Peer Review Report
Coastal Slope Instability Hazard Study
Various Sites Shoalhaven City Council LGA
Prepared for
Shoalhaven City Council
Project 72051
July 2011
Table of Contents
Page
1.
Introduction........................................................................................................................ 1
2.
Methodology...................................................................................................................... 1
3.
Site Observations.............................................................................................................. 1
4.
3.1
Racecourse Beach ..................................................................................................2
3.2
Rennies Beach.........................................................................................................3
3.3
Collers Beach Headland ..........................................................................................3
3.4
Bannisters Point.......................................................................................................3
3.5
Inyadda Point ...........................................................................................................4
3.6
Berrara Bluff.............................................................................................................4
3.7
Hyams Point.............................................................................................................5
3.8
Plantation Point........................................................................................................5
3.9
Penguin Head and Culburra Beach.........................................................................5
Comments ......................................................................................................................... 6
4.1
Items Arising from Community Consultation...........................................................6
4.1.1 Sea-Level Changes .....................................................................................6
4.1.2 Cliff Line Erosion Rates ...............................................................................7
4.2
Marine Cliff Line Locations Assessed by SMEC...................................................10
4.3
Hazards Within Cliff Lines and Slopes ..................................................................10
4.3
Australian Geomechanics Society (2007) Methodology.......................................11
4.4
Application of the AGS Methodology by SMEC....................................................12
4.5
Risk to Property .....................................................................................................13
4.5.1 SMEC Assessments ..................................................................................13
4.6
Risk to Life .............................................................................................................14
4.7
Suggested Methodology For Input to DCP 118 ....................................................15
5.
Conclusions..................................................................................................................... 16
6.
References ...................................................................................................................... 17
7.
Limitations ....................................................................................................................... 18
Appendix A:
Appendix B:
Appendix C
About this Report
Correspondence from Dr Brady
AGS Extracts
Peer Review Report Coastal Slope Instability Hazard Study
Various Sites - Shoalhaven City Council LGA
Project 72051
July 2011
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Report on Peer Review
Coastal Slope Instability Hazard Study
Various Sites - Shoalhaven City Council LGA
1.
Introduction
This report presents the results of a peer review by Douglas Partners Pty Ltd (DP) of the Coastal
Slope Instability Study (Draft Report Document Number 3001209-020, dated January 2008) carried
out at various sites within the Shoalhaven City Council Local Government Area (LGA) by SMEC
Australia Pty Ltd (SMEC) as part of the Shoalhaven Coastal Zone Management Study and Plan.
It is understood that, due to the high level of community concern regarding the SMEC study findings, a
review is required to assess whether the application of the methodology of the Australian
Geomechanics Society (AGS, 2007) is sound and the results provide Shoalhaven City Council (SCC)
with a valid framework to apply planning controls included within Development Control Plan 118 DCP
for Areas of Coastal Hazards.
2.
Methodology
The peer review methodology comprised:
•
An initial review of the SMEC documentation, together with relevant sections of Council’s
development control plan (DCP) and previous input from concerned residents.
•
Inspection of each of the sites (Racecourse Beach, Rennies Beach, Collers Beach Headland,
Bannisters Point, Inyadda Point, Berrara Bluff, Hyams Point, Plantation Point, Penguin Head and
Culburra Beach) included in the SMEC report, for assessment of slope conditions and likely
elements at risk in comparison with those included in the report. The inspections were carried out
from safe access points along publicly owned sections of cliff crests, access tracks down cliffs
and wave cut platforms.
•
Review of literature citing evidence of prior sea levels and related erosion features within the
Illawarra – South Coast areas of New South Wales.
•
Provision of opinions on the SMEC application of the AGS methodology.
•
Reassessment (where appropriate based on site observations) of risk levels.
3.
Site Observations
Each of the sites (refer Figure 1, following page) was inspected by a Principal Engineering Geologist
during 27 and 28 September 2010. Details of observed conditions directly relevant to the findings of
the SMEC report are summarised in the following sections. Additional observations, supplementary to
those directly related to the peer review, are included in a separate DP report (Project 72051-1, dated
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Project 72051
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May 2011). Notes describing classification methods and descriptive terms used in the following
descriptions are included in Appendix A.
Figure 1: Location of study sites (after SMEC, Figure 1.1)
3.1
Racecourse Beach
The main items of note are summarised below:
•
A landslide with a toe length of approximately 70 m and extending up to 50 m back from the
beach is located at the northern end of the SMEC assessed beach section (approximately 30 m
from the northern-most residential lot in Coral Crescent.
•
Cliff line sections are discontinuous within a mostly well vegetated slope north of the access ramp
to the beach. In comparison, the SMEC report indicates an “estimated cliff line” at the crest of the
vegetated slope. Only one short section of a 3 m high cliff has been directly eroded by wave
action and this has resulted in a < 0.5 m undercut.
•
1 m to 2 m thickness of debris accumulated from the slope and cliff sections forms a basal,
sloping pediment, the outer edge of which has been locally eroded by wave action.
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•
A 2 m thick by 10 m wide debris lobe extends onto the back of the beach. The lobe is mostly well
vegetated.
•
A continuous section of cliff line, directly eroded by wave action, extends southward from the
access ramp to the beach. With the exception of the south-westerly facing section of the cliff line,
the cliff line is seaward of the alignment shown in the SMEC report.
3.2
Rennies Beach
The main items of note are summarised below:
•
Two soil slumps affect the sides of a gully entrenched into aeolian sand underlain by a lateritic
weathering profile (possibly developed on Tertiary sediments). The lower slump appears to have
expanded since the SMEC assessment when small slumps (2 m3) and tension cracks were
recorded.
•
There is a short section of a 2 m to 3 m high cliff line at the rear of the beach. This is
approximately 50 m downslope of the “estimated cliff line” shown as lying at the crest of the
vegetated slope in the SMEC report.
•
The beach below SMEC’s estimated cliff line section is backed by a sand dune system, partially
stabilised by vegetation.
3.3
Collers Beach Headland
The main items of note are summarised below:
•
The cliff line is near-continuous but is significantly seaward of the “estimated cliff line” in the
SMEC report.
•
The eastern (approximately) 80 m to 90 m section of the cliff line is being directly eroded by wave.
This has resulted in minor undercutting of its base.
•
Two areas of colluvium at the base of the cliff line appear to represent debris from multiple slump
failures or individual, larger collapses within the cliff line (now well vegetated). The outer edges of
the debris piles have been locally eroded by wave action .
•
The cliff line dies out at the western end of the assessed section and the slope has stumps of
large trees, probably in excess of 100 years old. The toe of the slope is protected by an
extensive cover of natural rock armour.
