Tasmanian Landslide Map Series: User Guide and Technical ...

Tasmanian Geological Survey Record 2010/01

Tasmanian Landslide Map Series: User Guide and Technical Methodology

Department of Infrastructure, Energy and Resources Mineral Resources Tasmania

Tasmanian Geological Survey Record 2010/01

Tasmanian Landslide Map Series: User Guide and Technical Methodology

by Colin Mazengarb and Michael Stevenson

Tasmanian Geological Survey Record 2010/01 Department of Infrastructure, Energy and Resources Mineral Resources Tasmania

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MAZENGARB, C.; STEVENSON, M. D. 2010. Tasmanian Landslide Map Series: User guide and technical methodology. Record Geological Survey Tasmania 2010/01. Cover photo: Recently active landslide at Abels Hill, St Leonards, 1972. [Photo: MRT 234] While every care has been taken in the preparation of this report, no warranty is given as to the correctness of the information and no liability is accepted for any statement or opinion or for any error or omission. No reader should act or fail to act on the basis of any material contained herein. Readers should consult professional advisers. As a result the Crown in Right of the State of Tasmania and its employees, contractors and agents expressly disclaim all and any liability (including all liability from or attributable to any negligent or wrongful act or omission) to any persons whatsoever in respect of anything done or omitted to be done by any such person in reliance whether in whole or in part upon any of the material in this report.

Mineral Resources Tasmania

PO Box 56

Rosny Park

Tasmania

Phone: (03) 6233 8377 l Fax: (03) 6233 8338 Email: [email protected] l Internet: www.mrt.tas.gov.au

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CONTENTS INTRODUCTION

……………………………………………………………………………

An overview of the Tasmanian Landslide Map Series

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Intended users and application of landslide information

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Limitations of information maps and advisory maps … … … … … … … … … … … … … … … … … … …

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Origin of the methodology and the peer review process … … … … … … … … … … … … … … … … …

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PART 1: USER GUIDE … … … … … … … … … … … … … … … … … … … … … … … … … … … …

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Do landslides occur in Tasmania?

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What types of landslides occur in Tasmania? … … … … … … … … … … … … … … … … … … … … …

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Falls and topples … … … … … … … … … … … … … … … … … … … … … … … … … … … … …

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Flows

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Slides

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Spreads … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … …

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Transitional slope failures

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Is there a serious landslide risk in Tasmania? … … … … … … … … … … … … … … … … … … … … … What is a landslide zone? … … … … … … … … … … … … … … … … … … … … … … … … … … …

19 21

How to decide if a property is within a landslide zone?… … … … … … … … … … … … … … … … … …

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PART 2: TECHNICAL METHODOLOGY … … … … … … … … … … … … … … … … … … … … …

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Field and desktop mapping and data collection … … … … … … … … … … … … … … … … … … … …

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Geological mapping … … … … … … … … … … … … … … … … … … … … … … … … … … … …

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Geomorphological mapping … … … … … … … … … … … … … … … … … … … … … … … … … …

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Landslide mapping

…………………………………………………………………………

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Topographical mapping … … … … … … … … … … … … … … … … … … … … … … … … … … …

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Data storage and formats … … … … … … … … … … … … … … … … … … … … … … … … … …

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Landslide susceptibility modelling techniques … … … … … … … … … … … … … … … … … … … … …

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Rock fall susceptibility

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Shallow slide and flow susceptibility… … … … … … … … … … … … … … … … … … … … … … … …

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Deep-seated landslide susceptibility… … … … … … … … … … … … … … … … … … … … … … … …

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Statewide landslide susceptibility

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CONCLUSIONS … … … … … … … … … … … … … … … … … … … … … … … … … … … … … …

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ACKNOWLEDGEMENTS

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REFERENCES … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … …

