gully position, characteristics and geomorphic thresholds - hancock

HYDROLOGICAL PROCESSESHydrol. Process. 20, 29352951 (2006)Published online 8 June 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.6085

Gully position, characteristics and geomorphic thresholdsin an undisturbed catchment in northern Australia

G. R. Hancock1* and K. G. Evans1,21 School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia

2 Hydrological and Ecological Processes Program, Environmental Research Institute of the Supervising Scientist, Darwin, NorthernTerritory, Australia

Abstract:Gullying is a significant process in the long-term dynamics and evolution of both natural and rehabilitated (i.e. post-mining) landscapes. From a landscape management perspective it is important that we understand gully initiation anddevelopment, as it is well recognized that catchment disturbance can result in the development of gullies that canbe very difficult to rehabilitate. This study examines gully position using geomorphic statistics relating to featuressuch as depth, width and length in a catchment undisturbed by European activity in the Northern Territory, Australia.The results demonstrate that gullying occurs throughout the catchment and that a slopearea threshold does not existand that gully position broadly follows the catchment areaslope relationship. Simple relationships relating catchmentarea and slope to gully depth, width and length provide poor results, despite these relationships having been found toapply for ephemeral gullies in cropland. The results suggest that gully initiation thresholds are low as a result of anenhanced fire regime. A threshold model for gully position that uses catchment area and slope to switch between gullyand hillslope was evaluated and found broadly to capture gully position. Copyright Commonwealth of Australia,(2006) Published by John Wiley & Sons, Ltd.KEY WORDS gully; gullying; channelization; geomorphology; hydrology; areaslope relationship; modelling; digital

elevation model

INTRODUCTIONThe drainage network in a catchment integrates many factors in landscape development in response to theprevailing climatic and physical attributes. Channels route sediments and nutrients and also provide a localbase level affecting local landscape stability. Degradation or lowering of the channel may occur as a resultof structural and/or climatic changes associated with differences in topography, vegetation and lithology(Leopold et al., 1964). The extension of the drainage network by headward migration of headcuts or gullyingis a significant process by which the channel bed is lowered (Leopold et al., 1964). Stream down-cutting isgenerally associated with the development of convex hillslope profiles, whereas aggradation produces concaveprofiles (Knighton, 1998). The resultant hillslope form and overall catchment morphology is, therefore, aproduct of the linkages between all these factors.

In many catchments, gully incision and development are important factors in soil erosion (Vandekerckhoveet al., 1998; Nachtergaele et al., 2001a,b; Martinez-Casasnovas et al., 2003) and catchment development. Theinitiation of gullies, as well as the headward and lateral progression, releases large amounts of sediment andcan enhance rates of overall landscape lowering and evolution (Hancock and Willgoose, 2001, 2002; Alonsoet al., 2002). This can result in increased sedimentation and water quality problems in many catchments.Consequently, the position and stability of gullies and gully processes are important determinants of thedrainage network and landscape processes (Patton and Schumm, 1975; Knighton, 1998).

* Correspondence to: Dr G. R. Hancock, School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2308,Australia. E-mail: [email protected]

Received 24 January 2005Copyright Commonwealth of Australia, (2006) Published by John Wiley & Sons, Ltd. Accepted 9 June 2005

2936 G. R. HANCOCK AND K. G. EVANS

From a landscape management perspective it is important that we understand the process of gully initiationand development, as it is well recognized that catchment disturbance can result in the development of gulliesthat can be very difficult to rehabilitate (Saynor et al., 2004). This issue is also of critical importance inunderstanding the long-term stability of post-mining landforms, which often have steeper slopes than thesurrounding undisturbed landscape, are devoid of, or have only limited, vegetation and often may containhighly erodible material, such as tailings (or potentially acid-forming material), below the landscape surface(Hancock et al., 2000, 2002). Therefore, the prediction of where gullies may begin and end is very importantfor environmental management (Vandekerckhove et al., 2000a).

