geotechnical risk assessment

8/10/2019 Geotechnical Risk Assessment

http://slidepdf.com/reader/full/geotechnical-risk-assessment 1/23

William C. Cromer Pty. Ltd. – 74A Channel Highw ay Taroona, Tasmania 7053 Aust raliaMob. 0408 122 127 – Fax 03 6227 9456 – www.bil lcromer.com.au – email bil [email protected]

Geotechnical Risk Assessment

Storm surges causing foreduneerosion and flooding

Roches Beach, Lauderdale

WILLIAM C. CROMER PTY. LTD.ACN 009 531 613 ABN 48 009 531 613

E NVIRONMENTAL, E NGINEERING AND GROUNDWATER GEOLOGISTS

C C

W



Clarence City CouncilGeotechnical risk assessment, foredune erosion, Roches Beach, Lauderdale 17 February 2006


2

Cover photo. A view north along Roches Beach, 19 September 2005. The foredune ends abrupt lyin a 2m high erosion scarp exacerbated by a storm surge a fortnight earlier. This part of the coasthas receded about 5m since the mid 1970s. The foredune slopes left to lower-lying residentialland about a metre above mean sea level.

1 BackgroundThis report commissioned by Clarence City Council provides a preliminary assessment of thegeotechnical risks to property and human life presented by storm surges and attendant coastalerosion along Roches Beach at Lauderdale (Attachment 1). Council is particularly concernedto know whether immediate actions are required to treat (manage) the risks, or whether itwould be acceptable to defer any actions in the medium term (eg for six months or more) untilthey can be more thoroughly investigated.

2 Basis if assessment

This assessment is based on:

• My general experience in coastal sands deposits in Tasmania,

• A brief site inspection of Roches Beach on 19 September 2005,

• A scrutiny of aerial photographs of Lauderdale taken in 1957, 1965, 1973, 1975, 1977,1979, 1986, 1992, 1997, 2002 and 2004,

• A review of relevant previous reports1, and

• A risk assessment in general accordance with AS/NZS4360 – 2004 Risk Managementand Australian Geomechanics Society Subcommittee (2000) Landslide RiskManagement – Concepts and Guidelines. Australian Geomechanics 35(1) March2000, pp49 – 92.

It is stressed that this assessment is a preliminary or “first pass” review of the risks. For arelated and considerably more detailed discussion of the vulnerability of southeasternTasmanian coastal landforms, and the effects of sea level rise, refer to Sharples (2002, 2004)cited below.

3 Risk assessment

3.1 The risk management contextThis report is concerned only with the risk to property and infrastructure, and the individual riskto life of residents, occasioned by storm surges causing foredune erosion and flooding alongRoches Beach and impacting the residential strip immediately inland from the beach. Generalcomments only are made about the total risk to life of residents as a group in the potentiallyaffected area.

1 The reports were:

Cromer, W. C. (2001). Treating domestic wastewater in a shallow coastal sand aquifer near Hobart. Proceedings ofOn-site ’01 Conference: Advancing On-site Wastewater Systems by R.A. Patterson & M.J. Jones (Eds). Published byLanfax Laboratories, Armidale ISBN 0-9579438-0-6, 400 pages.Sharples, C. (2002). Geomorphology of the South Eastern Region of Tasmania. Explanatory report to accompanydigital mapping dataset. Prepared for the Integrated South East Coastal Management Authority.Sharples, C. (2004). Indicative Mapping of Tasmanian Coastal Vulnerability to Climate Change and Sea Level Rise:Explanatory Report. Prepared for the Department of Primary Industries, Water and Environment, Hobart (December

2004).





3

No other hazards and risks have been investigated.

For the purposes of this assessment, it is assumed that other than the rock sea wall at thesouthern end of Roches Beach, no measures are in place or planned to mitigate the flood riskfrom storm surges, and that residents and visitors have unrestricted access to houses, roadsand recreational areas at all times.

The extent to which this assessment might fit within a broader risk management context at anorganisational level is outside the scope of this report – except that, in accordance withCouncil’s brief, recommendations are made about the timing of possible actions to address therisks.