3.4
Bannisters Point
The main items of note are summarised below:
•
The cliff line is discontinuous and lies within a mostly well vegetated slope. The SMEC report
indicates an “estimated cliff line” at the crest of the vegetated slope, located variously a few
metres to approximately 25 m shoreward of the observed cliff sections. No cliff section is
currently being directly eroded by wave action.
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•
The cliff line comprises open jointed basalt and with isolated fallen blocks, mostly less than 1 m
greatest dimension, along the cliff line.
•
Within the western section of the assessed slope, the lower slope (below the discontinuous cliff
line) is mantled by 2 m to 3 m thick colluvium and/or marine (wind blown and probably wave
transported) debris. This slope is mostly covered by a mature vegetation assemblage. The toe of
the overburden deposit has been subject to wave erosion which has resulted in 0.5 m to 1 m high
exposures of these materials.
•
A distinctive deposit of natural rock armour mantles the rock platform and upper beach in the
central eastern section of the assessed slope. This deposit is assessed as being derived from
the collapse of a section of the adjacent cliff line which, together with the lower slope, is now well
vegetated and includes mature trees.
3.5
Inyadda Point
The main items of note are summarised below:
•
The are no distinct cliff lines within the section, of mostly vegetation covered slope, assessed by
SMEC.
•
The wave cut platform is either mantled by sand or natural rock armour that extends 5 m to 10 m
seaward of outcrop or soils at the toe of the slope. There is no evidence of current erosion in to
the vegetated area.
•
Localised embayments in the now well vegetated slope (such as at the eastern end of the
assessed section) may indicate previous slump and/or erosion gully features.
•
A cliff line characterised by an irregular rocky surface with patches of vegetation cover is present
along the northern section of hillside falling to the east-facing beach. This cliff line is adjacent to
or intersects residential lots indicated by SMEC to be subject to potential hazard.
3.6
Berrara Bluff
The main items of note are summarised below:
•
The marine cliff line is mostly continuous but is significantly seaward of the “estimated cliff line”
included in the SMEC report.
•
The hillslope above the level of the marine cliff line is mostly well vegetated and includes stands
of mature Banksia trees.
•
There is a partially clay cemented colluvial deposit overlying a rock plinth), approximately 2 m
above the current wave cut platform level. This may indicate the landward side of a previous
wave cut platform related to an earlier sea level.
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3.7
Hyams Point
The main items of note are summarised below:
•
A marine cliff line, up to 3 m in height, is present only in the southern half of the site and is
significantly seaward of the “estimated cliff line” included in the SMEC report.
•
The development of salt weathered features in a fallen block at the northern end of the cliff
suggests a long period of exposure prior to fall.
•
The rocks exposed in the wave cut platform are ironstained and apparently weathered in place
indicating a likelihood that wave energy is significantly reduced at this site. This is commensurate
with its location within Jervis Bay.
3.8
Plantation Point
The main items of note are summarised below:
•
A marine cliff line, ranging between 1 m and 6 m in height, is present in the 90 m long section of
the north-westerly facing side of Plantation Point assessed by SMEC. It also continues along the
north-facing side of the point. The marine cliff line is significantly seaward of the “estimated cliff
line” included in the SMEC report.
•
The cliff faces expose numerous ironstained or highly weathered joints within the siltstone profile
and these facilitate spalling of small blocks, mostly less than 0.4 m greatest dimension where
present on the rock platform at the toe of the cliff line.
•
The fretting of the rock faces results in localised spalling of the 1 m thick soil profile and reduces
the support to trees and shrubs along the cliff crest.
3.9
Penguin Head and Culburra Beach
The main items of note are summarised below:
•
A marine cliff line is developed discontinuously along north and south-facing slopes of Penguin
Head. Where present, the marine cliff line, and some sections of cliff line within the slope are
significantly seaward of the “estimated cliff line” included in the SMEC report.
•
Only minor sections of the cliff line along the north-facing section of Penguin Head are directly
affected by wave erosion which has resulted minor undercutting of the siltstone in the lower face
sections and subsequent loss of vegetation cover, spalling of siltstone and undercutting of
sandstone bands within the upper slope.
•
Much of the toe of the north-facing slope is at least partially protected against wave erosion by a
5 m to 10 m wide apron of sandstone blocks lying at the landward edge of wide, wave cut
platforms.
•
Blocky sandstone debris near the eastern-most section of Penguin Head mark the site of a 20 m
long by approximately 3 m wide area of cliff line collapse.
•
Three, mostly vegetation covered embayments in the south-facing slope of Penguin Head may
also mark the site of previous cliff line collapses.
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4.
Comments
4.1
Items Arising from Community Consultation
As part of the community consultation with respect to the introduction of Development Control
Plan 118, SCC has received correspondence from Dr Howard Brady providing comment on
interpreted wave cut platform levels related to previous sea-levels and coastal erosion rates within the
study area. Copies of the correspondence are included in Appendix B. In summary, the items
relevant to Dr Brady’s assessment of SMEC’s interpretation of the wave cut platforms, marine cliff
lines and adjacent hillslopes are that:
•
Incorrect sea-level assumptions for 6000 years ago were used to estimate erosion rates;
•
Assumptions that the cliffs were present at the present rock platform edges 6000 years ago were
incorrect;
•
SMEC erosion rates of cliffs as sea-level rises are incorrect by factors of 5 – 10.
4.1.1
Sea-Level Changes
A review of literature (Chappell, 1983) relating to sea level change in the Australian context indicates
that the coast line has been exposed to marine attack on three occasions during the last
340 000 years. An inferred sea-level curve (after Lambeck et al., 2002) extending to 150 000 years
BP is shown in Figure 2 (following page). It indicates sea-level rising to approximately 3 m to 4 m
above datum in the period 120 000 to 130 000 years BP (ie during the Pleistocene) and again
approximately 6000 to 6400 years BP (ie during the Holocene).
Years (1000s) BP
Figure 2: Sea-level curve after Lambeck et al., 2002.
Studies by various authors have provided more detailed models of the Holocene event which
continues to the present. Woodroffe et al (2005) includes analysis of data from sea-level index points
from South Eastern Australia (refer Figure 3, following page) which indicates establishment of a
maximum sea-level approximately 2 m above current datum between 4000 and 5000 years BP. This
was followed by an oscillating pattern of sea-levels declining to the present.
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Sea-level model curve
alternatives
Figure 3: Summary of Regression Models for Holocene Sea-level Change (after Baker, 2001).
Studies by the University of New England (UNE) have further refined the Holocene sea-level model,
which is assessed as including a 1 m to 2 m fall in sea level in less than 100 years, 3000 to
3500 years BP, in tandem with cooling of the climate.
The sea-level index points forming part of the basis of current Holocene sea-level curves include dated
(by UNE), semi-fossilised shellfish and calcareous coated tube worms on coastal rocks. This
evidence indicates that at least some sections of the sea cliffs have been only minorly affected by
erosion since the Holocene sea-level high-stand.
