Publications

CSIRO

intro | copyright | queries | user guide | publications | bug info | data program code | feedback | search | tour | around the world | workshops

CRCCH

Abstracts for papers on Erosion/Slope Stability Modelling

VERTESSY, R.A., WILSON, C.J., SILBURN, D.M., CONNOLLY, R.D. and CIESIOLKA, C.A. (1990): Predicting erosion hazard areas using digital terrain analysis. IAHS AISH Publication 192, pp. 298-308.

The application of an integrated terrain analysis and hydrologic modelling package to problems of erosion in semi-arid tropical pasturelands is described. The model, known as TOPOG, focuses on describing the complex inter-relationships which exist between topography, soils, climate and vegetation. The calculation of a steady-state erosion hazard index (H) is shown to be a powerful diagnostic for land managers.

DIETRICH, W.E., WILSON, C.J., MONTGOMERY, D.R., McKEAN, J. and BAUER, R. (1992): Erosion thresholds and land surface morphology. Geology, 20: 675-679.

We propose a graphical technique to analyse the entirety of landforms in a catchment to define quantitatively the spatial variation in the dominance of different erosion processes. High- resolution digital elevation data of a 1.2 km² hilly area where the channel network had been mapped in the field were used in the digital terrain model, TOPOG, to test threshold theories for erosion. The land surface was divided into ~20 m² elements whose shapes were then classified as convergent, planar, or divergent. The entire landscape plotted on a graph of area per unit contour length against surface gradient shows each planform plotting as a separate field. A simple steady-state hydrologic model was used to predict zones of saturation and areas of high pore pressure to mimic the extreme hydrologic events responsible for erosive instability of the land surface. The field observation that saturation overland flow is rare outside convergent zones provided a significant constraint on the hydrologic parameter in the model. This model was used in threshold theories to predict areas of slope instability and areas subject to erosion by saturation overland flow, both of which can contribute to channel initiation. The proportion of convergent elements predicted to exceed the threshold varies greatly with relatively small changes in surface resistance demonstrating a high sensitivity to land use such as cattle grazing. Overall, the landscape can be divided, using erosion threshold lines, into areas prone to channel instability due to runoff and stable areas where diffusive transport predominates.

DIETRICH, W.E., WILSON, C.J., MONTGOMERY, D.R. and McKEAN, J. (1993): Analysis of erosion thresholds, channel networks and landscape morphology using a digital terrain model. J. Geology, 101(2): 259-278.

To investigate the linkage between erosion process and channel network extent, we develop two simple erosion threshold theories driven by a steady state runoff model that are used in the digital terrain model TOPOG to predict the pattern of channelization. TOPOG divides the land surface into elements defined by topographic contours and flow lines, which can be classified as divergent, convergent and planar elements. The calibration parameter for the runoff model is determined using empirical evidence that the divergent elements which comprise the ridges in our study area do not experience saturation overland flow, where as the convergent elements in the valleys do during significant runoff events. A threshold theory for shallow landsliding predicts a pattern of instability consistent with the distribution of landslide scars in our 1.2 km² study site and confirms the interpretation, based on field observations, that indicate the steeper channel heads to be at least partially controlled by slope instability. Most sites of predicted and observed slope instability do not, however, support a channel head, hence landslide instability alone is not sufficient for channelization. In contrast, most elements predicted to be eroded by saturation overland flow coincide with the observed location of the channel network, In addition, areas of predicted downslope decrease in relative sediment transport capacity were found to correspond to locations where channels became discontinuous. The topographic threshold given by the saturation overland flow erosion theory varies with the third power of critical boundary shear stress, suggesting that critical shear stress, although difficult to quantify with much precision in the field, is a dominant control on the extent of the channel network where saturation overland flow is significant. Current extent of the channel network in our field site, for example, may best be explained as resulting from grazing-induced reduction in surface resistance.

COSTANTINI, T., DAWES, W., O'LOUGHLIN, E. and VERTESSY, R. (1993): Hoop pine plantation management in Queensland: 1. Gully erosion hazard prediction and watercourse classification. Aust. J. Soil and Water Cons., 6(1): 35-39.

Forest managers require a watercourse classification system which is capable of predicting gully erosion hazard. High intensity storms during March/April 1989 resulted in significant gully erosion, and stimulated a review of the watercourse classification system used in hoop pine plantations in Queensland. Indices of surface wetness and stream power, generated by the terrain analysis and steady-state hydrologic modelling components of the TOPOG package, were evaluated for their utility in predicting the location of gully erosion resulting from the storms. The potential of these indices for use as watercourse classification criteria in hoop pine plantation management is examined.

MONTGOMERY, D.R. and DIETRICH, W.E. (1994): A physically-based model for topographic control on shallow landsliding. Water Resour. Res., 30(4): 1153-1171.