Spatial variability of catchment behaviour and process is an intrinsic feature of natural hydrological systemsand can have a substantial influence on the overall behaviour of a catchment (Western et al., 2001). Gullyinitiation is expected to occur above a threshold flow shear stress within uniform geological and hydrologicalconditions, and different threshold values are expected where geology, soils, vegetation and climate differ(Vandekerckhove et al., 1998). Gullying has often been considered to be a threshold function inverselydependent on catchment area and slope and has been analysed as a predictive tool for location of gulliesor valley incision (Patton and Schumm, 1975). The areaslope relationship provides a simple tool forinvestigating the location of gullies in concentrated flow areas (Horton, 1945; Patton and Schumm, 1975;Dietrich and Dunne, 1993; Montgomery and Dietrich, 1994; Vandaele et al., 1996, 1997; Moore et al., 1998;Desmet et al., 1999; Vandekerckhove et al., 1998, 2000a,b; Nachtergaele et al., 2001a,b; Martinez-Casasnovaset al., 2003).

There has been considerable research into understanding gully development and channelization, but muchof this effort has been in disturbed or agricultural settings, examining ephemeral gullies or areas that have beencleared and subject to grazing (Patton and Schumm, 1975; Montgomery and Dietrich, 1988; Prosser et al.,1995; Vandaele et al., 1996; Vandekerckhove et al., 1998, 2000b; Nachtergaele et al., 2001a, 2002; Daba et al.,2003). There have also been many attempts to model gully position, features and development (Sidorchuk,1999; Hancock et al., 2000; Torri and Borselli, 2003); but, again, this has mostly been attempted for ephemeralgullies or in catchments that have been disturbed by grazing (or other agricultural) practices (Nachtergaeleet al., 2001a,b; Alonso et al., 2002). Consequently, the impact of humans is difficult to differentiate fromnatural (or non-human) processes. Consequently, many of these field and gully modelling studies haveproduced ambiguous results. If we are to model gully formation and development, then it is important thathydrology and erosion be linked (Nachtergaele et al., 2002).

This study examines gully characteristics in an undisturbed (by Europeans) catchment in the NorthernTerritory, Australia. There appears to be a lack of studies examining gullies in undisturbed (by Europeans)environments. This catchment has uniform geology, soils, vegetation and, because of its small size (50 ha),climate can be assumed to be uniform. The aims of this study are to investigate gullying processes in thecatchment by examining trends in gully head drainage area and slope characteristics, as well as gully depth,width and length. Gully position in the catchment will be examined using a digital elevation model and itslocation within the drainage network.

STUDY SITE

Tin Camp Creek is a site undisturbed by Europeans in Arnhem Land, Northern Territory, Australia (Figure 1).There has been no intense grazing or other agricultural practices ongoing within the area, as a result ofEuropean settlement. Consequently, the hydrology and erosion history that shaped the landform can bereasonably assumed to be as that observed today, subject to caveats about long-term climate fluctuations.The catchment has a geology very similar to the Energy Resources Australia (ERA) Ranger uranium mine.It is thought to be an analogue for the long-term rehabilitated post-mining landscape and has undergoneextensive examination in recent years as a result (Riley and Williams, 1991; Hancock et al., 2002; Moliereet al., 2002; Hancock, 2003, 2005; Willgoose et al., 2003).

Copyright Commonwealth of Australia, (2006) Published by John Wiley & Sons, Ltd. Hydrol. Process. 20, 29352951 (2006)

GULLY POSITION, CHARACTERISTICS AND GEOMORPHIC THRESHOLDS 2937

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Figure 1. Location of the Tin Camp Creek (TCC) study site and the ERA Ranger Mine (ERARM)

The site is located in the seasonally wet/dry tropical environment of northern Australia, with an annualrainfall of 1389 mm, falling mostly in the wet-season months from October to April. Short, high-intensitystorms are common; consequently, fluvial erosion is the primary erosion process (Evans et al., 1999; Townsendand Douglas, 2000; Saynor et al., 2004).