Authorities and stakeholdersParties which reasonably might have an interest in this risk assessment would include (but notnecessarily be restricted to) the following:

• The residents of Lauderdale, and in particular those living in the low-lying residentialstrip immediately inland from Roches Beach, in relation to risk to property, life or injury,

• Insurers of the houses within the same residential strip, in relation to property damageon the one hand, and injury or loss of life claims on the other,

• Investors, including lending institutions and landlords,

• Clarence City Council as the main authority permitting and controlling development,and as the owner of infrastructure and public amenities,

• other parties (eg the Integrated South East Coastal Management Authority), and

• Police, ambulance, fire and State Emergency Services personnel, in relation to siteaccess.

Risk evaluation criteriaProperty risk and Individual risk to LifeEvaluation criteria for property risk and individual risk to life are taken from the AustralianGeomechanics Society Subcommittee (2000) report Landslide Risk Management – Conceptsand Guidelines cited above. This guideline used the generic framework of AS/NZS4360 –1999 Risk Management. In particular, Appendix G is useful in assessing property risk, and ispresented here in Attachment 4

2.

ProcessRisk management includes risk analysis, evaluation and treatment, and involves answering thefollowing questions:

• What might happen? (Hazard identification)

• How likely is it? (Frequency or likelihood)

• What damage or injury might occur (Consequences)

• How important is it? (Level of risk; acceptability)

• What can be done about it? (Risk treatment)

The following sections discuss each of these questions in relation to Roches Beach.

2 The AGS (2000) guidelines have generated lively debate, and Landslide Risk Management in general is a constantly

evolving field. See, for example, Australian Geomechanics Vol. 37(2) May 2002; Australian Geomechanics Vol. 40 (4)December 2005. A useful discussion of quantifying Likelihood of a hazard occurring is in Moon, A. T. and Wilson, R.

A. (2004). Will it happen? – Quantitative judgements of landslide likel ihood. Proceedings of the Australian and NewZealand Conference on Geomechanics. University of Auckland, Vol. 2 pp 754 – 760.





4

3.2 Hazard identification

The main hazards arise from storm surges on Roches Beach (Figure 1) which are reasonablyexpected to increase in height (ie height of the maximum wave run-up or wave erosion scarpon foredunes), if not in frequency, in coming decades because of sea level rise. Two of many

scenarios

3

arising from this hazard are:

• Scenario 1. Limited flooding of residential land and roads from southeasterly stormsurges entering only existing breaches in the foredune – for example, at theLauderdale Canal boat ramp, and at several access tracks (Plate 8 in Attachment 2).The surges also erode the foredune but are not high enough to overtop it.

• Scenario 2. Flooding of residential land and roads from a single southeasterly stormsurge high enough to not only flood through existing breaches, but also to overtop thethen existing foredunes in numerous places along the length of the beach.

Figure 1. Terminology for storm surges on foredunes fronting flat coastal plains ofunconsol idated sands similar to Roches Beach. Reproduced from Sharples (2004, page 16, inturn reproduced from McInnes et al (2000). Impact of Sea Level Rise and Storm Surges onCoastal Resorts; Final Report for CSIRO Touri sm Research, by CSIRO Atmospheric Research.)

With respect to Scenario 1, storm surges have previously flooded up the boat ramp onto andover the road linking North and South Terraces. It is not clear whether access tracks have alsobeen breached, but Plate 8 suggests recent storm surges in June and September 2005 did nodamage to at least some of these.

Scenario 2 has yet to arise, but every time Scenario 1 happens, the foredune is also eroded.Without natural net replenishment of sand (the situation since 1975 at least) the foredunecontinues to recede and becomes lower in height, so that Scenario 2 becomes increasinglylikely. Sea level rise is another positive feedback enhancing the chances of Scenario 2happening.

In relation to these scenarios, the following are relevant:

3 Other related hazards at Lauderdale involve flooding of low lying land caused by high tides and winds in Ralphs Bay,

on the opposite side of the Lauderdale isthmus. These have occurred in recent decades (see Sharples, 2004) and will

occur again. However, they are outside the brief for the present report.





5

• Sharples (2004, page 30) states “Beach erosion has also been a long term problem atRoches Beach (Lauderdale) in south-eastern Tasmania, where many houses havebeen built only metres behind the beach, on or just behind the low foredune. Ananalysis of historical air photos from 1948 to 1984 was stated to have revealed nodetectable net beach or dune front recession over that period ....although the degree of

resolution of that analysis was not stated. However a vertical wave erosion scarp up to2 metres high has now been present along much of the foredune front for well over 5years, and has not exhibited significant natural wind-blown sand accretion rebuildingthe dune front as would normally occur on an equilibrium beach in the course of thenatural "cut-and-fill" cycle of beach erosion and accretion. Instead, the exposure of theroots of large trees in the scarp suggests a degree of erosion that has not occurred fordecades previously, at least.”

