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Coastal erosion

The power of the sea has shaped Ireland’s coast into what we can see today. Two main processes are responsible for this; erosion and deposition. Coastal erosion is the breaking down and carrying away of materials by the sea. Deposition is when material carried by the sea is deposited or left behind on the coast.

Swashes and backswashes

The swash is when a wave washes up onto the shoreline and the backwash is when the water from a wave retreats back into the sea.

Destructive Waves

Coastal erosion takes place with destructive waves. These destructive waves are very high in energy and are most powerful in stormy conditions. Destructive waves have stronger backwashes than swashes. This strong backwash pulls material away from the shoreline and into the sea resulting in erosion.

Constructive Waves

Constructive waves, on the other hand, are low energy waves that result in the build-up of material on the shoreline. Constructive waves are low energy and have stronger swashes than backwashes. This means that any material being carried by the sea is washed up and begins to build up along the coastline. The material that is deposited by constructive waves can most often be seen by the creation of beaches.

How Waves Erode

Destructive waves erode through four main processes; Hydraulic Action, Compression, Abrasion and Attrition.

Hydraulic Action
Hydraulic Action is the sheer force of water crashing against the coastline causing material to be dislodged and carried away by the sea.

Compression
Compression occurs in rocky areas when air enters into crack in rock. This air is trapped in cracks by the rising tide, as waves crash against the rock the air inside the crack is rapidly compressed and decompressed causing cracks to spread and pieces of rock to break off. Compression is one of the main processes that result in the creation of caves.

Abrasion
Abrasion is when rocks and other materials carried by the sea are picked up by strong waves and thrown against the coastline causing more material to be broken off and carried away by the sea.

Attrition
Attrition is when material such as rocks and stones carried by waves hit and knock against each other wearing them down. As these materials are worn down sand and rounded beach pebbles are formed.

Evidence including shell middens and rock shelters suggest that this area was utilised significantly by

indigenous Australians who visited for the marine resources and as a centre for trade.

In the 1940s the dune system of Dee Why / Long Reef beach was flattened to allow an open vista of the

ocean for defense purposes during WWII. The Long Reef Surf Club had a clear view of the ocean, until

the 1980s when the dunes were reshaped and the vegetation regenerated. In 1973 Dee Why Lagoon was

declared a Wildlife Refuge followed by Long Reef Headland becoming an Aquatic Reserve in 1980.

Collaroy and Narrabeen Beaches together form a beach 3.6km in length. Fishermans Beach is 800m long

and is protected from wave action to a large extent by Long Reef headland and extensive offshore reefs.

Together, Fishermans-Collaroy-Narrabeen beaches have the most intense shoreline development in the

Northern Beaches Council Area and are the most at risk of storm damage in NSW.

Biophysical processes

In geography it is important to understand the interactions and processes in the natural world before we

can understand human impacts and management. In this coastal environment biophysical interactions

and processes are occurring in the lithosphere, hydrosphere, atmosphere and biosphere.

1. Identify the main components of the four spheres at this study site.

2. What are the main two geographical processes that drive the formation of coastal landforms?

Hydrospheric (waves) and aeolian (wind)

Study site

Today’s weather conditions

Forecast summary:

____________________________________________________________________________________

Temperature (°C): _____ Rainfall (mm): _____ Wind speed (km/h) and direction: ________________

Swell height (m) and direction : _________________________________

Figure 2: Long Reef Beach Figure 3: Collaroy Beach

Lithosphere Hydrosphere Atmosphere Biosphere

Sand dunes Ocean Wind Coastal vegetation

Biophysical processes

Swash and backwash

Swash carries sand up / down the beach, and

backwash moves by gravity to bring sand up / down.

Constructive waves are high / low energy waves that

deposit / erode material on a coast, as the swash /

backwash is more powerful than the swash /

backwash.

Destructive waves are high / low energy waves that

deposit / erode material on a coast, as the swash /

backwash is more powerful than the swash /

backwash.