The area is presently a tectonically inactive or stable area. Tin Camp Creek is part of the Ararat Land System(Story et al., 1976) and developed in the late Cainozoic by the retreat of the Arnhem Land escarpment, whichhas resulted in a landscape that is actively being dissected. In this study, a smaller, geologically uniform50 ha catchment was selected for this investigation, and is representative of others in the area (Figure 2). Thecatchment consists of closely dissected short steep slopes 10100m long and gradients generally between 15and 50%. The soils are red loamy earths and shallow gravelly loam with some micaceous silty yellow earthsand minor solodic soils on alluvial flats (Riley and Williams, 1991).

The native vegetation is described as open dry-sclerophyll forests and, although composed of a mixture ofspecies, is dominated by Eucalyptus and Acacia species (Story et al., 1976). Melaleuca spp. and Pandanusspiralus are also found in the low-lying riparian areas with an understorey dominated by Heteropogoncontortus and Sorghum sp. There is vigorous growth of annual grasses during the early stages of the wetseason. These grasses often fall over during the wet season, providing a thick mulch that causes reductionsin erosion rates of bare soil.

Cover afforded by vegetation is often reduced by fire during the dry season, which enhances the potentialfor fluvial erosion (Prosser et al., 1995; Evans et al., 1999; Townsend and Douglas, 2000; Saynor et al.,2004). Wild fire in the late dry season is common, with the area experiencing fire on an annual to biennial



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Figure 2. Digital elevation model (10 m 10 m grid) of the Tin Camp Creek catchment, Northern Territory, Australia

basis (Townsend and Douglas, 2000). Fires are initiated both naturally and by land managers, following thepractice of the Australian Aborigines, who have burnt large tracts of land in the north of Australia for at least40 000 years.

Previous studies have examined gullies at Tin Camp Creek (Riley and Williams, 1991) and other close-bylocations (Hancock et al., 2000; Saynor et al., 2004). Reasons for the existence of such gullies are speculative,ranging from the presence of feral animals, such as water buffalo (introduced in the 1800s and removed inthe 1970s), pigs (introduced in the 1900s) and wild horses, as well as an enhanced fire regime due to thepresence of aboriginals in the area over the last 40 000 years (Townsend and Douglas, 2000). Nevertheless,there has been no intense grazing or other agricultural practices within the area as a result of the presenceof Europeans. Other studies in the region have examined gully development on the waste rock dumps ofthe Scinto 6 former uranium mine (Hancock et al., 2000) (Figure 1). This study demonstrated that numericalerosion models can capture the behaviour of individual gullies on an unrehabilitated mine waste-rock dump.

A high-quality digital elevation model of the area exists and has been used extensively in past hydrologicaland geomorphological studies (Hancock et al., 2002; Hancock, 2003, 2005; Willgoose et al., 2003) (Figure 2).The digital elevation model for Tin Camp Creek was created using digital photogrammetry by AIRESEARCHPty Ltd, Darwin, and was supplied as 240 000 irregularly spaced data points within an irregularly shapedboundary. The data had an eastings/northings and elevation positional accuracy of better than 01 m and05 m respectively. To place the data onto a regular grid, a gridding program was used to interpolate thelandscape elevation data on to a 10 m 10 m grid, producing a data set of approximately 82 000 points. Thisspacing was equivalent to the average spacing of the original AIRESEARCH data over the study catchment.All pits were removed from the digital elevation model using the Tarboton et al. (1989) method.

FIELD MEASUREMENT OF GULLIES AND CHANNELSIn August 2002, a field campaign mapped the Tin Camp Creek catchment for its gullies (Figures 3 and 4).At this time the entire catchment had been burnt, resulting in a near-complete absence of groundcover, thusgreatly enhancing the ability to locate and measure gullies. To locate and measure gullies, the catchment wassystematically walked along all drainage lines to the catchment divide.



