Hillside Stability Study
ANZAC PARADE / PUTIKI DRIVE,
WANGANUI
Wanganui District Council
Hillside Stability Study
Anzac Parade / Putiki Drive,
Wanganui
Prepared By
Jayne Flack
Engineering Geologist
Reviewed By
P Brabhaharan
Principal Geotechnical Engineer
Approved for
Release By
Mark Frampton
Senior Geotechnical Engineer
© Opus International Consultants Limited 2011
Opus International Consultants Limited
Wellington Office
Level 9, Majestic Centre, 100 Willis Street
PO Box 12 003, Wellington 6144,
New Zealand
Telephone:
Facsimile:
+64 4 471 7000
+64 4 471 1397
Date:
Reference:
Report No:
Status:
June 2011
5-W1100.08
GER 2011/19
Final
Anzac Parade / Putiki Drive Hillside Stability Study
&RQWHQWV
Executive Summary ...................................................................................................................... ii
1
Introduction .......................................................................................................................... 1
2
Site Location and Topography............................................................................................ 1
3
Geology ................................................................................................................................ 2
4
Desk Study ........................................................................................................................... 2
5
Engineering Geological Mapping ....................................................................................... 2
6
Stability Assessment ........................................................................................................... 3
6.1 Causes of Instability...................................................................................................... 3
6.2 Slopes below Hipango Terrace / Rees Street (Area 1) .................................................. 4
6.3 Slopes above Mount View Road (Area 2) ..................................................................... 6
7
Qualitative Risk Assessment .............................................................................................. 7
8
Landslide Hazard Zonation ................................................................................................. 8
9
Recommendations ............................................................................................................... 9
10
Limitations of the Assessment ......................................................................................... 10
11
References ......................................................................................................................... 11
Figures
Figure 1
Landslide hazard zonation – Area 1 (Hipango Terrace)
Figure 2
Landslide hazard zonation – Area 2 (Mount View Road)
Appendix
Appendix A
Engineering geological maps
Appendix B
Australian Geomechanics Society (2007) risk assessment tables
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Anzac Parade / Putiki Drive Hillside Stability Study
Executive Summary
Opus International Consultants Ltd has been engaged by Wanganui District Council to
carry out a stability assessment and define zones of unstable slopes not suitable for
development for two areas above Anzac Parade / Putiki Drive, Wanganui. A desk study and
reconnaissance level engineering geological mapping were undertaken, to allow a risk
assessment for slopes within the study areas. The risk assessment has been used to define
two landslide hazard zones.
Areas classified as Zone A is land that is both steep and shows evidence of instability. No
development should be permitted in these areas, as the risk of further instability and
damage to property or life is very high.
Areas classified as Zone B are marginal slopes, which have shallower slope angles but are
still prone to instability. Geotechnical investigations to allow a landslide hazard and risk to
development assessment should be carried out prior to consideration of any development
in these areas, as part of the resource consent process. The investigations will determine
the risk to property from landsliding, and therefore whether the land can be further
developed, with mitigation measures implemented, or whether it is unsuitable for further
development.
It is recommended the landslide hazard zones should be incorporated into Wanganui
District Council’s district plan, by way of a landslide hazard overlay.
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1
Introduction
Opus International Consultants Ltd (Opus) has been engaged by Wanganui District Council
(WDC) to develop zones of unstable slopes not suitable for development for two sections of
hillside along Anzac Parade / Putiki Drive in the Durie Hill / Bastia Hill areas of Wanganui.
The slope stability assessment of the two areas will allow WDC to better understand the
risk posed by the slopes and to limit or impose restrictions on further development within
areas assessed to be of high risk of slope instability. The study provides WDC with a map
showing areas unsuitable for development, and those that would require further
geotechnical investigation before future development is allowed to proceed.
This report details our investigations which included a desk study and reconnaissance level
engineering geological mapping of the study areas. It provides an assessment of the
stability, zonations and recommendations for future development.
2
Site Location and Topography
The two study areas are shown on Illustration 1. The first is bound by Anzac Parade / Putiki
Drive, Portal Street, Rees Street and Hipango Terrace, in Durie Hill, Wanganui. The second
is between Anzac Parade, Mt View Road and Wairere Road in Bastia Hill.
