South Sister St. Marys, Tasmania

south sister hydrology

- d. leaman



A forestry proposal for working coupe NI114A, said to involve 240 ha of 50% thinning, has been declared. This coupe extends along the south face and around the east face of South Sister in the Nicholas Range above St. Marys (see map). As is often the case with these proposals and indicative plans no definite or clear plan is available for timely public information or comment and the entire proposition can be subject to change. This is a weakness in State laws and the forest practices system which suits the industry.

The following comments are general and apply regardless of the details of the final working plan - whenever that becomes available. If that plan does not provide for all, or any, of the factors listed here then it is either irresponsible or not in accord with the Forest Practices Code. It would certainly not be in the interests of local residents who draw water from the area.

There are several concerns within the upper slopes of South Sister which must be properly allayed. These include matters of slope stability and associated operational factors, and preservation of water quality and volumes.


The stability and hydrology of South Sister cannot be appreciated without some understanding of the geology and history of the landscape.

The Nicholas Range is composed primarily of Triassic coal measures and some minor production has occurred above the intersection of Dublin Town and German Town Roads. Carbonaceous mudstones are exposed in recent washouts along the Dublin Town Road and feldspathic sandstones crop out in the cuttings. Triassic basalts crop out below this level and mark an ancient Triassic land surface, later buried by the coal measures

The basaltic lavas vary in thickness and character but a substantial volume extends down slope toward St. Marys. These flows have been exhumed by relatively recent erosion and their resistance to erosion means that they either form the surface rocks or shape steps in the terrain. Jurassic dolerite later intruded the pile of rocks and, after much erosion, now caps the top of the range

The present land forms reflect this rock mix and active recent erosion - especially through the late Pleistocene ice age period of the last million years or so. The alternation of glacial times and warmer inter-glacials has shaped the stream system and the range itself. The dolerite has been almost removed by erosion and dolerite debris is broadly spread over the upper slopes. The slope debris varies greatly in thickness (from nil to many metres) and the scale of the deposits, in any detail, can only be inferred from some of the past failures within them or from drilling or geophysical testing. Slope angles reflect the variation in the underlying rocks: the dolerite above and the basalts below producing steeper slopes and steps in the terrain. The shallowest slopes occur where the slope debris is dominant.


The slope deposits derived from the dolerite, which include blocks of rock of various sizes and a matrix of clays and smaller fragments, depend on the balance of vegetation, total rainfall, and drainage capacity for their stability. The material is capable of mass failure and much of the humpiness evident on the southern and eastern slopes of South Sister is due to previous failures. Movement and failure is possible after times of exceptional rainfall. Few, if any, of the large past failures appear recent or within the life of the existing trees. Failures which have occurred on the slopes of South Sister range from massive, rock-involved to very surficial and small.

Southern slopes

The entire section south of the pinnacle, but just north of the coupe boundary, which is several hundred metres long, is a large scale failure which has involved the dolerite cap of the mountain and, also, a section of the underlying Triassic rocks The failure surfaces, and there are almost certainly several of these, may approach surface south of the east-west track where they are covered by thinner debris deposits. At least four major scoop (rotational) failures have occurred in these materials (approx. 100 to 150 m across with movements of 50 to 100 m) as well as many smaller creeping zones

Eastern face

The eastern face of the mountain is of similar constitution but has steeper slopes. There is no evidence of any very large scale bedrock-involved structures but there are several large (up to 100 m across with movements of similar scale) rotational failures and many smaller surface failures wholly within the slope debris.

The most recent failures on either slope consists either of single blocks or column collapse from high points or minor creep and block rotation on the slopes.

Most of these structures, on either face, appear fairly stable in the present forested regime but large changes in either forest cover or local climate could be expected to lead to changes in failure frequency.

Typical modern slope angles for the surface deposits within the proposed coupe area are of the order of 7 to 10° on the south face and upper parts of the east face but 13 to 25° on the lower sections of the east face.

The Forest Practices Code requires that areas which exceed 'acceptable' slope threshold angles be inspected by a geotechnical specialist before operations may proceed (Section D1, pages 53-54, Forest Practices Code, 2000). The relevant angle is quoted, with some caveats, as 19° for dolerite soils and debris and 15° for everything else present here. The Code wholly ignores the reality of bedrock failure involving either dolerite or Triassic rocks but it does occur - and has occurred on South Sister - with consequences for hydrology (below). No allowance is made for the significance of previous failure and it is possible to argue about the validity of the quoted angles generally. Some would be unacceptable for land subdivision in other applications. The tertiary sedimentary materials, for example, quoted in the Code at 11°, are generally considered unstable at 7° (e.g., Telfer, 1988). The value assigned to dolerite slope deposits is directly relevant to issues at South Sister. These materials are notoriously variable in rock and matrix content and proportion, and failures have clearly occurred on South Sister at angles much lower than the Code advisory and we should take that indication as reality. Such failures, especially if large, may be extremely rare.

The message here, however, is to tread very carefully on these slopes - after detailed inspection.

