Gully Initiation Research Articles

Development and controlling factors of gullies and gully complexes, East Coast, New Zealand

AbstractSequential aerial photographs of a small headwater catchment in the Waiapu basin, East Coast Region, North Island, New Zealand, were interpreted to measure and analyse temporal changes in active area of gullies and gully complexes for a longer time span (1939–2003) and with higher temporal resolution compared to previous studies. We focus on the conditions leading to the development of gullies and gully complexes under pasture and forest by using topographic thresholds (slope–area relationships) of catchments for the initiation of gullies and gully complexes. In addition, the influence of two different lithologies as well as the occurrence of major rainfall events was related to gully activity.Twenty gullies and four gully complexes (occupying 62·5 ha or 12·5 per cent of the catchment area) occurred in the study catchment between 1939 and 2003. However, the majority of these were not active at all of the dates studied. Gullies developed in the sandstone‐dominated Tapuwaeroa Formation tended to attain their maximum size by 1957 with a mean catchment area of 2·1 ha. Gullies developed in mudstone of the Whangai Formation attained their maximum size in 1939 with a mean catchment area of 4·31 ha. Exceptions are gullies which developed into mass movement deposits or into an earth flow deposit as well as gullies developed under indigenous forest. Topographic threshold values for gullies under pasture and indigenous forest show that values for gullies under forest plot far above the threshold line of gullies under pasture, indicating that the topographical threshold for gully development under forest is higher compared to under pasture. A threshold value of 9·4 ha in catchment area is needed for the development of gully complexes under pasture, all located in the Whangai Formation and with the same orientation as the strike of the mudstones. Gully‐complex area and dominance of mass‐movement erosion increased with larger catchment area. A decreasing distance to the threshold line for gullies under pasture indicates a later development for gully complexes. No gully complexes developed under indigenous forest, indicating that the threshold value for gully‐complex development is higher than for gully complexes under pasture and was not reached in the study area.A model of shifting topographical threshold for gully development for a given catchment is developed which depends on land use. When a catchment has an indigenous forest cover the topographical threshold is very high. After conversion to pasture, threshold values decrease drastically. With the invasion of scrub, the threshold slowly increases and returns to a similar level to that under indigenous forest after reforestation.Development of gullies and gully complexes is a highly dynamic phenomenon, and phases of expansion and inactivity indicate that models describing only unidirectional advancing stages without periods of inactivity are not suitable. Therefore, this study adds more phases to models of gully and gully‐complex development in the East Coast Region. The threshold line for gully initiation under pasture and a value of 9·4 ha in catchment area for gully‐complex initiation permits one to predict which catchments, under similar environmental settings, develop gullies and gully complexes on a physical basis. This enables land managers to implement sustainable land‐use strategies to reduce erosion rates of gullies and gully complexes. Copyright © 2006 John Wiley & Sons, Ltd.

Paraglacial response of steep, sediment‐mantled slopes to post‐‘Little Ice Age’ glacier recession in the central Swiss Alps

AbstractThis research assesses the morphological consequences of recent (post‐‘Little Ice Age’) paraglacial reworking of valley‐side sediment mantles in the European Alps. It aims to identify the extent and conditioning factors of slope adjustment at sites in the Swiss Alps, model the temporal pattern, and assess the rates of sediment reworking involved. Gully systems have cut into steep, high‐level lateral moraines, and debris cones have accumulated downslope. Debris flow is the dominant agent of sediment transfer. Factors controlling the extent of this activity include moraine slope gradient, relief and moisture availability. Gullies appear to have reached their maximum dimensions within ca. 50 yr of deglaciation, after which gully relief is reduced by removal of inter‐gully slopes and gully infilling (within 80–140 yr). On the most recently deglaciated terrain, minimum erosion rates average ca. 95 mm yr−1 since gully initiation, greatly exceeding ‘normal’ erosion rates in other environments. Mean annual accumulation of a single debris cone since ice retreat was calculated to be ca. 30 mm yr−1. Implications of these findings are applied to patterns of paraglacial sediment‐mantled slope adjustment, conceptualising paraglacial landscape response in terms of a sediment release exhaustion model, and paraglacial landform succession. Copyright © 2005 John Wiley & Sons, Ltd.