Illustration 1
Locations of two study areas.
Both areas are hillsides with slope angles of up to 56o, alongside the Whanganui River.
Residential development of the hillsides and crests began in the early 1900s, and became
more concentrated following the installation of the Durie Hill elevator to allow better access
to Durie Hill, in 1916.
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3
Geology
The New Zealand Geological Survey geological map for the Wanganui area indicates the
site to be underlain by rocks of the Rapanui Formation, comprising marine sand, dune
sand, volcanic sand and lignite bands with basal conglomerate of Pleistocene age (2.5
million – 12,000 years old). This is underlain by early Pleistocene Castlecliffian Stage rocks,
comprising shell beds, sand, conglomerate and siltstone (New Zealand Geological Survey,
1959).
Observations made during the reconnaissance level engineering geological mapping were
that the Castlecliffian Stage rocks are exposed within the study areas. These rocks are
overlain by colluvium of maximum 2 m thickness.
4
Desk Study
The desk study consisted of a review of aerial photograph stereopairs from three flights –
1942, 1993 and 2011. The use of stereopairs allowed viewing of the photographs in three
dimensions, using a stereoscope, to identify evidence of slope instability and investigate the
general topography of the study areas.
A report documenting slope stability issues on the adjacent Shakespeare Cliff was also
reviewed to understand expected types of slope failure in the area (Opus, 2009). Published
geological maps were reviewed to understand the regional geology of Wanganui.
Approximately 50 archive photographs were obtained from both the Wanganui Regional
Museum and from books. These commonly show the hillsides above the Whanganui River
before they were heavily developed with housing, and therefore enable mapping of
landslide and slope instability features which are often now obscured by housing and
vegetation.
Conversations with Wanganui Opus personnel allowed documentation of slope instability
events over the last approximately 20 years within the study areas.
5
Engineering Geological Mapping
Reconnaissance level engineering geological mapping was carried out by an Opus
engineering geologist on 30 and 31 May 2011. The mapping of the hillside was undertaken
from the streets bounding the study areas, with no access gained to private properties
within the study areas.
The mapping involved identification of areas of slope instability, typically from landslide
scarps, hummocky ground or exposed soil. Some older landslide features were also
identified, from degraded scarps and evacuated slopes.
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Existing slope mitigation measures, such as retaining walls, were also mapped as they
indicate a precedent for past slope instability.
The engineering geological maps are given in Appendix A.
6
Stability Assessment
6.1
Causes of Instability
6.1.1 Slope Materials
The Castlecliffian Stage soft rocks from which the hillslopes within the study area are
composed of are described as soft rock, or colloquially as ‘papa’. Where slope instability is
not restricted to the surficial zone of highly weathered rock or overlying colluvium, soft rocks
typically fail by two progressive failure modes:
1. Slabbing, where slabs 300 mm – 400 mm thick fail along planes subparallel to the
slope. Slabbing is typically observed on siltstone slopes of angles greater than 45o.
2. Slaking, where the surface disaggregates, or frets, to form fragments ranging from silt
to gravel sized. Slaking is more common in finer-grained rocks (Read and Millar, 1990).
Failure of both surficial materials and deeper failure of the soft rock by slabbing was
mapped within the study areas, as detailed below.
6.1.2 Slope Angle
Engineering geological mapping of landslides within the study areas has shown instability is
apparent on hillsides with slope angles of greater than 40o, and is almost always present on
slopes of greater than 50o, such as along Putiki Drive between the former “Totalspan”
building and below 72 Hipango Terrace (Appendix A).
A few slopes of less than 40o also showed some shallow instability, however this was
generally localised to less than 5 m2 and was generally found on modified slopes, e.g.
Mount View Road and Hipango Terrace road cuttings. Slopes steeper than 35o are
marginally stable and pose a risk to development.