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Figures 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Relationship diagram for the Information and Advisory maps and how they support decision processes … … Known landslide distribution in mainland Tasmania … … … … … … … … … … … … … … … … … … General landslide types … … … … … … … … … … … … … … … … … … … … … … … … … … … Coastal cliff receding through mainly rock fall processes, Fossil Bluff, Wynyard … … … … … … … … … … Small rock fall of Parmeener Supergroup sedimentary rocks in a coastal cliff setting, Tasman Peninsula … … A dolerite column presenting an obvious potential danger above a walking track, Cataract Gorge … … … … Conceptual model of solitary rock topple and rock fall processes on a columnar jointed dolerite escarpment Large block topple failure in Jurassic dolerite, Mt Arthur, Hobart … … … … … … … … … … … … … … Conceptual model of a large block topple-block slide failure on a dolerite scarp … … … … … … … … … Debris flow deposit from a rainfall event in 2008, Alexander Creek, Burnie … … … … … … … … … … … Aftermath of a debris flow in 2007, Philps Peak, western Tasmania … … … … … … … … … … … … … Source area of the large 1872 Glenorchy debris flow on the flanks of Mt Wellington … … … … … … … … Conceptual cross-section demonstrating how shallow debris slides and debris flows form in the mid-slope flanks of dolerite capped mountains underlain by Parmeener Supergroup sedimentary rocks … … 14. Conceptual model for shallow slides and flows on the basaltic former coastal escarpment of NW Tasmania 15. Large, deep-seated, rotational landslides on the active coastal escarpment, east of Boat Harbour Beach … … Tasmanian Geological Survey Record 2010/01

7 9 10 11 11 11 11 12 12 13 13 13 14 14 15 3

16. Grooms Landslide, between Ulverstone and Penguin, northwest Tasmania … … … … … … … … … … … 17. True scale profile of a large, deep-seated landslide in Tertiary basalt at Gawler River with interpreted failure plane and major components shown … … … … … … … … … … … … … … … … … … … … … 18. An extremely slow moving, deep-seated landslide in Tertiary and Quaternary sediments at Taroona … … … 19. Recent, shallow earth slide in Launceston Group sediments at Evandale … … … … … … … … … … … … 20. Conceptual diagram of shallow earth slides at Beauty Point, Tamar Valley … … … … … … … … … … … 21. Large spread-like landslide in Tertiary basalts and sediments west of Wynyard … … … … … … … … … … 22. True scale profile of a spread-like landslide west of Wynyard with interpreted features … … … … … … … 23. Rotational earth slide transitional into an earth flow at Home Hill, Huon Valley … … … … … … … … … 24. A near-new house being demolished in 1990 after extensive damage from landslide movement, Rosetta … … 25. An outbuilding moved off its foundations when a small landslide caused the failure of the adjacent retaining wall, West Hobart … … … … … … … … … … … … … … … … … … … … … … … … … … 26. Boulder fallen from an adjacent quarry wall and tumbled downslope to a house, Dynnyrne … … … … … … 27. Location of Proclaimed (Declared) Landslip Areas … … … … … … … … … … … … … … … … … … … 28. Coastal recession impacting on coastal development, St Helens… … … … … … … … … … … … … … … 29. Conceptual map illustrating the difference between mapped landslides and landslide susceptibility zones … … 30. Example of an orthophoto draped over a Digital Elevation Model showing landslide features … … … … … 31. Example of geomorphological map of similar area to Figure 30 … … … … … … … … … … … … … … … 32. Geomorphic units mapped in the coastal zone, northern Tasmania … … … … … … … … … … … … … 33. Geomorphic units mapped along the coastal escarpment, northwest Tasmania … … … … … … … … … … 34. Geomorphic units mapped in valley settings … … … … … … … … … … … … … … … … … … … … … 35. Conceptual model of the basaltic coastal escarpment, NW Tasmania, showing major geomorphic units … … 36. Simplistic end-member slope retreat models of hillsides … … … … … … … … … … … … … … … … … 37. Conceptual cross-section showing the incision of streams and parallel retreat in response to long term uplift … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … 38. Conceptual cross-section involving more competent basalt overlying clay, such as in the Tamar Valley … … 39. The progressive landslide mechanism on hillsides composed of over-consolidated clay … … … … … … … 40. Components of a typical rotational, deep-seated landslide that has not experienced degradational processes … … … … … … … … … … … … … … … … … … … … … … … … … … … 41. Effects of erosion on the components of a typical rotational, deep-seated landslide … … … … … … … … 42. Spatial representation of slide-type landslides stored in the landslide database … … … … … … … … … … 43. Spatial representation of earth and debris flows stored in the landslide database … … … … … … … … … 44. Spatial representation of rock falls stored in the landslide database … … … … … … … … … … … … … 45. Conceptual diagram of the rock fall modelling process … … … … … … … … … … … … … … … … … 46. Conceptual map demonstrating runout paths for two hypothetical rock fall source cells … … … … … … … 47. Conceptual map relating rock fall susceptibility to observed landslides … … … … … … … … … … … … 48. Modelled debris flow susceptibility zones on Mt Wellington, Hobart … … … … … … … … … … … … … 49. Population distribution of representative source slope values for all known shallow slides and flows in weathered basaltic soils, North West Coast mapping … … … … … … … … … … … … … … … … … 50. Conceptual map showing relationship between observed landslides and relative susceptibility zones … … … 51. Conceptual diagram illustrating the modelling method for determining runout and regression areas from source areas for deep-seated landslides … … … … … … … … … … … … … … … … … … … … … … 52. Conceptual diagram of a hillside showing pre-failure and post-failure profiles for deep-seated landslides … … 53. Example of deep-seated landslide susceptibility map … … … … … … … … … … … … … … … … … … 54. Conceptual diagram illustrating the construction of the statewide Landslide Susceptibility map … … … … …

