ESPL State of Science Draft What Type of Gully Is That? I. the Need

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

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

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

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 that it is a major global environmental problem (Vanmaerke et al., 2016). However, gullies continue to be described in the literature variously in terms of morphology (e.g. size, planform shape or pattern, depth, head shape, cross-section size and shape), processes of formation and development, stage of development, landscape location, location in the drainage basin, soil/substrate material, vegetation, and climatic environment. This is because gullies and similar erosional features can differ substantially in their appearance, erodibility, and erosion processes, in different environmental contexts, such as hydrological and climatic regimes, and are interpreted differently within different precepts and premises for different purposes. It is also clear from this expanding body of literature that, while a great assortment of gully forms has been described in different parts of the world, there is no systematic approach for defining and describing these diverse types of gullies that is firmly groundedDRAFT in geomorphological principles. Fundamentally, a standardised terminology and classification is required to ensure that there is a common understanding of what is being described and analysed. Beyond this, we also believe that it is essential that we have an appreciation of the diversity of gully types in the landscape to underpin their management.

The following is an example of this confusion, both in concept and definition, in some of the literature that sets out to define gullies and gully erosion, without

THWAITES ET AL. intending any criticism of the authors, under the debatable premise that there is a definable threshold between a continuum from rills to gullies.

According to Torri and Borselli (2000) a rill can be ‘upgraded’ to a gully if it cannot be removed by normal tillage operations, and then that it is ‘ephemeral’ if it is removed shortly after its formation (which still sounds like a rill feature by their definition – and by others). They offer a refined, more technical definition, irrespective of tillage equipment, whereby any channel with ephemeral run-off can be called a gully with a “maximum cross-sectional area equal or above 0.1 m2” (Torri and Borselli, 2000, p. G-183). Morgan (2005, p. 30) has also stated that a “… widely recognized definition …” (without citing any sources) to separate gullies from rills is a cross-sectional area of “… 1 m2”, while pointing out that the value is more accurately 929 cm2 (which actually equates to 0.1 m2, thereby appearing confounding). Whether this is a presumed typographical error or not, it can add to the confusion and incongruity in the reader’s mind when this 929 cm2 metric is then used by Poesen (1993) and others as to distinguish between rills and ‘ephemeral gullies’, rather than gullies in general. This precise metric value is the conversion of 1 ft2, the specification of which as a convenient threshold originates from Hauge (1977, cited in Poesen, 1993). This serves to highlight that not only can the stated measurements for thresholds be adopted somewhat unconditionally between different environments, and then rubberized by conversion of units, but also can appear confusing in what they really represent.

‘Ephemeral’ gullies were initially expounded by Foster (1986) to distinguish them from ‘classical’ (or ‘permanent’) gullies that all occurred in natural drainage lines. These ephemeral relatives of the permanent gullies are restricted to cultivated lands only, continually occurring in natural drainage lines (in contrast to ‘classical’ rills), only to be removedDRAFT by tillage operations – yet they are seen to be ‘not-rills’. The classical gullies are only really defined by implication by not being ‘ephemeral’ and not being ‘rills’ (e.g. Zheng and Huang, 2002). Poesen (1993, et seq.) has taken this further to distinguish ‘rills’ from ‘Ephemeral Gullies’, ‘classical gullies’, and consequent ‘Bank Gullies’, which are presumed to be also ‘permanent’ or ‘classical’ as they apparently extend by headward migration and are not easily repaired. Casalí et al. (1999) have then distinguished ‘classical ephemeral gullies’ from two other forms (‘drainage ephemeral’ and ‘discontinuity ephemeral’ gullies) that they

THWAITES ET AL. identified in Navarra, Spain. The concept of ‘classical’ is vague and its application as a term is generalised. There is a need to refer to a more formal classification to describe and define these different types of gullies.

To allay further confusion, we have to grammatically differentiate the term ‘Ephemeral Gullies’ here as proper name or class, from the expression of ‘ephemeral’ gullies, which, although may be uncommon, exist naturally outside of cultivated lands. They can occur in soft or unconsolidated sediments – to become or return to zero-order, or first order drainage valley features, or open depressions (Dietrich et al., 1987). Bull and Kirkby (1997)prefer to call these gullies ‘impermanent’, meaning that the morphology of the channel is periodically destroyed by human intervention or natural processes.

While discussing rill and gully erosion generically Grissinger (1996) explicitly sections gullies into ‘ephemeral gullying’, ‘upland gullying’, and ‘valley floor gullying’, inferring landscape process rather than form or types, and viewing them all from a cultivation and soil-loss perspective. Natural gullying and sediment yields impacting water quality are not discussed. This landscape-position categorisation of gullies was first expressed by Brice (1966), where he also included a ‘valley-head’ category of gullies, as he saw a distinct grouping of gully-forming processes providing different patterns of gullying.

Similarly, Morgan (2005) introduces gully erosion generically in the third edition of his book on Soil Erosion and Conservation with the differentiation of essentially the two gully types of ‘valley floor’ and ‘valley side’ (in this case, inferring form more than process). He then states that valley-floor gullies generally take the form of ephemeral gullies in topographic swales, with run-off generated from nearby hillslopes and developmentDRAFT largely by headcut retreat and sidewall collapse. Valley- side gullies, meanwhile, are seen to be dominated by headward retreat and channel-floor incision and being either ‘continuous’ or ‘discontinuous’ (a status originally identified by Leopold and Miller (1956), applying to all gully environments). Grissinger (1996) from the outset discusses only ‘discontinuous’ gullies in the valley-floor group of gullying. ‘Continuous’ gullies are not otherwise mentioned, except to state that both forms also occur in ‘ephemeral gullying’ (and assumed to be in the ‘upland gullying’ and ‘valley-floor’ categories also).

THWAITES ET AL.

To add more conceptual complexity to this mix, Higgins et al. (1990) had already made a comprehensive and convincing argument to include continuous/discontinuous gullies in all three of Brice’s (1966) valley-position gully types, as well as separately identifying the ‘upland gullies’ favoured by Grissinger (1996). These “very common” upland gullies are defined by Higgins et al. (1990) as similar to Brice’s valley-bottom gullies in form but only in an upland environment. They are described as initiating on stream-channel banks or other localised, steep slopes, and erode headward into a near-level plain. These gullies are not to be confused with the ‘bank gullies’ of Poesen and Govers (1990) and Poesen (1993), which are not stated to erode stream banks but rather to incise artificial earthen banks or cut banks (for tracks and roads). These latter, bank, gullies do not initiate at the bank but are extensions of upslope (ephemeral) gullies, or slope-wash / concentrated flow from cultivated land through, and down, constructed or formed banks – aided by piping and mass movement. So, although these gullies are similar to valley-bottom gullies in form, they are distinct by their nature of landscape position (and process to a degree), which is clearly explained by Higgins et al. (1990). The labelling of these features as ‘upland gullies’ is freely admitted as being for want of a better, accepted name.

On the other hand, Torri and Borselli (2000) in the Handbook of Soil Science (Sumner, 2000) mention neither discontinuous/continuous gulling nor valley- side/valley-floor gullying in their definitions within ‘water erosion’, but they do state that gullies have been classified in “several ways”. They then follow up by stating that gullies “… can be described as digitated …; axial …; or frontal…” (Torri and Borselli, 2000: G-183) citing Morgan’s (1995) categorisation (presented only in the second edition of his book), which is the essence of De Ploey’s original classification (De DRAFTPloey, 1973).

It seems little has changed from when Rose (1993: 153) summed up the situation concerning the differentiation of rill and gully erosion, that, “(b)ecause the terminology used to describe soil erosion is not standardised, there may be some differences in judgement or definition …”. The same can be still said for discussing differences in gullies and the transition from gullies to stream channels.