Evidence of Pleistocene age clastic deposits situated behind shore platforms at Austinmer and
Coledale on the northern lIIawarra coast is cited (Brooke et al., 1994) as supporting the claim by Bird
et al (1966) that platforms, at least in eastern Australia, are Pleistocene relicts reworked during the
Holocene, especially by slightly higher seas. This proposition that the current marine cliff lines and
wave cut platforms have considerable inheritance from previous still stands is supported (pers. comm.)
by Professor Colin Woodroffe (School of Earth and Environmental Sciences, University of
Wollongong).
The correspondence from Dr Brady indicates that observations of raised (by approximately 2 m) rock
platforms near the head of the Crookhaven River and at Penguin Head, where there is more than one
ancient sea level etched into the cliffs. While the brief site inspections for this review were carried out
prior to receipt of Dr Brady’s correspondence, at least one, apparently old, rock plinth/platform was
noted at Berrara Bluff.
4.1.2
Cliff Line Erosion Rates
SMEC determined an approximate recession rate of the coastal headlands by dividing the maximum
width of rock platform at each site by a 6500 year “still-stand” during the Holocene. The SMEC data
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assessment is summarised in Table 1 and is qualified by a statement that the actual rate of the
physical weathering process is more irregular than the 10 – 18 mm per year specified and further
relates to the quality of the headland rock type and susceptibility of the rock to weathering and erosion.
Table 1: Summary of SMEC Average Regression Rates
Site
Range in Width of Wave Cut
Platforms (m)
Average Regression Rate
(mm/year)
Racecourse Beach
68 - 75
12
Rennies Beach
42 - 66
10
Collers Beach Headland
42 - 79
12
Bannister Point
70
11
Inyadda Point
48 - 87
13
Berrara Bluff
53 - 95
15
Hyams Point
-
-
Plantation Point Headland
33 - 118
18
Penguin Head
15 - 70
11
Culburra Beach
15 - 70
11
It is noted that the SMEC report does not include wave cut platform data for Hyams Point. Site
observations indicate that a wave cut platform is present and ranges from approximately 15 m to 40 m
in width. It is however noted that the wave platform is ironstained and weathered in place indicating
that the current (and presumably Holocene) erosion rate is very slow, as could be reasonably
expected for this relatively sheltered site within Jervis Bay.
Crozier and Braybrooke (1992) carried out an analysis of erosion rates for shale and sandstone
dominated cliff lines (ie similar to the range of lithologies within the Shoalhaven region) affected by
marine erosion between Broken Bay and Port Jackson. Using the same assumptions as SMEC (ie an
approximate recession rate of the coastal headlands determined by dividing the maximum width of
rock platform by 6500 years), that study indicated average regression rates of 4.3 mm/year and
6.2 mm/year, respectively, for sites where sandstone or mudstone/shale formed the rock type at sea
level.
Taking into consideration recent evidence (refer Section 4.1.1) of old (at least Holocene) wave cut
platforms and dated, semi-fossilised shellfish and tube worms and clastic deposits, it is considered
that the values estimated by SMEC (refer Table 1) and Crozier and Braybrooke (1992) may, in many
cases, only reflect the rate of erosion of the remnant wedge of material lying between the Holocene
and/or Pleistocene wave cut high-stand surfaces and the current wave cut platform rather than the
retreat of cliff lines from the outer edge of the current wave cut platforms.
For comparison of wave-induced regression rates, Crozier and Braybrooke (1992) assessed literature
references and measured regression rates related to weathering within marine cliff line sections above
the level of wave action and of shale and sandstone exposures within natural formations and road
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cuttings at sites in the Shoalhaven to Gosford areas. A summary of the measured or estimated rates
of weathering is given in Table 2.
Table 2: Summary of Measured or Estimated Rates of Weathering (after Crozier et al., 1992).
Location
Lithology
Sassafras
(Shoalhaven
Valley)
Nowra
Sandstone
Liverpool
Hawkesbury
Sandstone
Defect Type and Facies
Scarp retreat
Time Period
(years)
Maximum
Weathering Rate
(mm/year)
30 million
0.012 to 0.025
106
0.24
Salt weathering of sea cliff
Bondi
Hawkesbury
Sandstone
Limonite cemented
sandstone
100
Resistant sandstone
1 to 2
‘Softer sandstone’
Warringah
Road, Beacon
Hill
Gosford
Up to 5
Sandstone
Differential weathering rate in
weathered fine grained clayey
sandstone
Sandstone
Differential weathering rate
between sheet facies
Shale
Between sheet sandstone
and mudstone
5 to 8.5
Shale
Differential weathering rates
between sandstone and shale
23 to 43
Oxford Falls
F3 Freeway
Berowra to
Hawkesbury
River
1
Sandstone
Sandstone
Narrabeen
Sandstone
Salt decay in sandstone
caves
15
10 to 17.4
1 to 4.6
13
30
1 to 3.3
0.1 to 0.2
The data included in Table 2 emphasises the significant variation in weathering and resulting erosion
rates within individual lithologies, which combined with spacing of rock mass defects (bedding, joints
and faults) result in collapse of individual undercut cliff line sections. Crozier et al (1992) concluded
that cliff face undercutting by wetting/drying and salt weathering of weaker sandstones and shales
appeared to be controlling the cliff line regression rates with the importance of sea action generally
being the removal of rockfall material.
The removal of rockfall material by storm events should also be expected to expose materials,
physically softened or weathered by water included in the debris, to direct wave erosion (including
abrasion by the mobile materials). Brooke et al (1994) indicates that the importance of infrequent,
high-energy events in the formation of high-tide platforms.
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4.2
Marine Cliff Line Locations Assessed by SMEC
The locations of the marine cliffs as estimated by SMEC are shown as the crest of the slope falling to
the wave cut platform or beach, whether or not a cliff (in geography and geology; a cliff is a significant
vertical, or near-vertical, rock or primarily rock, exposure) is present.
The inspections carried out as part of this review indicate that:
•
the SMEC and DP locations of marine cliff correspond only over the southern-most end of the
Racecourse Beach site.
•
Elsewhere at the assessed sites, cliff line sections recorded by DP as being directly exposed to
marine erosion are variously discontinuous or near-absent and typically significantly seaward of
the SMEC locations.
•
There are additional sections rocky cliff lines at Racecourse Beach, Collers Beach Headland,
Bannisters Head and Penguin Head within vegetated and partially soil (probably colluvial)
covered slopes that are not currently exposed to marine erosion and which can be reasonably
interpreted as being related to earlier sea-level high-stands.