Figure 3. Large gully in main channel (top) and a medium-sized gully in a tributary (bottom)

When walking along the drainage lines, gully positions were located by a satellite global positioning system(GPS; Magellan Meridian). Once located, gully depth, width and length were measured by hand tape measureto the nearest 001 m. In this study, a gully is defined as an incision in a drainage line that is clearly degradingwith a well-defined break in slope in the channel with a vertical or near-vertical headcut. The headcut, whetherthe incision was continuous downslope or not, had a height greater than 02 m.

The positional accuracy of the hand-held GPS was deemed sufficient for this study. Easting and northingaccuracy of the instrument was better than 7 m (95% RMS; Thales Navigation, 2001), which is suitable forthe 10 m digital elevation model used in this study. Additionally, the accuracy of the GPS was assessedby placing the unit at the same position within the catchment at different times during the field campaign.The results from 19 individual positional readings demonstrated that the standard deviations of eastings andnorthings coordinates were 38 m and 51 m respectively.



Figure 4. Position of gullies (represented by dots) in Tin Camp Creek with drainage support area of 1 pixel (top) and 20 pixels (bottom).Dark dots indicate that two gullies are close by and are overlaid on the digital elevation model

Table I. Statistics for the 140 gullies measured at Tin Camp Creek

Field data

Depth (m) Width (m) Length (m)

Average 055 14 59Standard deviation 04 13 80Median 04 08 3Minimum 02 02 05Maximum 25 14 50

RESULTS

The field investigation found 140 individual points of incision within the 50 ha Tin Camp Creek catchment(Table I). A description of gully field and statistical properties is discussed below.

Gully field propertiesGully depth ranged from 02 to 25 m, with an average incision depth of 055 m (Table I). In this study, the

smallest gullies measured were little more than rills that largely existed in the catchment headwaters and werethe terminal positions of the gully heads in the channel. The deepest gullies occurred in the main drainageline and developed in depositional material, whereas the widest gullies developed in the middle reaches ofthe catchment (Figure 3).

The gullies occurred at points of concentrated flow, both on the erosional hillslope areas and within thedepositional main channel areas, and appeared to be distributed over the catchment area (Figure 4). The



incisions occur over a range of contributing areas from a few pixels to many hundreds of pixels, with themajority of incisions occurring in the catchment headwaters (Figure 4).

Examination of the gully headcut and sidewalls revealed that the gullies on the hillslope areas developedlargely on hillslope colluvium (i.e. on the intact soil mantle) and many were underlain by bedrock at their base.The incisions all had well-defined headcuts and sidewalls and either merged with the surrounding channeldownstream, making it difficult to determine their length, or were well-defined incisions where the headcutand tail could be well defined. The majority of the incisions with well-defined headcuts and well-definedsidewalls occurred in depositional material farther down the hillslope, whereas the incisions that merged withthe surrounding hillslope occurred in the upper reaches of the catchment and developed on colluvium.

Many of these incisions were discontinuous (Leopold et al., 1964), in that they commenced with a headcutwith vertical sidewalls but rapidly lost definition moving downslope and merged with the hillslope only tocommence again further downstream with another headcut. Often, a depositional fan occurred at the terminusof the incision. It was clear in all cases that the headcut was retreating upslope, with the main erosional actionbeing overtopping and scour at the base.

Also, in the major drainage lines, there were many well-defined incisions or pot holes that werediscontinuous incisions created in depositional material at points of concentrated flow. Consequently, manyof the gullies observed and measured during this field campaign appeared to be transient in nature, withgully development occurring at one point while at another point downstream a previous incision could beaggrading. Examination of the headcuts and sidewalls of the gullies in the main channels revealed that thesegullies formed in depositional material that was constantly being reworked and transported downslope. Themajority of the main drainage lines displayed considerable aggradation. In some cases the passage of a headcutdestabilized the channel banks, resulting in slumping of the hillslope. Consequently, it is likely that gullyinghas played a major part in the landscape development of this catchment. There was no evidence of sappingor tunnelling in the catchment (Leopold et al., 1964; Knighton, 1998).

