South Sister St. Marys, Tasmania

South Sister

erosion possibility

[text of letter from Owen Ingles recommending caution over logging on South Sister]

Owen Ingles Pty. Ltd.
Tasmania, 7275


Dear Lesley,

I received your letter with the detailed proposed logging coupe (NI 114A) map and photos of nearby areas. I note that the top of South Sister itself is reserved, and a further narrow reserve lies to the east thereof. It was very difficult to read the legend at lower left, but I think I am correct in saying that the average slope in the proposed logging coupe is about 12° (but very variable), so that landslip hazard should only be an issue in respect of the logging roads, which would need to be carefully designed and maintained (i.e. the Code should be strictly followed).

The real concern on this site will be the SOIL. Within the logging area the soil has formed from a dolerite scree over Permian sand/silt/mud stones, and is quite shallow; mostly less than 1 m thick (sometimes half that), and is a brown gradational loam (Northcote Gn 4.31), moderately porous and susceptible to sheet erosion. In such circumstances, it is absolutely imperative that a good ground cover be maintained at all times - in other words, clear felling is quite out of the question (and may be the reason why selective logging has been proposed). But what must also be out of the question is any BURNING after logging, as that would be just as bad as the clear fell if it were done just before heavy rains (this area has an annual rainfall of 1000 - 1200 mm).

The dolerite baserock is EXTREMELY slow weathering, so there is a major risk in this coupe that erosion (principally sheet) will destroy the soil cover and hence prevent forest regrowth within a matter of a few years - perhaps as little as 50 years (see my paper in Upper Catchment Issues). In this regard, I would call your attention to Forestry's own publication Geomorphology Manual (Kieman, K., 1990) which says on p. 21 'Forests represent a renewable resource, but the soils in which they grow are not renewable over less than centuries' Correct (my emphasis)! That Manual continues on p. 21 to say 'it takes at least 2-4 years of regeneration before runoff returns to pre-logging levels. erosion risk is aggravated during this time'. And on p. 23 is the specifically relevant phrase 'From the perspective of forest management, the loss even of dolerite soils is effectively irrevocable'.

I suggest that you use this letter in submissions to the local Council or local Parliamentary representative seeking Forestry's justification for ignoring its own assessments. If so, please attach a copy of my own technical paper referred to above, as the soil erosion is a very insidious one which is not perceived at an average few millimetres a year, but can be quite deadly for the welfare of the next generation.

Yours sincerely,


Owen G. Ingles (Dr.)

Owen G. Ingles, B.A., M.Sc., Ph.D., F.R.S.C., F.I.E.Aust., M.I.E., M.A.I.E., C.Eng., C.Chem., C.P.Eng.
Owen Ingles P/L,
Soil Engineering and Risk Management Consultants,
Swan Point, Tasmania

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