•
Where cliff lines are absent at the rear of the current beaches, steep scarps developed by marine
erosion of mostly colluvial, but potentially some marine alluvium (eg wind blown sands and cobble
boulder deposits) are locally developed and are typically less than 2 m in height.
•
The vegetated slopes above cliffs directly exposed to marine erosion and those both above and
below cliff sections or soil slopes not directly affected by marine erosion, previous clearing or
stormwater drainage channels, have mature vegetation assemblages including large trees (or the
stumps thereof) indicating slow erosion and retreat of these slopes.
4.3
Hazards Within Cliff Lines and Slopes
A range of slope instability hazards (eg soil creep, rock fall, rock toppling, slumping) within the cliff
lines and slopes were identified by SMEC and are mostly consistent with observations during the site
inspections carried out by DP. However, several additional hazards having a bearing on subsequent
risk assessments, have been observed or inferred by DP within, or adjacent to the slope sections
assessed by SMEC. A summary of the additional DP assessed or inferred slope instability hazards
are included in Table 3 (following page).
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Table 3: Summary of Additional Slope Instability Hazards
Site
Racecourse Beach
Additional Hazards Identified or Inferred by DP
Landslide (70 m toe length extending 30 m to 50 m from beach)
adjacent to northern end of SMEC section
Soil slumps to 10 m width from slope
Minor slumps of colluvium or marine debris at slope toe
Rennies Beach
Bannister Point
Rock falls to approximately 12 m3 from cliffs or roofs of sea caves in
areas of pedestrian traffic, east of the section assessed by SMEC
Slumping of soil and upper weathered rock profile, mainly as a result
of the disposal of street and building stormwater
Possible reactivation of probable ancient slump by uncontrolled
discharge of street stormwater
Slumping of soil and upper weathered rock profile, mainly as a result
of the disposal of street and building stormwater
Inyadda Point
Berrara Bluff
Penguin Head and
Culburra Beach
Rock fall or topple from rocky cliff at northern end of the headland,
within area nominated as requiring detailed geotechnical assessment
for Development Application
Slumping of soil and upper weathered rock profile above the level of
the marine cliff as a result of direct rainfall and the disposal of building
stormwater
Erosion and/or slumping (triggered by disturbance of vegetation cover)
of sand profile overlying bedrock above the western section of the
north-facing embayment and beach
Landslide (20 m toe length and 2 m to 3 m wide) at eastern end of
Penguin Point
4.3
Australian Geomechanics Society (2007) Methodology
The methodology of AGS (2007) for description and assessment of risk levels associated with
landslides, rock falls and soil slumps is based upon inputs including:
•
Identification of landslide susceptibility, landslide hazards including potential triggers (eg
undercutting, saturation, earthquake), and frequency (or likely range of frequency) of occurrence.
•
Probability of the effects of a hazard on the element at risk (ie property, services or site including
occupants), requiring assessment of the translational mode of landsliding (rate of movement and
run out distance).
•
Probability of occupation of the element of risk at the time of the event.
•
Vulnerability, the probability and cost of damage of the property or loss of life given the impact of
the particular hazard.
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The AGS (2007) methodology is routinely employed for assessment of coastal slopes and cliff lines
where regression is by rock fall or landslide activity, whether triggered by marine erosion or other
factors. It is essential that assessment is based on the best estimates available and that expert
judgment be applied to answers so derived. Understanding the slope forming process is critical before
a frequency assessment can be made.
The AGS (2007) documents indicate that for most assessments, the use of knowledge based expert
judgment may be the only suitable option for estimating frequency due to the lack of objective data.
The judgment or ‘degree of belief’ method, which combines experience, expertise and general
principles, relies to a large degree on subjective assessment of available data where other more
rigorous methods are not available or viable. The method still requires some degree of research to
obtain relevant data and an understanding of the geological model to qualify the judgment of
likelihood. The approach requires proposing various possible scenarios followed by the systematic
testing and elimination of options as a result of investigation, discussion and judgment leading to the
development of an estimate of frequency (Lee et al., 2004).
The result is conditioned by the ‘degree of belief’ of the practitioner. Typically, the resulting accuracy
for a frequency assessment and, perhaps, a consequence assessment could vary from half an order
of magnitude at best, to one or perhaps two orders of magnitude. As a result, the risk assessment
should clearly display its sensitivity.
4.4
Application of the AGS Methodology by SMEC
DP’s assessment of the appropriateness of the application by SMEC of the AGS (2007) methodology
to the assessment of risk levels to the sites indicates that:
•
Erosion rates for marine cliff lines have been determined by SMEC assuming average rates
based upon the width of wave cut platforms assumed to have been formed in the Holocene under
still stand conditions, however it is highly likely that these platforms have been formed over
multiple erosion periods within the Pleistocene and Holocene (when there was fluctuating but
generally declining sea-levels).
•
While erosion rates for exposed, weathering susceptible rock bands (refer Table 2, Section 4.1.2)
may be similar to the average regression rates determined from wave cut platform widths,
individual cliff sections include strata of variable lithology, strength, rock mass defects and
weatherability which necessitate detailed assessment of individual cliff sections to provide best
estimates of the risk of instability. SMEC has applied an average marine cliff erosion rates to all
sections of the individual sites regardless of whether or not that section is directly affected by
marine erosion. Examples of the methodology of AGS (2007b) for subdivision of sites for
assessment of landslide susceptibility, landslide hazard and resultant risk to property for detailed
planning purposes are given in Appendix C.
•
Slopes above cliff lines, directly affected by marine erosion, and most slope sections elsewhere
include mature vegetation assemblages which would not be present if the projected 50 year
regression line reflects erosion and regression rates of the recent past.
•
The locations of individual cliff sections subject to marine erosion (ie forming the nominal starting
point for estimated regression distances) are mostly incorrectly located on figures included the
SMEC report.
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From all the above points, SMEC’s Potential Cliff Regression Limits are judged to be conservative.
While most failure mechanisms in sea cliffs and adjacent slopes have been identified, several
additional minor hazards and at least three large landslides (at Racecourse Beach, Bannisters Head
and Penguin Head) within or adjacent to the study sections, have not been identified or confirmed by
SMEC.
4.5
Risk to Property
4.5.1
SMEC Assessments
No systematic recalculation of risk to property has been carried out as part of this review. It is,
however, considered that the mostly slower slope retreat rates and the re-setting of the origin (the
existing marine cliffs) to the actual locations seaward of those indicated on figures included in the
SMEC report will result in maintaining or reducing the very low to low risk levels (ie acceptable levels)
to property which were assessed for all but two hazards identified by SMEC.
The two hazards assessed by SMEC as having greater than low risk levels are:
•
A > 20 m3 slump failure affecting the upper gully slopes and adjacent private properties at
Rennies Beach, assessed by SMEC as resulting in a moderate risk level.