Gully statistical propertiesThe gullies were examined for their areaslope characteristics. For catchments, the areaslope relationship

is the relationship between the area draining through a point versus the slope at the point. It quantifies thelocal topographic gradient as a function of drainage area. A relationship of the form

AS D constant 1where A is the contributing area to the point of interest and S is the slope of the point of interest. The areasloperelationship is considered to be a fundamental geomorphic relationship with the value of ranging between 04and 07 for natural catchments (Hack, 1957; Flint, 1974; Montgomery and Dietrich, 1988, 1989, 1994; Guptaand Waymire, 1989; Willgoose et al., 1991ad; Tarboton et al., 1992; Montgomery and Foufoula-Georgiou,1993; Willgoose, 1994; Moglen and Bras, 1995a,b).

Two distinct regions of the relationship are typically observed in catchments. Small catchment areas aredominated by rainsplash, interrill erosion, soil creep or other erosive processes that tend to round or smooththe landscape. As the catchment area becomes larger, a break in gradient of the curve occurs. This is whereslope decreases as catchment area increases. This region of the catchment is dominated by fluvial erosiveprocesses, i.e. those processes that tend to incise the landscape.

Gully head position was plotted against its slope and area draining through the headcut. Figure 5demonstrates that there is considerable scatter in gully slope and area in the catchment (as found by others,such as Prosser and Abernethy (1996), Montgomery and Dietrich (1988), and Vandekerckhove et al. (1998)).The data largely follow the catchment areaslope relationship both in the loglog-linear (fluvial) componentof the curve (at areas approximately greater than 10 pixels) and in the diffusive (or convex) region of thecurve at areas less than 10 pixels (Figure 5). A greater proportion of incisions (96 in total) occurred at areasless than 11 pixels (diffusive region), compared with 44 incisions for areas greater than 11 pixels (fluvial



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region). The value of the exponent for the loglog-linear section of the gully areaslope data (i.e. areasgreater than 11 pixels) is 028 and fits within the range reported in many other studies investigating ephemeralgullies, cropland or land subject to grazing (Vandekerckhove et al., 1998). This value is also very close tothe value of D 032 for the catchment areaslope data for the same (fluvial) region of the data set.

To evaluate the impact of potential position errors in the gully coordinates and digital elevation model,the digital elevation model was regridded to a 20 m 20 m spacing and gully position replotted. This20 m 20 m grid is well within the accuracy of the gully coordinates and reduces the impact of any gullyposition error (over that of the 10 m 10 m digital elevation model). Hancock (2005) demonstrated that a20 m 20 m grid contains adequate hillslope detail for the Tin Camp Creek digital elevation model to bereliable. The catchment areaslope and gully areaslope relationship for the 20 m 20 m grid was equivalentto the 10 m grid, therefore providing confidence in the digital elevation model data and gully coordinates(Figure 5).

Examining continuous and discontinuous gully areaslope data separately produced the same pattern asobserved for the entire data set (Figure 6). Examination of gully depth and width versus area and of gullydepth and width versus slope revealed no statistically significant relationships (Figures 7 and 8), nor dida comparison of gully width and depth, of width and depth or of length and width (Figure 9). The lackof strength of some of these relationships is similar to that found in other studies (Vandaele et al., 1996;Vandekerckhove et al., 1998, 2000a,b).



DISCUSSION

Many studies have examined ephemeral gullies in agricultural fields that have developed during a singlestorm event/events or over a season. These ephemeral gullies are often removed by tillage in the followingseason. At Tin Camp Creek the gullies, although measured at a single point in time, are the product of manyyears of erosion. This study recognizes that plotting gully position and measuring its features is indicativeof their status at the time of measurement, not of how or when the gully started. Nevertheless, the fact thatwell-defined incisions exist in the catchment (and surrounding catchments) is indicative that gullying is animportant process in the area. Also, the fact that gullying is present in the absence of European agriculturalpractices in the area is an important finding.