THWAITES ET AL. 2. Gully definition and terminology

Before we can consider a systematic way in which to categorise and classify gullies, we need to determine a definition of the entity that we call a gully. Not only does it need to encompass the varieties of land-surface incisional forms that have been described as gullies in the past, but it also needs to accommodate the different viewpoints and terminology that fully interprets our understanding of what we all perceive as a ‘gully’, both from a geomorphological landscaping perspective as well as that from land conservation management.

The earliest known usage of the word ‘gully’ is in the 1650s (?1657), originating from the French word goulet (a diminutive form of goule: throat) in the meaning of a narrow entry, passage, neck, or opening (Collins English Dictionary, 2014). The word stems reasonably from the figurative sense of a narrow passageway, derived from the Latin gula, also meaning throat, leading to the French goulet, thence the Middle English golet, and, later, gullet (Wells, 2004; Collins English Dictionary, 2014).

There have been many different attempts to define gullies and rills in both the agricultural and geomorphological landscapes, too numerous to list and reproduce here. Initially the US Soil Conservation Service (SCS) apparently provided the guideline that a gully was the feature that was too wide for a prairie dog to jump across! (Fullen and Catt, 2004). Nowadays, the USDA-Natural Resources Conservation Service (NRCS) and National Soil Survey (NSS) are more assiduous in their guidelines by defining a gully as, “(a) small channel with steep sides caused by erosion and cut in unconsolidated materials by concentrated but intermittent flow of water usually during and immediately following heavy rains or ice / snow melt. A gully, generally, is an obstacle to wheeled vehicles and too deep (e.g. > 0.5 m) to be obliterated by ordinaryDRAFT tillage; (a rill is of lesser depth and can be smoothed over by ordinary tillage).” (NSSH, 2008, p 38; SSSA, 2020).

The only standard definition of a gully in Australia, reflecting much of the rest of the world, is that it is an, “(o)pen depression with short, precipitous walls and a moderately inclined to very gently inclined floor or small stream channel, eroded by channelled stream flow and consequent collapse and water aided movement” and, that it is, “… a channel more than 0.3 m deep” (The Australian Soil and Land 9

THWAITES ET AL. Survey Field Handbook: NCST, 2009, pp 39 & 136). This is by no means a clear definition, and it confuses the concept by stating that gullies are “… eroded by stream flow …” and is ambiguous as to what is meant by ‘short’ walls.

The above are particularly agricultural or practical definitions of a gully, rather than geomorphological or technical. They relate largely to inter-rill and rill erosion, and to time periods measured in seasons or years in environments suited to the operation of agricultural machinery. We have also seen in the past 30 years the growth of interest in ‘ephemeral’ gullies, as contrasted to ‘permanent’ gullies at a high order of differentiation. This is not only a distinction between gullies from a land management perspective but also a potential ambiguity between the defining erosion processes producing rills and gullies.

None of the (agricultural) definitions offered explicitly include what is probably the most discerning and critical feature of a gully, that is the ‘head wall’ or ‘head scarp’ that is seen to erode upslope extending in a headward direction through mass- movement processes: tunnel collapse, wall collapse, or basal sapping by groundwaters (e.g. Leopold and Miller, 1956, Heede, 1970, de Ploey, 1989; Wells, 2004) as well as from rain splash and surface erosion (Ireland et al., 1939; Higgins, et al. 1990; Wells, 2004; Morgan, 2005, Kirkby and Bracken, 2009). This is a geomorphological perception that is essential to the identification of gullies and gully processes.

Summarising some of the major authors in the gully erosion literature from a more geomorphological perspective, Radoane et al. (1995) suggest that a gully is an unstable landform that forms part of a drainage network, and is characterized by: i) a steep, incised channel, often with a sharp headcut, ii) many headcutsDRAFT on their floors; iii) prior or present rapid head growth; iv) cross-sections that are V-shaped where the subsoil is fine textured and is relatively resistant to rapid cutting, but U-shaped where the subsoil is more erodible; v) ephemeral streamflow; vi) not obliterated by normal tillage operations.

THWAITES ET AL. We may not all agree with that summation and others that have been offered (e.g. Geyik, 1986; Bergsma, 1996), and the fact that these definitions still need to be qualified as comprehensive general-purpose descriptions, underlines their inconsistency and inadequacy (Schumm et al., 1984).

Bennett and Wells (2018) highlighted the dichotomy between geomorphological and agricultural engineering perspectives as ‘disciplinary fragmentation’. They believe that terms such as gully, ephemeral gully, and permanent or classic gully can refer to distinctly different forms and processes to researchers within specific disciplines. This disciplinary fragmentation may therefore hinder scientific advancement. Understanding, interpretation, and description of gullies in the geomorphological community are conceived within the models of landscape evolution, where timescales and hillslope gradients are greater than those contemplated by the agricultural and productive land management community (Bennett and Wells, 2018). They therefore state that gullies that develop on agricultural hillslopes may not be (and often are not) equivalent in form and process to those developed on hillslopes and landscapes in the broader geomorphic context. This has led the authors to suggest, very sensibly, classifying gullies primarily into ‘Field Gullies’ and ‘Landscape Gullies’ (Bennett and Wells, 2018).

2.1 Definition: What is a gully? In some ways it may be best to define gullies by determining what they are not. However, this notion presumes that features that are not gullies have robust definitions themselves. Besides rills, it is best to assume that this is not so.

i) Not a Rill

A rill can be described as shallow linear run-off erosion channel, not more than 0.3 m deep, formed byDRAFT concentrated runoff with flow during and immediately after rain, with steep sides when recent, rejuvenated by each (modal) rain event and presents no obstacle to normal tillage operations (Bennett 1939, 1955; Geyik, 1986; Houghton & Charman 1986; Wells, 2004, USDA, 2007). Rills affect only the depth of the plough layer (Bennett, 1955), hence the adherence to the, now anachronous, one-foot (0.3 m) average depth for a rill.

THWAITES ET AL. Foster (1986, p. 93 – Table 1) offers the definitive characteristics that make up rill erosion(emphases added): • rills are normally erased by tillage [note: no depth threshold given]; • rills usually do not recur in the same place; • rills may be of any size but are usually smaller than ephemeral cropland gullies; • cross-sections tend to be narrow relative to depth; • flow pattern develops as many small disconnected parallel channels ending at ephemeral cropland gullies, terrace channels, or where deposition occurs; • rills are generally uniformly spaced and sized; • occur on smooth side-slopes above drainageways; • soil [in rills] is removed in shallow channels but annual tillage causes the soil profile to become thinner over the entire slope.

Poesen (1993) distinguishes rills from (ephemeral) gullies by using Hauge’s (1977) objective criterion of a critical cross-sectional area of 929 cm2 (or the ‘one square foot’ rule). Other criteria that have been used include a minimum width of 0.3 m, or 1 ft – originating from Bennett (1939), the founding director of the US Soil Conservation Service from the ‘dust bowl’ of the 1930s, and a minimum depth of 0.3 m, also from Bennett (1939), or about 0.6 m (2 ft?) (Brice, 1966), or a minimum depth of 0.5 m (Imeson and Kwaad, 1980) and, “... e.g. > 0.5 m...” (USDA_NRCS, 2008). There seems to be no upper limit, or maximum depth, for the definition of gullies (see also Table 2 in ‘General-purpose concepts of gully classification’ later in this article). Therefore, the boundary between a large gully and an ephemeral stream, stream or river channel, or an arroyo, gulch, or ravine, can be very vague, as well as that between a gully and a rill. Under modern tillage regimes in agricultural environments and employing universal SI units, it seems appropriate to use a 0.5 m boundaryDRAFT between rills and gullies if the tillage rule is to be continued. To use it for the natural/permanent/classical gullies is even more arbitrary, but an average 0.5 m retreating headscarp is more convincing as a gully than a 0.3 m one. To adopt a 0.5 m lower limit of depth for gullies, may mean that some gully-like rills will remain as ‘rills’, and some rill-like gullies may also now be under the ‘rill’ heading, at least in the agricultural environment

THWAITES ET AL. So, the distinction of rills from gullies seems to be that of ephemeral gullies and rills in the agricultural context, and is one largely erasure by tillage; ‘size’ (commonly regarded as the incisional depth, or width/depth ratio of traversibility), being too small to be called ‘gullies’ (e.g. Foster, 1986; Poesen, 1993); and non-recurrence in the same place. However, rills may also be differentiated on the basis of process.