Tasmanian Geological Survey Record 2010/01

15 16 16 17 17 18 18 19 20 20 20 22 23 23 25 25 26 26 27 27 28 29 29 29 31 31 32 32 32 34 34 35 36 37 37 39 39 40 42

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INTRODUCTION All societies are affected by natural hazards in one form or another. Whilst landslides do not present the most significant danger in Tasmania (e.g. Gilmour, 2003) the cost to the community over time from economic and social perspectives is considerable. Fortunately, few if any people in Tasmania have lost their life to landslides, but the potential exists for catastrophic failures and possibly lethal consequences. With the raised awareness of the consequences of landslide activity in Australia resulting from the Thredbo disaster in New South Wales, strong recommendations for improved land use planning practice were recommended in the Coroner’s report (Hand, 2000). Following on from this in 2005, the Ministerial Council for Local Government and Planning endorsed Emergency Management Australia’s publication Planning Safer Communities (EMA, 2002). According to this guideline, the creation of safer, sustainable communities requires land use planning strategies, in regard to risks in general, to consider:

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avoiding those areas where development will increase the likelihood of risk and/or the level of impact;

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creating incentives for removing or modifying structures in areas that increase risk; and

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prohibiting ways of undertaking development that are more likely to contribute to increased risk.

The guideline also recommends that zoning, with associated planning overlays, be established to create a continuum along which risks increase, and controls on the use and development of land also increase, such that the planning schemes:

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prohibit development in high risk areas through zoning and overlay controls;

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limit the types of development allowed in high to moderate risk areas — zoning such areas for recreation or other forms of public use can reduce the potential impacts of hazard events; and

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establish and apply appropriate development controls based on the assessed risk in moderate and lower risk areas. These controls can include minimum elevations, setbacks and lot sizes, as well as maximum densities and site coverage.

The Australian Geomechanics Society has also been active in producing landslide risk management guidelines (AGS, 2000; 2007a; 2007b; 2007c; 2007d; 2007e) in response to the Thredbo disaster. These guides provide geotechnical practitioners and regulators with tools and information to assist the development of best practice in site investigations, landslide zoning and regulation. Considerable valuable landslide research has been undertaken over a number of years by Mineral Resources Tasmania (MRT), and its predecessor the Department of Mines. The resulting landslide maps that were produced largely concentrated on urbanised areas of Tasmania. The research, over time, employed various methodologies and maps were produced at various scales. Some of these maps Tasmanian Geological Survey Record 2010/01

simply depicted known landslides or generalised zones of known instability. Maps of this type do not attempt to identify slopes that may be susceptible to new, first-time failures in the future. Other maps have built on these maps by providing slope categories that can aid the judgement of relative landslide susceptibility outside of known landslides. For some areas with notable landslide problems more predictive Advisory Landslide Zoning maps were produced, which extended the zoning to identify areas of potential future failure. The Advisory Landslide Zoning maps produced were the 1:25 000 scale Tamar Valley series (6 maps) and 1:12 500 scale maps for the Burnie, Penguin and Lilydale–Karoola areas. Of these, the Tamar Valley series had the most developed methodology, with a five zone scheme that included known landslides and adjacent areas (class V – active landslides and class IV – old landslides), combined with three broad susceptibility classes (class III – susceptible geology and slopes >7°, class II – ‘soft’ geology and slopes 42° slope 30° 34° travel angles