The DP inspection indicates that discharge from Council stormwater and possibly the adjacent
sewer pumping station or sewer line is likely to significantly contribute to the likelihood of the
hazard. At least the extension of the stormwater system to beach level, improvement of surface
drainage from the adjacent carparking area and access stairs, together with repairs (if required) to
sewer lines would be needed to reduce the risk associated with this hazard to an acceptable
level.
•
A < 200 m3 slump failure affecting the upper slopes and adjacent private properties at Inyadda
Point, assessed by SMEC as resulting in a high risk level.
The DP inspection indicates that discharge from Council and private stormwater lines are
enhancing entrenchment of the drainage path (possibly into Tertiary sediments or the weathered
bedrock profile) with resulting minor slumping of gully sides. At least one drainage path has
exposed intact bedrock of at least medium strength approximately 20 m downslope of the rear
property boundaries. The presence of the underlying bedrock is likely to limit the scale of any
slumping and probably the resulting risk level. At least the extension of the Council and private
stormwater systems to the water front combined with restoration of previously damaged areas
would be needed to reduce risk associated with this hazard to an acceptable level.
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4.6
Risk to Life
For loss of life, the individual risk can be calculated from:
R(LoL) = P(H) x P(S:H) x P (T:S) x V(D: T)
Where:
R(LoL) is the risk (annual probability of loss of life (death) of an individual).
P(H)
is the annual probability of the landslide.
P(S:H) is the spatial probability of spatial impact of the landslide hitting a building (location)
taking into account the travel distance and travel direction for a given event.
P(T:S) is the temporal probability (eg of the building or location being occupied by the individual)
given the spatial impact and allowing for the possibility of evacuation, if there is warning
of the landslide occurrence.
V(D: T) is the vulnerability of the individual (probability of loss of life of the individual given the
impact).
The assessment of risk to life is significantly constrained by the accuracy of estimates of the each of
the partial probabilities which SMEC has provided ranges and adopted values. In particular, there are
no data relating to duration of occupation (the temporal probability) for each of the sites.
A summary of adopted values and estimates of annual probability of loss of life of an individual
determined by SMEC for the hazards (with inferred greatest risk) are given in Table 4 (following page)
together with preliminary estimates, based on brief site inspection, of the same or similar hazards by
DP.
The R(LoL) values (refer Table 4) estimated by DP are generally within or one order of magnitude less
than assessed by SMEC. Of particular note are the temporal probability values indicated by SMEC.
For example, in the Rennies Beach case, a 0.5 probability of occupation has been adopted for the
garden area at the crest of the slope subject to slump hazard. It is clearly unreasonable to assume
12 hours per day occupation of this area.
Each of the slopes included in the study are classified as ‘existing slopes’ of the AGS (2007) which
suggests a tolerable risk level to life (person most at risk) as 10-4 per annum. On the basis of the brief
site inspections and preliminary assessment, DP considers that it is likely that only the southern,
overhanging cliff line section of Racecourse Beach has less than tolerable risk levels for life.
Measures to reduce risk at that site may include signage of rock fall danger, periodic inspection and
scaling of loose or open jointed blocks.
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Table 4: Summary of Estimates of Probability of Loss of Life
P(H)
P(S:H)
P(T:S)
V (D:T)
R(LoL)
Site
SMEC
DP
SMEC
DP
SMEC
DP
SMEC
DP
SMEC
DP
Racecourse
Beach
10-2
10-1
0.2
1
0.2
10-3
0.5
0.5
2x10-4 *
5x10-4
Rennies Beach
10-5
10-2
0.1
0.1
0.5
10-3
0.3
0.1
1.5x10-7
1x10-7
Collers Beach
Headland
10-1
10-1
0.1
1
0.05
10-4
0.5
0.5
2.5x10-4
5x10-5
Bannister Point
10-3
10-2
0.1
1
0.1
10-3
0.1
0.1
1x10-6
1x10-7
Inyadda Point
5x10-3
5x10-3
0.2
0.1
4**
(0.1)
0.1
1
1
1x10-4 **
5x10-5
Berrara Bluff
10-2
10-2
0.2
1
0.1
10-3
0.5
0.5
1x10-4
5x10-6
Hyams Point
2x10-1
10-1
0.05
1
0.1
10-3
0.1
0.1
1x10-4
1x10-5
Plantation Point
- ***
1
-
1
-
10-4
-
0.5
-
5x10-5
Penguin Head
<10-1
10-1
0.1
1
0.1
10-3
0.5
0.5
5x10-4
5x10-5
Notes: * computational error
4.7
** computational error – P(T:S) cannot be > 1
*** Rock fall was not considered by SMEC
Suggested Methodology For Input to DCP 118
DP considers that the rapid variation in rock mass, vegetation cover and drainage conditions in the cliff
and coast slopes, together with variable slope aspects to wave action, protection of the slope toe by
natural rock armour or reduction in wave energy over wide wave cut platforms do not allow the
definition of smooth regression lines as shown by SMEC in Figures 4.1 – 4.9. The prediction of
regression rates over 1000 years is considered to be unrealistic.
AGS (2007a and 2007b) include detailed recommendations for landslide susceptibility, hazard and risk
zoning for land use planning. Extension of the SMEC study into a similar format is suggested to
provide more detailed breakdown of risk levels to individual properties for input to DCP 118.
It is suggested that the development application and construction certificate processes simply include
requirement for site specific, detailed geotechnical assessment of allotments (as already nominated on
some included SMEC figures) on or adjacent to the cliffs or coastal slopes. For sites above cliff line
sections, the minimum set back for structures should be related to the maximum depth of undercutting
present within the cliff line and anticipated during the life of the structure, together with a factor relating
to joint/fault dip (minimum Tan 70°) times the cliff height. It is noted that, while current standards for
the design of structures are based on a 60 year period, a realistic ‘life of structure’ period may be
longer (Pittwater Council nominates 100 years) requiring a program of monitoring and remedial works
to be undertaken by owners.
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July 2011
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5.
Conclusions
The review of the SMEC documentation, a literature search for relevant data on sea-level variation
and erosion rates within South Eastern Australian, and brief site inspections have led to the following
conclusions:
•
The methods of the AGS (2007) are valid for assessment of the long-term stability of marine cliffs
and slopes but require careful assessment of regression rates and failure modes (resulting from a
wide variation of rock mass and environmental considerations), for selection of appropriate input
parameters to the risk assessment process.
•
The application of the methods of AGS (2007) by SMEC has included average cliff regression
rates, most probably significantly in excess of the actual values, to all cliff lines and slopes
regardless of rock mass and environmental considerations. Their application of the calculation of
risk to life includes some computational and input errors.
•
No detailed mapping of subdivision of landslide susceptibility, landslide hazards and risk zones
has been included in the study.