Poesen et al., 1993 argues “… the transition from rill erosion to ephemeral gully erosion to classical gully erosion and to river channel erosion represents a continuum”, while Grissinger (1996) concludes that any classification of hydraulically-related erosion forms into separate classes, such as “microrills, rills, megarills, ephemeral gullies, gullies”, is subjective to varying degrees. Under this concept of a continuum Nachtergaele et al. (2002) then go on to distinguish three different types of ‘channels’ by an equation based on their width-to-flow discharge relationship. These three ‘domains’ are those of the Rill, Gully and (small) River, the transition from one to the other being an order of magnitude discharge (in m3 s-1). Nevertheless, this is very dependent upon the homogeneity and erosion resistance, or otherwise, of the soil materials, antecedent moisture conditions, and the unchanging nature of the channel during flow events (Nachtergaele et al., 2002; Poesen et al., 2003).

However, this premise of a continuum from rills to gullies is debatable when dealing with non-agriculturally induced gullies. Imeson and Kwaad (1980) proffer the interesting and appropriate analogy that rills In the natural environment act more like river channels, while gullies act more like the river valley. Gao, (2013) states that ‘rills’ are self-stabilizing features, i.e. displaying negative feedback – their location and size change with different rain events. ‘Gullies’ are self-perpetuating, however, displaying positive feedback, and remaining at the locations in which they initiated (both naturallyDRAFT and by cultivation). In addition, rills are formed almost exclusively by overland flow and can be described by field measurement and process-based modelling. This viewpoint supports Bull and Kirkby’s (1997) assertion that neutral or negative feedback exists for rills as they may not form in the same position once drainage is improved.

This, then could be a basis for distinction between the two forms and processes of rills and gullies. A threshold is exceeded for a rill to develop, which may not occur

THWAITES ET AL. for all runoff events (Bull and Kirkby, 1997), and a threshold in feedback (from negative to positive) is reached for rills to be then seen as gullies. The reliance on arbitrary depths, width-depth ratios, or discharges is then unnecessary.

Gullies, on the other hand, are more complex to model than rills (as overland flow- generated features) because they involve both surficial and sub-surface causal processes in their formation and development (i.e. seepage, piping, sub-surface saturation, subsoil dispersion, structural failure). Hillslope and in-channel processes, as well as groundwater interactions obviously vary widely with differences in environmental factors such as climate, soil or other eroding material, landscape, and land use history, even in the agricultural environment. Often, we can witness the rapid onset of a positive feedback mechanism in gully development.

ii) Not a stream channel

It is important to determine whether the feature is a gully or a stream channel, or at what point a gully transitions to something that is better-defined as a stream or stream channel. Carey et al. (2015) venture some differences between creeks (stream channels) and gullies in Queensland that are criteria to bear in mind:

• “Gullies can retreat to the very top of their catchments.

• Gullies generally have far more capacity than they need to carry sediment from the flow they receive.

• Gullies may be short and isolated on hillslopes.

• Tree roots help to stabilise saturated streambanks, but trees have a limited role in stabilising gully heads [or walls].

• Gullies would not normally be a habitat for fish and any structures built in them should notDRAFT need to cater for fish passage.” (Carey et al. 2015; p.13-8)

The boundary between gullies and stream channels can still be a fuzzy one, both physically and conceptually. The physical boundary may better be viewed as a transition zone. Nonetheless, the definition is important for environmental policy, with legislation often specific to streams. In the Queensland context of gully rehabilitation, inappropriate legislated requirements for fish passage, riparian zone

THWAITES ET AL. and aquatic ecology management, and bank retention for example would apply, if a gully/stream transition is not adequately delineated or misinterpreted.

iii) Not an ephemeral gully

Foster (1986) was the first to comprehensively describe characteristics of ‘ephemeral gullies’ (also described as ‘megarills’ or ‘concentrated flow erosion’) as channelised flow concentrated in low swales on the hillslope that recur in the same position – in cultivated land: in contrast to rills that can occur in different position on the hillslope and do not normally coalesce or concentrate (Foster, 1986; see above). Thus, he aligns them more with rills rather than what he refers to as ‘classical’ gullies. He makes no mention of the presence or absence of a headcut or headward retreat.

Foster also offers a definitive characterisation of both ephemeral gullies and ‘classical’ gullies, all in the context of cultivated land, in terms of their erosion.

Table 1 Characteristics of ephemeral gully erosion and classical gully erosion abbreviated from Foster (1986, p. 93 – Table 1). Emphases added in italics; added text shown in brackets.

EPHEMERAL GULLY CLASSICAL GULLY • temporary features, usually obscured by • [Permanent features,] not obscured by tillage normal tillage operations • recur in the same location • Usually larger than ephemeral cropland • may be of any size but are usually larger gullies than rills and smaller than permanent • Cross-sections of many gullies tend to be gullies narrow relative to depth; sidewalls are • Cross sections tend to be wide relative to steep; headcut usually prominent depth; sidewalls frequently are not well • Tend to form a dendritic pattern along defined; DRAFTnatural water courses; nondendritic • headcuts are usually not readily visible patterns may occur [elsewhere] and are not prominent because of tillage • Generally occur in well-defined • Usually forms a dendritic pattern along drainageways depressional water courses; occurs along • Soil may be eroded to depth of the shallow drainageways upstream from profile and can erode into soft incised channels or gullies [weathered] bedrock

THWAITES ET AL. • Soil is removed along a narrow flow path, typically to the depth of the tillage layer where the untilled layer is resistant to erosion, or deeper where the untilled layer is less resistant

In general, ephemeral gullies and classical gullies have been distinguished on their persistence, size and active erosional features (particularly a headcut and steep walls). Once formed, classical, or permanent, gullies develop by the common process of eroding the gully floor with head-cut migration and erosion of the gully walls – usually by a significant mass movement component in a perceived positive feedback mechanism (see above). With (agricultural) ephemeral gullies, not only are head-cut migration and gully wall erosion far less significant, but also, by definition, ephemeral gullies can be removed only to recur and repeat the process of incision (Foster, 1986; Bull and Kirkby, 1997; Nachtergaele et al., 2002).

It may need to be established on a case-by-case basis as to what trajectory ephemeral gullies follow if they are not removed. Would they all display positive feedback processes and develop significant headcut retreat (of over 0.5 m depth), and develop mass-wasting activity?

Nevertheless, although ephemeral gullies may sit better in a classification of rills (should one eventuate), their variety and importance and affinities with some gully forms and processes, particularly the possibility of developing a retreating headcut/scarp, they can be considered as a separate type of gully within the ‘field gullies’ of Bennett and Wells (2018).