• These comments by Sharples are supported by the aerial photographic evidence in Attachment 2, at least for sections of Roches Beach north of the Lauderdale Canal.The photos show no obvious net coastal erosion between 1957 and 1975 but about5m of foredune has been eroded between 1975 and 2002. Significantly, no obviousnet accretion of sand has occurred to arrest the erosion.

• Tide gauge records show global sea level has risen 0.1 – 0.2m during the twentiethcentury (Sharples, 2002, page 23). Moreover, the trend appears to be accelerating(Sharples 2002, Figure 8c). The average global rise is supported by local tide gaugeswhich indicate sea levels relative to the land in southeastern Tasmania have risen byabout 0.14m since the 1840s, and most of that possibly since the 1880s (Sharples,2004, page 8). Assuming the rise is linear, the average rate since the late 1880s hasbeen 1.2mm per year. If this is applied to Roches Beach, sea level relative to theforedunes has risen about 0.05m (50mm) since the mid 1960s. The global averagesea level rise projected to 2100 is between 0.08 and 0.88m (Sharples, 2004, page 65)but based on historical Tasmanian data the low end of this range would appear to betoo low.

Figure 2. A surveyed profil e of the foredune at the eastern end of Aragoon Street. Note the similar profilein Plate 7 in Attachment 2. The houses at the end of the street are about 1m above mean sea level (AMSL).Since this survey was done in 1997 (Cromer 2001), the storm surges of June and September 2005 probablyreached a wave setup (Figure 1) of at least 1mAMSL, producing a vertical wave erosion scarp on theforedune (Plate 6 in Attachment 2). About 5m of the foredune has been lost since 1975.

3.3 Likelihood of the scenarios occurring

No official records exist for the frequency and intensity of storm surges along the southeasternTasmanian coastline. Sharples (2004, page 17 and 18) discusses anecdotal and historicalevidence for storm surges at Lauderdale and Roches Beach, and others have been added(pers. comm. G. Rumbold and P. Rapley) to the following list from late 1967:

• 1967, November. Flooding of properties and houses in South Terrace and Bayview

East

20100

A rox. metres V=H

West

m above a rox. mean sea levelWater table

Surveyed land surface

House

0

3

Foredune

Sand Beach





6

Road

• 1970, October/November. Flooding.

• Early 1980s. Storm surges reportedly worse that those of mid-late 2005. In early1980s, 150,000m

3 of sand placed by State Government at southern end of beach; sea

wall erected.

•

1986, January• 1988, July. High tide caused flooding in Lauderdale

• 1991, August. Waves breaking on BP service station on South Arm Road; debrisacross road.

• 1994, May. Main South Arm Road closed

• 1994, November.

• 2005, June

• 2005, September

Other, lesser surges may have gone unreported.

It cannot be stated from this information whether or not the intensity (ie height) or frequency ofthe storm surges is increasing. However, other evidence is available: for example, Sharples

(2004, page 15) reports that tide gauge records for Sydney and Fremantle show storm surgesreaching a given height have occurred more frequently over the last 50 years compared to thepre-1950 period. This is considered to be a result mainly of sea level rise. Accordingly, it islikely that the frequency of storm surges of a given height at Roches Beach, and the meanwave erosion height with respect to the foredunes on Roches Beach, are both increasing.

Based on the foregoing, it is assumed that for the purposes of this preliminary risk assessment,

• The likelihood of storm surges causing Scenario 1 is “ Almost certain4”, with an

indicative annual probability (IAP) of 0.1 or higher 5 (Attachment 4). This implies

flooding through breaches in the present foredunes is expected on average once in tenyears. (Surges are more common than this, but not all result in flooding through thebreaches). Table 2 recognises the inherent uncertainty of “once in ten years” by

instead adopting a range for the IAP of 0.05 to 0.5 ie flooding is expected betweenonce every 20 years and once every two years.

