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save the Blue Tier

soil engineering report

- owen g. ingles, geotechnical & geomorphological consultant

Concerned Citizens, Pyengana,
c/- L. Nicklason,
167 Forest Lodge Road,
PYENGANA, Tas. 7216
2003/03/20

Re: Proposed Logging Coupes, Blue Tier

In response to your request, I inspected an area of the Blue Tier in the catchment of the Groom River which has been proposed for clearfell logging in the near future; on the 14th March, in the presence of representatives of your group, I took undisturbed rock and soil samples from forest and stream in the vicinity of newly built forestry roads in the proposed logging area. This report describes my subsequent examination of those samples, and conclusions therefrom.

The geology of the whole area is coarse-grained granite (porphyritic leucocratic adamellite, rich in muscovite, biotite and plagioclase phenocrysts). The slope of the bedrock averages 18°, but varies about ±5° so that some localised slopes would exceed 20°.

The grain size in fresh rock was observed to be, on average 0.5 mm; but 1 mm size grains are common and the topsize is around 3 mm. It is significant that the depth of weathering on surficial rock varies between 0.5 and 4 mm, corresponding quite closely with a single grain depth (even in joints). This observation is in line with the known weathering resistance of the mineral suite in this granite, which is high.

The slopes examined were overlain by a stony gradational brown soil, which might be described as a gravelly sandy loam having an average depth of about 900 mm (less on the higher slopes). Three soil samples were taken, two from undisturbed profile at different depths and localities, and one from the bed of a running class 4 stream feeding to the Groom River. Northcote (1960) has described the local soils as "Brown, friable, porous earth, no A2 - Gn 4.31" and this writer wholly concurs with that description, although adding that the porous nature of the soil is largely due to fine fibrous organic matter.

Under the microscope, it is clear that the soil components are essentially the primary weathering products, more or less unchanged, of the parent rock. Thus in both samples (one from near the surface, one from near the bedrock) the maximum grain size was around 1 mm and the average grain size around 0.1 mm, and no clay was present. In the sample taken from the stream bed there was likewise no clay, but the grain size was slightly coarser, namely a topsize of some 3 mm and an average size of 0.5 mm; very similar to the parent rock.

It might be added that there is a strong tendency for the finer particles in the soil to aggregate near the surface to about 0.2 mm size.

CONCLUSIONS

  1. Because of the mineral composition of this granite its rate of weathering is extremely slow despite being in an area of high rainfall (1250 - 1500 mm per annum).
  2. The grain size range, both in soil and parent rock, corresponds very closely with that liable to maximum erodibility (erosion at minimum flow velocity). Transportation will occur at velocities as low as 4 cm/sec (Hjulstörm, 1935).
  3. In the absence of clay, the soil, when saturated, may be subject to sliding at slope angles of 18° and above, but landslip is not expected.
  4. It follows that this soil is held in place, not by the large trees (which might be safely felled) but by the understory, which must at all costs be retained if severe erosion is to be avoided. Indeed, removal of the understory to bare earth, even for a short time in such a high rainfall zone, is likely to lead to a very rapid loss of soil cover over the base rock; and regeneration will quickly prove impossible on those slopes at or above the average slope. Clear felling and burning would present a grave risk of destroying the asset in a quite short time. My conclusion on this is corroborated by the assessment of the Department of Agriculture (Land Systems of Tasmania, Region 4 (1980), Land System 641231, component 1) who state that the area is liable to "severe sheet erosion".
  5. Some relevant quotations from Forest Practices own Geomorphology Manual (1990) are:- p.22 "Some of the most dramatic examples of accelerated soil erosion to be seen in Tasmania are in areas of granite and granodiorite ..... coarse textured soils in high intensity rainfall areas, such as some of the granite soils in NE Tasmania, are highly prone to erosion". Agreed! p. 38 recommends that a geomorphologist be consulted "in a very steep area ..... in a granite area" AND to "stick to the Code ..... in the different sections that deal with soils and soil erosion and geomorphology."

To use words from one of Forestry Commission's own publications "solutions that may be economically cheap may be environmentally very costly" - and this might well express the distinction between selective logging and clear felling at these sites on the Blue Tier. However, in my opinion following inspection and testing, I consider that the most damaging action would be removal of the understory - not the tall trees - either by fire or by mechanical means. Special attention will need to be paid to roadside drainage and its disposal

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

[Republished with permission of the author and editors of Upper Catchment Issues Tasmania, which first published this in Vol.2#1A, ISSN 1444-9560. A more detailed report is available.]

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