The effect of forest thinning or fire can only be surmised but some slope changes are inevitable if disturbance is followed by extreme rain events.

There can be no doubt, however, about the lower section of slopes above Dublin Town Road; these clearly exceed the Code specification and offer evidence of both past failure and present activity. The critical zone adjacent to the road is marked on the map and it is my professional opinion (as a qualified engineering geologist) that these lower slopes should not be disturbed by any industrial operations.

The nature of the debris on some slope segments will also influence the detailed planing and manner of operation; either by removal, shifting of large blocks or avoidance of such blocks for access. Observations elsewhere suggest that this will lead to considerable ground disturbance by a 50% thinning. The effect of such disturbance on either stability or infiltration within the matrix materials and soils present is unclear. Greater water run off is the most likely scenario which would assist stability, provided the water escaped, but reduced groundwater recharge - with ramifications for supply (below). There are short and long term effects. Greater run off rates after rainfall commonly leads to short term reductions in water quality. Increased run off often means loss in storage or delay in recharge.

This effect must be contrasted with the water exchange involved in tree removal.

Research on the effect of reduction in vegetation cover also shows that the amount of water involved may represent a significant proportion of rainfall For example a 10% reduction in eucalypt cover may release between 20 nd 65 mm of rain equivalent and it is likely in the high rainfall area of South Sister that the value will be at the higher end of this range (Bosch and Hewlett, 1982). This will mean more springs but also higher risk of failure and the critical time is in the first few years after thinning (Brooks et al, 2003). Note that a 50% thinning implies a water change of perhaps 200 mm and this, coupled with the surface disturbance effects, will lead to major changes in water circulation and supply conditions. It may well destabilize some slopes, or parts of slopes.

Combination of the two effects in the highly porous and fractured media on South Sister is likely to have undesirable effects - changes in water quality and, at least, more surficial failures.


The discussion about stability indicates some of the connections between forest management and hydrology. It is simply not possible to change land use without some impact, either positive or negative, on the hydrological equation.

The upper part of South Sister is typically 'dry' Tasmanian mountain. It may provide sheet run off during and immediately after rain events but it possesses no well defined streams above an elevation of 600 metres. This means that subsurface distribution of the water is important. It also means that many of the surface water protections, such as afforded under the Forest Practices Code, do not apply. The Code is not noted for its wisdom in dealing with groundwater systems. The hydrology of the mountain depends on the constituent rocks and their groundwater storage.

Several properties depend on water supplies from the forested area on the mountain. A major intake point is within the coupe area along Dublin Town Road. An older intake point is nearby. I have also been advised that several properties depend on spring and seepage water from the region between Struggle Gully and the saddle at Cheeseberry Hill. Supply conditions along the south face have not been inspected.

Although my inspection followed some weeks of limited and abnormally low Spring rain input, many seepage points were found. These were mainly within, or at the base of, dolerite slope debris but two seepages involved the coal measures directly. Some pools, with vegetational variation, are clearly perennial.

Permanent springs can be correlated with failed areas and may reflect bedrock involvement and, or, fracturing due to previous failures. The observed water quality, of 70-90 ppm TDS, is excellent. The water flow from all springs was clear. The level of total dissolved solids indicates that the coal measures are acting as the main storage medium with rapid recharge through the dolerite-derived slope cover. Disturbance of that cover may affect both recharge and quality.

The intake points, and pools, must be protected by the forest operation plan - either in terms of wet and marshy areas (Section C.4.3, p. 48) where a 30 m protection zone may be applied, or as significant springs which must be treated as Class 3 or 4 watercourses at least (Section D.2.1, p. 57; Forest Practices Code, 2000). The latter conditions require a protection zone of 10 to 20 metre width plus a minimal disturbance of both channels and soils (p. 55). The real nonsense in all of this is that the Code then allows timber to be removed from any of the protection zones provided the equipment does not enter them (see pages 46 and 48). There is no thought about what effect tree removal near such sites might have upon them because groundwater implications are almost totally ignored by the Code.

There is, however, a discretionary clause in Section D.2.2 (page 58) which is more demanding although I have yet to see it applied. It is quoted in full here.

'Within 2 km upstream of known domestic water intakes measures in addition to the standard provisions of this Code may be prescribed in the Forest Practices Plan. In particular, measures may be required where a domestic water supply is derived wholly or predominantly from within an area of forestry operations. Consult with a specialist if disturbance is likely to significantly affect water quality.'

Consider the implications of this clause.
The intakes on Dublin Town Road, and perhaps some of those along the south face of South Sister, have a claim to this clause.
The Code, as always, emphasizes quality factors and evades the even more serious issue of quantity. Even so, it is clear that every water supply around South Sister falls within the 2 km envelope however the word 'upstream' is interpreted. If it be argued that groundwater is not included in this then the Forest Practices Code should be wholly dismissed as irrelevant and impractical notwithstanding the treatment of springs as subsurface conduits on page 57. Some of the conduits here must be the fracture systems or failure surfaces associated with previous instability.