Longshore drift

Beach material almost always moves along the beach in a particular direction which is determined by the

prevailing winds and swell. As this process is repeated, sand gradually shifts along the beach, a process

called longshore drift.

Sand dune anatomy and behaviour

Landward aeolian processes (onshore winds) transport loose sediment, which is deposited as sand

dunes. This occurs continuously and is dependent on weather conditions.

A vegetated dune system will have a definite primary and secondary dune. The start of the primary dune

is defined by the start of vegetation as you progress landward from the beach profile. The dune vegetation

exhibits distinct differences as it changes from primary to secondary and finally tertiary species.

1. In which direction does long shore drift generally act on this section of the coast, and why?

In a northerly direction, because the prevailing winds and swell are from the south.

2. Name the landforms that have been created by biophysical processes from the sand here.

Sand dunes (berm, dunes and swales) including barrier dunes

Secondary

Dune

Permanent,

highly

independent

vegetation

surviving in the

protection of

dune and

secondary

species

Adaptive

reseeding

intermediate

shrubs of

secondary

zone

Very tolerant,

rapid regrowth of

vegetation on

active dune

Primary

Dune

Swale

Berm

Swash zone

Figure 5: Generalised pattern of vegetation on sand dunes

Figure 4: Movement of sediment via Longshore drift,

(Zhu et. al., 2010)

Backwash

Coastal survey techniques

Dune profile

Use a clinometer and a tape measure to collect data that will enable you to draw a 1:500 scale profile of

the sand dune.

1. Divide the dune into sections, beginning and ending with each obvious change in slope.

2. To measure the slope angle, stand at the beginning of

one section, opposite someone standing at the end of

the section who is approximately your height.

3. Look through the clinometer and align the horizontal

line with the equivalent point on that person.

4. Record the angle, paying attention to whether it is

positive (uphill) or negative (downhill). If you are having

trouble determining the angle, a third person can read it

off from the outside.

5. Measure the distance of the section of the dune at the

same time, using a tape measure.

6. Repeat for each section of the dune, moving towards or

away from the sea. Record all the data in the table as you go, noting whether each section is in the

swash, berm or primary dune.

7. At school or at home use a ruler and protractor to draw your dune profile to scale on the grid paper.

1. How can dune profiling be used as a coastal management tool?

Dune profiling can help track the evolution of the sand dune system over time.

Studies can be carried out before and after management strategies are

implemented, to gauge their effect, or after an erosion event such as a storm, to

gauge its impact. They could also be done periodically to track long-term trends.

Figure 6: Side view of a clinometer showing scale

used to measure degrees of the slope

Swash Berm Primary Dune

1 2 1 2 1 2 3 4

Figure 7: An example of a dune profile showing different sections

Profile position Measurement Distance (m) Slope (degrees)

Swash 1

Swash 2

Berm 1

Berm 2

Berm 3

Berm 4

Primary Dune 1

Primary Dune 2

Primary Dune 3

Primary Dune 4

Coastal survey techniques

Data sheet: Dune profile

Coastal survey techniques

Biophysical processes

Please take care of all the equipment and make sure it is all returned, clean, dry and packed away into its

box or case, at the end. Don’t put the equipment on the ground.

Wind speed

Use the anemometer to record the wind speed.

1. Turn it on with the power button and check that it is on the MAX setting and recording in km/h.

2. Hold it up high, facing directly into the wind. You may need to rotate it to find the best direction.

3. After 1 minute, or when the reading has stopped increasing, write down your result. Repeat by turning it

off and on again (to reset the device) and measuring a total of 3 times. Turn it off when you are finished.

4. Calculate the mean.

Wind direction

Use the compass to record the wind direction.

1. Place the compass flat on your hand and turn it so the arrow on the straight edge is pointing towards

the wind. Turn the bezel so that the “N” is aligned with the red side of the needle.

2. Read off the direction in degrees that aligns with the straight edge. Convert this to a cardinal direction

using the card attached to the compass.

