South Sister St. Marys, Tasmania

Landslips

comment on dispute

[We now have a geologist, David Stapledon, with 53 years experience in the field including work on the Snowy Mountains Hydro-Electric Authority, commenting on the dispute]

INITIAL FOREST PLAN, SOUTH SISTER – NICHOLAS RANGE COMMENTS ON REPORTS BY D.E. LEAMAN AND P.D. MCINTOSH

I have read the report dated 22 November 2203 by Dr. Leaman, some comments on it prepared by Dr P.D. McIntosh, dated 28 November 2003 and Dr Leaman’s reply to those comments.

I am not familiar with the local topography and geology of the area at issue, the Initial Forest Plan and the Forest Practices Code. Therefore I am unable to comment on most of the individual points of argument relating to these. However I have for many years been involved professionally in developmental projects on moderately to very steep slopes. Present and significant past climatic conditions have varied from alpine to tropical. From this experience I am aware of:

The high frequency of slope instability and erosion problems associated with such projects, and
The range of causes and reasons for those problems

Regarding the frequency of such problems, Fell et al (1992) wrote, in regard to old landslip scare 'Experience has shown that in more than 50 per cent of cases, disturbance of an old landslide, e.g. by cutting into or inundating its toe, causes reactivation'. More recently, the International Commission of Large Dams (2002) produced a figure of 'at least 75%' of reactivation, from statistical data at 145 known reservoir landslides. Those figures include slides affected by actual inundation by a reservoir, but at most of those in my experience, reactivation started during or after the cutting or clearing of vegetation, and excavation for access tracks or roads. Some examples are given in Fell et al (1992).

However, disturbance-induced landsliding has not been limited to old landslide scars. It has been most common at or near sites showing localised topographic evidence of past landsliding, but has occurred also in areas from of such evidence.

Regarding causes, there are short and long term effects, but usually, removal of trees causes increases in the water content in the ground, lowering its stability. Greater runoff and exposed soil in access tracks contributes to erosion, which in many cases has been negligible, prior to the disturbance.

In the light of the above, if involved with planning a project in the area under consideration I would not place any reliance on the following argument by Dr McIntosh, in his comment on Page 2, Para 3 of Dr Leaman’s report:

'It must also be pointed out that almost all parent materials on slopes in Tasmania have formed in part by previous erosion under a different climate from that now prevailing and past 'failure' or erosion is not necessarily an indication of present risk'

David Stapledon, B.Sc., M.Sc
October 6, 2004

References

Fell, R., MacGregor, P and Stapledon, D (1992). Geotechnical Engineering of Embankment Dams. Balkema, pp 675

International Society for Large Dams (2002). Reservoir landslides – Guidelines for Investigation and Managements