•
The locations of marine cliffs are generally incorrectly depicted in the SMEC figures and the
incorrect locations have been used as origin points for definition of average regression lines.
•
The SMEC assessment of the rate of cliff line retreat has not included consideration of the age of
vegetation now present within many sections of the cliff lines, the inferred relic Pleistocene and
Holocene wave cut features or existing cliff line and marine slopes features. There are no
observable features within vegetated slopes or no recent history (ie the last 50 years) which
would support the proposed 50 year overall retreat of most of the sites included in the study.
•
Several significant landslide features within or adjacent to the study sites have not been
described by SMEC. Taking these features into account may lead to a High risk level over a
section of the Racecourse Beach site. Additional site work site is recommended to provide an
appropriate level of understanding of conditions for assessment of risk to property and to life.
•
Two locations, Rennies Beach and Inyadda Point, include hazards assessed by SMEC as
respectively Moderate and High risk. Improvement of site drainage is suggested to reduce risk to
property to acceptable levels.
•
Of the hazards identified by SMEC, preliminary DP assessment of risk to life indicates that only
the southern section of Racecourse Beach marine cliff line is likely to have a risk level less than
‘tolerable’ for existing slopes; the variation between SMEC and DP assessments for this area
appears to be largely the result of over-estimation of temporal probability by SMEC. Remedial
and precautionary measures to reduce risk at this site may include signage of rock fall danger,
periodic inspection and scaling of loose or open jointed blocks.
•
On the basis of the above conclusions, it is considered that the SMEC 50 year (50%) regression
line is inaccurate over most of the length of the marine slopes and marine cliffs within the sites
included in this review. As such, it is considered not to be a sound basis for hazard planning
incorporated within DCP 118/draft LEP.
Peer Review Report Coastal Slope Instability Hazard Study
Various Sites - Shoalhaven City Council LGA
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6.
References
AGS (2007a). Guideline for landslide susceptibility, hazard and risk zoning for land use planning.
Australian Geomechanics, Vol. 42, No. 1, March 2007
AGS (2007b). Commentary on guideline for landslide susceptibility, hazard and risk zoning for land
use planning. Australian Geomechanics, Vol. 42, No. 1, March 2007
AGS (2007c). Practice note guideline for landslide risk management. Australian Geomechanics,
Vol. 42, No. 1, March 2007.
AGS (2007d). Commentary on practice note guideline for landslide risk management. Australian
Geomechanics, Vol. 42, No. 1, March 2007.
Baker, R G V, Haworth, R J and Flood, P G, 2001. Inter-tidal fixed indicators of former Holocene sealevels in Australia: a summary of sites and a review of methods and models. Quaternary International
83 – 85, pp. 353 – 365.
Brooke B P, Young R W, Bryant E A, Murray Wallace C V, and Price D M, 1994. A Pleistocene origin
for shore platforms along the northern Illawarra coast, New South Wales. Australian Geographer,
Vol. 25: pp. 178 – 185.
Chappell, J M A, 1983. Geology of coral terraces on Huon Peninsula, New Guinea: a study of
Quaternary tectonic movements and sea level changes. Bull. Beol. Soc. Amer. Vol. 85, pp. 553 – 570.
Crozier P J and Braybrooke, J C, 1992. The Morphology of Northern Sydney’s Rocky Headlands,
Their Rates and Style of Regression and Implications for Coastal Development. Publ. Proc. 26th
Newcastle Sympodium.
Douglas Partners Pty Ltd, 15 November 1990. Project 14057 Report on geotechnical investigation
Lot 376 Surfers Avenue, Narrawallee via Ulladulla.
Douglas Partners Pty Ltd, 10 June 1992. Project 12436B Report on assessment of stability of cliff
lines, Bannisters Point.
Douglas Partners Pty Ltd, 16 September 1993. Project 19404.. Report on geotechnical investigation
Lots 374 and 375 Surfers Avenue, Narrawallee.
Lambeck, K, Esat, T M, and Potter, E, 2002. Links between climate and sea levels in the past three
million years. Nature 419, pp. 199 – 206.
Lee, E M, and Jones, D K C, 2004. Landslide Risk Assessment. Thomas Telford, 454p.
Snowy Mountains Engineering Corporation, January 2008. Coastal Slope Instability Hazard Study,
Draft report, Document Number 3001209-020.
Woodroffe, S A, and Horton, P H, 2005. Holocene sea-level changes in the Indo-Pacific. Journal of
Asian Earth Sciences, Vol. 25, No. 1, pp. 29 – 43.
UNE Publicity Office, 1999. Scientists crack the climate and sea-level code near a Sydney Beach.
Release 117/99.
Peer Review Report Coastal Slope Instability Hazard Study
Various Sites - Shoalhaven City Council LGA
Project 72051
July 2011
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7.
Limitations
Douglas Partners (DP) has prepared this peer review report of sites within the Shoalhaven City
Council LGA in accordance with DP's proposal dated 31 August 2010 and acceptance received from
Mr Ray Massie of Shoalhaven City Council on 21 September 2010. The report is provided for the
exclusive use of Shoalhaven City Council for this project only and for the purpose(s) described in the
report. It should not be used for other projects or by a third party. In preparing this report DP has
necessarily relied upon information provided by the client and/or their agents.
The results provided in the report are indicative of the sub-surface conditions only at the specific
observation locations and at the time the work was carried out. Sub-surface conditions can change
abruptly due to variable geological processes and also as a result of anthropogenic influences. Such
changes may occur after DP's field work has been completed.
DP's advice is based upon the conditions encountered during this investigation. The accuracy of the
advice provided by DP in this report may be limited by undetected variations in ground conditions
between sampling locations. The advice may also be limited by budget constraints imposed by others
or by site accessibility.
This report must be read in conjunction with all of the attached notes and should be kept in its entirety
without separation of individual pages or sections. DP cannot be held responsible for interpretations
or conclusions made by others unless they are supported by an expressed statement, interpretation,
outcome or conclusion given in this report.
This report, or sections from this report, should not be used as part of a specification for a project,
without review and agreement by DP. This is because this report has been written as advice and
opinion rather than instructions for construction.
Douglas Partners Pty Ltd
Peer Review Report Coastal Slope Instability Hazard Study
Various Sites - Shoalhaven City Council LGA
Project 72051
July 2011
Appendix A
About this Report
Introduction
•
These notes have been provided to amplify DP's
report in regard to classification methods, field
procedures and the comments section. Not all are
necessarily relevant to all reports.
•
DP's reports are based on information gained from
limited subsurface excavations and sampling,
supplemented by knowledge of local geology and
experience.
For this reason, they must be
regarded as interpretive rather than factual
documents, limited to some extent by the scope of
information on which they rely.