• The likelihood of storm surges causing Scenario 2 above is “Possible”, with anindicative annual probability of around 0.01 (Attachment 4). Like Scenario 1, Table 2adopts a range of values for the IAP, in this case 0.005 to 0.05 ie Scenario 2 isexpected to occur between one every 200 years and once every 20 years. Thisreflects the potential for continued foredune erosion, coupled with the possibility of sealevel rise in the next century, resulting in storm surges individually high enough to overtop the (remaining) foredunes and flood part of the Lauderdale isthmus. The areapotentially affected by Scenario 2, based on 2004 sea levels, is shown in Figure 3,reproduced from Sharples (2004).

3.4 Consequences to property

4 Terms like “Almost certain”, “Likely”, “Possible”, etc. are defined in Attachment 4. Terms used to describe

“Likelihood”, are, however, the subject of debate. For example, Moon and Wilson (2004; cited above) have suggestedmodifications (eg Extremely Likely” instead of “Almost certain”) to the definitions in Attachment 4.5 An IAP of 0.1 for an event means that the event has a 10% chance of occurring in any one year. Another way to look

at it is to imply that the event will recur on average once every ten years (this is the “Implied indicative recurrenceinterval”). Similarly, an IAP of 0.02 means an event has a 2% chance of occurring in any one year (or an impliedindicative recurrence interval of 50 years). An IAP of 0.001 means an event has a 0.1% chance of occurring in any oneyear (or an implied indicative recurrence interval of 1,000 years).





7

Scenario 1The consequences of storm surges flooding through access tracks and similar breaches in theforedune will depend on the height of the surge and its duration. For all but extreme events,flooding up and over the boat ramp is not considered significant for Hazard 1 since the waterwill discharge harmlessly into Lauderdale Canal and because of the local topography would bevery unlikely to impact houses. As a preliminary estimate, the consequences are assumed tobe limited to localised flooding of properties and houses immediately behind and adjacent tothe dunes breaches (neighbouring houses would not be affected). The consequences arerated as “medium” (moderate damage to some houses, and parts of the foreshore requiringstabilisation works).

Scenario 2The consequences of Scenario 2 are rated “Major” to “Catastrophic” (several hundred housesalong the foreshore strip extensively damaged (some possibly destroyed), with medium tomajor damage extending to houses and infrastructure further inland, resulting in extensiverepairs, some rebuilding, and major engineering works for stabilisation. Some residents mightconsider abandonment instead of repair of their dwellings.

Figure 3. Reproduct ion of Figure 17 from Sharples (2004, page 90). The blue areas are potentiallyat risk of flooding at 2004 sea levels. Along the coastal strip behind Roches Beach, there are atleast 300 hundred houses at risk.

3.5 Consequences to human life





8

The consequence to human life of Scenario 1 are estimated to be quite low, or perhaps non-existent, given that the flooding through breaches in the foredune is assumed to be localised,only a few houses might be affected, only a portion of these will be occupied at the time,evacuation ought to be readily achievable, and vulnerability

6 would probably be low because of

early warnings of the flooding. Nevertheless, fatalities cannot be ruled out altogether (egelderly or invalided person or persons living alone, in the pathway of flooding).

The consequence to human life of Scenario 2 are estimated to be potentially greater thanScenario 2, because many more houses would flood in a short period of time, the chances of100% evacuation are much less, and the vulnerability is perhaps higher because of shorterwarning time, or reduced access for rescuers.

3.6 Level of risk to property

Based on the foregoing, Table 1 summarises the level of risk to property (high) as a result ofScenarios 1 and 2.

Table 1. Level of risk to property, and possible risk treatments, for Scenarios 1 and 2

A B C D

Scenario Likelihood ofoccurrence

Consequencesto property

Level of risk toproperty

Risk treatment

1 Limited flooding ofresidential land and roadsfrom southeasterly stormsurges entering onlyexisting breaches in theforedune; foredunerecedes; sand notreplenished

Almost certain Locally Medium;elsewhereInsignificant

Locally Highnear foredunebreaches;elsewhereModerate

Block off (ie fill in) access tracksthrough foredune and constructstyle-type accesses instead;consider the construction of a flood-proof gate at boat ramp near canal,although this is seen as less of apriority.