DRAFT iv) Not badlands

There is some confusion in the literature as to whether ’badlands’ should be regarded as synonymous with gully erosion. Badlands are typically highly eroding landscapes formed in soft rock terrain, such as marl or shale sedimentary rock (Gallart et al., 2002; Harvey, 2004) which has often experienced some form of uplift or re-exposure due to base level change (Bryan

THWAITES ET AL. and Yair, 1982). The major badlands landscapes described around the world, as a whole, do not conform to the central concept of a gully or gully system, but gullies can be perceived to form within badlands (Lee and Charman, 2005; Howard and Kerby, 1983). Extensive alluvial gully systems, such as those described in northern Australia by Brooks et.al. (2009) have sometimes been described as badland-type erosion. However, Brooks et al. (2009, p.1954) contend that ‘…describing alluvial gully complexes as ‘badland erosion’ does not help to explain the processes driving this form of erosion, and only serves to further cloud the literature on badland erosion.’ Therefore, we believe that alluvial gullies, cannot be regarded as ‘badlands’ per se on the basis of perceived processes rather than just form. By extension we would conclude that badlands are not a type of gully per se, although it is highly likely that badland areas contain gullies within them, particularly of the alluvial form that occur within alluvial fills in Badlands.

v) Not a ravine

In a review of the world literature Raj et al. (1999) find the technical use of the term ‘ravine’ is limited to south and east Asia, primarily India and China. Otherwise, elsewhere ravine is used as a synonym for gully. Dagar (2018) sees that the use of the terms ‘gully’ and ‘ravine’ largely overlap, and reminds us that the French ‘ravine’ includes both gullies and larger valleys. However, the preferred definition according to Dagar (2018) is that a ravine, “… will have steep, weakly consolidated side slopes, flat channel bottoms characterized by a heavy sediment load, and a clear break of slope with the surface above.” Most importantly, he also states that ravines can be cut into regolith or bedrock. This indicates that (Indian) ravines, at least, can be construed as stream channels, with more similarity to arroyos and wadis, and therefore do not readily come under the umbrella of gullies. DRAFT Moeyersons (1991) identifies ‘ravines’ in Rwanda, central Africa, with a ‘second form’ of gully that are deep incisions occurring on steep slopes, initiated by overland flow and progress downslope. Thus, he describes them where the “head is the oldest, and most times the deepest and largest point of the ravine” (Moeyersons, 1991: 311), and are normally disconnected from the valley bottom drainage. Because of this last attribute, they are considered gullies rather than first order drainage, being likened to the second form of gully identified by De Oliviera 17

THWAITES ET AL. (1989) in Brazil. These are steep-slope incisions initiating by concentrated overland flow at the breaks of slope. The dominant incisional, downward erosion into deep unconsolidated materials is a characteristic attribute, but the downward progression of the erosion with little if any headwall retreat puts them into disconnected stream channels rather than gullies proper.

With the USDA-NRCS (2008) also describing ravines (syn. gulches) as stream channels we believe that ravines are problematic to include in the definition of a gully. It also seems that there are many similarities between the Indian ravines and Badlands.

2.2 Definition: what a gully is

From the above discourse we can assert that gullies can be defined by elimination as not rills, not stream channels, not all ephemeral gullies, not badlands in themselves, and possibly not ravines. We turn now to defining gullies by inclusion, albeit that we cannot move completely away from consideration of ‘not gullies’. Brooks, et al. (2018) have provided an objective definition of a gully distinguished from a stream channel that is pertinent to consider here. They stress that the identification and categorisation into one or the other definitions is by weight of evidence – not all criteria are necessary, nor is each characteristic in isolation a defining feature of a gully. It is presented here in modified form from Thwaites et al. (2020).

An active Gully (See Figure 1): i) is not a ‘stream channel’ (or watercourse); ii) is a persistent erosional feature, with active head or walls on average > 0.5 m deep, and has multiple modes of expansion, but always including headwardDRAFT retreat into an otherwise un-dissected landscape; iii) erodes unconsolidated materials and saprolite, but not bedrock (otherwise it is a ‘stream channel’ or a low-order drainage line); iv) must have an active head scarp or head wall at the upslope limit of the gully (which may or may not be a clear nick point): • sometimes a series of head scarps may occur;

THWAITES ET AL. • a ‘scalded’ or desiccated area (i.e. an area stripped of its topsoil with degraded vegetative cover) may often fringe the upslope area of the head scarp and head walls; • has an erosional gully wall scarp, or clear erosional cut – the point one can stand on the edge of and be sure that, “this is the gully margin (rim, edge)”. • some exceptions, such as the expanding, shrink-swell, cracking clays (i.e. Vertosols† ≡ Vertisols, Vertic Cherno-, Kastano-, Phaeo- zems, ‘blacksoil’; and friable Black Dermosols ≡ Udolls, Cherno-, Kastano- zems, Prairie Soils, ‘blacksoil’) that may have convex walls and a head ‘ramp’ need considering for evidence of other active erosion present in these cases, e.g. piping, tunnels or slumping; v) has the head (head scarp, head wall), or heads, marking the upstream boundary of concentrated water flow and sediment transport between definable banks; vi) has a cross-sectional shape (U-shaped, V-shaped, trapezoidal, slot, or tiered‡) that is permanently recognizable without flow; vii) has a straight bed long-profile, rather than a curved one (of stream channels); viii) has a dominant proportion of a hard margin (a comparatively sharp break of slope from the unbroken land surface to the incisional feature, that represents a gully head scarp and wall scarp of active erosion, commonly referred to as a rim, edge or scarp), active or otherwise, or equivalent; ix) has active erosional walls of at least moderately steep gradient (~ 30o; ~ 60 %), and gully walls are dominantly bare soil materials; x) are autonomous – having the active sediment source predominantly within the gully (a clear autocthonous (or ‘internal’) erosional zone); xi) may haveDRAFT land upslope of the head, or beyond, that may be a drainage depression (swale), or marshland in keeping with the incisional caveats above; xii) is typically driven by proximal ephemeral flows (i.e. associated with rainfall directly in the gully and in the gully catchment). † Australian Soil Classification (Isbell and NCST, 2012).

THWAITES ET AL. ‡ Defined in Thwaites et al. (this volume)

Figure 1 The concept of differentiation between gullies and steam channels – essentially autochthonous vs allochthonous sediment sources.

In contrast, a stream channel is considered to be a permanent feature formed by flowing water (either ephemeral or perennial) through both erosion and deposition. It is a ‘fluvial’ feature that shows features such as terraces, channel bars, and deposited materials derived from distant up-stream sources. Channels also show evidence of intermittent overbank flooding. Definition overall is again by weight of evidence – not all criteria are necessary, nor is each characteristic in isolation a defining feature of a channel.

DRAFT A stream channel (see Figure 1) has: i) defined channel bed and banks; ii) evidence of fluvial processes and dynamics, e.g. they are dynamic and are both erosional and depositional systems (i.e. not autonomous: comprising both autocthonous (internal) and allocthonous (external) sediment sources) without an obvious, active upslope head scarp (other than the rare case of active knick point migration triggered by downstream or base-level changes);

THWAITES ET AL. iii) features that indicate fluvial processes and dynamics, for example: • the presence of a self-formed floodplain; • a distinct assemblage of fluvially derived depositional geomorphic units, e.g. point bars, lateral bars, benches. • bed material load that is distinctly different to the locally available sediment source (i.e. from head or wall erosion) • a width/depth ratio > 1 • meander development of the bed (not just within the bed).

For the purposes of the classification that is the outcome of this paper, a working definition of a gully can be derived from the foregoing. We propose a generic definition of a gully that can serve as a basis to the construction of a classification framework:

A gully is a persistent, incised erosion feature with walls and/or head scarp on average > 0.5 m deep (or that cannot be generally be removed by conventional tillage methods for Field Gullies) and cannot be defined as a stream channel (1st order or greater). The feature erodes residual soils, unconsolidated materials and saprolite, but not solid bedrock, and it has multiple modes of expansion. It always includes extension by headward retreat into an otherwise un-dissected landscape by way of a clearly-defined head scarp or head wall. It has erosional side walls of at least moderately steep gradient (~ 30o; ~ 60 %), and gully walls are dominantly bare soil materials with a distinct break of slope from the uneroded hillslope surface, that exhibit erosion by means of mass movement, with or without sapping by seepage. The feature overall is typically driven by ephemeral flows associated with direct rainfall into the gully and in DRAFTthe gully catchment. Alluvial gullies, however, are commonly affected by backwater, and in some cases complete over topping, by floodwaters from the stream/river they adjoin. The active sediment source is predominantly from within the gully as a clear, bounded, internal erosional zone.