Copyright
This report is the property of Douglas Partners Pty
Ltd. The report may only be used for the purpose
for which it was commissioned and in accordance
with the Conditions of Engagement for the
commission supplied at the time of proposal.
Unauthorised use of this report in any form
whatsoever is prohibited.
Borehole and Test Pit Logs
The borehole and test pit logs presented in this
report are an engineering and/or geological
interpretation of the subsurface conditions, and
their reliability will depend to some extent on
frequency of sampling and the method of drilling or
excavation.
Ideally, continuous undisturbed
sampling or core drilling will provide the most
reliable assessment, but this is not always
practicable or possible to justify on economic
grounds. In any case the boreholes and test pits
represent only a very small sample of the total
subsurface profile.
Interpretation of the information and its application
to design and construction should therefore take
into account the spacing of boreholes or pits, the
frequency of sampling, and the possibility of other
than 'straight line' variations between the test
locations.
Groundwater
Where groundwater levels are measured in
boreholes there are several potential problems,
namely:
•
In low permeability soils groundwater may
enter the hole very slowly or perhaps not at all
during the time the hole is left open;
•
A localised, perched water table may lead to
an erroneous indication of the true water
table;
Water table levels will vary from time to time
with seasons or recent weather changes.
They may not be the same at the time of
construction as are indicated in the report;
and
The use of water or mud as a drilling fluid will
mask any groundwater inflow. Water has to
be blown out of the hole and drilling mud must
first be washed out of the hole if water
measurements are to be made.
More reliable measurements can be made by
installing standpipes which are read at intervals
over several days, or perhaps weeks for low
permeability soils.
Piezometers, sealed in a
particular stratum, may be advisable in low
permeability soils or where there may be
interference from a perched water table.
Reports
The report has been prepared by qualified
personnel, is based on the information obtained
from field and laboratory testing, and has been
undertaken to current engineering standards of
interpretation and analysis. Where the report has
been prepared for a specific design proposal, the
information and interpretation may not be relevant
if the design proposal is changed. If this happens,
DP will be pleased to review the report and the
sufficiency of the investigation work.
Every care is taken with the report as it relates to
interpretation of subsurface conditions, discussion
of geotechnical and environmental aspects, and
recommendations or suggestions for design and
construction.
However, DP cannot always
anticipate or assume responsibility for:
•
Unexpected variations in ground conditions.
The potential for this will depend partly on
borehole or pit spacing and sampling
frequency;
•
Changes in policy or interpretations of policy
by statutory authorities; or
•
The actions of contractors responding to
commercial pressures.
If these occur, DP will be pleased to assist with
investigations or advice to resolve the matter.
July 2010
Site Anomalies
In the event that conditions encountered on site
during construction appear to vary from those
which were expected from the information
contained in the report, DP requests that it be
immediately notified. Most problems are much
more readily resolved when conditions are
exposed rather than at some later stage, well after
the event.
Information for Contractual Purposes
Where information obtained from this report is
provided
for
tendering
purposes,
it
is
recommended that all information, including the
written report and discussion, be made available.
In circumstances where the discussion or
comments section is not relevant to the contractual
situation, it may be appropriate to prepare a
specially edited document. DP would be pleased
to assist in this regard and/or to make additional
report copies available for contract purposes at a
nominal charge.
Site Inspection
The company will always be pleased to provide
engineering inspection services for geotechnical
and environmental aspects of work to which this
report is related. This could range from a site visit
to confirm that conditions exposed are as
expected, to full time engineering presence on
site.
July 2010
Appendix B
Correspondence from Dr Brady
Appendix C
AGS Extracts
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: LANDSLIDE RISK ASSESSMENT
QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY
QUALITATIVE MEASURES OF LIKELIHOOD
Approximate Annual Probability
Indicative
Value
Notional
Boundary
10-1
10
-2
10
-3
10
-5
10
-6
Note:
(1)
10 years
Description
The event is expected to occur over the design life.
The event will probably occur under adverse conditions over the
100 years
design life.
200 years
5x10-3
1000 years
The event could occur under adverse conditions over the design life.
2000 years
5x10-4
The event might occur under very adverse circumstances over the
10,000 years
design life.
20,000 years
5x10-5
The event is conceivable but only under exceptional circumstances
100,000 years
over the design life.
5x10-6
200,000 years
1,000,000 years
The event is inconceivable or fanciful over the design life.
The table should be used from left to right; use Approximate Annual Probability or Description to assign Descriptor, not vice versa.
5x10-2
10-4
Implied Indicative Landslide
Recurrence Interval
20 years
Descriptor
Level
ALMOST CERTAIN
A
LIKELY
B
POSSIBLE
C
UNLIKELY
D
RARE
E
BARELY CREDIBLE
F
Descriptor
Level
QUALITATIVE MEASURES OF CONSEQUENCES TO PROPERTY
Approximate Cost of Damage
Indicative
Value
Notional
Boundary
200%
100%
60%
40%
20%
10%
1%
5%
0.5%
Notes:
(2)
(3)
(4)
91
Description
Structure(s) completely destroyed and/or large scale damage requiring major engineering works for
stabilisation. Could cause at least one adjacent property major consequence damage.
Extensive damage to most of structure, and/or extending beyond site boundaries requiring significant
stabilisation works. Could cause at least one adjacent property medium consequence damage.
Moderate damage to some of structure, and/or significant part of site requiring large stabilisation works.
Could cause at least one adjacent property minor consequence damage.
Limited damage to part of structure, and/or part of site requiring some reinstatement stabilisation works.
Little damage. (Note for high probability event (Almost Certain), this category may be subdivided at a
notional boundary of 0.1%. See Risk Matrix.)
CATASTROPHIC
1
MAJOR
2
MEDIUM
3
MINOR
4
INSIGNIFICANT
5
The Approximate Cost of Damage is expressed as a percentage of market value, being the cost of the improved value of the unaffected property which includes the land plus the
unaffected structures.
The Approximate Cost is to be an estimate of the direct cost of the damage, such as the cost of reinstatement of the damaged portion of the property (land plus structures), stabilisation
works required to render the site to tolerable risk level for the landslide which has occurred and professional design fees, and consequential costs such as legal fees, temporary
accommodation. It does not include additional stabilisation works to address other landslides which may affect the property.