2 Flooding of residentialland/roads from a singlesoutheasterly storm surge

high enough to top thethen existing foredunesas well as flood throughexisting breaches

Possible Major tocatastrophic

High Not specified. Might include acombination of engineering options(eg rock walls), planning options

(building constraints) and publicawareness options (eg evacuationplans)

3.7 Level of risk to human life

Table 2 is a risk management table which estimates the quantitative individual risk to humanlife arising from Scenarios 1 and 2. The process of estimating the risk is set out in thefollowing numbered steps, which correspond to the steps in column 4 of the table. All theadopted values are subjective. In every case, others could be used in the assessment, withdiffering results.

Step 1. Identify the hazard(s)This report is concerned only with the two stated hazards in Section 3.2, but many others couldbe envisaged.

Step 2. Qualitatively estimate the frequency of the hazard(s) occurringScenario 1 is expected to be Almost certain, and Scenario 2 Possible. As discussed, thesedescriptive terms for likelihood are taken from Appendix G of the Landslide Risk Management – Concepts and Guidelines in Attachment 4.

Step 3. Quantitatively estimate the frequency of the hazard(s) occurringBased on what is already known, but recognising the inherent uncertainty of forecasting, Table2 provides a range of IAPs for Scenarios 1 and 2. It is perhaps reasonable to assume thefollowing:

6 Vulnerability attempts to describe the chances of dying if impacted by the hazard. Death is certain for vulnerability = 1.





9

• IAPs in the range 0.05 to 0.5 imply an indicative annual recurrence interval of 20 to 2years ie Scenario 1 might occur between once every 20 years and once every 2 years,and

•

IAPs in the range 0.01 to 0.05 imply an indicative annual recurrence interval of 100 to20 years ie Scenario 2 might occur between once every 100 years and once every 20years

However, as already discussed, the process of sea level rise which appears to be continuing ifnot accelerating suggests that the IAP for Scenario 2 ought reasonably to be closer to 0.05than 0.01.

Step 4. Use of landResidential, because non-residential land is excluded from this assessment. Residentialincludes houses on both sides of Balook and Bangalee Streets north of Lauderdale Canal, andhouses on both sides of Bayview Road south of the canal. It also includes the coastalforeshore reserve and other public land (including streets) within the residential area.

Step 5. Probability of the scenario affecting houses A value of 0.25 (ie 25% chance) is adopted for Scenario 1. This is the same as saying thatthree out of four breaches do not affect any houses. For Scenario 2, it is assumed it is acertainty (value of 1) that at least some houses will be affected.

Step 6. Occupancy (number of people affected) A relatively small number of people (say, in the range of 10 to 20) are assumed to be presentin houses affected by Scenario 1, and between say, 150 and 300 people for Scenario 2.These assumptions are highly subjective, and in any case are not used to calculate total riskfrom individual risk.

Step 7. Proportion of time the house(s) are occupied

A value of 1 is allocated for a person occupying a house for 24 hours per day. It is assumedthat for Scenarios 1 and 2, all the potentially affected house occupiers are present half thetime, so that the individual occupancy is simply 0.5.

Step 8. Probability of not evacuatingIt seems reasonable to expect a fair degree of warning of impending danger for Scenario 1. Avalue of 0.01 has been adopted for this factor, which means there is a 1 in 100 chance of notevacuating (or a 99% chance of evacuating). The slightly lesser degree of warning forScenario 2 is reflected by the corresponding figure of 0.05, meaning that there is a 95%chance of evacuating.

Step 9. VulnerabilityThis factor attempts to describe the chances of dying if impacted by the hazard. A value of 1

means death is certain. Values of 0.01 and 0.05 are suggested for Scenarios 1 and 2respectively, indicating 1% and 5% chances of a fatality (ie 99% and 95% chances of survival).

Step 10. Risk for person most at riskThe risk for a single person is calculated by multiplying the values in Steps 3, 5, 7, 8 and 9. Arange of risk will result because a range of IAPs has been adopted for Step 3. For example,adopting the lowest IAP in Step 3, the risk to life for a single person from Scenario 1 is 0.05 x0.25 x 0.5 x 0.01 x 0.01 = 0.000000625, or about 6 x 10

-7. This is less than one chance in a

million7, or about one in 1,600,000, of the person most at risk dying from Scenario 1. If the

higher of the two IAP values is used for Step 3, the risk increases tenfold, to 6 x 10-6

, or about1 in 160,000.

7 Numbers like 6 x 10-6 are often written in Scientific Notation, where the “x10” is replaced by the capital letter “E”, and

the-6

is written as -06 on the same line ie 6E-06. So, 0.00002 = 2 x 10-5 = 2E-05.