3. Classifications, categorisations, groupings, typologies: what type of gully? 21

THWAITES ET AL. Many authors have attempted to both classify gullies and define them in some way by different criteria (e.g. Ireland et al., 1939; Schumm, 1954; Frevert at al., 1955; Brice, 1966; Heede, 1970; Hilsky, 1973; DePloey, 1974; Tejwani et al., 1975; Bradford and Piest, 1980; Imeson and Kwaad, 1980; Schumm et al., 1984; Foster, 1986; Geyik, 1986; Planchon et al., 1987; Ezechi and Okagbue, 1989; Poesen, 1990; Poesen and Govers, 1990; Poesen et al., 1993; Wells, 1996; Casalí et al., 1999; Oustwoud, et al., 2001; Franklin et al. 2004; Brooks et al., 2009). There is even more than one classification for gullies on Mars (Aston et al., 2011; Auld and Dixon, 2016). Previous efforts of classification have usually been for specific purposes of investigation and are usually related to location or specific environment. They are therefore difficult to transfer or extrapolate for different purposes or to different parts of the world. Above all, besides being location- specific, these various categorisations and definitions have been in many cases unsystematic, thus limiting their applicability and transferability. The first clear attempt at classifying gullies (by their form or shape – their morphology) was by Ireland et al, (1939), their terminology and other aspects of which are still used today (Fig. 2). They considered the gully head as an important aspect to the process of gully formation and an aid to identifying gully type (Fig. 3).

However, gullies have been classified in many other ways than by profile or plan form. The following section specifies those features deemed useful for a general- purpose classification of gullies.

DRAFT

THWAITES ET AL.

Figure 3 An initial, and enduring attempt at classifying gullies by Ireland et al. (1939: p 55). Their caption included: A; B; C; D; & F are all from field surveys. Scale is 300 ft per inch. E is from a sketch: scale about 75 ft per inch.

Figure 2 The identificationDRAFT and classification of differences in gully head profile and planiform by Ireland et al. (1939: p 57).

3.1 General-purpose concepts of gully classification

Hilsky (1973) proposed that classification of gullies should be based on i) gully form (in plan), ii) gully-side morphology, and iii) forms of the longitudinal and transversal profiles. This concept was followed up by de Ploey (1974) in his three

THWAITES ET AL. morphological types of gullying identified in the Mediterranean environment of Tunisia: i) V- or U-shaped axial gullies with a single headcut and approximately constant width – equivalent to the ‘Linear’ gullies of Ireland et al. (1939); ii) digitate- form gullies with complex patterns and several headcuts – similar to the ‘digitate’, and ‘dendritic’ gullies of Ireland et al. (1939) and Schumm (1956) among others; and iii) ‘frontal’ gullies starting at riverbanks and vertical stabilised gullies – adopted by Morgan (1995) and adapted by Brooks et al., (2009) and Shellberg and Brooks (2012; 2013) as their ‘continuous scarp-front’ gullies. We see this as a good basis for a morphological component of a generic classification.

Imeson and Kwaad (1980) tender a useful set of four gully ‘types’, specifying that both morphology and processes distinguish the various types of gullies recognised. Their four types are, however, described – rather than defined – by sets of conditions, with the only real diagnostic criterion being the cross-sectional shape (an attribute rather than a condition) – the other conditions being ‘position in the landscape’, ‘principal source of run-off’, ‘material in which the gully is developed’, and other ‘favourable conditions’ (to the development of those specific gully types). This scheme is more of a Typology than systematic classification (hence the use of the term ‘type’), and although designed to be applicable globally (with examples from Europe, northern and southern Africa, and Australia) it appears to fall short of conditions that can be encountered in other gully erosion environments. Nonetheless this can be a useful foundation for various typologies that can emanate from a general-purpose classification.

Similarly, the Indian approach to classification can be seen to be more of a typology as it is directed purposely at reclamation and control (Haigh, 1984; Dagar, 2018). Essentially it relies on eight parameter criteria to classify ‘ravines’ into five suitability groups (for reclamation).DRAFT No classification of gully form (sensu Ireland et al., 1939) is considered for Indian ravines, as the focus is on what is deemed most important for control and reclamation, such as depth, width, and either side-slope gradient (Tejwani et al., 1975) or channel bed slope (Haigh, 1984; Raj et al. 1999; Dagar, 2018). As such, this parameterized classification (typology) has little relationship to a general-purpose classification.

THWAITES ET AL. Oostwoud(-Wijdenes and Bryan (2001) identified a hitherto unrecognised form of gully on alluvial plains in Kenya that has become to be more well known since, especially in Australia (Brooks et al., 2009) as “… deep badlands systems … enhanced by dispersive soils and localised piping…”. Brooks et al (2009) adopted this type of gully description to their extensive alluvial gully systems identified in northern Australia but argued to differentiate them from ‘badlands’ for the reasons mentioned earlier.

Wells (1996) specifies several characteristics for grouping gullies under both morphological and managerial properties. Although his factors are biased towards those features occurring in cultivated land, they can be modified to apply to the ‘Landscape Gully’ scenario: i) Morphological - a. gully depth (e.g. Frevert et al., 1955; Houghton and Charman, 1986); b. gully length; c. gully length per unit area; d. gully cross-section – most commonly the U-shaped, and V-shaped classes of Ireland et al. (1939); e. gully pattern, i.e. plan-form shape, though only dendritic and parallel forms are referred to); f. gradient of channel; g. drainage area (contributing catchment area). ii) Managerial – a. hazard of extension (e.g. Imeson and Kwaad, 1980); b. need for conservation (measures); c. possibilities for control.

This appears to be a sound basis both for morphometric and modelling studies of all gully types as wellDRAFT as being practical for conservation or sediment control planning.

The following is a categorisation of the discriminatory characteristics that have been used to classify gullies.

3.2 Plan-form shape

THWAITES ET AL. As noted earlier Ireland et al. (1939) were the first to document and widely communicate their findings from gully erosion on the Piedmont of South Carolina. Their classes and graphical example are given in Figures 2 and 3. From this scheme we still see the common usage of the dendritic, linear, and parallel patterns terminology in the literature to describe gully forms.

The classification of Brooks et al. (2009) and Shellberg and Brooks (2012; 2013), emerging from their work on the Mitchel River alluvial megafan in northern Queensland, is both form- and process-based and includes the variations in plan- form shape – associated with dominant erosional processes in predominantly alluvial landscape environments:

• Linear gullies are often incipient, commonly associated with land use disturbances that tend to concentrate overland flow. (e.g. stock tracks, roads, and fences); • Dendritic gullies, associated with well-defined drainage networks, separated by distinct interfluves with often less distinct head scarps; • Amphitheatre gullies that are often as wide as, or wider, than they are long due to the lack of structural control, have well-developed head scarps, and drain into relatively narrow outlet channels; • Continuous scarp-front gullies, paralleling the main channels and developed from the coalescence of laterally expanding amphitheatre gullies, and/or from river bank erosion on meander bends.

Thwaites et al., (2020) have since grouped the first two gully types into ‘Extensional’ gully systems for their dominant process of longitudinal axial advance largely by head scarp retreat, and the latter two into ‘Expansional’ systems, based on their predominantDRAFT advancement by irregular retreat of walls along the lateral axis, that have become multiple head scarps. These are known as ‘axial advancement forms’ (Thwaites et al., 2020) and are descriptive of their processes only in the phase in which the gullies are currently observed, but there are many instances where one advancement process form can transform over time from dominantly one axis to dominantly the other, especially in dispersive and other susceptible host materials.

THWAITES ET AL.

3.3 Head, head-cut and wall (bank) morphology

Ireland et al. (1939) were also the first to highlight the importance of the gully head, and its form being a result of distinctive processes, by producing a classification of the head profile- and plan-form into four classes of each (see Fig. 3).

Brice (1966) identified three types of steep scarp features that occur in all of his three valley positions (valley-bottom, valley-side, valley head) as ‘head scarps’ at the gully heads, ‘channel scarps’ within the long profile of the gully (mainly valley- bottom gullies) and ‘side scarps’ being the actively eroding (steep) gully banks. These were all viewed as seepage-induced features.