A model for the topographic influence on shallow landslide initiation is developed by coupling digital terrain data with near-surface through flow and slope stability models. The hydrological model TOPOG (O'Loughlin, 1986) predicts the degree of soil saturation in response to a steady state rainfall for topographic elements defines by the intersection of contours and flow tube boundaries. The slope stability component uses this relative soil saturation to analyse the stability of each topographic element for the case of cohesionless soils of spatially constant thickness and saturated conductivity. The steady state rainfall predicted to cause instability in each topographic element provides a measure of the relative potential for shallow landsliding. The spatial distribution of critical rainfall values is compared with landslide locations mapped from aerial photographs and in the field for three study basins where high-resolution digital elevation data are available: Tennessee Valley in Marin County, California; Mettman Ridge in the Oregon Coast Range; and Split Creek in the Olympic Peninsula, Washington. Model predictions in each of these areas are consistent with spatial patterns of observed landslide scars, although hydrologic complexities not accounted for in the model (eg. spatial variability of soil properties and bedrock flow) control specific sites and timing of debris flow initiation within areas of similar topographic control.

PROSSER, I. P. and ABERNETHY, B. (1996): Predicting the topographic limits to a gully network using a digital terrain model and process thresholds. Water Resources Research, 32: 2289-2298.

A digital terrain model is used with process thresholds to predict the extent of a stable gully network in a 5 km² catchment of the south-eastern highlands of Australia. The model, developed by Dietrich et al. [1992, 1993], predicts the topographic controls on channel networks and interprets these in terms of a critical shear stress for channel incision (t_c) applied by saturation overland flow. We adapt the model slightly to compare the shear stress applied by Hortonian overland flow to that applied by saturation overland flow. The limits to gully erosion in the catchment are controlled strongly by a topographic threshold that has an inverse relationship between upslope catchment area and local gradient. The topographic threshold for channel incision is reproduced using a simple model of Hortonian overland flow and a t_c appropriate for incision into a degraded grass surface (t_c= 245 dyne/cm²). This is consistent with historical evidence for the timing of gully erosion. The study confirms a strong topographic control on the extent of the channel network in a catchment significantly different from the western North America catchments where the topographic threshold was first demonstrated. Despite its simplicity, the model for incision by overland flow appears capable of distinguishing the hydrological processes responsible for channel incision when these are reflected in the relationship between channel network and landscape morphology. The model requires relatively simple inputs, suggesting it may be useful for mapping gully erosion hazard in actively eroding catchments.

PROSSER, I. P. and W. E. DIETRICH (1995): Field experiments on erosion by overland flow and their implication for a digital terrain model of channel initiation. Water Resources Research, 31: 2867-2876.

Dietrich et al. (1992, 1993) proposed a digital terrain model for predicting the location of channel heads on the basis of the assumption that they occur where saturation overland flow exerts a boundary shear stress in excess of a critical value. Flume experiments were conducted in the modelled field site to evaluate the threshold hypothesis and to constrain critical shear stress and flow resistance parameters. Under complete grass cover, microtopography and grass stems were found to prevent significant sediment transport at all but the highest flows. When the grass stems were cut close to the ground, flow resistance and critical shear stress for significant sediment transport were reduced by up to an order of magnitude, but the remaining dense root mat prevented deep scour. These field experiments support the threshold assumption and the model estimations of the critical shear stress if local topographic convergence of flow is taken into account. The experiments also support the interpretation that significant degradation of vegetation cover is required for channel incision.

PROSSER, I. P. and ABERNETHY, B. (1999). Increased erosion hazard resulting from log-row construction during conversion to plantation forest. Forest Ecology and Management 123: 145-155.

During forest clearing operations, embankments of logs are often constructed approximately parallel to contours. These log-rows resemble contour banks used in agriculture and it is expected that they perform a similar function of erosion control. Gully erosion was observed in a forest cleared for pine plantation in SE Australia. The gullies appeared to be associated with flow diversion around log-rows, particularly where log-rows do not follow the contour precisely. A digital elevation model (DEM) of the plantation was used to investigate how the distribution of erosion hazard, as a function of upslope contributing area and local gradient, had been changed by the log-rows. We found that benefits gained by decreased erosion hazard on many parts of the landscape were outweighed by increased erosion hazard at the end of log-rows and in hillslope valleys. This is because of the non-linear increase in erosion hazard with increasing upslope catchment area. The log-rows increased the probable extent of gully erosion by 1.5 – 1.7 times and the total sediment transport capacity, summed over the landscape, by 1.5 times. The results lead us to suggest techniques for improved log-row construction and plantation preparation.

PROSSER, I. P. and SOUFI, M. (1998). Controls on gully erosion following forest clearing in a humid temperate environment. Water Resources Research, 34: 3661-3671.

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 y. The chronology suggests that daily rainfall of 80 to 100 mm, which has a recurrence interval of 1.4 to 2 y, 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 to 15 y. 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 (DEM), 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 sub-soils.