Vegetation species

Use the vegetation diagrams to identify three of the dominant plants in each location. Don’t pick or dam-

age the plants whilst doing so.

Vegetation profile

Estimate the height of the vegetation at each location using the comparison technique.

1. Choose a person whose height you know (estimated to the nearest half metre).

2. The person stands next to the vegetation you are measuring, acting as a measuring-stick.

3. Compare the plant to the person, and estimate the plant height to the nearest half metre.

4. Draw the appropriate-sized symbol on your vegetation profile diagram. An example is provided below.

Figure 8: Example of a vegetation profile from UTexas (2010)

Coastal survey techniques

Data table: Biophysical processes

Vegetation Profile

Measurement Rear of Primary Dune (swale) Front of Primary Dune

Wind direction

Wind speed

(km/h)

Mean: Mean:

Vegetation

species

Coastal Tea Tree Hairy Spinifex

Coastal Banksia Pigface

Mat Rush Sea Rocket

Plant Type

Plant height in metres

Rear of primary dune (swale)

Sea Rocket

Pennywort

Hairy Spinifex

Pigface

Coastal Wattle

Coastal Banksia

Coastal Tea Tree

Mat Rush

Front of primary dune

Groundcovers Shrubs Trees

Discussion of primary data results

1. Where was the wind speed highest, and what factors may have been responsible for this? Consider the

prevailing wind direction as well as any windbreak effects.

With a southerly/south-easterly wind, the wind speed is highest at the front of the

primary dune. The swale is more sheltered due to the windbreak effects of the

vegetation and the dune itself.

With a northerly/westerly wind, the swale may be more windy than the front of

the dune.

2. Sand grains can be transported by wind upwards of 16km/h (Bagnold, 1941). Describe the likely

deposition patterns at this site today, considering wind speed and direction. Can you see evidence of this?

With wind speeds <16km/h, little movement of sand is taking place. Above this

speed, sand grains will be transported in a leeward direction by creep/reptation or

saltation. Evidence of this may include visible wind-blown sand grains (hitting

one’s legs) or speedy filling-in of footprints by sand.

3. How does the height and plant type differ between the front and rear of the primary dune?

The front of the dune supports small plants that can resist the effect of salt-laden

wind by being flexible, tough and low-growing. At the rear of the dune, taller plants

including trees are able to grow in the more sheltered conditions.

4. List at least 3 important functions that vegetation performs on a dune.

• The roots stabilise the sand, reducing erosion

• The canopy reduces wind speed, sheltering other plants and increasing

deposition of wind-blown sand

• Plant remains also increase the nutrient level of the soil, encouraging growth

of other plants

5. If you were in charge of rehabilitating the front and rear of the dunes at Cronulla, what would you plant?

Describe the characteristics of species you would select for the two locations.

a) Front

Hardy plants such as Hairy Spinifex that can resist salt-laden winds and grow in

sand that is low in moisture and nutrients, and have extensive root systems to

stabilise the dune. (NB: Native plants, not exotic species such as Bitou Bush that

may become invasive.)

b) Rear

A wide range of plants including trees, shrubs and groundcovers to increase

biodiversity and ecosystem stability. They will provide habitat for other species of

plants and for animals, and help to stabilise the dunes. Species might include Mat

Rush, Coastal Banksia and Coastal Tea Tree.

Coastal management

Past practices

1. Examine the aerial photographs of the Long Reef/Collaroy area. How has the appearance of the dune

area changed between 1930 and 1998?

In 1930, there was little or no vegetation on the Long Reef sand dunes, particularly

on the barrier dune.

2. What kinds of human activities were responsible for the difference in the appearance of the dunes in

1930 vs 1998?

There was unrestricted access over the dunes for recreational purposes (walking,

riding horses) which damaged coastal vegetation. People would also remove sand

for building purposes.

3. List some strategies you observed today, that Northern Beaches Council has put in place to preserve

the vegetation on the Long Reef sand dunes.