The results demonstrate that there is no strong statistical relationship between gully depth and widthand area draining through the gully or slope at the gully head. The data demonstrate that, at best,gully position can be broadly predicted by the catchment areaslope relationship. Simple relationshipsrelating gully depth and width to discharge (or its surrogate catchment area), such as that proposed byLeopold and Maddock (1953) and Nachtergaele et al. (2002), do not appear to apply in this catchment(Figures 79).

There does not appear to be a critical slope or drainage area for the commencement or termination of gul-lies, as found for studies examining ephemeral gullies (Vandaele et al., 1996; Vandekerckhove et al., 2000b;Nachtergaele et al., 2001a,b). Consequently, a topographic threshold does not appear to exist, other than thedata follow the catchment areaslope trend. The result here also differs to that of Vandekerckhove et al.(1998), who demonstrated that the areaslope properties of gullies was loglog-linear for the entire dataset for ephemeral gullies in Mediterranean Europe. Published research investigating where and how gulliesend is scarce (Nachtergaele et al., 2001a). The finding that there is little difference between the area andslope properties of the continuous and discontinuous gullies (Figure 6) suggests that the same mechanics areoperating in both cases.

The results obtained in the Tin Camp Creek catchment demonstrate that whereas the majority of gulliesoccur in the upper reaches of the catchment (i.e. the diffusive region at areas approximately less than 11 pixels),gullies can occur throughout the entire drainage network and that the whole drainage network of the catchmentis at risk. Gullying occurred in both the diffusive- or rain-splash-dominated and the fluvial-dominated areasof the catchment. Montgomery and Dietrich (1994) stated that a drainage areaslope threshold for soil watersaturation can be used to predict the spatial extent of saturation overland flow, which can then be used topredict the spatial extent of a gully. This is based on the idea that there is a critical slope and/or catchment areato cause gully incision. At Tin Camp Creek, the threshold concept (such as that of Montgomery and Dietrich(1988), Patton and Schumm (1975) and Moore et al. (1998)), does not readily apply (Vandaele et al., 1996).Although this study cannot make comments on physical values for thresholds required for the initiation ofgullies, such as that done for ephemeral gullies, the data from this study demonstrate that the whole catchmentis potentially subject to gullying either by the initiation and or the passage of a gully, this being predicted bythe catchment areaslope relationship.

Nevertheless, it is very likely that gully initiation thresholds have been reduced by both the enhancedfire regime and the presence of feral animals. Studies in the region have shown that, in the absenceof vegetation as a result of fire, infiltration is reduced, a surface seal develops and runoff is enhancedwith an increase in both peak discharge and hydrograph volume (Evans et al., 1999). It is also wellrecognized that the soils of the region are fragile and that the passage of vehicles along bush trackscan initiate gullies (Saynor et al., 2004). Consequently, it is likely that the combination of enhanced fireand feral animals has reduced gully initiation thresholds to low values that place the whole catchment atrisk.

Figure 5 demonstrates that there is considerable scatter in the area and slope properties of the gullies.Nevertheless, this scatter is similar to that observed by other studies (i.e. Montgomery and Dietrich, 1989;Vandekerckhove et al., 2000a,b; Nachtergaele et al., 2001a,b). There have been many explanations for the



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scatter in the relationship between gully area and slope, ranging from different soil properties to differentsurface and subsurface rock content (Vandekerckhove et al., 1998). In this case, geology, soil catena andassociated vegetation are uniform down the hillslope, and we believe that this catchment and those surroundingit are largely similar.