Gully erosion and environmental change: importance and research needs

Assessing the impacts of climatic and, in particular, land use changes on rates of soil erosion by water is the objective of many national and international research projects. However, over the last decades, most research dealing with soil erosion by water has concentrated on sheet (interrill) and rill erosion processes operating at the (runoff) plot scale. Relatively few studies have been conducted on gully erosion operating at larger spatial scales. Recent studies indicate that (1) gully erosion represents an important sediment source in a range of environments and (2) gullies are effective links for transferring runoff and sediment from uplands to valley bottoms and permanent channels where they aggravate off site effects of water erosion. In other words, once gullies develop, they increase the connectivity in the landscape. Many cases of damage (sediment and chemical) to watercourses and properties by runoff from agricultural land relate to (ephemeral) gullying. Consequently, there is a need for monitoring, experimental and modelling studies of gully erosion as a basis for predicting the effects of environmental change (climatic and land use changes) on gully erosion rates. In this respect, various research questions can be identified. The most important ones are: (1) What is the contribution of gully erosion to overall soil loss and sediment production at various temporal and spatial scales and under different climatic and land use conditions? (2) What are appropriate measuring techniques for monitoring and experimental studies of the initiation and development of various gully types at various temporal and spatial scales? (3) Can we identify critical thresholds for the initiation, development and infilling of gullies in different environments in terms of flow hydraulics, rain, topography, soils and land use? (4) How does gully erosion interact with hydrological processes as well as with other soil degradation processes? (5) What are appropriate models of gully erosion, capable of predicting (a) erosion rates at various temporal and spatial scales and (b) the impact of gully development on hydrology, sediment yield and landscape evolution? (6) What are efficient gully prevention and gully control measures? What can be learned from failures and successes of gully erosion control programmes? These questions need to be answered first if we want to improve our insights into the impacts of environmental change on gully erosion. This paper highlights some of these issues by reviewing recent examples taken from various environments.

Threshold conditions for initiation of valley-side gullies in the Middle Veld of Swaziland

Gullied and ungullied catchments in four areas of deeply weathered saprolite in the Middle Veld of Swaziland can be successfully separated ( p<0.01) using discriminant analysis based on area and slope of their basin heads. The discriminant scores for the catchments indicate that those on the Manzini land system differ in their behaviour from those on the Lobamba land system. No differences exist within each land system with respect to land use when the basins are classified as arable, rangeland or mixed. The discriminant functions indicate that gullies on the Lobamba land system have developed by Hortonian overland flow whereas those on the Manzini land system should be attributed to subsurface processes. Since no evidence exists for subsurface activity, it is suggested that gullying on the Manzini land system has been more active and, though initiated by overland flow, has developed to a stage where seepage at the base of the saprolite is now the main process. Erosion control measures should be based on dense ground vegetation to reduce flow velocity on the Lobamba land system and deep-rooted vegetation with high evapotranspiration rates to reduce flow volumes on the Manzini land system.

Thresholds for gully initiation and sedimentation in Mediterranean Europe

In Mediterranean areas the dynamics of gully development act as an important indicator of desertification. However, little is known about the influence of climate and land-use changes, and almost no field data exist to assess the sensitivity of a landscape to gully erosion. Two important components of gully erosion studies are the prediction of where gullies begin and where they end. To address some of these issues, topographical thresholds for gully initiation and sedimentation in six different Mediterranean study areas were established. Field measurements of local soil surface slope (S) and drainage-basin area (A) at the point of initiation of ephemeral gullies in intensively cultivated fields (five datasets) and permanent gullies in rangelands (three datasets) were carried out. A negative power relationship of the form S = aA−b was fitted through all datasets, and defined as the mean topographical threshold for gullying in the respective area. Topographically controlled slopes of sedimentation at the gully bottom were also measured. Compared to theoretical relationships for channel initiation by overland flow, relatively low values for b are obtained, suggesting a dominance of overland flow and an influence of subsurface flow. The influence of landsliding at steeper slopes appeared from the flattening of the overall negative trend in the higher slope range (S > 0·30) of the integrated dataset. Comparing the threshold lines of our datasets to the average trend lines through data found in literature revealed that vegetation type and cover could better explain differences in topographical thresholds level than climatic conditions. In cultivated fields, soil structure and moisture conditions, as determined by the rainfall distribution, are critical factors influencing topographical thresholds rather than daily rainfall amounts of the gully-initiating events. In rangelands, vegetation cover at the time of incision appears to be the most important factor differentiating between topographical thresholds, overruling the effect of average annual rainfall amounts. Soil texture and rock fragment cover contributed little to the explanation of the relative threshold levels. Differences in regression slopes (b) between the S–A relationships found in this study have been attributed to the soil characteristics in the different study areas, determining the relative importance of subsurface flow and Hortonian overland flow. Sedimentation slopes where both ephemeral and permanent gullies end were generally high because of the high rock fragment content of the transported sediment. A positive relationship was found between the rock fragment content at the apex of the sedimentation fan and the slope of the soil surface at this location. Copyright © 2000 John Wiley & Sons, Ltd.