Designated pathways to restrict access to dune vegetation

Signage to inform visitors of the need to protect the dunes

4. From the aerial photographs, or from your own observations, why might Collaroy Beach be more

vulnerable to coastal erosion than Long Reef Beach or Narrabeen Beach?

There is significant residential development on the primary dune at Collaroy Beach.

Coastal development

Hard structures built on sand dunes interfere with natural build-up and erosion cycles. Sand dunes absorb

wave energy, whereas hard structures reflect the wave energy back out to sea, taking the sand with it.

Sea walls are built to protect development behind them, but some kinds of sea walls actually cause more

erosion of the beach itself due to greater loss of sand from the reflected wave energy.

5. Consider the natural beach build-up

and erosion cycle illustrated here. How do

sea walls change the behaviour of the

sand?

By reflecting the energy of the

backwash, sea walls increase

erosion and result in a negative

sand budget in the long term.

6. Coastal management is a challenge in Collaroy due to historic development on the primary dune,

where buildings take best advantage of the ocean views and proximity to the beach. List some

developments you observe that are at risk from coastal erosion.

Surf lifesaving clubs, houses, café, restaurants, carpark, apartment blocks

Pre-storm profile Pre-storm profile

Storm profile Storm profile

Post-storm profile Post-storm profile

Coastal management

Current practices in use

Use the information below to complete the coastal management transects on the following page.

Mark the options that you observe with their equivalent location on the transect photo. Use the additional

information to evaluate the success of each strategy in limiting damage from coastal erosion.

Management option Description Advantages/Disadvantages

Piled foundations Building foundations that extend down below any

possible area of movement in the dunes, usually

to bedrock.

Cost: $250,000+ in addition to construction costs

• Effective

• High construction

costs when building

Beach scraping After severe storms the Council runs a tractor

along the beach to level off any dangerous

escarpments that may have formed.

Cost: $90,000 per effort

• Speeds up natural

beach stabilisation

• Only temporary

Relocation Buildings at risk of damage by coastal processes

are dismantled and rebuilt elsewhere.

Cost: Significant

• Building character can

be maintained

• Significant cost

Development

controls

Future development is prevented in areas at risk

of damage.

Cost: Opportunity cost to owner of owning

undevelopable land

• Prevents future

development problems

• Reduces land values,

comes too late here

Repurchase

(acquisition)

Private land in high-risk areas is purchased from

the owner by council or state government, then

buildings are demolished and redeveloped for

public use.

Cost: $1.1m for units, $3.5m for houses (2023)

• Landowners are

compensated

• Expensive (seafront

property values high)

high

Sand nourishment Sand is imported from elsewhere to replenish

lost sand on the beach. Required approximately

every 7-10 years.

Cost: $11-30m per effort depending on

equipment, source and quantity required

• More natural form of

coastal protection

• Temporary and costly

Vertical sea wall A hard, vertical structure built parallel to the

beach to protect the buildings or assets behind it.

Cost: $2,000 to $11,000 per linear metre

• Protects assets

• Increases erosion

elsewhere

Curved sea wall A sea wall engineered to dissipate wave energy,

with a curved seaward profile.

Cost: Significantly greater than vertical sea walls

• Protects assets

• Less increase to

erosion

Mounded sea wall

(revetment wall)

A sloping sea wall constructed of porous material

such as boulders, designed to dissipate and

absorb wave energy.

Cost: Dependent on materials, average $2,200

per linear metre

• Effective; looks nicer

• Still costly

Coastal vegetation Areas of loose or exposed material are stabilised

by planting with coastal vegetation. Mulches or

geo-textiles may be used during establishment.