topples decreasing susceptibility talu

s

Figure 45 Conceptual diagram of the rock fall modelling process. The setting in this example is based on a dolerite talus slope. order to improve the cartographic result the raster is simplified to remove features such as small donut holes and solitary outlier cells using geoprocessing tools such as ‘expand and shrink’ and focalsum. Runout paths were modelled from each source cell, travelling in the direction of maximum downhill slope as defined by an aspect grid. This is referred to as the travel angle method and is superior to the shadow angle method (defined by straight line travel paths), as the former honours the topography in the path of the runout (i.e. the path is not necessarily a straight line — see Figure 46). Despite the advantages of the travel angle method it is somewhat simplistic as in reality the actual path of material may deviate from this to some degree. The modelling does not take into account obstacles and small scale topography that are beyond the resolution of the input layers, such as trees, structures and protective fences. For cartographic reasons the runout cells have been simplified in a similar manner to the source cells. The extent of each runout has been defined using an angle limiter, 34° and 30°, representing decreasing susceptibility respectively (fig. 45). These values are based on field studies of dolerite talus fan slope angles in Tasmania. For rock falls occurring in weaker rock units, the travel angle values chosen may, in many cases, be too low and thus over-estimate the runout distance. For other rock types and settings the converse may also be true. Relative or quantified susceptibility descriptors of Very Low, Low, Moderate and High, as defined in the AGS (2007a) guidelines, were not adopted because of insufficient Tasmanian Geological Survey Record 2010/01

co n

Base of slope tou

rs

Top of slope

Figure 46 Conceptual map demonstrating runout paths for two hypothetical rock fall source cells (small squares). Runout paths are not straight lines and follow maximum slope direction. field evidence in the study areas. Instead the guidelines allow an alternative approach, although not entirely satisfactory, of susceptible or not susceptible. The three areas identified on the susceptibility map (source area and the two runout areas) can be considered as susceptible to rock fall (fig. 45). Outside these areas are generally considered not susceptible. The conceptual relationship between mapped rock falls and susceptible areas is shown in Figure 47. 34

fallen boulder in database rock fall fan deposit 30° 34°

Very low susceptibility or not susceptible

Area susceptible to rock fall runout

source point in database

top of cliff rock fall travel path in database Susceptible source area where rock falls may originate

Figure 47 Conceptual map relating rock fall susceptibility to observed landslides. The users of the maps should be warned that because the modelling is not perfect there may be locations outside of the mapped susceptible areas where rock fall could occur. This is a situation where a suitably qualified and experienced practitioner should be consulted if doubt exists. The frequency and consequences of rock fall events are difficult to quantify and need further work for site specific investigations.

Shallow Slide and Flow Susceptibility Debris flow susceptibility maps in the Glenorchy and Hobart studies were produced prior to the release of the AGS guidelines. In light of the new guidelines this map theme has been expanded to include shallow slides and earth flows because the source areas for all these landslide types are modelled in the same way. Aspects of the methodology have been adjusted to suit local conditions, as described below. The susceptibility zones shown on these maps apply to two types of landslides — shallow slides and earth or debris flows. Shallow slides by definition are typically small, i.e. 1000 m3 in volume (as defined by AGS, 2007a). These landslide features are deep seated, in that they describe failures where the failure plane extends well below any surficial soil horizons into deeply weathered regolith and/or underlying geological units. The depth of these landslides usually exceeds five metres although this is only an estimate with little direct field evidence. They mainly consist of the following types; deep rotational slides, deep translational slides, deep slides that are transitional into flows, and block or complex spreads. The method for modelling deep-seated landslide susceptibility zones is described in Mazengarb (2005) but it has been necessary to modify some aspects to suit local conditions in other mapped areas and to satisfy the AGS (2007a, b) guidelines as much as practical. A brief review of the methodology is provided below.

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The original Baynes approach (Baynes, 2001) involved the determination of source areas, based on a nominated threshold slope angle for each of the major geological units (simplified from the 1:25 000 scale geological map series). The values chosen were essentially based on expert judgements. Within these areas a subset of areas conducive to failure was identified, as determined from geomorphological criteria. An area uphill of the latter (regression area) is included in the susceptibility zone by projecting lines upslope at the threshold slope angle from each identified source cell. The method used in the Hobart and Glenorchy areas (Mazengarb, 2004c, d) refined the Baynes approach by not employing the geomorphological subset step. The principal justification for this modification is that the earlier method failed to identify the Rosetta landslide, a feature whose movement is the most significant landslide disaster in Tasmania in recent years (Donaldson, 1991). Given the widespread distribution of the geological unit involved in the failure, the revised method was required to identify similar areas. In the Launceston study (Mazengarb, 2006), the previous approach required further modification for the Tertiary sediments of the Launceston Group. In this unit, the uphill projection method (regression) identified what were regarded as unrealistically large areas upslope of the areas above the threshold angle. The Launceston study differed in that the landslide susceptibility map was designed to identify areas susceptible to both shallow Tasmanian Geological Survey Record 2010/01

slides and deep-seated landslides. The modification applied for the Launceston Group sediments, and deeply weathered dolerite, in the Launceston study involved creating a 20 m buffer around the modelled source areas. This buffer therefore includes areas of potential regression (upslope) and runout (downslope).