Gully-wall failure is due to an interaction of several soil and topographic properties; the stress state within the soil mass and the conditions that precipitate the failure may vary appreciably from one gully to the next (Bradford et al., 1973, Crouch, 1987). Dietrich and Dunne (1993) acknowledged the work of Ireland et al. by producing a more detailed version resulting in the definition of no less than 10 different types – based on the depth of erosion, signifying distinct material and process differentiation. Essentially, this resulted in variations of the gully head into three types, described as ‘gradual’, ‘step’ and ‘headcut’ , the last requiring a drop of more than one metre).

The morphology and dynamics of the gully walls (banks, side-walls) has received less attention than head walls and head-wall retreat. Blong et al. (1982; 1985) make a case for the importance of side-wall processes in gully development, gully description and conservation management from their work in New South Wales, Australia. They estimateDRAFTd that more than half of the sediment volume transported from the gully is from the gully walls, and that active erosion in many gullies corresponds to particular side-wall forms, dominated by mass failure and fluting (from dispersive, sodic soils). Interestingly, they found that seepage, Brice’s (1966) driver of gully bank failure, appeared to be negligible in these instances. A finding compatible with that of Brooks et al. (2009) and Shellberg and Brooks (2012; 2013) for the gully system in northern Queensland.

THWAITES ET AL. In a further development of their earlier work Crouch and Blong (1989) introduced a four-factor classification comprising criteria relating to i) side-wall erosion activity (3 classes), ii) side-wall morphology (4 classes), iii) dominant side-wall process (4 classes with sub-classes), and iv) ‘special feature’ (no classes: open-descriptive, e.g. rock outcrops, piping, animal tracks). This classification suits a special- purpose, i.e. conservation management, typology approach very well and should be considered in any such undertaking. However, the categorizing of erosion side-wall activity observations and morphology could be developed usefully for a general- purpose classification system.

3.4 Continuity / Connectedness

Leopold and Miller (1956) originally classified gullies into two types: ‘continuous’ and ‘discontinuous’. They relate that continuous gullies start in the headwater area from many rills that coalesce to form the main gully on the valley floor. Discontinuous gullies (or ‘independent’ gullies – Geyik, 1986) on the other hand form on the backslope and lower slopes of the valley side, which was a major, classifying factor introduced by Heede (1970). They do not join up with other gullies or stream networks but, instead, distribute their flow as fans over gentle slopes, or in valley-fill aggradation in ponds or swamps, e.g. the headwater ‘swampy meadows’/ ‘chain-of-ponds’/’cut-and-fill’ landscapes (Eyles, 1977; Prosser et al. 1994; Fryirs and Brierly, 1998; described in S E Australia . Their discontinuity is explained by changes in slope gradient affecting the hydraulic capacity of the gully flows to deposit the eroded sediment from upslope. This observation of discontinuity is important, hydrologically, when considering delivery and fate of eroded sediment.

3.5 Landscape position

Originally, Brice (1966)DRAFT had identified the ‘valley-side’ and ‘valley-bottom’ gullies in the loess landscapes of Nebraska and included a third type: ‘valley-head’ gullies in the upper catchment regions. Higgins et al. (1990) adopt this grouping of gully types and go on to describe them in detail. More recent definitions of gully types appear to have dropped the concept of valley-head category of gullies, e.g. Grissinger (1996); Harvey et al. (1985).

THWAITES ET AL. Lee and Charman (2005) differentiated badlands from gullies, by low-resistance material, steep forms, and a high degree of instability. Badlands can contain gullies. They identified just two types of gullies: ‘valley-floor’ gullies and ‘valley-side’ gullies. The former occurs with concentrated run-off in valley alluvium, ranging from ephemeral features to entrenched arroyos and wadis, while the latter form from concentrated run-off on hillslopes, sub-surface piping, and mass movement. This distinction between the two gully types (valley-floor / valley-side) and the concept appears to be well-accepted for basic gully types, albeit in modified forms, and for different purposes. Valley floor gullies are described as valley-fill gullies by Eyles (1977) and Prosser et al. (1994) in the ‘swampy meadows’ landform and cut-and-fill landscapes of the substantial Holocene aggraded valleys in the southern Tablelands of SE Australia. Deep (up to 10 m ) and rapid gully erosion in these permanently or periodically saturated fills has been caused through re-activated swamp drainage through protective valley-floor vegetation degradation (Prosser and Slade, 1994), probably triggered by human activities in the past 3,000 – 6,000 years (Prosser et al., 1994), but substantially since European settlement (Eyles, 1977; Prosser et al., 1994; Fryirs and Brierly, 1998). From the authors’ descriptions, these gullies appear to be more of an arroyo-form by distinct drainage channels, albeit discontinuous drainage rather than autonomous gullies. These palustrine valley fills, or ‘swampy meadows’, are largely alluvial (Eyles, 1977; Prosser, et al., 1994), which is, by far, the most common material for these types valley-bottom/-fill gullies.

Gullies on the ‘valley-side’ hillslope in hillslope materials, i.e. colluvial soils, or deep residual soils, aeolian deposits (e.g. loess), saprolite, and soft, weathered rock material, whether cultivated or not, are the most commonly described around the world (Brooks et al., 2009). They are also common in northern, eastern, and southeastern Australia DRAFT(e.g. Blong et al., 1982; Crouch, 1987; Olley and Wasson, 1993; Rustomji, 2006; Hancock and Evans, 2006; Bartley et al., 2007; Saxton et al., 2012) where they have not been grouped into any particular type of gully but described by the materials in which they occur (see Soil Materials below). These gullies have been studied for largely hydrological process and landscape development purposes, as well as palaeoclimate reconstruction.

THWAITES ET AL. Higgins et al. (1990) added an ‘upland gully’ class (see earlier discussion) and describe it as being similar to Brice's valley-bottom gullies but in an upland setting. Although reference to this type of gully has been maintained in more recent literature from the USA, it is seen here to be only a variation of ‘valley-bottom’ gullies. Brooks et al (2009) describe the ‘hillslope’ gullies studied in Australia as occurring in the upland parts of catchments (referring to low stream-order headwater settings) but refrain from calling them upland gullies as the emphasis is on the hillslope processes and hillslope materials as well as providing ‘valley fill’ environments (e.g. Eyles, 1977; Prosser et al., 1994; Fryirs and Brierly, 1998; Olley and Wasson, 2003), where their ‘alluvial’ gullies can also occur. The features described by Eyles (1977) and Prosser et al. (1994), are described as gully incision into upland valley fills (‘swampy meadows’) in the Australian southern highlands. These features are clearly not hillslope gullies, in that they form within largely alluvial fills, and so technically might be better described as a type of alluvial gully.

Higgins et al. (1990) also described the ‘upland gullies’ as occurring on stream banks or proximal steeper slopes (hence the valley-bottom analogy). Although these upland gullies are insufficiently defined as a class of gullies and can be lumped with valley-bottom gullies, Brooks et al. (2019) and Thwaites et al. (2020) identify a type of alluvial gully that does occur in the stream bank position, called ‘stream bank gullies,’ that are distinct from those ‘bank gullies’ defined by Poesen and Govers (1990) and Poesen (1993), discussed in the Introduction, for their typology of agriculturally induced incisional erosion. The stream bank gullies occur as features proximal to the drainage line or stream network (i.e. continuous gully features connected to the drainage system) and are initiated at the bank either hydraulically by localised floodplain overland flow concentrating in swales, or by repeated saturation and mass movement from flooding events, and hydraulic erosion from floodDRAFT drainage (Brooks et al., 2019; Thwaites et al, 2020).