The table should be used from left to right; use Approximate Cost of Damage or Description to assign Descriptor, not vice versa
Australian Geomechanics Vol 42 No 1 March 2007
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
APPENDIX C: – QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY (CONTINUED)
QUALITATIVE RISK ANALYSIS MATRIX – LEVEL OF RISK TO PROPERTY
LIKELIHOOD
A –
B -
CONSEQUENCES TO PROPERTY (With Indicative Approximate Cost of Damage)
Indicative Value of
Approximate Annual
Probability
1: CATASTROPHIC
200%
2: MAJOR
60%
3: MEDIUM
20%
4: MINOR
5%
5:
INSIGNIFICANT
0.5%
10-1
VH
VH
VH
H
M or L (5)
10
-2
VH
VH
H
M
L
-3
VH
H
M
M
VL
ALMOST CERTAIN
LIKELY
C -
POSSIBLE
10
D -
UNLIKELY
10-4
H
M
L
L
VL
10
-5
M
L
L
VL
VL
10
-6
L
VL
VL
VL
VL
E F -
RARE
BARELY CREDIBLE
Notes:
(5)
(6)
For Cell A5, may be subdivided such that a consequence of less than 0.1% is Low Risk.
When considering a risk assessment it must be clearly stated whether it is for existing conditions or with risk control measures which may not be implemented at the current
time.
RISK LEVEL IMPLICATIONS
Risk Level
VH
VERY HIGH RISK
H
HIGH RISK
M
MODERATE RISK
L
LOW RISK
VL
VERY LOW RISK
Note:
92
(7)
Example Implications (7)
Unacceptable without treatment. Extensive detailed investigation and research, planning and implementation of treatment
options essential to reduce risk to Low; may be too expensive and not practical. Work likely to cost more than value of the
property.
Unacceptable without treatment. Detailed investigation, planning and implementation of treatment options required to reduce
risk to Low. Work would cost a substantial sum in relation to the value of the property.
May be tolerated in certain circumstances (subject to regulator’s approval) but requires investigation, planning and
implementation of treatment options to reduce the risk to Low. Treatment options to reduce to Low risk should be
implemented as soon as practicable.
Usually acceptable to regulators. Where treatment has been required to reduce the risk to this level, ongoing maintenance is
required.
Acceptable. Manage by normal slope maintenance procedures.
The implications for a particular situation are to be determined by all parties to the risk assessment and may depend on the nature of the property at risk; these are only
given as a general guide.
Australian Geomechanics Vol 42 No 1 March 2007
COMMENTARY ON GUIDELINE FOR LANDSLIDE SUSCEPTIBILITY, HAZARD AND
RISK ZONING FOR LAND USE PLANNING
APPENDIX CA - EXAMPLES OF LANDSLIDE ZONING MAPPING
58
Australian Geomechanics Vol 42 No 1 March 2007
COMMENTARY ON GUIDELINE FOR LANDSLIDE SUSCEPTIBILITY, HAZARD AND
RISK ZONING FOR LAND USE PLANNING
LEGEND
Mapping
Area
Landslide Classification
Landslide Susceptibility
(1)
Landslide Hazard
(2)
Landslide Risk for Life Loss
C1
Rock falls from cliff
High
High
Negligible (4)
C2
Rock falls from cliff
High
Moderate
Negligible (4)
S1
Rock fall travel path
Moderate
Moderate
Moderate (5)
S2
Rock fall travel path
Moderate
Low
Low (5)
M1
Rock fall deposition zone
Low
Low
Low (5)
M2
Rock fall deposition area
Low
Very Low
Very low (5)
F1
Area above cliff
Not susceptible
No hazard
No risk
F2
Area beyond rock fall deposition
zone
Negligible
Negligible
Negligible
( 3)
Notes
(1) Likelihood that rock falls will reach the area if they occur.
(2) The number of rock falls per annum/ km of cliff which will reach
this area. The frequency of rock falls is an order of magnitude lower for
areas, C2, S2 and M2 than for C1, S1 and M1.
(3) Accounting for the landslide hazard and the persons within the area.
(4) Because there are no elements at risk.
(5) Within the area to be developed for housing, otherwise negligible.
(6) H=high; M=moderate; L=low; VL=very low; N=negligible.
Figure CA1 Example of landslide zoning for rock fall
Australian Geomechanics Vol 42 No 1 March 2007
59
COMMENTARY ON GUIDELINE FOR LANDSLIDE SUSCEPTIBILITY, HAZARD AND
RISK ZONING FOR LAND USE PLANNING
60
Australian Geomechanics Vol 42 No 1 March 2007
COMMENTARY ON GUIDELINE FOR LANDSLIDE SUSCEPTIBILITY, HAZARD AND
RISK ZONING FOR LAND USE PLANNING
LEGEND
Mapping
Area
Landslide
Landslide Classification
Susceptibility
3
(1)
Landslide Hazard
(2)
Landslide Risk for
Life Loss
( 3)
(4)
High
High
Negligible
Moderate
Moderate
Negligible
Debris flow deposition areas
Moderate
Moderate
Moderate
D2
Debris flow deposition areas
Low
Low
Low
E1
Debris flow deposition areas-fan deposits
Moderate
Moderate
High
E2
Debris flow deposition areas-fan deposits
Low
Low
Moderate
E3
Debris flow deposition areas-fan deposits
Very low
Very low
Low
F
Outside area affected by landsliding
Very low
Very low to negligible
Low to Very low
S1
Rapid earth slides and debris flows up to 200m
S2
Rapid earth slides and debris flows up to 2000m
D1
3
Notes
(1) Number of small slides per square km
(2) Number of small slides per square km/annum
(3) Accounting for the landslide hazard and the persons within the area.
(4)
( 5)
( 5)
( 5)
( 5)
( 5)
( 5)
(4) Because there are no elements at risk.
(5) Within the area to be developed for housing, otherwise negligible
(6) H=high; M=moderate; L=low; VL=very low; N=negligible.
Figure CA2: Example of landslide mapping for small landslides.
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COMMENTARY ON GUIDELINE FOR LANDSLIDE SUSCEPTIBILITY, HAZARD AND
RISK ZONING FOR LAND USE PLANNING
LEGEND
Landslide
Landslide
Mapping
Area
Landslide Classification
A
Active very slow earth slide
High
Very high
Very high
AT
Slope onto which ‘A’ may travel
Moderate
High
High
AR
Slope into which ‘A’ may retrogress
Moderate
Moderate
Moderate
B
Inactive earth slide
Moderate
High
High
BT
Slope onto which ‘B’ may travel
Low
Moderate
Moderate
BR
Slope into which ‘B’ may retrogress
Low
Low
Low
BW
Slope into which ‘B’ may widen
Low
Low
Low
D
Slopes with no geomorphologic
characteristics of landsliding
Not susceptible
Very low
Very low
Susceptibility
(1)
Landslide Hazard
(2)
Risk
Property Loss
for
( 3),( 4 )
Notes (1) Likelihood large landsides may occur in this area given the topography, geology and geomorphology
(2) Annual probability of active sliding
(3) Accounting for the landslide hazard and the persons within the area. It is assumed that the whole area is available for development
(4) Life loss risk is very low for all areas because of the very low slide velocity
Figure CA3: Example of landslide mapping for large landsliding.
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Australian Geomechanics Vol 42 No 1 March 2007