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In the Tertiary basalt and associated Tertiary sediments of the North West Coast region, the determination of the upslope regression area was able to be refined from the Mazengarb (2005) approach given the large number of landslide features available. Here, the potentially susceptible uphill extent (regression area) was identified by utilising a series of radiating upslope projections from each source cell in a fan shape. The added benefit of this approach was to improve the smoothness and continuity of the regression area boundaries. Through trial and error the angle of uphill projection was increased, from the threshold slope angle, and the amount of fanning adjusted to a point where the results appeared similar to known adjacent landslides (fig. 51, 52). A further modification was employed to reflect the fact that significant runout areas exist for the known large landslides in Tertiary basalt (and intercalated sediments). A fan-type method similar to the regression model was applied (using a straight line shadow angle method) with parameters adjusted to achieve a result that matched observed landslides. An additional limiter was also implemented to ensure the total runout distance travelled would not exceed a maximum of 3.5 times the width of the source area traversed. This value was estimated from a statistical distribution of modelled runouts. This limiter ensured that unrealistic runout distances were not created in circumstances where small susceptible bluffs sat above deeply incised, steep-sided valleys composed of older, more competent basement geology.

Before modelling the runout and regression, a manual masking is applied, in some cases, to specific parts of the modelled source area known from limited field observation to be unlikely to be susceptible to deep-seated failure, for example some raised former shore platforms, large stable areas of outcropping unweathered rock, etc. A final processing step involved an expand-and-shrink routine (one cell dimension) that filled in minor holes and smoothed ragged edges. This has further ensured that an acceptable cartographic output has been produced for the output scale of the maps. It is important to note that known past deep-seated landslides are considered separately from the modelled susceptibility zones (susceptible to first-time failure), as described above. It is well known that past deep-seated landslides can be reactivated by a variety of mechanisms. Therefore mapped landslides should be considered as areas that are susceptible to reactivation, whether or not they are totally within modelled susceptibility zones. Mapped landslides are shown on the landslide susceptibility maps as additional susceptibility zones (e.g. fig. 53). Susceptibility modelling will often not successfully identify all of the areas of an existing deep-seated landslide as the surface morphology is usually significantly altered by 38

p la

re Exi stin

gr

sio es

g la

rge

n

tea

u

slo p

ea ar

l an d

e

valley flo o

slid e

Figure 51 Conceptual diagram illustrating the modelling method for determining runout and regression areas from source areas. Note that the zones are adjusted to match the approximate extent of typical deep-seated landslides occurring in this geological and geomorphic setting.

source area

n ru

Very Low to No Susceptibility

t ou

ea ar

Regression area

r

Runout area

Source area

Very Low to No Susceptibility

regression line

a (runout shadow angle) original (pre-failure) hillside profile runout line landslide toe

b

Figure 52 Conceptual diagram (cross section) of a hillside showing pre-failure and post-failure profiles for a deep-seated landslide. Runout and regression lines for a hypothetical landslide are defined with their relationship to the modelled susceptibility zones for the pre-failure landscape.

deep-seated movement. However susceptibility modelling will often identify parts of an existing landslide that could fail as new, parasitic landslides. One deep-seated landslide type that has not been modelled is spreads, such as those near Wynyard. These landslides are problematical to model given that they have failed on extremely low slopes and if the techniques used above were applied, they would create unacceptably large susceptibility zones. Given the lack of definitive evidence of recent movement in these spreads it has been decided not to model them until more is understood about their origin, age and triggering mechanisms. Tasmanian Geological Survey Record 2010/01

Determination of source areas The determination of source areas for deep-seated landslides requires further discussion as there are three significant problems encountered:

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The AGS guidelines provide examples of various GIS based modelling techniques available for identifying potential source areas for future first time failures. Most of the methods provided rely on obtaining easily extracted parameters associated with the landslides (a training set), analysing these to develop an algorithm and applying a rating (based on the algorithm) back to the remaining landscape. This may be a valid approach where 39