6.1.3 Storm and Earthquake Events
Several of the mapped landslide events occurred during the wetter winter months or
following storm events, or during a period when more than usual water was entering the
slope (e.g. a landslide below 19 Putiki Drive which damaged pipes and the wooden
staircase behind the Braeburn was likely caused by a sprinkler left on adjacent to the slope
crest). This indicates the hillslopes are vulnerable to rises in groundwater level, which may
cause instability.
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Ground shaking caused by earthquake events could also trigger slope instability. There are
records of earthquake-induced landslides from Shakespeare Cliff (adjacent to the current
study areas).
6.1.4 Modification of Natural Slopes for Housing and Infrastructure
Excavation into natural slopes may cause instability, particularly if excavation is into the toe
of a slope. The formation of fill embankments may also contribute to landsliding, due to the
increased load on slope crests.
Conversations with local Opus personnel confirmed several property owners have required
assessment of landslides from cut slopes and below fills. Examples are a landslide at 4
Rees Street which caused damage to the house, and failure of a slope below a new fill at 5
Hipango Terrace required a retaining wall to stabilise. Landsliding at 20 Taylor Street
destabilised the house, which was eventually removed and fill placed as a buttress to
stabilise the slope.
6.2
Slopes below Hipango Terrace / Rees Street (Area 1)
Instability of the slopes immediately above Putiki Drive is common between Portal Street
and below 74 Hipango Terrace, as shown on the engineering geological map in Appendix
A. The slopes south of 74 Hipango Terrace generally appear currently stable.
Instability of the hillside below Hipango Terrace and Rees Street is typically evident by
shallow failures of topsoil, vegetation and soils (Photographs 1 and 2). Deep-seated failures
involving the underlying siltstone are rare. Creep-type failures of soil and vegetation are
also common, and can be seen on old photographs of the slopes north of 30 Hipango
Terrace.
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Photograph 1 Surficial soil failures behind Red Lion Inn, Anzac Parade. The red
dashed line shows the approximate extent of instability identified from archival
photographs (1905-1915).
Photograph 2 Shallow instability to south of Braeburn Hotel, and recently installed
retaining wall to stabilise slope in front of blue house (72 Hipango Terrace).
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6.3
Slopes above Mount View Road (Area 2)
Two types of instability are typical on the slopes above Mount View Road / Anzac Parade.
Shallow failure of soil and vegetation (as per previous section) are common, and there is
also evidence of deeper-seated failures involving the underlying siltstone, as shown on the
engineering geological map in Appendix A.
Deeper failures were mapped on the slope between 7-9 Wairere Road and Anzac Parade
below (Photograph 3). The landslide below 7 Wairere Road failed slowly as a mudflow-type
failure for approximately five months over winter, with a run-out zone covering half of Anzac
Parade below.
Shallow instability is evident by small failures of soil and vegetation from steep sections of
hillside (40o – 50o), such as below 13 Wairere Road (Photograph 4).
Photograph 3 Landslide below 7 Wairere Road which failed slowly over winter 2006.
Siltstone is exposed in the headscarp on the right side of the photograph.
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Photograph 4 Surficial failures in soil and vegetation on Mount View Road, circled in
red.
7
Qualitative Risk Assessment
A Qualitative Risk assessment to property of failure of the hillslope has been undertaken
following the process described in ‘Practice note Guidelines for Landslide Risk
Management 2007’ (Australian Geomechanics Society, 2007). The risk assessment tables
and terminology are given in Appendix B.
For areas showing precedent for instability and having steep slope angles of greater than
40o, the recurrence interval for failure is expected to be approximately 10 to 50 years, giving
a likelihood of failure of likely to almost certain. Such a failure is likely to cause large scale
property damage and would likely require major engineering works for stabilisation, giving a
consequence of failure of catastrophic. The risk rating for such areas is therefore very high,
and is unacceptable.
Some areas have shallower slope angles of 30o to 40o but still show some evidence of
instability. Failures may occur less frequently on these marginal slopes, depending on sitespecific conditions, such as colluvium thickness and type and groundwater level. Therefore
the recurrence interval will be variable for marginal slopes - perhaps 50 to 1000 years,
giving a likelihood of failure of possible to likely. Property damage is likely to be less severe,
and the structures may not be completely destroyed, giving a consequence to property of
medium to major. The level of risk to property is therefore moderate to very high.