Paraglacial modification of slope form

The morphological consequences of paraglacial modification of valley-side drift slopes are investigated at six sites in Norway. Here, paraglacial slope adjustment operates primarily through the development of gully systems, whereby glacigenic sediment is stripped from the upper drift slope and redeposited in debris cones downslope. This results in an overall lowering of average gradient by up to 4·5° along gully axes. In general, slope profile adjustment appears to be characterized by a convergence of slope profiles towards an ‘equilibrium form’ with an upper rectilinear slope gradient at 29°± 4° and a range of concavities of approximately 0·0 to 0·4. After initial rapid incision, further gully deepening is limited, but gullies become progressively wider as sidewall gradients decline to c. 25°, after which parallel retreat appears to predominate. The final form of mature paraglacial gully systems consists of an upper bedrock-floored source area, a mid-slope area of broad gullies whose sidewalls rest at stable, moderate gradients, and a lower slope zone where gullies discharge onto the surfaces of debris cones and fans. Some gullies appear to have attained this final form and have stabilized following exhaustion of readily entrainable sediment within decades of gully initiation. At most sites, paraglacial activity has transformed steep drift-mantled valley sides into gullied slopes where an average of c. 2–3 m of surface lowering has taken place. At the most active sites, these average amounts imply minimum erosion rates averaging c. 90 mm a−1 since gully initiation, which highlights the extreme rapidity of paraglacial erosion of deglaciated drift-mantled slopes. Copyright © 1999 John Wiley & Sons, Ltd.

Shelfbreak gullies; products of sea-level lowstand and sediment failure; examples from Bowser Basin, northern British Columbia

ABSTRACT The transition from cyclothemic shelf to shale-dominated slope deposits (Callovian to Oxfordian) that accumulated along the northern convergent margin of Bowser Basin is delineated by spectacular, conglomerate-filled channels, or gullies. The gullies, representing the shelf-slope break, formed in two stages: (1) Slumping of upper slope-outer shelf sediment created topography, which determined (2) the course of fluvial, distributary-type channels and focused gravel-dominated sediment transport during relative sea-level lowstands. Gully fill consists predominantly of debris-flow deposits, locally incorporated into small (lowstand) fan deltas that prograded from gully margins. Lowstand fluvial channels overlie shelf cycles, incise the underlying highstand deposits, and are overlain by fossiliferous, transgressive sandstone. However, the shelfbreak gullies deposits are up to 10 times thicker than those of the associated lowstand fluvial channels. Although the lowstand fluvial channels likely acted as bypass conduits for gravel and sand during times of low sea level, the initiation and overdeepening of the shelfbreak gullies by slumping probably took place during both high and low relative sea levels. Deposition within gullies may also have continued during relative highstands, from sediment in storage on the outer shelf. The implications for sequence stratigraphic models are: shelf-break gullies can form at any stage of sea level fluctuation; gully thickness may not be related to the magnitude of lowstand incision on the adjacent shelf; and gullies formed by sediment gravity failure do not provide quantitative information about the magnitude of sea-level fall.

Importance of slope gradient and contributing area for optimal prediction of the initiation and trajectory of ephemeral gullies

Ephemeral gully erosion rates are controlled by various factors out of which topography plays an important role. This study investigates the possibility to predict the location of ephemeral gullies using two topographic attributes only: i.e., local slope gradient ( S) and upslope contributing area per unit length of contour ( A s). An inverse relationship between S and A s, in which the relative importance of the area exponent was varied, was evaluated as to its performance in predicting the location of ephemeral gullies for three intensively cultivated catchments in the Belgian loess belt. This model is easily applicable as it only uses information on topography, which can be derived from a digital elevation model, and land use. Predicted locations of ephemeral gullies were confronted with the locations recorded over 5 years of field observations. For all catchments a high relative area exponent (relative to the slope exponent) was required to yield good results. The optimal relative area exponent ranged from 0.7 to 1.5; however, the quality of the prediction did not vary significantly when changing the exponent within this range. An average relative area exponent of ca. 1 is appropriate for all three study areas and may therefore be proposed for similar areas under similar conditions. A striking discrepancy was found between the high relative area exponent required to predict optimally the trajectory of the gullies and the low relative area exponent (0.2) required to identify the spots in the landscape where ephemeral gullies begin. This indicates that spots in the landscape where gullies start are more controlled by slope gradient, while the presence of concavities control the trajectory of the gullies until the slope gradient is too low and sediment deposition starts. No relationship was found between the frequency of the occurrence of ephemeral gullies and the percentage of predicted gully pixels for each frequency class. This indicates that frequently occurring gullies at the same location are not easier to predict than accidental gullies.