Cost: $2,500 per ha, plus any earthworks

• Can be combined with

other strategies

• May not work on its

own

© Auseco Education 2024 13 Management key Collaroy Beach satellite image Management key Long Reef Beach satellite image Management Strategies at Collaroy Beach Management Strategies at Long Reef Beach © Google Earth, 2022 Satellite images taken 1,246m above sea level Management option Symbol Piled foundations Beach scraping Relocation Repurchase $ Sand nourishment Vertical sea wall Curved sea wall Mounded sea wall Coastal vegetation

Coastal management

Evaluating management options

Notable erosion events have occurred at Collaroy in 1945, 1967 and 2016. Answer the following questions

using the historic photographs provided, and your observations of the sites.

1945 erosion event: Where were these cottages located

at Collaroy? What management strategy was used to

prevent further damage, and was it effective?

The cottages were located where the current

council carpark is now. They were

repurchased and demolished instead of being

repaired, and a low-value public asset was

built in their place. A mounded sea wall was

built for long-term stabilisation.

1967 erosion event: What management strategy

prevented critical damage to the “Flight Deck” apartment

building during this storm? Was it effective in the long

term?

Piled foundations prevented damage in 1967,

however they were deemed not effective

enough for future storms and were

augmented by the addition of a mounded sea

wall and coastal vegetation, which are visible

today.

2016 erosion event: A variety of structures have been

installed to protect these properties from further damage.

Name two of these and comment on their pros and cons.

• Mounded sea wall with coastal vegetation

(individual houses on southern end): costly,

takes up more space (no room for pool

now), looks nicer, effective.

• Vertical sea wall: protects valuable

properties, highly controversial, may

increase erosion elsewhere. Costs shared

between apartments (lower individual

cost).

Figure 10: Damage to beachside cottages,

1945

Figure 11: Scouring beneath “Flight Deck”

apartments, 1967

Figure 12: Damage to waterfront properties,

2016

Coastal management

Case study: Collaroy sea wall

Severe storms in 2016 and 2020 caused excessive damage to beachfront homes in this area, and as a

result a $25M sea wall was proposed. In early 2021, construction of a 1.3km long sea wall began,

stretching from Collaroy to South Narrabeen to protect private and public land including parks and the surf

club. A portion of the costs (80%) are being paid by local residents of the 49 homes as well as Northern

Beaches Council (10%) and NSW State Government (10%). It is estimated that each property owner will

have to pay $282,000 for their stretch of wall.

The completed 130m section of the wall measures 6m above the sand, extends 7m below the sand, and

is fronted by a quantity of basalt boulders buried beneath the sand to improve storm resistance.

This has been a controversial development in the Northern Beaches area with many strong opinions for

and against the structure.

1. Note some possible opinions from parties who might object to, or be in favour of, this strategy.

a) The residents or landowners of the properties protected by the sea wall: Significant value of

coastal real estate is preserved, residents continue to enjoy views and beach

access, properties are uninsurable without protection.

b) Other residents or ratepayers in the Northern Beaches Council area: Ugly wall destroys

amenity, cost to ratepayers who don’t benefit, erosion to beach increases.

2. Is this structure likely to be effective in limiting coastal erosion in the long term?

Eventually a long-term negative sand budget could mean that no sand remains on

Collaroy Beach in front of the wall, unless repeated costly sand nourishment takes

place (ongoing cost to ratepayers). It may be inadequate to cope with sea level rise.

3. What alternative management strategies could be considered?

Repurchase/demolition of residential buildings, mounded sea wall

4. Consider the strategy of “planned retreat”. Is this a feasible option for this stretch of coastline?

Planned retreat (where development is demolished to make way for the advancing

sea) is not a feasible option because eventually the sea would cause damage to

critical infrastructure such as Pittwater Road (a major arterial route) and utilities.

Figures 12 & 13: Collaroy beach sea wall construction (Auseco, 2021)

Glossary

Accretion A build-up of sand.

Backwash The downwards movement of waves from the beach into the sea.

Bar A mound of sand located in the surf zone which is normally parallel to the shore.

Berm The section of the beach which is located between the swash zone and primary dune.

Coastal processes Waves, winds, currents etc and their interaction with and effects on the coastal

environment.