Although the above results demonstrate that simple approaches to modelling gullying may not be applicableat Tin Camp Creek, the SIBERIA catchment evolution model (Willgoose et al., 1991ad), when calibratedfor Tin Camp Creek, can capture the long-term impact of gullying by correctly predicting catchmentgeomorphology (Hancock et al., 2002; Hancock; 2003; Willgoose et al., 2003). The modelling of catchmentevolution at Tin Camp Creek required erosion model parameters that were highly suggestive of gullying(i.e. m D 2 and n D 21) (Kirkby, 1971). When using less incisive erosion model parameters (i.e. m D 18,n D 21), a poor geomorphic match with the field catchment resulted (Hancock et al., 2002). For a reviewof the influence of these parameters see Kirkby (1971). The modelling of the evolution of Tin Camp Creekcatchment demonstrates that, at least for the recent geomorphic history, gullying has been a significantprocess and that correct erosion model parameters are needed to capture catchment geomorphology (Hancocket al., 2002). Using erosion model parameters that are indicative of less incisive erosion processes producescatchments with visually less incised hillslopes and geomorphologically do not match the field catchment.Previous modelling indicates that small changes in erosion model parameters can have a large impact oncatchment morphology. Further, a model of gully position was evaluated. This model (Willgoose et al.,



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1991ad) isAmSn

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where A (m2) is the area draining through a point, S (m m1) is slope, m (D 040), n (D 03) and (D 25)(Willgoose et al., 1989) are constants and at is a gully differentiation threshold. On using an at value of 25,although not able to capture individual gully position measured in the field (Figure 10), the model is able todemonstrate that gullying occurs throughout the stream network similar to the field data (Figure 11, top). Themodel also demonstrates that gullying is patchy and that, similar to the field data, there are some areas thatare not affected by gullying that have low slopes or small catchment areas. The gullying also extends highup into the channel network. Examination of the areaslope relationship for the modelled gullies (Figure 11,bottom) shows that the gullies occur throughout the drainage network and compare well to the field data.

The threshold for gullying is able to be increased by reducing the value of at. Figure 10 (top) shows thatwhen using at D 20 the predicted gully position is lower down the stream network and that less area ispredicted to be impacted by gullying. A threshold is also observed in the areaslope data, with increased arearequired to initiate a gully (Figure 11, top). Using smaller at values reduces the catchment area affected by



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gullying and increases the upslope area threshold needed for gullying. Although the parameter values usedhere match the field data, and we believe they are applicable to surrounding catchments with the same geologyand climate, considerable further work is needed to evaluate fully the sensitivity of these values both at TinCamp Creek and in other environments.

The above findings provide confidence in numerical catchment evolution models and the calibration processto predict catchment geomorphology (Hancock et al., 2002). Also, although it does not appear that simplemathematical relationships can capture gully features such as depth, width and length over the catchment(Figures 69), SIBERIA, when calibrated, is able to capture the overall landscape evolution process that inthis catchment includes gullying. Further, a model that utilizes the relationship between area and slope is ableto capture gully position and is likely to provide a measure of risk assessment. This is an important finding,as there has been little work done to test such gully initiation models in Australia in relation to post-miningerosion risk.

Understanding gullying at Tin Camp Creek is a difficult task, as the initial surface conditions of thelandscape are not known, the time-scale is not known, nor is the influence of introduced animals and achanged fire regime (there are no control catchments where fire and feral animals have been excluded). Suchinformation is difficult to resolve. Examination of the gully walls in some of the deeper incisions in the mainchannels reveals a stratigraphic record that demonstrates a cycle of erosion and deposition likely to predatethe introduction of feral animals. Consequently, gullying may be the result of a changed fire regime in the area



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over the last 40 000 years as a result of human (aboriginal) land management practices. Although it is difficultto estimate the impact of feral animals on the landscape, the catchment has little to offer in terms of fodderwhen compared with the much richer floodplains available in the region. Also, the area of the catchmentdisturbed in any one season by animals is likely to be very small compared with that of a continually grazedpaddock or annually ploughed field. This is an area for considerable further research.

