Week 3 - Coastal Hazards

Exam focus (lecturer's advice)

Know key definitions and concepts (e.g. what is a storm surge, what is a tsunami, how do they differ), not memorised examples. You may be given an image of an event and asked to interpret how or why it happened using lecture and practical knowledge. Natural Hazards in Australia is a recommended book.

Coastal hazards

Many different processes that interact within the coastal zone (e.g. storm surge, coastal erosion, sea level rise, tsunami).

Coastal zone

The interface between land and sea: beaches, near shore and offshore areas (shelves), and estuaries. It can extend tens to hundreds of kilometres inland via estuaries and low lying areas during storm surge and tsunami.

Estuary

Where a river meets the sea.

What percentage of the Australian population lives within 50 km of the coast?

87%

What percentage of the global population lives within 100 km of the coast?

40%

What is the global trend in coastal population?

Increasing. More people are moving to the coastal zone worldwide.

Why is it important to understand coastal hazards and coastal risk?

Not only humans but also all the economic assets that go with them are concentrated in the coastal zone, so exposure is high and rising.

Active beach profile

The part of the beach where geomorphic change occurs, from the foredune out to the intertidal and subtidal zone.

Risk equation

Risk = hazard × exposure × vulnerability. Exposure = the elements at risk (the people, buildings and assets in the affected area). Vulnerability = how susceptible they are to damage. (The exam may say "elements at risk"; check your slides that it means exposure.)

Why does where people build matter in the coastal zone?

Building within the active coastal zone increases exposure and vulnerability, so you need to understand coastal processes.

Setback limits

Rules on how close to the current shoreline people are allowed to build. Sea level rise projections (e.g. 2050, 2100) must be considered when setting them.

Hydrodynamic conditions

What the water is doing: storm surges, east coast lows, wave properties, sea level.

Morphological conditions

What the land is doing (the setting): rocky or sandy coast, what engineering structures are present.

Wave crest vs wave trough vs wavelength

Crest = highest part of the wave. Trough = lowest part. Wavelength = distance between two crests.

Significant wave height vs maximum wave height

Significant wave height is the average of the largest 1/3 of waves. It is what is thought to cause the greatest damage (particularly coastal erosion), not the maximum wave height.

Three factors controlling the height of wind waves

Wind speed, wind duration, and fetch.

Fetch

The distance of water that wind can move over in a constant direction to generate waves.

Wave motion in deep water

Particles move in circular orbits as the wave passes (smaller circles with depth). The base of the wave does not interact with the sea floor, so waves are regular.

Shoaling

As waves enter shallower water they start to feel the bottom: orbits become elliptical, wave height and steepness increase, and wavelength decreases.

Why do waves break?

In shallow water the bottom slows the wave, so the crest moves faster than the rest and overturns. Most energy is not reflected, so breaking is sudden and violent. The depth controls wave form and how waves break.

Ocean waves vs tsunami waves (so far)

Ocean waves only interact with the sea floor as they approach shore (shoaling). Tsunami waves interact with the sea floor all the way across the ocean. (Add more after Week 4.)

Why is sea level rise the baseline for all other coastal hazards?

It raises the base level on which all other hazards interact, so inundation and severity increase (e.g. a surge on top of sea level rise is far worse).

Eustatic sea level rise

A GLOBAL change in the volume of ocean water, from melting ice and snow plus thermal expansion.

Thermal expansion

Warmer water expands, increasing ocean volume and contributing to eustatic sea level rise.

Isostatic sea level change

REGIONAL to LOCAL change caused by changes in the land surface: rebound (uplift after an ice sheet melts) or subsidence (e.g. at subduction zones).

Rebound

Gradual uplift of land after the weight of an ice sheet is removed by melting.

Relative sea level

Sea level at a local level, combining eustatic and isostatic effects. Global eustatic change alone does not tell you what is happening locally.

Eustatic vs isostatic (quick check)

Eustatic = global, change in water volume. Isostatic = regional/local, change in land height.

Glacial vs interglacial

Interglacial: warmer temperatures, higher CO2, higher sea level. Glacial: lower temperatures, lower CO2, lower sea level (water locked up in ice).

What controls glacial-interglacial changes in temperature and sea level?

Strongly controlled by CO2 concentrations (CO2, global temperature and sea level are coupled).

Last glacial maximum sea level

Around 120 to 130 metres below present, with much more land available. Sea level has risen gradually since.

Is today's CO2 normal?

No. It is unprecedented in the last 400,000 years (and 800,000 years in other records), due to human CO2 emissions. Modern sea level rise rates are also higher than in the past and increasing.

IPCC Physical Science Basis report (2021)

The most recent IPCC source on sea level in the lecture. It shows relative sea level has risen at a rate higher than the global average in recent decades in many parts of Australia.

Is sea level rise avoidable?

Now unavoidable, but its magnitude can be reduced by cutting greenhouse gas emissions.