Figure 53 Example of a deep-seated landslide susceptibility map. Mapped landslides (black and grey outlined polygons — susceptible to reactivation) and modelled susceptibility zones (comprising red source areas, orange regression areas and yellow runout areas — susceptible to first-time failure) are depicted. Blue dots are mapped springs or seeps, which have a known association with landslides in many cases. the area under each landslide gives us meaningful information about the landscape prior to failure. This condition is not true for the large rotational slides of the North West Coast region, especially when many of the slides are significantly eroded. For example, slope analysis of digital terrain models on these landforms does not readily provide reliable pre-failure slope conditions.

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For some geological units there are limited numbers of landslides from which to derive reliable characteristic parameters. While this problem may reduce as more areas are studied, judgements must be made for immediate projects and could subsequently prove unreliable and necessitate refining in later years. Mazengarb (2005) developed a slope analysis method to determine threshold slopes that roughly matched the expert judgements of Baynes (Baynes, 2001; 2002). The slope analysis approach is influenced by the early work of Carson (1975) and Stevenson (1977), but subsequent Tasmanian mapping projects indicate that the technique may not be sufficiently reliable and should be revisited in the future. From a conceptual basis, the development of simplistic modelling techniques for what is in effect a complex three-dimensional geometrical problem is fraught with pitfalls. Further, given that some key parameters such as Tasmanian Geological Survey Record 2010/01

groundwater are not well understood for each landslide, the identification of susceptible areas cannot be expected to be a precise determination. Note that mapped springs and seeps are also included on the susceptibility maps produced after 2006. Notwithstanding these issues, the published literature typically considers geology and slope to be the two most important factors for predicting landslide susceptibility. Given these criteria a slope value must be chosen that is roughly equivalent to a hillside threshold slope concept (Carson, 1975; Montgomery and Brandon, 2002). The approach adopted here involves consideration of the characteristic slopes that develop in various geomorphic and geological settings, the material properties (particularly shear box tests) of analysed soils where available, previous studies (largely based on expert judgements) and the analysis of the mapped landslides. Ultimately the actual choice of slope threshold is an expert judgement. Tables 1 to 3 summarise methods and parameters used for specific study areas and rock types to date. It is realised that there are some minor inconsistencies in methods and parameters between the study areas. These will eventually be reviewed with the aim of producing a standard table for the entire State. 40

Table 1 Parameters used for Hobart and Glenorchy deep-seated failures (2004) Geological sub-unit

Method

Parmeener Supergroup

Permian mudstone Permian sandstone Triassic sandstone Triassic mudstone Undifferentiated

Slope analysis Slope analysis Slope analysis Slope analysis

32 41 41 32

32 41 41 32

Slope analysis Slope analysis Rosetta Landslide Taroona Landslide

41 38 10 6.5

41 38 10 6.5

Jurassic Dolerite Tertiary basalt Tertiary sediments

Rosetta Taroona

Source determination parameter (°)

Regression angle (°)

Runout area

not modelled

Geological unit

Table 2 Parameters used for Launceston deep-seated failures (2006) Geological unit Jurassic dolerite Weathered Jurassic dolerite Unweathered Tertiary basalt Tertiary sediments (Launceston Group)

Geological sub-unit

Method

Source determination parameter (°)

Geotechnical report Undifferentiated

Landslide slope analysis

Regression area

Runout area

20 m buffer

20 m buffer

20 m buffer

20 m buffer

Source determination parameter (°)

Regression angle (°)

Runout angle (°)

16

20

12 (shadow angle)

14

20

14

20

16 (shadow angle) 16 (shadow angle)

50 12, 15 50 7 (minimum), 12 (median)

Table 3 Parameters used for North West Coast region deep-seated failures (2010) Geological unit Parmeener Supergroup

Geological sub-unit

Method

Permian mudstone

Jurassic dolerite Weathered Jurassic dolerite Weathered Tertiary basalt and sediments Basaltic colluvium

Not modelled Not modelled

The deep-seated landslide susceptibility map has a number of inherent limitations and must be regarded purely as an indication of instability from a regional perspective. The values chosen represent a pessimistic or worst-case scenario that may, with further site investigations, be found to be better than indicated in some cases. The model does

Tasmanian Geological Survey Record 2010/01

not account for spatial variations in groundwater levels, pore pressures, weathering, lithology, fractures, structural orientation, etc., all of which may have a significant effect on local instability but which are often poorly understood at a regional scale.