Dynamic and static models of gully erosion

The main causes of gully formation are anthropogenic factors: the clearing of native forests, tilling of fallow lands and associated change of the hydrological conditions in the rainfall–runoff system. Gully channels formation is very rapid during the period of gully initiation, when morphological characteristics of a gully (length, depth, width, area, and volume) are far from stable. This period is relatively short, about 5% of a gully's lifetime. For the most part of a gully's lifetime, its size is near stable, maximum value. These two stages of gully development led to two types of gully erosion models: (1) dynamic models to predict rapid changes of gully morphology at the first period of gully development; (2) static models to calculate final morphometric parameters of stable gullies. The dynamic gully model is based on the solution of the equations of mass conservation and gully bed deformation. The model of straight slope stability was used for prediction of gully side walls inclination. The static gully model is based on the assumption of final morphological equilibrium of a gully; when averaged for several years, elevations and width of gully bottom do not change. This stability is associated with a negligible rate both of erosion and sedimentation at the gully bottom. It means that flow velocity is less than the threshold value for erosion initiation, but is more than the critical velocity of wash load sedimentation. The dynamic and static gully models were verified on the data on gullies morphology and dynamics from Yamal peninsula (Russia) and New South Wales (Australia).

Controls on gully formation following forest clearing in a humid temperate environment

We have constructed a chronology of gully initiation, forest clearing, and rainfall events for gullies eroded into pine plantations near Bombala, southeastern Australia, over the last 15 years. The chronology suggests that daily rainfall of 80–100 mm, which has a recurrence interval of 1.4–2 years, can initiate gully erosion on areas cleared of native forest within the previous year. Massive gully erosion was experienced from a daily rainfall of 200 mm with a recurrence of 10–15 years. Resistance to channel initiation effectively recovers within a year of disturbance, allowing only a limited opportunity for erosion. Analysis of the spatial pattern of gully erosion, using a digital elevation model, shows that gullies were initiated across all landscape positions. In contrast to previous studies, there is no clear topographic threshold that limits the extent of the gully network. We infer that the weak topographic threshold results from low resistance to scour, allowing local flow convergence to dominate over topographic accumulation of flow. Although resistance to scour is low relative to previous studies, a process threshold for gully initiation is still a useful simplification of the erosion processes. For the soils that we studied, the threshold for gully erosion relates to intense scour exposing erodible subsoils.

Topographical thresholds for ephemeral gully initiation in intensively cultivated areas of the Mediterranean

Ephemeral gullying is an important erosion process in Mediterranean areas suffering from land degradation because of increased drought and human impact. An ephemeral gully is formed when a hydraulic threshold for incision into a resistant soil surface is exceeded. The flow intensity at the soil surface of any landscape position is controlled by topographical parameters, such as local slope ( S) and drainage-basin area ( A). The objective of this study was to investigate to what extent one could use field measurements of S and A to predict the location of ephemeral gully initiation in intensively cultivated Mediterranean environments. Two relatively homogeneous study areas in Southeast Spain and Northeast Portugal with widespread ephemeral gullying were selected. S and A were measured by standard methods at the initiation point of 50 ephemeral gullies in each study area. The relationship between S and A was statistically investigated. On a double logarithmic scale, the S and A data showed considerable scatter. Orthogonal regression resulted in the threshold relationship S= aA − b with b=0.133 for Spain, and b=0.226 for Portugal. The analysis also showed that S and A were weakly correlated for the Spanish dataset, and more strongly correlated for the Portuguese dataset. The correlation was considerably strengthened by including additional information on planform curvature and on tillage direction relative to the gully direction. Greater soil stoniness and the presence of a less permeable horizon generally increased the threshold level in both study areas. The threshold relationships resulting from this study were compared with literature data. The indices A c S and ln( A c/ S) were applied to our study sites but neither relationship individually described the general trend between the data of S and A. Therefore, an alternative index, A c b S, with 0.133≤ b≤0.226, according to the empirical trend between drainage basin area and local slope, was proposed to describe the topographical threshold conditions for ephemeral gully initiation in similar Mediterranean areas.