In the cultivated land case, ‘bank’ gullies are one of two types of gully, the other being ‘ephemeral ‘ gullies, sensu Foster (1986), which are defined in the cultivated land environment to “… form where a wash-line, a rill or an ephemeral gully crosses an earth bank (Poesen et al. 2003), e.g. “… a terrace bank, a lynchet, an exploitation talus [undefined] or a sunken lane bank ...” (Poesen, 1993: 221; 226). These are also described as discontinuous gully features controlled eventually by

THWAITES ET AL. piping and mass movement from upslope runoff, rather than by gully catchment size (Poesen and Govers, 1990). However, these gullies are further described by Poesen et al. (2002) and Poesen et al. (2003) as retreating by headcut migration to gentler sloping surfaces of pediments, or river and agricultural terraces, implying that these are permanent or classical gullies, or non-ephemeral gullies at least, and are not readily removed, but can be controlled. For the purposes of this article and the proposed gully classification these are designated ‘field bank’ gullies.

Brooks et al. (2009) and Shellberg and Brooks (2012; 2013) distinguish proximal gullies, that drain towards the main drainage network channels, from ‘distal gullies’ that drain away from the main channel towards distally draining creeks or lagoons, or indirectly to the main drainage network – if connected to a drainage channel or network.

3.6 Cross-sectional shape Based on cross-sectional morphology, gullies have commonly been recognized as having different types, such as V-shaped, U-shaped, rectangular, triangular, trapezoidal, and wedge-shaped (Bennett 1939, 1955; Deng et al., 2015).

Shape index (si) is used to distinguish between U-shaped and V-shaped cross- sections, where low values are related to V-shaped cross-sections and high values are U-shaped ones (Gabet and Bookter, 2008). Gully channels have also been classified as triangular cross-sections (shape factor (sf) = 2.0), parabolic cross- section (sf = 1.5), and rectangular cross-section (sf = 1.0) (Heede, 1970; Deng et al., 2015). Heede (1970) perpetuated Ireland’s et al. (1939) belief that the shape factor expresses the gully evolutionary ‘stage’, developing from one to another over time in the Davisian paradigmDRAFT of landscape evolution. Ireland et al. (1939) expressed the stages as: i) V-shaped rill/gullies incising resistant clay B-horizons if the soil after A- horizon removal; 2. Incision breaking through the less-resistant C horizon or weathered rock material creating a clear headcut, and developing a U- shaped channel profile; ii) Re-adjustment with development towards a graded profile and shaping of the walls by weathering and erosional processes;

THWAITES ET AL. iii) The slowing of energy inputs and colonisation by plants towards what was understood to be stability.

These stages of gully growth towards inactivity (stability) have been expressed by several authors around the world but without the reference to cross-sectional shape per se and with and without the reference to cyclicity (e.g. Sobolev, 1948 (cited in Haigh, 1984); Tejwani et al., 1975; Schwab et al. 1981; Dagar, 2018).

Boucher (undated) recounts that Downes (1946; 1949), working in north-eastern Victoria, Australia, associated the cross-sectional shape of gullies to differences in particular soil characteristics: a 'U'-shaped cross-section was associated with highly erodible, dispersive sodic subsoils whereas 'V'-shaped gullies formed in soils with a high level of aggregate stability and where the subsoils exhibited less sodicity. Much of the recent literature tends not to make these distinctions and, instead, records cross-sectional shape as either stages of growth within the same eroding materials, or as a result of different erodible materials: V-shaped in more cohesive, highly aggregated materials, U-shaped (and trapezoidal) in more erodible, unstable, poorly structured materials – with substantial floor deposition. From our experience, cross sectional shape is both a function of soil material type (see below), and gully evolution (single or multi-incisional phases).

3.7 Depth

We have seen that depth is a major differentiating criterion between rills and gullies, and it has been seen by some authors to be a useful discriminant from a land management and rehabilitation viewpoint. Different gully depth classes have been suggested by a few authors for simple classification purposes, or as typologies for land management guidelines (e.g. Frevert, 1955; Tejwani et al., 1975; Schwab et al., 1981; Narayana and Shastri, 1985; Wells, 1996; Franklin et al., 2004). Table 2 provides a summaryDRAFT of the variable depth class limits for gullies, for different purposes and reasons but mainly for control and rehabilitation, from a selection of authors, including a definition for ravines.

THWAITES ET AL. Table 2 Depth classes for gullies specified by authors from different environmental regions. Where it is not stated a lower limit of 0.3 m for a gully, distinguished from a rill, is assumed.

Shallow Deep Very Deep Ravine Source < 1 m < 3 m 3 - 9 m > 9 m Tejwani et al., (1975)

0.3 – 1 m 1 – 5 m > 5 m n/a Schwab et al., (1981)

< 3 m 3 – 9 m > 9 m n/a Narayana & Sastry, (1985)

< 1.5 m 1.5 – 3 > 3 m n/a Houghton & Charman, m (1986) < 1 m 1 – 5 m > 5 m > 10 m or Wells, (1996) >20 m Slight Moderate Severe Very severe < 1 m 1 – 3 m 3 – 5 m > 5 m SARCCUS (1981)

Class D Class C Class B Class A < 1.5 m 1.5 – 3 3 – 6 m > 6 m Franklin et al., (2004); m South East LLS, (2018)

The variation in classes and their boundaries and the purposes for which they were devised obscures a clear understanding of different gully types. It is our opinion that a categorisation of depths for incisional features is of more use for a conservation and land management typology rather than a generic classification.

3.8 Soil material

Surface soils and soil materials (including sediments) are often overlooked when considering drivers of gully formation and development. Soil and soil forming materials are mentioned in most publications concerning gully and rill erosion, mostly as a descriptiDRAFTon of the materials in which the features form, rather than being diagnostic of the types or forms of the features described. Higgins et al (1990) cite Fenneman (1923[2]) as being among the first to describe the effect of differential resistance of materials on erosion forms and rates of erosion.

Ireland et al. (1939) presented an in-depth description of the gully-forming environment, including the soil series recently identified in the region. However, soil

THWAITES ET AL. materials only feature in their stages of gully development rather than the classification of their gullies.

Subsequently, attention has been given to various aspects of the soil material that affect gully development, e.g. soil crusts, resistant clay B-horizons, absent A horizons, weak C-horizons, soil crusting, soil cracking, piping, sodicity (thence dispersibility), and other resistant layers, both in cultivated lands and in grazing or natural landscapes. These and other many, independent characteristics, are reported in an ad hoc manner and therefore cannot be encapsulated into a few soil ‘types’ or groups. Successfully categorising soil material for gully classification and description in detail proves elusive.

Schumm (1961) drew attention to the effects of a resistant hard surface soil, which, when breached would create and maintain a headcut for gully initiation. Soil crusts can have the effect of inhibiting infiltration and promoting runoff with the greater capacity to cause sheetwash/rainsplash erosion and rilling downslope (Sumner and Stewart, 1992) . Thus, crusted soils are eroded more by gully erosion than surface erosion (Valentin et al., 2005) as any susceptible, erodible subsoil, or sediment material beneath the crust will rapidly erode and undermine the crust of hard-setting topsoils by head and wall retreat. This is viewed as an issue of a ‘duplex’ property in soil material fabric (texture and structure) which also occurs in subsurface layering by fragipan formation or induration, or in different juxtaposed strata.

Soil piping is a feature especially of dispersive sodic soils and is a major component of erosive sub-surface flow (Rengasamy, 2002). It has been related to gully initiation and development (Heede, 1971; Crouch, 1976) and is prevalent in many badlands environments (Faulkner, 2013) as well as certain alluvial gullies in sub-tropical and tropicalDRAFT semi-arid climates (Brooks et al. 2009; Reinks et al. 2000) . The effect of soil piping then leading to tunnelling and tunnel collapse and cavitation, thus exacerbating the erosion process, is significant to gully development (Crouch, 1976; Bryan and Jones, 1997; Bernatek-Jakiel and Poesen, 2018). Piping can also occur in very leached soils (coarse sands, and unstructured clays) as well as the well-structured shrink-swell ‘cracking clays’, with high smectite clay content (Kasanin-Grubin and Bryan, 2007).