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Statewide Landslide Susceptibility A statewide landslide susceptibility map is being developed that may eventually be used as the basis for a statewide landslide planning overlay. This statewide coverage is produced at a coarser, less accurate scale than used in the current 1:25 000 scale outputs for individual regional study areas. This map merges together the modelled susceptible areas for rock fall, shallow slides and flows, and deep-seated landslides, along with all known landslides from the MRT landslide database, to present a generalised statewide landslide susceptibility map.

have more reliable susceptibility modelling at larger scales. The map will also depict the few locations that are covered by proclaimed Landslip A and B areas, which have associated legislated controls and so take precedence over any other zoning (fig. 54). The regional studies for the Tasmanian Landslide Map Series have added to our understanding of landscape evolution and landslide susceptibility, in a range of geomorphic and geological settings within Tasmania. As a result MRT has been able to refine its landslide susceptibility modelling with each new regional study, and has now applied this modelling to examples of most of the major landslide susceptible settings in Tasmania. The statewide landslide susceptibility modelling utilises this knowledge and applies these modelling techniques in a consistent way across the remainder of the State that has not been mapped as part of the Tasmanian Landslide Map Series.

Where more detailed regional studies have already been conducted, as part of the Tasmanian Landslide Map Series, these will be merged with the statewide coverage and take precedence over the less accurate coverage. As new regional studies are completed they will be added in this way to the Statewide Landslide Susceptibility map. The map is attributed in such a way that it is clear what areas are covered by more detailed regional studies, and therefore

Statewide Landslide Susceptibility Map

B A

B A

Proclaimed Landslip Areas

Landslide database

Combined Landslide Susceptibility Maps

Figure 54 Conceptual diagram illustrating the construction of the Statewide Landslide Susceptibility map.

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CONCLUSIONS A landslide zoning methodology for Tasmania is outlined that supersedes the earlier approach of Mazengarb (2005). A number of changes have been implemented to allow general improvements to be made to the overall methodology making it more applicable to a wider range of land instability, to conform to the new Landslide Risk Management Guidelines (AGS, 2007a, b) as much as possible, and to make the maps more accessible to planning authorities. Among the significant changes from the previous publication are:

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the landslide database has been enhanced, allowing full spatial depiction of landslide features on the maps;

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an expanded set of geomorphic features are mapped;

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development of a non-technical user guide (Part 1) to better inform the planning community, with the aim of improving landslide risk management.

various adjustments to the modelling techniques; development of a statewide landslide susceptibility map, that may be used as the basis for a statewide landslide planning overlay;

The reader is reminded of the caveats that apply to the maps. With ongoing studies there may be additional refinements required to reflect improved knowledge and advances in landslide zoning science.

ACKNOWLEDGEMENTS We wish to acknowledge the financial support of the Natural Disaster Mitigation Programme (Federal and Tasmanian governments) in making this report and the associated maps possible. Several people have been involved in reviewing this manuscript and in particular we wish to acknowledge Dr Adam Beer (Coffey Mining), Wendy Saunders (GNS

Tasmanian Geological Survey Record 2010/01

Science), Tony Miner (Australian Geomechanics Society) and Carol Bacon (MRT). A range of former and present staff of MRT and external colleagues have contributed to the ideas presented in the report to whom we are extremely grateful.

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REFERENCES AGS, 2000. Landslide risk management concept and guidelines. Australian Geomechanics 35:49–92. AGS, 2007a. Guideline for landslide susceptibility, hazard and risk zoning for land use planning. Australian Geomechanics 42(1):13–36. AGS, 2007b. Commentary on guideline for landslide susceptibility, hazard and risk zoning for land use planning. Australian Geomechanics 42(1):37–62. AGS, 2007c. Practice note guidelines for landslide risk management. Australian Geomechanics 42(1):63–114. AGS, 2007d. Commentary on practice note guidelines for landslide risk management. Australian Geomechanics 42(1):115–158. AGS, 2007e. Australian GeoGuides for slope management and maintenance. Australian Geomechanics 42(1):159–182.