Disturbance may affect water quality; it will certainly affect water quantity.
Section D.2.2 therefore demands that extra and particular measures are required of the forest plan.

It is worth noting here that the Forest Practices Code offers no justification for any of the widths used as recommendations. None make any sense hydrologically. This was stressed by Leaman (2002) in a review of dolerite properties where it was suggested that much wider zones were required. Other forest services are much more generous and buffer strips in the western US require trees to be left with the buffer and generally specify widths of 30 to 300 metres (belt et al, 1992). This is much more reasonable and ecologically sensible. It may, of course, not be as commercially lucrative. It would, however, protect other users and their rights.

Review of spring distribution in the general area shows that the coal measures sequence is the main storage and permeability differences between slope cover and underlying basalts provide the primary outlets. Variations in content (sandstones v mudstones) or structure (stratification, fracturing) impose secondary elements. The best springs, in terms of flow and probably quality, are likely to be located near intersections of some or all of these elements - hence the link to past or present failure zones.


The comments in this brief report are predicated on the assumption that the forest operation will amount to a 50% thinning as presently indicated. A modest variation in thinning density would not affect the conclusions which follow but any major change, such as to clearing and regeneration/plantation, would be another matter entirely. Such a change would totally disregard values upon which residents depend. One would also hope that should thinning proceed anywhere on this coupe that it might be undertaken with more care and responsibility than I have observed in many other coupes in the region.

Nothing should be done which damages the infiltration capacity of the materials in a broad scale manner or which encourages failure of slopes.

No activity should be entertained which might risk the water supply to the many residents in the area. This means quantity as well as quality.
It has to be appreciated, for example, that water supply to the east is controlled by the groundwater divide (see map) which is barely 500 metres from the principal intakes.

The Forest Practics Plan is required to consider

It should also consider

These factors essentially mean that the eastern face of South Sister, east of the divide, should not be worked at all. About one half of this area has slopes requiring a discretionary judgment using the Code specification, and which I consider too high, and the remainder includes spring sources and failed ground. All the intake conditions require sensitive planning and operation in any event and this may render operations impractical - unless no one cares to keep to the Code or Plan. I have seen too many cases of complaints rendered after the event but then, of course, it is too late: the damage has been done.

I believe it would be irresponsible and neglect of any relevant duty of care to work the area above Dublin Town Road. It is time to use the discretion of clause D.2.2.

The situation around the southern face of the mountain is more complex. Water supplies may be affected but the risks cannot be properly evaluated without further review and location of intakes. A better understanding of the hydrology of this area could be obtained after a wet period when all seepages could be mapped and related to surface conditions. All eight considerations listed above should still be applied to any plan.

Operations should not commence until, or unless, such appraisals have been completed and linked to the following recommendations.


Some basic recommendations have been included as part of the conclusions above; such as completion of adequate inspections and application of relevant discretionary powers, including the non working of at least one face of the mountain.

Some other information should be collected.

  1. How many properties are connected to the intakes visited (a census is required).
  2. How many properties draw water from the southern face (a census is required).
  3. How much water is supplied on an annual basis from intakes on the east face and any supply points on the south face (meter the main pipes).
  4. What is the regularity of flow or variability by season or rain event (monitor the meter with adequate frequency to determine this).
  5. How does water quality vary according to rain event or season (sample at time of a few events and then at least monthly). Basic measures such as turbidity and total dissolved solids will provide a sound guide.

Records of this type will establish the nature and extent of any loss, or change in supply, should the forest operation proceed. A replacement cost or value could then be defined if necessary. The Forest Practices Code does not provide any guarantee concerning water supply; only that it will be 'protected'. This claim requires great care and much information, and Forest Plans are not usually based on such information.

While each of the recommendations should be completed by Forestry Tasmania before the sealing of a Forest Plan I further recommend that it also be done independently by local residents in any event. Observation should commence immediately and continue indefinitely. Information should be available in advance of any forest working. Observations should be maintained in a secure location and include dates, details of observer and records of measurements and how done.


Belt, F. H., O'Laughlin, J. and Merrill, T., 1992. Design of forest riparian buffer strips for the protection of water quality: analysis of scientific literature. Idaho Forest Wildlife and Range Policy Analysis Group Report 8, Moscow, ID. Univ. Idaho

Bosch, J. M. and Hewlett, J. D., 1982. A Review of catchment experiments to determine the effect of vegetation changes on water yield and evapotranspiration. J. Hydrology, 55, 3-23

Brooks, K. N., Ffolliott, P. F., Gregerson, H. M. and DeBano, L. F., 2003. Hydrology and The Management of Watersheds 3rd ed. Iowa State Press.

Forest Practices Code, 2000. Forest Practices Board, Hobart.

Leaman, D. E., 2002. The Rock which makes Tasmania. Leaman Geophysics, Hobart.

Telfer, A. L., 1988. Landslides and land use planning. Geol. Surv. Tasm. Bull. 63

Report prepared by:
D. E. Leaman, Ph.D., Hydrologist
Leaman Geophysics, Hobart, Tasmania
23 November, 2003

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