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8
Landslide Hazard Zonation
Based on the risk assessment above, two landslide hazard zones are proposed to reduce
the risk to property to low risk (Figures 1 and 2). Preventing or restricting development in
landslide hazard zones treats the risk by avoiding it. These zones are:
•
Zone A: Areas of very high landslide risk that are unsuitable for future
development.
The zone comprises three parts, which are shown in Illustration 2:
1. A setback zone at the crest of the cliff, where failure of the slope below will undermine,
or to allow natural regression of the cliff to a more stable slope angle.
2. The landsliding zone – the steep section of the cliff, where landsliding occurs, or has
occurred in the past.
3. The run-out zone, where landslide debris collects at the toe of the slope. The width of
this zone has been determined based on anecdotal evidence of past landslide events in
the area.
It is recommended no further development is allowed within Zone A, as the risk of
further instability and damage to property or life is very high.
•
Zone B: Marginal land requiring prior geotechnical investigation to confirm
suitability for development. This zone has a significant landslide hazard. The
landslide hazard and risk to development assessment should be carried out prior to
consideration of any development as part of the resource consent process. It is
recommended that building consents not be issued unless prior resource consents are
obtained for development including geotechnical investigations and assessment that
prove their suitability for development with a low risk to the properties and life.
The outcome of geotechnical investigations will determine if the risk to property is
moderate, high or very high, depending on factors such as the thickness and type of
colluvium and groundwater levels. An outcome of very high risk will mean the land will
be unsuitable for development (Zone A), whereas moderate risk may mean the land
can be developed, with mitigation measures designed and implemented.
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Anzac Parade / Putiki Drive Hillside Stability Study
Illustration 2 Typical cross-section through a cliff section below Hipango Terrace, showing
determination of a building set-back line at the top of the cliff and run-out zone at bottom of
cliff.
The zonation is given on Figure 1 and 2. All areas not given either of the zonations
described above have been assessed as having a low risk of instability and the risks of
slope stability can be managed using existing consenting processes.
The engineering geological mapping was carried out at approximately a 1:2500 scale;
therefore the zone boundaries are approximate only. The landslide hazard maps should be
used only at the scale provided.
9
Recommendations
We recommend:
1. No future development should be permitted within the zone of very high landslide risk
(Zone A).
2. Any future development on marginal slopes (Zone B) should be preceded by sitespecific geotechnical investigation and assessment prior to consideration of any
development as part of the resource consent process, to determine the risk of instability
and identify treatment measures.
3. The landslide hazard zones should be incorporated into Wanganui District Council’s
district plan. This may be best achieved by a landslide hazard overlay in the district
plan.
4. The areas surrounding current study areas and other areas in the city where slope
instability has caused issues in the past should be assessed in a similar way as this
study to provide uniformity in how these areas are treated in the district plan.
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10
Limitations of the Assessment
The slope stability assessment for this study covers only the areas shown in Figures 1 and
2. No assessment of hillside stability has been made for properties outside of the two study
areas.
Engineering geological mapping within the study areas was carried out from aerial
photography and observations from streets within the study areas. No access was gained
to properties, and therefore individual property stability assessments have not been made.
This is not a property specific assessment, but an area-wide assessment to assist with
development of land development controls.
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11
References
Australian Geomechanics Society, 2007. Practice Note Guidelines for Landslide Risk
Management 2007. Journal and News of the Australian Geomechanics Society, 42:1.
Frampton, M. and Moran, T. Personal communication. Opus International Consultants,
Wanganui office, 30 and 31 May 2011.
New Zealand Geological Survey, 1959. Geological Map of New Zealand: Sheet 10 –
Wanganui. Department of Scientific and Industrial Research, Wellington.
Opus International Consultants Ltd. 2009. Shakespeare Cliff, Wanganui – Building Line
Restriction. Report for Wanganui District Council, November 2009. Reference No.
5W1100.04/1WC.
Read, S.A.L. and Millar, P.J. 1990. Strength characteristics of New Zealand Tertiary age
sedimentary rocks and their stability in cut slopes. RRU Bulletin 84 Bridge Design and
Research Seminar 1990 Volume 4: Road Engineering in Soft Rock Materials. Wellington:
Transit New Zealand.