MAZENGARB, C. 2004a. Tasmanian landslide hazard map series. Glenorchy. Map 3. Potential debris flow hazard. Mineral Resources Tasmania. MAZENGARB, C. 2004b. Tasmanian landslide hazard map series. Hobart. Map 3. Potential debris flow hazard. Mineral Resources Tasmania. MAZENGARB, C. 2004c. Tasmanian landslide hazard map series. Glenorchy. Map 5. Potential deep seated landslide hazard. Mineral Resources Tasmania. MAZENGARB, C. 2004d. Tasmanian landslide hazard map series. Hobart. Map 5. Potential deep seated landslide hazard. Mineral Resources Tasmania, Hobart. MAZENGARB, C. 2005. The Tasmanian landslide hazard map series: Methodology. Record Tasmanian Geological Survey 2005/04.

BAYNES, F. J. 2001. Hobart land stability project. Consultants report to Mineral Resources Tasmania. Baynes Geologic 138/21.

MAZENGARB, C. 2006. Tasmanian landslide hazard map series. Launceston. Map 5. Potential landslide hazards. Mineral Resources Tasmania.

BAYNES, F. J. 2002. Land stability project Windermere/Pleasant Hills. Consultants report to Mineral Resources Tasmania. Baynes Geologic 138/1/30.

MONTGOMERY, D. R.; BRANDON, M. T. 2002. Topographic controls on erosion rates in tectonically active mountain ranges. Earth and Planetary Science Letters 201:481–489.

CAINE, N. 1983. The mountains of northeastern Tasmania : a study of alpine geomorphology. A. A. Balkema : Rotterdam.

SAUNDERS, W.; GLASSEY, P. 2007. Guidelines for assessing planning policy and consent requirements for landslide prone land. Miscellaneous Series GNS Science 7.

CARSON, M. A. 1975. Threshold and characteristic angles of straight slopes, in: YATSU, E.; WARD, A. J.; ADAMS, F. (ed.). Mass wasting: 4th Guelph Symposium on geomorphology. 19–34. Geo Abstracts Ltd : Canada.

SCHWAB, J. C.; GORI, P. L.; JEER, S. 2005. Landslide hazards and planning. Report American Planning Association Planning Advisory Service. 533/534.

CRUDEN, D. M.; VARNES, D. J. 1996. Landslides types and processes, in: TURNER, A. K.; SCHUSTER, R. L. (ed.). Landslides investigation and mitigation. Special Report Transportation Research Board National Research Council Washington DC 247:36–75.

SELBY, M. J. 1993. Hillslope materials and processes (Second edition). Oxford University Press : Oxford.

DONALDSON, R. C. 1991. Rosetta landslide, geological investigation and slope risk assessment. Report Division of Mines and Mineral Resources Tasmania 1991/20.

STEVENSON, P. C. 1975. A predictive landslip survey and its social impact. Proceedings 2nd Australia–New Zealand Conference on Geomechanics. 10–15. Institution of Engineers, Australia.

EMA, 2002. Planning Safer Communities. Land use planning for natural hazards. Australian Emergency Manuals Series. Part II Approaches to Emergency Management. Volume 2 – Mitigation Planning Manual 1. Emergency Management Australia : Canberra.

STEVENSON, P. C. 1977. An empirical method for the evaluation of relative landslide risk. Bulletin International Association Engineering Geology 16:69–72.

FELL, R.; MOON, A. T. 2007. Final report on the review of Mineral Reso urces Tasmania Landsl ide Haz ard Zoning, M t Wellington–Hobart area debris flows. Report to Hobart Water by UNSW Global Pty Ltd and Coffey Geotechnics Pty Ltd. [available online: www.ses.tas.gov.au]. GILMOUR, R. (ed.). 2003. State Summary. The Tasmanian emergency risk management project — a community perspective. State Emergency Service Tasmania : Hobart. HAND, D. W. 2000. Report of the inquest into the deaths arising from the Thredbo landslide. [State Coroner, Glebe, NSW]. KIERNAN, K. 1990. Geomorphology manual. Forest Practices Unit, Forestry Commission Tasmania : Hobart.

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STEVENSON, P. C. 1972. Examination of a landslip at Beauty Point. Technical Reports Department of Mines Tasmania 15:61–63.

TWIDALE, C. R. 1968. Geomorphology with special reference to Australia. Thomas Nelson Australia : Melbourne.

Publications and maps issued by Mineral Resources Tasmania and Tasmania Department of Mines are available for downloading from the MRT website using the document search facility: www.mrt.tas.gov.au – Search – Document Search. AGS publications can be downloaded from the Australian Geomechanics Society website: www.australiangeomechanics.org – Publications and Resources – Downloads

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