The results of this study demonstrate that whereas the positions of gullies can be broadly predicted bythe area and slope properties of the gully head, other features (such as depth, width and length) are not



Figure 10. Predicted gully position for Tin Camp Creek for gully initiation value at of 20 (left) and 25 (right). Positions of gullies arerepresented by grey

easily predicted, and simple modelling approaches will not work in this catchment despite these relationshipshaving been found to apply for ephemeral gullies in cropland (Nachtergaele et al., 2002; Torri and Borselli,2003). This result suggests that, even in catchments with uniform geology, soils, vegetation, climate and landmanagement, simple modelling approaches cannot capture all gully properties and factors other than slopeand area influence gullies (Torri and Borselli, 2003), and approaches investigating texture of soil, alluviumand microtopography may provide insights (Evans and Willgoose, 2000; Vandekerckhove et al., 2000b).

It is possible that gully initiation is driven by minute surface irregularities that concentrate flow (Desmetand Govers, 1997) lower down in the catchment and that the gullies migrate headward, merging with thehillslope in the stream headwaters. Figure 4 demonstrates that many gullies were discontinuous along thedrainage lines and that this incision process is likely to be cyclical, with erosion/incision occurring at onepoint with deposition at another. The observation that the passage of a gully destabilizes a hillslope and causesslumping suggests that gullying is a major landscape evolution process.

The study of gullies at Tin Camp Creek will continue, as the position and features of 40 gullies representativeof those occurring in the catchment have been recorded. This will allow an examination of gully featuresthrough time in an undisturbed environment.

CONCLUSIONS

It is critical that we understand the link between hydrology and erosional processes if we are to understandand predict catchment processes successfully. Many studies have examined ephemeral gullies in agriculturalfields that have developed during a single storm event/events or over a season in previously disturbed areasor in fields currently subject to agriculture. These ephemeral gullies are often removed by tillage in thefollowing season. The results of this study in a catchment undisturbed by agricultural practices demonstrates



0.01

0.1

1

10

1 10 100 1000 104

model dataTin Camp Creek

slop

e (m

/pixe

l)

area (pixels)

0.01

0.1

1

10

1 10 100 1000 104

model dataTin Camp Creek

slop

e (m

/pixe

l)

area (pixels)Figure 11. Areaslope relationship for observed and modelled position for the Tin Camp Creek catchment gullies using a value of at D 20

(top) and at D 25 (bottom)

that gullying is a complex dynamic process and that there is no strong statistical relationship between gullydepth and width and area draining through the gully or slope at the gully head. There does not appear to bea critical slope or drainage area for the commencement or termination of gullies; and although the majorityof gullies occur in the upper reaches (i.e. the diffusive region at areas approximately less than 11 pixelsor 1100 m2) of the catchment, gullies can occur throughout the entire drainage network and that the wholecatchment is at risk. The finding that there is little difference between the area and slope properties of thecontinuous and discontinuous gullies suggests that the same mechanics are operating in both cases. The datademonstrate that, at best, gully position can be broadly predicted by the catchment areaslope relationship.The findings demonstrate that, in this catchment with uniform geology, soils, vegetation, climate and landmanagement, simple modelling approaches cannot capture all gully properties, and factors other than slopeand area influence gullies.

ACKNOWLEDGEMENTS

The traditional owners of the land where the study site is located, Parks Australia North, The Northern LandCouncil and Supervising Scientist Group staff, especially Bryan Smith are thanked for their cooperation andassistance. The advice and support of Garry Willgoose, Dene Moliere together with review comments by Tony



Wells and David OBrien is very much appreciated. The helpful suggestions made by the anonymous reviewersare appreciated. The support offered by AFMECO Mining and Exploration Pty Ltd (John Fabray) and CamecoAustralia Pty Ltd (Ted OConnor and Jennifer Parks) at Myra Camp is also gratefully acknowledged.

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