10

The risk to life from Scenario 2 can be calculated in a similar fashion, first adopting the lowervalue for Step 3 ie 0.01 x 1 x 0.5 x 0.05 x 0.05 = 0.00001 = 1 x 10

-5 = 1E-05, or about 1 in

80,000. Using the higher value in Step 3 increases the risk tenfold, to 1 in 8,000.

Step 11. Total riskThis preliminary assessment does not attempt a quantitative assessment of the total risk to life.Instead, because of the uncertainty surrounding the number of people which might be at riskfrom either scenario, it is suggested that until more detailed analysis is attempted, reasonablestatements are:

• The total risk to residents presented by Scenario 1 is higher than the individual risk iehigher than the range 6E-07 to 6E-06, and

• The total risk to residents presented by Scenario 2 is higher than the individual risk iehigher than the range 1E-05 to 6E-05.

12. Uncertainty and sensitivity analysis As discussed, most of the inputs to the risk to life assessment are uncertain and judgemental.

Different assessors would most likely choose sets of numbers which differ from thosepresented here, with different results. Some values might reasonably be changed (up ordown) by a factor of 10 or more. There are no right or wrong inputs, but the resulting riskcalculation should be reasonable, and consistent with common sense and/or experience. Assessment includes repeated checking and revision of the data to arrive at realisticconclusions.

This row indicates those inputs which are considered most uncertain.

13. Risk evaluationSee Section 3.8.

14. Risk treatment

Acceptance usually requires the risk to be acceptable or tolerable. Avoiding the risk requiresabandoning the project. Reducing the likelihood requires rivulet hydraulic works (eg enlargingthe culvert). Reducing the consequences involves relocation of the project, or minimisingpotential exposure of people or property to the hazard. Transferring the risk means requiringanother authority to accept the risk.

3.8 Acceptability of the level of risk

General commentThe risk management process recognises that the acceptability of a particular risk is likely tovary between stakeholders. For example, a Lauderdale resident may be relaxed about takingno actions to treat the risk of flooding from storm surges, or indeed, might object to having

beach access modified. Council, arguably with a duty of care to ratepayers, might judgeotherwise. Insurers, too, may view inaction to treat a recognised risk as unacceptable.

It is also commonplace for authorities to ignore known risks because treatment would be toocostly or unacceptable to residents. A good example is the widely recognised high flood risk toNew Orleans. Others include the high earthquake risk presented by the Alpine Fault zonethrough the middle of Wellington, New Zealand, and the San Andreas fault zone in California.

Table 2. Assessment of the individual risk to human life for Scenarios 1 and 2

Step Scenario 1 Scenario 2





11

1

H a z a r d I d e n t i f i c a t i o n

Limited flooding ofresidential land and roadsfrom southeasterly storm

surges entering onlyexisting breaches in the

foredune; foredunerecedes; sand not

replenished

Flooding of residentialland/roads from a single

southeasterly storm surgehigh enough to top the then

existing foredunes as well asflood through existing

breaches

2 Likelihood Almost certain (event

expected to occur)Possible (event could occurunder adverse conditions)

3 F r e q u e n c y

a n a l y s i s

Indicative annual probability 0.05 to 0.5 0.01 to 0.05

4 Use of land Residential Residential

5Probability of scenarioaffecting houses

0.25 1

6 Occupancy (no. of people)Uncertain; perhaps 10 to

20Uncertain; perhaps 150 to

300

7Proportion of time occupied(24 hours x 1 person = 1)

0.5 0.5

8 Probability of not evacuating 0.01 0.05

9 C o n s

e q u e n c e a n a l y s i s

Vulnerability 0.01 0.05

10 Risk for person most at risk 6E-07 to 6E-06 1E-05 to 6E-05

R i s k A n a l y s i s

11 R i s k

c a l c u l a t i o n

Total riskHigher than the risk for a

person most at riskHigher than the risk for a

person most at risk

12

U n c e r t a i n t y

a n a l y s i s

Most uncertainty probably

relates to Items 6 to 9Most uncertainty probably

relates to Items 6 to 9

R i s k A s s e s s m e n t

13 R i s k

e v a l u a t i o n

See report See report

Accept risk? No No

Avoid risk? No No

Reduce likelihood? Not feasible Not feasible

Reduce consequences? Yes Yes

Transfer risk? NoPart transfer possible?