THWAITES ET AL. A lack of attention applies to detailing types of material into which gullies are eroding at depth, (e.g. granitic materials, alluvial sands, alluvial silts, sandstone or mudstone materials) other than ‘sandy’, ‘clayey’, ‘silty’ colluvial or alluvial materials, weathered bedrock, or loess materials. Reinks et al. (2000) have appreciated the central role of soil materials in South African dongas by focusing on the stratigraphic layering of colluvial materials and buried soils (palaeosols). They specifically target which properties contribute most to i) erosion vulnerability, ii) the vulnerability of the layer sequences, and iii) understand the relationship between erosion susceptibility and where the dongas occur in the landscape. We conclude from this and our own experience with Australian soil environments that any categorization of soil materials should at least include material texture (e.g. gravelly, sandy, silty, clayey), presence of crusts or hardsetting top layers, predisposition to dispersivity and piping, and a recognition of sub-surface layer properties and their spatial effects.

4. Classification Summary

Gullies have been classified on the basis of size, outline (plan-form/pattern), cross- section shape (and area), head form (both plan and profile), bank wall morphology, depth, stage of development, continuity, and location within the drainage basin landscape, but rarely by their dominating processes (i.e. a genetic definition), for example a sidewall-eroding gully extending laterally compared to a headwall eroding gully extending longitudinally, or a gully receiving erosive energy from upslope channelised flow compared with one that is eroding internally from in-gully rainsplash and surface flow and groundwater sapping. Ezechi and Okagbue (1989) valiantly attempted a four-class genetic classification of gullies in eastern Nigeria, identifying baselevel; scarp; fracture; and incidental classes of gullies. This is, essentially, still a morpho-genetic scheme, but predominantly genetic. However, it is too localised andDRAFT environmentally specific to be applied to other gully environments for testing, or adoption within a general-purpose scheme.

4.1 An Inceptive Classification framework

After having sieved out the common themes and factors favoured by previous and current workers from these numerous methods and approaches to describe and define gullies, we can now consider a rationale and potential framework for a

THWAITES ET AL. generic, general-purpose gully classification. As a summation of the common themes and factors gleaned from the prior analysis, an ‘Inceptive Classification’ framework of gullies within all incisional land-surface forms is given graphically in Figure 4. The structure reflects a scalar order of criteria relative to the scale in the landscape at which they can be applied, arranged for minimal repetition and indirect association. i) Definitions of arroyos (e.g. Schumm & Hadley, 1957, Wells, 1996) and ravines (e.g. Haigh, 1984, Wells, 1996), as presented here, are open to question to be considered under the Gullies banner, and therefore are currently not included; ii) The high-level distinction of Field Gullies from Landscape Gullies is crucial. The classification of each can diverge here. The precept behind Field Gullies is that they are both formed by cultivation management practices and largely defined by them. Landscape Gullies drive the classification for the most part from here onwards, but Field Gullies are re-incorporated into the body of the classification framework for the sake of simplicity, even though some aspects of the lower level criteria may not usually apply. iii) Bank gullies suffer from blurred definitions and can be considered as extensions of (linear) Field Gullies (or rill flow) through constructed banks, and as a separate class of Landscape Gullies initiating on natural stream banks and extending by headward retreat into the floodplain alluvium beyond. These latter are similar in concept and process to the upland gullies described by Higgins et al. (1990), which are seen as an optional inclusion here, as they can be incorporated into alluvial Valley-bottom gullies (in uplands or lowlands). iv) Valley-head gullies were described by Brice (1966) in the same way as Valley-side gullies, except for their position in the valley. Higgins et al. (1990) seeDRAFT this as really a doubling up gully types and descriptions and recommends that valley-head gullies be treated as special cases of valley- side gullies. v) The plan-form or pattern of gully margins has always been one of the most apparent criteria by which to describe as well as define gullies. Both confusion and amalgamation of suggested types has been apparent in the literature since Ireland et al. (1939) expressed their form types in detail, but

THWAITES ET AL.

Figure 4 An Inceptive Classification framework of incisional land-surface forms, specifically the Gully-form pathway. This is seen as an intermediate stage for the GGC, constructed as a synthesis of the most relevant and useful criteria taken from the analysis of the gully erosion literature, as interpreted by the lead author. This forms an inceptive basis for the more comprehensive generic classification scheme.DRAFT

the two basic patterns, of Linear and Dendritic (or more accurately, ‘axial’ and ‘digitate’) have endured. We have included DePloey’s (1973) Frontal form as he describes and defines this as this appears to align with Brooks et al. (2009) concept of ‘continuous scarp-front’ gullies.

THWAITES ET AL. vi) Much has been made of gully cross-sectional shape as an indicator of soil material type and erodibility or as a stage of gully development. Only three main types have really been defined, with a fourth, ‘rectangular’, being mentioned occasionally as a special case of either a straight-sided Trapezoidal, or a flat-bottomed U-shaped gully. vii) After the cross-sectional shape the classification can get untidy and involved, with more detailed and specific observations of both processes and morphology relating the erosion and erodibility of the soil materials and erosion activity. Key erodibility factors can be reduced to the effects of the Dispersive nature of the material, the surface Crust (algal/cryptogamic or from slaking and/or dispersion) or Hard-set topsoil (from slaking, dispersion, and or fine infilling), or a sub-surface Limiting Layer (inhibiting infiltration and resisting hydraulic erosion). These are the only non-mutually exclusive classes in this inceptive classification scheme, and a gully could possibly possess all four. So these factors only serve to qualify the gully type rather than aid classifying it. viii) Distinctions in soil material form and texture can be made, along with the gully head and wall morphology (inferring process), but it is polemical as to what are the salient attributes of these criteria, of what worth they are to a general-purpose classification, and how they should fit in systematically. Hence, they are included at the lowest level of classification in a flexible form of arrangement. The dominant foci of attention for soil materials and processes, and conceptions of categorization, have been synthesised here in order to maintain some form of systematic progression from soil materials to the most relevant erosion processes. ix) From the literature the main interests in erosion processes and activity are directed at the gully Head as well as the Banks/Walls, which can define a dominant DRAFTerosion types that can then go towards defining a gully type by process. Both the Dominant Process (Failure, Seepage, Overfall) and Erosion Activity (Active, Semi-active, Stable) for both components of the gully are represented by those presented by Crouch and Blong (1989) that gives a good overview of the various aspects they investigated. Their morphology classes of gully walls (Vertical, Sloping, Faceted) is adopted here along with those of Dietrich and Dunne (1993) for the gully head (Gradual, Stepped, Headcut).

THWAITES ET AL.

5. Concluding remarks

Erosion gullies have been perceived, described, measured, and modelled in a variety of ways for a variety of purposes in different environments. Attempts at transferring the knowledge gained and regionalising, or environmentally correlating, those studies have been largely thwarted by the fact that it is difficult to convey and explain the complexity and variety of forms and behaviours of gullies and their environments without a lingua franca and systematic method of definition. It is obvious the vast amount of information gathered for each erosion gully setting requires some form of organisation and management.

From this review and discussion, we believe a rational and methodological framework can be developed for categorizing erosion gullies. The goal is taken to be a globally-acceptable, broad scale to relatively fine scale, general-purpose classification system. Although this aim may serve to complicate the selection of criteria and differentiating properties to classify gullies, it is seen as being the basis for more special-purpose classifications and typologies. The conceptualisation and rationale of such a gully classification system is the subject of the second paper in this series, where the structure and content of a comprehensive generic classification framework is presented.

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Acknowledgements The initial part of this literature review work was part-funded by the National Environmental Science Program (NESP) for Project 4.9: Gully Characterisation Framework to underpin GBR Catchment Water Quality Management. The authors would like to thank Julie Carmody, Damien Burrows and Sheriden Morris from the NESP Tropical Water Quality Hub for their support of the project. We also thank the other team members of the (Precision Erosion and Sediment Management) PrESM Research Group: Dr James Daley, Dr Justin Stout, and Graeme Curwen for their inputs to the earlier Queensland Gully Classification and the concepts.

The authors declare that they have no conflict of interest in undertaking this work. DRAFT