SSPs (shared socioeconomic pathways)

Scenarios of how society might develop (continue emissions vs shift to renewables). By 2100 sea level rise is at least 0.3 m above 2000 levels, and up to 2.5 m in the highest emission pathway.

Why is more sea level rise locked into the system?

Greenland and Antarctic ice sheets respond to warming over decades to centuries to millennia, not immediately.

How is sea level measured?

Tide gauges (since the 1800s), satellites (since the 1970s, global, finer resolution, lower error margins), and proxies for the past (corals, mangrove sediments, isotopes from foraminifera in marine sediment cores).

Which places are most vulnerable to sea level rise?

Subsiding or sinking areas (e.g. river deltas like the Mekong), low lying land and estuaries, properties near the high tide mark, and coral reef islands/atolls.

Why are deltas vulnerable to sea level rise?

They are sinking, and sea level rise also causes salination, making good agricultural land unusable.

Salination

Salt entering land and water, making land unsuitable for agriculture.

Melbourne and sea level rise

Melbourne is on an estuary (the Yarra flowing into Port Phillip Bay) with low lying areas, so planners and emergency managers decide where to build.

Why are atolls so vulnerable to sea level rise?

They are only 2 to 3 metres above sea level with no higher ground to move to, so the issue may be abandonment, not just adaptation.

Why are the Solomon Islands a sea level rise hotspot?

Sea level has risen about 3 times the global average (about 7 to 10 mm/yr since 1993), due to global sea level rise PLUS trade winds pushing more water into this part of the Western Pacific.

Solomon Islands island loss study

Aerial photos and satellite imagery (1940s to 2014) show land shrinking in most areas. Of 33 islands studied, 5 were lost completely and 6 more lost a significant proportion of land.

Why is the Torres Strait vulnerable to sea level rise?

Sea level rise is 2 to 3 times the global average (about 6 to 8 mm/yr), the islands are low (e.g. Saibai, Boigu), and it is compounded by storm surges, monsoons and king tides. It floods homes, exposes burial grounds and destroys cultural sites.

What did the Torres Strait Eight establish?

Internationally (UN Human Rights Committee), inadequate climate adaptation can violate human rights (e.g. rights to culture, home and family).

What do the Pabai and Kabai case test?

Whether climate harms can create an enforceable duty of care in Australian law. The Federal Court found no duty of care; the decision is under appeal.

Adaptation limits (IPCC)

For atoll islands and Arctic communities, risk stays high to very high even with adaptation measures (e.g. seawalls, soft nourishment).

Why does the lecture bring in human stories?

A natural disaster is really a human created thing: in some modes of thinking disasters do not exist unless humans are there to experience them.

Storm surge

A rapid rise in sea level caused by a low pressure system (pressure driven) and strong winds (wind driven). About 95% is accounted for by wind. Caused by cyclones and ordinary storms (e.g. east coast lows). Causes inundation and erosion.

Two components of a storm surge

Pressure driven and wind driven. Wind accounts for about 95%.

Effects of a storm surge

Extreme flooding, damage to buildings, erosion of roads, overturned boats, deposition of sand, and destroyed shorelines.

Storm surge vs storm tide

Storm surge = the rise above normal sea level. Storm tide = storm surge + the astronomical tide (the total increase in water above normal levels).

Set up vs run up

Set up (wave setup) = rise in water level at the shore from the surge. Run up = the highest elevation the water reaches. Run up is also used for tsunamis.

Predicted tide vs observed tide vs surge

Predicted tide = expected high and low tides. Observed tide = what was actually recorded. Surge = the difference (height above the predicted tide).

Why does tidal timing matter for a storm surge?

The base sea level determines how far onshore the surge reaches. At high tide the impact is much larger than at low tide. Sea level rise raises the base level in the same way.

King tide

The highest of high tides.

Saffir-Simpson scale

Rates cyclone/hurricane intensity from category 1 to 5 (highest), based purely on wind speed.

Category 4 and 5 hurricanes and global warming

Since 1975 there has been a substantial observable increase in the proportion of category 4 and 5 hurricanes, and it is set to increase (though some areas may see a decrease).

Factors that make a storm surge WORSE

High tide or king tide, sea level rise, wide shallow continental shelf, low lying coast, perpendicular approach to the coast, intense low pressure and strong winds, heavy rainfall and river runoff, estuaries (water pushed tens of km inland).

Factors that reduce storm surge impact

Low tide, steep continental shelf and steep shoreline (energy reflects away), and barrier islands (dissipate energy).

Continental shelf and storm surge

Wide, shallow shelf = surge runs up and inundates much further inland, so more of the dunes and backshore are hit. Steep shelf/shoreline = energy reflects away (less onshore surge, but cliffs may erode).

Angle of approach and storm surge

A storm approaching perpendicular to the coast produces a BIGGER storm surge (concentrates energy).

Barrier islands and storm surge

They buffer the mainland by dissipating surge energy. Settlements built on them lose the buffer and take the full impact (washover, overtopping).