(eg share with local, state,federal govts)

Treatment plan?

Include (but not

necessarily be limited to)the following: public

education; block off (ie fillin) access tracks throughforedune and replace bysteps instead; remove

unsafe trees on edge offoredune; consider theconstruction of a flood-proof gate at boat ramp

near canal.

Not specified. Seek

specialist advice.Treatment might include acombination of engineering

options (eg rock walls),planning options (building

constraints), publicawareness program andoptions (eg evacuationplans). Will be part of a

wider strategy of managingsea level rise.

Implement plan? Yes. Now.Review options now.

Implement as required.

R i s k M a n a g e m e n t

R i s k T r e a t m e

n t

14

R i s k t r e a t m e n t

Monitor and review? Yes Yes

Acceptabil ity of the risk to property behind Roches Beach

Based on Appendix G of Landslide Risk Management – Concepts and Guidelines, it is





12

suggested that the acceptability of the risk to property presented by Scenario 1 might varybetween stakeholders as follows:

To residents at risk acceptable to unacceptableTo insurers acceptable to tolerableTo Council tolerableTo police, SES, etc acceptable

For Scenario 2, the range of acceptabilities might be

To residents at risk acceptable to unacceptableTo insurers unacceptableTo Council unacceptableTo police, SES, etc tolerable to unacceptable

Acceptabil ity of the risk to l ife at Roches Beach As a preliminary discussion point, it is suggested to stakeholders that the individual risk to lifepresented by Scenario 1 is low enough to be acceptable, and that the uncertain but reasonablyhigher total risk from Scenario 1 where a relatively small number of residents are exposed isalso possibly acceptable.

It is also suggested that the individual risk to life presented by Scenario 2 is reasonably at leastan order of magnitude higher than the individual risk from Scenario 1 but possibly stillacceptable to most stakeholders. However, perhaps the higher total risk from Scenario 2where a relatively large number of residents are exposed is tolerable rather than acceptable,and to some stakeholders, possibly unacceptable.

3.9 Risk treatment

General commentsTable 2 includes subjective comments on whether the risks to property and life occasioned bycoastal erosion and attendant flooding at Roches Beach ought to be accepted, avoided,reduced or transferred. This goes to the heart of Council’s concerns about whether somethingought to be done now or later. Acceptance usually requires the risks to be acceptable ortolerable. Avoiding the risks is simply not possible – they demonstrably exist. Reducing thelikelihood does not seem feasible given that the causes of storm surges are climatic andglobal. Reducing the consequences minimising the potential exposure of people and propertyto the hazard Transferring the risk means requiring another authority to accept the risk. Partialtransfer means sharing the risks (and the costs of treatment) with others eg state and federalgovernments.

Table 2 suggests that the Scenario 1 risks to life and property at Roches Beach should be

accepted by Council, and that a low cost treatment plan should be put in place. For Scenario2, Council should also accept the risks, investigate ways to treat them and (depending on theoutcome) perhaps instigate moves to transfer part of the risks.

It is worth noting that if the rate of sea level rise observed in the past century in southeasternTasmania continues unabated, Scenario 2 becomes increasingly likely in the medium term (ieperhaps within the next 50 years, or within the accepted life of a house being built today). If inthe next 50 – 100 years sea level rises to nearer the higher end of the 0.08m and 0.88mpredicted, Lauderdale and all similar low lying areas will become increasing uninhabitablewithout major engineering works (sea walls, extensive pump out of shallow groundwater, etc).These solutions are possible, but the costs may be prohibitive

4 Conclusions





13

Based on this assessment, it is concluded that:

1. Storm surges which cause coastal erosion and flooding are a fact of life at Lauderdaleand Roches Beach, and are a geotechnical hazard.

2. Sea levels relative to the land in southeastern Tasmania have risen by about 0.14msince the 1840s, and most of that possibly since the 1880s. The global average sealevel rise projected to 2100 is between 0.08 and 0.88m.

3. The storm surge hazards are likely to increase in intensity (storm surge height), andpossibly in frequency, as a result of sea level rise.

4. Two of many possible flood scenarios have been considered:

• Scenario 1. Limited flooding of residential land and roads from southeasterlystorm surges entering only existing breaches in the foredune. The surges alsoerode the foredune but are not high enough to overtop it.

