equate to the commonly held understanding of what constitutes the ‘typical’ gully. Adding to this muddle is that, unwittingly, we may not be talking about theDRAFT same thing. Arroyos may be seen to be the same as barrancos in the SW USA (Schumm & Hadley, 1957), although arroyos may be considered to be similar to the wadis of north Africa and the middle east (Goudie, 2004), or not. On the other side of the world, Haigh (1984) includes arroyos in his terminology of ravines in India, as well as the Brazilian vossorocas and bocorocas and the, almost unique, Madagascan lavakas. Wells (1996) prefers to differentiate ‘ravines’ from ‘gullies’ by depth and bedrock incision criteria, whereas the USDA-NRCS (2008) recommends from the National Soil Survey Handbook that we use the term ravine 2 THWAITES ET AL. to be synonymous to the, less-favoured, gulch – which is described as a small stream channel, and larger than a gully. Thus, it is often not clear whether the features being described share similar characteristics, or processes even, with differentially named features in other parts of the world. It is tempting to simply take the advice of Wells (2004, p 503) and say that, “… gullies, ravines and gorges are perhaps best separated by their implications of lethality: a fall into a gorge could easily be fatal, whereas only the terminally unlucky would die by falling into a gully, and falls into ravines are unpredictable!” We are left to wonder what the lethality index is for an arroyo or wadi, although we can guess at it being close to zero for a rill. Levity aside, we need to bring focus to the knotty problem of defining a gully and the terminology to describe the different types – and what actually is a gully anyway? In order to understand and manage gully erosion, we need to comprehensively characterise the gully – its morphology and its genetic processes. Despite over a century of investigations in to gullies and gully erosion there is still ambiguity and incongruity in the characterisation and definition of different gullies. The varied categorisations, nomenclatures, and terminology used has hindered the understanding and clear communication of the inter-relationships of gully forms and processes that have been investigated around the world. It is unclear where the bases of this disorder in characterising gully forms and processes generically may lie. 1.2 One solution: a gully classification scheme With a recent effort to map and characterise gullies within the catchments draining to the Great Barrier Reef in Australia came the realisation of a distinct need for a gully classification.DRAFT This article is the first of a pair in which we set out the challenges faced in developing a gully classification and the approaches we have taken in overcoming these challenges. The companion paper (Thwaites et al., this volume) provides detailed description of the classification scheme itself. Although development of the classification was instigated by our work in Queensland, Australia, we anticipate that the classification scheme will have global utility. The classification framework is designed to be generic and comprehensive, whilst still able to be expanded as new examples are observed or a new understanding 3 THWAITES ET AL. gained. The benefits of a generic, or ‘general-purpose’ classification framework expounded here are seen to be: i) to provide a scientific framework for standardizing gully observations and assessment: description, characterization, and interpreting erosion activity, for all examples of gullies identified throughout the world’s different erosional environments; ii) to provide a means of organising and explaining the complexity of gully materials, form, and processes in research information and the communication of research and assessment results. That is, a lingua franca that, allows international correlation of terms, concepts, assessment and mapping across erosional landscapes, and to frame new theories to their nature and development for further investigation; iii) to provide an understanding of the relationships of the selected properties and their significance to the gully erosion management, control and rehabilitation; iv) to act as a technology-transfer tool whereby knowledge and understanding of the behaviour of erosion gullies in response to interventions may be transferred to gullies or gully types considered to be similar (in classification terms). Such a classification scheme will also serve a number of practical purposes in land and water management, primarily: DRAFT i) providing a desktop and field decision-support tool for gully management decision-making – for gully erosion control, and gully remediation and rehabilitation. ii) aiding in identifying the gullies or gully systems that constitute the greatest hazard for soil and nutrient loss, and/or sediment and nutrient loading to the 4 THWAITES ET AL. drainage network for risk assessment and further conservation management attention; iii) aiding the process of identifying and quantifying sediment point sources for catchment hydrological modelling of sediment and nutrient flows; iv) enhancing the mapping, modelling and rehabilitation of gullies and gully systems by enabling gullies to be mapped to type and modelled, or managed, in a type-specific manner. 1.3 Why do we need a standardised terminology and classification of gullies? From a recent review by Castillo and Gomez (2016), it is very clear that, going back for over a century, there is a large and rapidly increasing literature on gully erosion throughout the world, which is associated with an increasing appreciation