Critical slope gradient for compulsory abandonment of farmland on the hilly Loess Plateau

Scientific evidence for the upper limit of farmland is proposed. Shallow gully erosion is one of erosion types distributed extensively on sloping farmland in the hilly regions of the Loess Plateau. Field observation and aerial photos interpretation, as well as laboratory experiments show that the shallow gully erosion occurring on the steeper farmland in the hilly regions of the Loess Plateau is an important factor leading to intensive erosion on slope, because of its extensive distribution and intensive runoff collection. The data on the formation, development and distribution of shallow gullies on sloping farmland indicate that critical slope gradient for shallow gully initiation ranges from 15 to 20 degrees with an average of 18.2 degrees. Therefore, it is suggested that critical slope gradient for compulsory abandonment of farmland on the hilly Loess Plateau should be kept within the critical slope gradient for shallow gully initiation to prevent shallow gully formation in order to control soil loss more effectively. But as the first step, the cultivation on the slopes with slope larger than 25 degrees where the maximum erosion occurs should be strictly forbidden.

Geomorphic threshold conditions for ephemeral gully incision

Ephemeral gully erosion in cultivated land is an important source of sediment that is frequently being overlooked and not accounted for in soil erosion studies. Furthermore, little is known about the factors controlling ephemeral gully erosion. In this paper the available information on the initiation and location of (ephemeral) gullies is summarised, focusing on the relationship between the upslope drainage area ( A) and the critical slope gradient ( S cr) for ephemeral gully initiation. By plotting the non-linear relationship between critical slope gradient (measured immediately upstream of the incision head) and drainage area (at the incision head) for gullied sites it was possible to draw a straight line on log-log paper through the lower-most points for each of the available datasets, representing a critical slope-area relationship for incision. Consequently, below this line no incision occurs. This line or critical relation can also be written as a power function between critical slope and area. Although many factors vary between the different datasets, the exponent of the drainage area relationship (−0.4) showed very little variation. The observed critical slope-area relationship can be related to a simple model of channel initiation by overland flow. Furthermore, this relationship can be used to identify potentially unstable sites where control measures should be undertaken.

Initiation and evolution of gullies along the shoreline of Lake Huron

Over one hundred gullies are deeply incised into Algonquin Bluff along the eastern shoreline of Lake Huron. Aerial photographs between 1930 and 1992 were used to measure temporal changes in area and length of a sample of forty-four gullies. Regression analysis shows that gully area and length have increased in a linear fashion, and that gully initiation occurred in the nineteenth century. Gully initiation coincides with deforestation and settlement of the area. Surface drainage was redirected into drainage ditches, concentrating the flow and enhancing gully erosion. Natural processes associated with climatic fluctuations are probably insufficient to cause gully initiation, but may contribute to ongoing gully expansion.

Gully erosion associated with the expansion of unterraced almond cultivation in the coastal Sierra de Lujar, S. Spain

AbstractSince Moorish times, the lower slopes of the Sierra de Lujar in southern Spain have been intensively cultivated. Despite steep slopes and the highly erodible nature of the local micaschist/quartzite bedrock and derivative Quaternary sequences, accelerated erosion has largely been contained throughout this history by substantial, well‐maintained rock terraces. However, since 1987, European Union (EU) agricultural policy has been to encourage the cultivation of certain crops by subsidising their expanded cultivation on a ‘per hectare’ basis. This has resulted in hasty clearance of native Mediterranean matorral on sites really unsuitable for the local staple crop (almonds), and erosion‐prevention terraces are often absent or rudimentary. Gullying is now a very serious problem on some of these newly cleared sites.This paper identifies the lithological, topographic and agricultural policy controls on the consequent gully distribution, and then explores the pedological and hydrological controls on gully initiation on a single unterraced slope. Using these results alongside morphological data from gully surveys, a gully initiation and growth model is presented. This suggests that two types of gully are present in the area. ‘Type 1’ or upper‐slope gullies are initiated by overland flows, which are relatively infrequent. By contrast, ‘type 2’ or slope‐base gully initiation involves saturation and collapse in fossil landslip scars in Holocene colluvium. These two very different morphological systems may integrate through the terminal fan of the upslope gullies, in a manner described by Tiexeira de Oliveira 1990. The implications of this are that in the study area, sediment removal is associated with a very few highly productive sites. Contemporary management is briefly assessed against this model of geomorphic change in the area.