Constructive waves Low energy waves that deposit material on a coast, as the swash is more powerful than

the backwash.

Creep (reptation) The rolling movement of sand grains or pebbles propelled by the wind along the surface.

Destructive waves High energy waves that erode material on a coast, as the backwash is more powerful

than the swash.

Erosion escarpment A near vertical step formed by wave action at the back of the beach. Usually indicates the

landward extent of wave incursion during recent storms.

Primary dune A vegetated sand dune located at the back of the beach. With the exception of the

incipient primary dune (if present), the primary dune is the most seaward of the dune

system.

Prevailing wind The direction from which winds blow most often.

Saltation The bouncing movement of sand grains propelled by the wind, which loosen other sand

grains on impact so they are transported too.

Secondary dune A more mature dune, inland from the primary dune.

Shoreline recession Landward migration of the beach profile.

Suspension The airborne movement of particles propelled by strong winds. Small suspended soil

particles, such as clay dust, may be carried for hundreds of kilometres.

Swale A trough between ridges in the dunes.

Swash The upwards movement of waves onto the beach from the sea.

There is a long history of informal seawalls being in place at Collaroy-Narrabeen Beach. During historic storms as far back as the 1930’s rocks and other material were placed on the beach to manage the impacts of erosion.

Over time this has created an almost continuous informal seawall from Collaroy to South Narrabeen. The footage in video above was taken in 2016 and shows how much rock there is on the public beach. Peoples ability to get onto and walk along the beach as well as enjoy it safely is impacted when this informally placed rock is exposed following storms. Building properly designed seawalls within private property enables this rock to be removed.

The Coastal Zone Management Plan for Collaroy-Narrabeen Beach and Fishermans Beach, first adopted by Council in 2014 and then amended in 2016, enables residents at Collaroy-Narrabeen Beach to submit development applications for seawalls that protect their properties.

To conduct a thorough analysis of Collaroy’s environmental changes, management practices, and their effectiveness, follow these steps:

### 1. Environmental Changes at Collaroy

Historical Context (Last 100 Years):

- Early 20th Century: Collaroy, located on the northern beaches of Sydney, Australia, has traditionally been known for its scenic coastal environment. In the early 1900s, it was primarily residential and undeveloped with a natural coastal dune system.

- Mid-20th Century: Post-World War II, Collaroy experienced significant development. The boom in population and real estate led to increased coastal infrastructure, such as houses and roads, which began altering the natural landscape. Coastal erosion became a notable issue due to construction and human activities.

- Late 20th Century to Early 21st Century: Increased awareness of environmental issues led to more detailed studies of coastal erosion and habitat loss. Climate change began to have a more noticeable impact, with rising sea levels and more frequent extreme weather events exacerbating erosion and threatening property and infrastructure.

Key Environmental Changes:

- Erosion: Coastal erosion has been a significant issue, accelerated by human activities and climate change.

- Development Pressure: Increased urbanization has led to the modification of natural coastal areas.

- Climate Impact: Sea level rise and increased storm intensity have further stressed the coastal environment.

### 2. Goals and Nature of Management Practices

Historical Management Practices:

- Early Practices: Initially, there were minimal formal management practices. Coastal development and infrastructure were generally constructed without considering long-term environmental impacts.

Modern Management Practices:

- Erosion Control Measures: This includes the installation of seawalls, groynes, and other structural interventions designed to protect the shoreline from erosion.

- Beach Nourishment: Adding sand to the beach to replenish eroded areas and maintain the beach profile.

- Dune Stabilization: Planting vegetation and constructing sand dune fences to reduce erosion and protect against wind and water.

- Integrated Coastal Zone Management (ICZM): A holistic approach considering environmental, social, and economic factors in coastal management.

- Climate Adaptation Strategies: Adapting infrastructure and policies to address the impacts of climate change, such as elevated sea levels and extreme weather events.

Goals of Management:

- Protect Property: Prevent damage to residential and commercial properties.