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Figures
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±
Allis
on
Legend
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Existing Hillside Protection Zone
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Zone A (No Development)
Zone B (Further geotechnical investigation
required prior to development)
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Project:
Anzac Parade/Putiki Drive
Hillside Stability Study
Title:
t Pla
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Landslide Hazard Zonation - Slopes
Below Hipango Terrace (Area 1)
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Project No:
5-W1100.08
Approved By:
M. Frampton
Date:
April 5, 2012
Status:
FINAL
Map:
Figure 1
±
Legend
Zone A (No Development)
Zone B (Further geotechnical investigation
required prior to development)
Study Area
Parcel
Roads
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or
Ge
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ett
Ro
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Wairere Road
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Hillside Stability Study
Title:
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Landslide Hazard Zonation - Slopes
Below Mount View Road (Area 2)
Client:
Prepared By:
J. Callosa-Tarr
Project No:
Consultant:
Approved By:
M. Frampton
Date:
1:1,000
0
10
20
40
Meters
Projection: NZ Transverse Mercator
5-W1100.08
April 5, 2012
Status:
FINAL
Map:
Figure 2
Anzac Parade / Putiki Drive Hillside Stability Study
Appendix
A
Engineering Geological Maps
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Anzac Parade / Putiki Drive Hillside Stability Study
Appendix
B
AGS (2007) Risk Assessment Tables
5-W1100.08
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(1)
10 years
20 years
Implied Indicative Landslide
Recurrence Interval
91
Notes:
0.5%
5%
20%
60%
200%
(4)
(3)
(2)
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.)
Description
4
5
INSIGNIFICANT
3
2
1
Level
F
E
D
C
B
A
Level
MINOR
MEDIUM
MAJOR
CATASTROPHIC
Descriptor
BARELY CREDIBLE
RARE
UNLIKELY
POSSIBLE
LIKELY
ALMOST CERTAIN
Descriptor
Australian Geomechanics Vol 42 No 1 March 2007
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
10%
1%
40%
100%
Notional
Boundary
Approximate Cost of Damage
Indicative
Value
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.
-3
200 years
5x10
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
Notional
Boundary
QUALITATIVE MEASURES OF CONSEQUENCES TO PROPERTY
Note:
10
-6
10
-5
10-4
10
-3
10
-2
10-1
Indicative
Value
Approximate Annual Probability
QUALITATIVE MEASURES OF LIKELIHOOD
QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY
APPENDIX C: LANDSLIDE RISK ASSESSMENT
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007
UNLIKELY
RARE
BARELY CREDIBLE
D -
E -
F -
92
L
M
H
VH
VL
L
M
H
VH
VH
VL
L
L
M
H
VH
VL
VL
L
M
M
H
4: MINOR
5%
VL
VL
VL
VL
L
M or L (5)
5:
INSIGNIFICANT
0.5%
MODERATE RISK
LOW RISK
VERY LOW RISK
M
L
VL
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.
Australian Geomechanics Vol 42 No 1 March 2007
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.
HIGH RISK
H
Note:
VERY HIGH RISK
VH
Risk Level
RISK LEVEL IMPLICATIONS
(7)
10
-6
10
-5
10
-4
10-3
10
VH
VH
-2
10-1
3: MEDIUM
20%
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.
POSSIBLE
C -
(5)
(6)
LIKELY
B -
Notes:
ALMOST CERTAIN
A –
2: MAJOR
60%
CONSEQUENCES TO PROPERTY (With Indicative Approximate Cost of Damage)
1: CATASTROPHIC
200%
Indicative Value of
Approximate Annual
Probability
LIKELIHOOD
QUALITATIVE RISK ANALYSIS MATRIX – LEVEL OF RISK TO PROPERTY
APPENDIX C: – QUALITATIVE TERMINOLOGY FOR USE IN ASSESSING RISK TO PROPERTY (CONTINUED)
PRACTICE NOTE GUIDELINES FOR LANDSLIDE RISK MANAGEMENT 2007