• Scenario 2. Flooding of residential land and roads from a single southeasterlystorm surge high enough to not only flood through existing breaches, but alsoto overtop the then existing foredunes in numerous places.

5. Although the acceptability of risk is sometimes highly subjective, the risks to propertyand individual life for Scenario 1 are judged to be generally acceptable to moststakeholders. The risks to property and life for Scenario 2 are judged to range fromacceptable to unacceptable, depending on the stakeholder.

5 Recommendations

Based on this preliminary assessment, it is recommended that:

1. Council should implement now the treatment plans suggested for Scenarios 1 and 2 inTable 2. For Scenario 1, Council should:

• Instigate a public awareness programme.

• Block off, with sand, all access tracks that have been cut through the RochesBeach foredune and replace all or some of them with steps to the beach.

• Remove all substantial trees whose roots have been exposed by foreduneerosion and which are considered in danger of falling.

• Consider whether it would be advisable to construct flood-proof gates whichcould quickly be put in place across the gap at the boat ramp. The gates maybe judged unnecessary at this stage, but are likely to be important in future.

2. Start reviewing options for addressing the risks associated with Scenario 2.Incorporate in broader municipal and/or state strategies for managing risks associatedwith sea level rise.





14

Bill CromerPrincipal

This report is and must remain accompanied by the following Attachments:1. Location map of Roches Beach and Lauderdale (1 page)2. Site photographs, 19 September 2005 (4 pages)3. Comparative aerial photography (3 pages)4. Evaluation criteria for risk to property (1 page)





15

At tachment 1(1 page)

Location map of Roches Beach at LauderdaleSource: Tasmanian Towns Street Atlas, Edition 6 (2001), page 110. Dept. Primary Industries, Water andEnvironment, Hobart

5000

Approx. metres

GN

AGD66 5249500mN

AGD66 540000mE

RochesBeach

Area covered by

aerial photos in Attachment 3





17

Plate 3 (above). A view looking south towards the southern corner of Roches Beach. The rock seawall inthe middle ground was reportedly erected in the early 1980s to mitigate against coastal erosion and localflooding during storm surges. It appears to have been successful, judging by the 4 – 5m of net coastalrecession at right, where the wall is absent. The wall also does not appear to have significantly affectedsand accretion on the beach in front.

Plate 4 (below). Detail of the basalt boulders of the sea wall.





18

Plate 5 (above). A view looking north on Roches Beach, north of the Lauderdale Canal, where recent netcoastal recession has been most marked. The fences erected to minimise beach erosion (evident at left)have been partly effective, but like those further south they have also prevented natural sand accretionfrom being deposited right up to the base of the eroded foredune.

Plate 6 (below). Recent coastal recession exposing tree roots north of Lauderdale Canal.





19

Plate 7 (above). A composite viewover the foredune (right) and lowerbeach ridge system (left) near thephoto location for Plate 6. Theforedune stands perhaps 2mabove HWM, whereas theunconsolidated sands behind areabout 0.5 –1m above HWM.

Plate 8 (left). An example of anaccess track to the beach, nearphoto location 7. The track is atleast a metre lower than the top ofthe foredune, and offers readyaccess for storm surges throughthe gap into the built-up and lower-lying residential area behind.





20

At tachment 3(3 pages)

Comparative aerial photography

10 20 30 40 50 60 70 80 90 1000

GN

Approx. metres

1957

30 JanuaryRoches Beach betweenfuture Balook andNerang Streets

All three photos are of the samescale and orientation. The red

triangle is the same size and in thesame position in all three photos.Between 1957 and 1975, no netrecession of the foredunes isdemonstrable. Between 1975 and2002 about 5m of net recession isapparent.

RochesBeach





21

197522 December

10 20 30 40 50 60 70 80 90 1000

GN

Approx. metres




William C. Cromer Pty. Ltd. – 74A Channel Highw ay Taroona, Tasmania 7053 Aust raliaMob 0408 122 127 – Fax 03 6227 9456 – www bil lcromer com au – email bil lcromer@bigpond com

23

At tachment 4(1 page)

Evaluation criteria for risk to property

Source: Appendix G of Australian Geomechanics Society Subcommittee (2000). Landslide Risk Management –Concepts and Guidelines. Aust. Geomechanics 35(1) March 2000, pp 449-92.

geotechnical risk assessment

Documents