- Preserve Natural Landscapes: Maintain the aesthetic and ecological value of the coastal environment.

- Enhance Resilience: Increase the coastal zone's resilience to future erosion and climate impacts.

### 3. Assessment of Management Practices

Effectiveness of Management Practices:

- Erosion Control Measures:

- Strengths: Seawalls and groynes can effectively reduce the rate of erosion in specific areas.

- Weaknesses: They can lead to increased erosion downstream and may disrupt natural coastal processes.

- Beach Nourishment:

- Strengths: Can effectively replenish eroded beaches and provide temporary protection.

- Weaknesses: It requires regular maintenance and can be costly. Its effectiveness diminishes over time as natural processes continue to erode the replenished sand.

- Dune Stabilization:

- Strengths: Effective in stabilizing sand dunes and reducing wind and water erosion.

- Weaknesses: Requires ongoing maintenance and monitoring to ensure continued effectiveness.

- Integrated Coastal Zone Management (ICZM):

- Strengths: Provides a comprehensive approach that integrates various aspects of coastal management.

- Weaknesses: Implementation can be complex due to the need for coordination among multiple stakeholders and balancing different interests.

- Climate Adaptation Strategies:

- Strengths: Proactively addresses the challenges posed by climate change, helping to future-proof coastal areas.

- Weaknesses: May require significant investment and long-term planning. Effectiveness depends on the accuracy of climate projections and the adaptability of implemented measures.

### Conclusion

The environment of Collaroy has undergone significant changes due to urban development, increased erosion, and climate change impacts. Management practices have evolved from reactive to proactive, incorporating a range of strategies from structural interventions to integrated management approaches. While each practice has its strengths, effectiveness often depends on the specific context and ongoing adjustments based on environmental changes and new information.

Coastal erosion is related to: wave height, direction, period, duration and set-up; and tidal range and the tidal anomaly generated by storm surge (wind and pressure set-up). Serious erosion, such as occurred at Collaroy during the D-Day storm, was caused by a combination of locally generated storm waves and elevated water levels due to wave setup, “King Tides” and storm surge. Experience dictates that it is possible to have minimal erosion when large waves, from a distant storm, combine with low tidal conditions. However significant erosion can result from even moderate wave attack from a local storm that induces storm surge that also coincides with times of high tidal ranges, particularly if the duration of the storm is more than 2 days and hence encompasses two of the peak high tides. Because there are so many independent or partially dependent factors involved, it is difficult to meaningfully assign return periods to erosion events. It is argued that it is more reasonable to adopt the simpler approach of ranking the outcomes rather than the components that made up the event. It is known that in the last 100 years there have been at least 5 events that have produced similar or worse erosion than the D-Day storm (PWD, 1986) and hence it is not unreasonable to assign it a ranking of 1 in 20 years, that is an ARI of 5%. By nightfall on Sunday 5th significant erosion began to develop at Collaroy, particularly in front of 10 properties between Stuart and Ramsay Street. They had no seawall and had been identified as being within the “Immediate Impact Zone”. Initially the waves were coming from NE, and although refracting, were still at an angle to the beach, giving rise to a strong southerly sweep in the surfzone. The longshore current was estimated to be up to 2 metres/sec. Over the period centered on 2 to 3 hours either side of the predicted high tide of 2.05m (+0.16 storm surge) at 2030hrs on Sunday 5th 2016, major beach erosion occurred along the 100m fronting the Stuart/Ramsay Street sector. Based on aerial photography before and immediately after the event it is estimated that between 12,000 and 14,000 m3 was lost from this region at this time. Erosion continued, albeit at an abated rate, during the 6th as the morning high tide was only 1.54m. However the next high tide of 2.04m (+ 0.15 storm surge) at 2120hrs also corresponded with some local intensification of the storm (the second centre). Again the erosion tended to be maximum around 2 to 3 hours either side of high tide but this time, with the more shore-normal wave direction, and the slumped seawall crests, the overtopping was noticeably greater. Over the next two days, as the ECL tracked south, the focal area of erosion shifted north. Progressively many of the existing rock revetments, along a 1km front, slumped and were overtopped resulting in a back-of-wall erosion escarpment developing which threatened residences that were thought to be protected by revetments.