GULLYING WITHIN WETLANDS IN ZIMBABWE: GULLY DEVELOPMENT AND FACTORS INFLUENCING GULLY GROWTH

ABSTRACT The initiation and growth of gullies within dambos were examined by means of sequential aerial photography for the period 1951 to 1986 and field monitoring during the 1986/87 and 1987/88 rainy seasons. Gullies were generally not present in the 1950's, many were initiated in the early 1960's, most grew rapidly in the 1970's, and typically displayed irregular growth during the 1980's associated, in particular, with variations in rainfall. Growing pressures from rapidly increasing human and livestock populations, especially the latter after 1962 when destocking ceased, contributed towards the erosion of dambos, while the progressive extension of dryland cultivation reduced the area available for grazing so confining livestock to dambo areas for much of the year. Estimates of runoff indicate that about half of the runoff entering gullies is likely to come from the dambos themselves, although these comprise, on average, only about one third of the land area. Peak flow estimates, based on 1: 2 year flo...

Gully initiation in Quaternary volcanic environments under temperate subhumid seasonal climates

Gully erosion processes occurring in a Quaternary volcanic landscape under a temperate subhumid seasonal climate are analyzed. It is postulated that gullying starts as a consequence of shallow mass-movements produced by non-concentrated throughflow, and that the fluvial, cyclic approach does not hold in this kind of landscape. The paper aims at formulating a conceptual model of gully initiation for the environments mentioned above.

The initiation and growth of gullies in Madagascar: are humans to blame?

Lavaka (Madagascar's unusual gullies) require thick lateritic regolith on smoothly convex hills, and they are initiated locally by a variety of cultural and natural proximate causes, such as paths, roads, defensive trenches around hillforts, steep fields, unchanneled to slightly concentrated storm run-off, springs, mass movements, etc. Growth continues by rain attack, incision by run-off, earthfalls caused by wetting and drying cycles, and especially basal sapping by diffuse groundwater flow above bedrock, somewhat in that order. Lavaka are less directly permitted and promoted by (1) hardening of exposed laterite surfaces, which creates a hard-soft-hard profile from bedrock to laterite, which favors incision relative to lateral erosion, (2) the superimposition of concave run-off profiles onto convex hills, and, locally, (3) re-equilibration of watersheds after stream piracy and faulting. Run-off and hardening are interrelated with devegetation, because hardening results from exposure following devegetation, because both hardening and devegetation increase run-off relative to infiltration, and because hardening, rain attack, and run-off impede vegetation. Devegetation is certainly worsened by harmful human activities (such as range burning, forest cutting, and/or overgrazing), but tectonism and natural climatic aridification are probably ultimately more important, given that many lavaka are simply a natural part of the landscape's evolution and that some lavaka clearly predate primary (uncut) rain forest.

A Comparison of Past and Present Episodes of Gully Erosion at Wangrah Creek, Southern Tablelands, New South Wales

Erosion gullies at Wangrah Creek expose walls of at least two previous episodes of gully erosion. Past gullies were of a similar size to those of today but were not as widespread across the region at any particular time. The causes of past and present erosion are examined by mapping points of gully initiation and comparing the geomorphic history with an independent palaeo‐environmental history. Past gullies were the result of intrinsically unstable conditions, possibly caused by the expansion of swampy deposits increasing runoff. In contrast, the present gully erosion is the result of major environmental change caused by European settlement, which had a greater effect on gully erosion than any other environmental change over the last 10,000 years. At Wangrah Creek, sites of gully initiation indicate that the present erosion was caused by localised disturbances to the valley floor.

An example of rapid gully initiation and extension by subsurface erosion: Coastal San Mateo County, California

Gully networks developed on coastal hillslopes in San Mateo County, California, extended nearly 300% from 1941 to 1978, primarily by subsurface erosion. In one drainage, subsurface erosion expanded the gully network by 80 m, nearly doubling the gully length from 1982 to 1987. Intense gullying is restricted to areas underlain by members of the Pliocene Purisima Formation, a unit rich in unstable volcanic lithic fragments, which weather to produce expansive smectite clays. Gully development proceeds through a sequence of lateral eluviation of clays near the base of the A horizon (piping), development of an open conduit in which erosion proceeds by turbulent, concentrated flow (tunneling), and finally, collapse of the tunnel roof, exposing an open gully. Measurements made in one gully system during a 25-yr storm showed that the subsurface drainage network carried four times as much flow as the surface drainage net and had average suspended sediment concentrations an order of magnitude higher.