On the Sunday evening, as the erosion escarpment approached the houses between Stuart and Ramsay Streets, a rapid assessment of coastal engineering, geotechnical and structural factors indicated that the residents had to be evacuated and the Emergency Sub Plan (WorleyParsons, 2012) put into effect. There was little opportunity for residents to take much with them. Police went house to house asking people to leave while at the same time the SES and Council were making arrangements to cordon off the area. One owner refused to leave, so the Police “arrested” him. Once clear of the danger he was 3 cooperative and the Police, recognizing that it was an emotional situation, “un-arrested” him; a very pragmatic action that built credibility. For the next several days there was good cooperation between the residents and the authorities. On the Monday morning of the 6th, with some easing of conditions, the residents were keen to re-enter their homes and recover precious items and changes of clothing. The Police requested the coastal engineers advice and as a result there was a need to consider the condition of each of the houses. An assessment of likely escarpment slumping and the setback required to account for the “Zone of Reduced Foundation Capacity” was compared with the pre-prepared dossier (Patterson Britton, 2007) of the foundations of all beachfront buildings. This assessment was the basis for decision-making regarding re-entry. The lesson being that, if there is an absence of access to specialist coastal engineers, a conservative setback table that can be applied to a specific beach situation and a readily available dossier on the foundations of all beach front properties should form part of any emergency action plan. It soon became apparent that there was potentially a new danger. The smell of gas could be detected in some of the houses. Many lights and electrical appliances were still active as a result of the emergency nighttime evacuations. Inspections were therefore temporarily suspended while all services to the 10 houses were disconnected. Water supply to each house was included in the disconnections as it was felt that if settlement/slumping broke a main it could add to the potential for collapse of the erosion escarpment. In the course of the inspections it was also found that on the seaward side of one of the apartment buildings, a fire hydrant was only half a metre away from the escarpment, so arrangements were made for NSW Fire and Rescue to isolate that particular outlet, which they were able to do while still retaining a viable fire fighting capability. The sewer was another challenge. The main sewer line servicing the houses had been laid in the dunes that once fronted the houses. The erosion had destroyed the sewer lines; they were strewn across the beach, spilling raw sewage into the surf zone. The solution was to locate the remaining serviceable manholes at the upstream and downstream ends and drop sandbags down to temporarily block the pipes. With all services secured it was possible to allow the residents to enter their houses to the limit of what had been assessed to be “safe”, albeit for just enough time to recover whatever precious and basic items they could. Entry to each house was generally restricted to 15 minutes and the residents were accompanied by an engineer and a Police officer. Again there was good cooperation and understanding of the seriousness of the situation and the residents patiently waited their turn. Detailed briefings and a forthright exchange of information were key to this cooperation. An interesting problem arose because, in a couple of cases, the owners of the residences were overseas and had asked friends to recover whatever they could. The Police had the challenge of checking their credentials. This was just as well as one person attempted to enter an apartment block, however the information he provided to the police proved false. Threatened with arrest, he quickly left the scene. On Monday Council acted expeditiously to arrange for the entire site to be secured. This involved the erection of temporary construction fencing from Ramsay to Stuart Street and beyond, to the north, where some properties with were starting to experience collapse of their revetments. The fencing not only made it far easier to control the large crowds but also helped manage media access. With many television, radio and newspaper journalists and cameramen gathered, and the potential dangers of providing access, the Police commander arranged for a briefing of the media and then asked them to nominate one journalist and one cameraman who they all trusted to bring back and share the footage 4 and story. Once elected the Police and the engineers provided controlled access to some key viewing areas. Again there was excellent cooperation by all parties

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