Washover

Storm surge rolling over a barrier island, moving lots of sediment onto it.

Rainfall and estuaries during a storm surge

Extra rain increases river runoff and inundation. A surge can reverse the flow of an estuary, pushing water tens of kilometres inland.

Storm and tsunami deposits

Sediments moved by storm surge or tsunami can be preserved in the geological record, and used to map how far the water flowed.

Compounding and cascading hazards

Hazards interacting so effects add up (e.g. sea level rise on top of a cyclone surge).

Case illustration: Tropical Cyclone Yasi

Costliest cyclone on the Australian coast ($3.5 billion, 7 m storm tide). Severe because of: intense low pressure, La Nina (warmer, expanded Coral Sea), wide shallow shelf (greater run up), low lying coast, perpendicular landfall.

Case illustration: South Australia storms (2025)

Several low pressure events one after another (storm clustering), hitting during a king tide: worst surge in a decade, with dune and road erosion.

Erosion

The transport of material away from a coastal profile.

Winter vs summer beach profile

Winter (more storms): steeper beach and steeper dune scarp. Summer (fewer storms): shallower profile as sand redistributes. This is natural seasonal variability.

What controls coastal erosion severity?

Wave height and period (wave energy), sea level and tide height (base level), lithology and sediment type, beach width and dune height, and dune vegetation.

Sediment size and erosion

Larger sediments are harder to move than smaller sediments.

Lithology and erosion (rocky coasts)

Rock type and structure matter: softer layers erode more easily than harder layers, and intensely bedded rock may erode more easily than amorphous rock.

Beach width and dune height

They have an effect on how much a coast is eroded and the kind of erosion that happens.

Dune vegetation and erosion

Vegetation traps sediment and dissipates wave energy, so vegetated dunes erode less than non-vegetated dunes.

Quiz: which combination most accelerates erosion on a sandy shoreline?

High wave energy, reduced sediment supply, and removal of dune vegetation.

Do higher waves always mean more extreme erosion?

No. Example: the 2016 Narrabeen east coast low, where wave DIRECTION was critical.

Case illustration: 2016 Narrabeen east coast low

Erosion was extreme because of: high tide, low pressure system, and wave direction about 45 degrees further anti-clockwise than usual (from east north east instead of the typical south-east). This generated an extreme rip current that ripped out sediment. It argues for including direction of approach in erosion and damage estimates.

Storm clustering

A series of storms within weeks to months, so the beach cannot recover to equilibrium, giving progressively more erosion (e.g. Gold Coast 1960s: 3 cyclones and 3 east coast lows in one year).

Five coastal erosion mitigation options

1. Do nothing. 2. Planned retreat. 3. Soft nourishment. 4. Hard engineering. 5. Abandonment.

Do nothing (erosion)

Let nature run its course. Suitable where there is little habitation; problematic in populated areas.

Planned retreat

Gradually rezoning and moving habitation, industry and structures away from the eroding area. Not palatable, but may be the most economically viable option. Considered when erosion and sea level thresholds modelled for 2050 are being reached now.

Not everything can be protected

Some structures (e.g. a heritage obelisk at Robe) are too expensive to protect, so councils may document and replicate them rather than protect them.

Soft nourishment

Working with natural processes: moving sand, sandbags, vegetation and dune stabilisation.

Beach nourishment (pro and con)

Pumping or trucking sand onto a beach. Pro: natural look, good for tourist beaches. Con: needs constant maintenance because storms keep removing sand.

Dune revegetation

Planting local, salt resistant foredune vegetation that traps sediment and stabilises dunes. It brings the community into mitigation (e.g. landcare groups).

Living shorelines

Mangroves, salt marsh, seagrass and oyster (shellfish) reefs that buffer wave and water energy, reduce inundation and erosion and trap sediment.

Mangroves, salt marsh and seagrass (limits)

Need lower energy environments (e.g. bays), so cannot be used on high energy coasts.

Oyster reefs / shellfish reefs

Used on higher energy coasts. Built from structures, crushed rock or gravel on which shellfish grow, buffering energy before it reaches the shoreline.

Benefits of living shorelines

Carbon storage (sequestration), habitat for plants and animals, and recreation and aesthetics (walking tracks, bird watching).

Hard engineering

Built structures: seawalls, rock armour, gabions, groynes (breakwaters), rock walls. Many shift the problem elsewhere.

Seawall (pros and cons)

Pro: can be built as high as needed to counter sea level rise; can make a promenade. Con: the beach is washed away (coastal squeeze); old walls erode at the base and can collapse; people can be trapped at high tide so escape stairs are needed; can be ugly.

Coastal squeeze

The beach is washed away in front of a seawall as wave energy hits it.

Rock armour

Rocks dumped at the base of a dune, cliff or seawall that absorb wave energy and protect what is behind. Relatively cheap and easy, local rock can be used, and rocks can be habitat.

Gabions (pros and cons)

Mesh cages filled with rocks. Pro: absorb wave energy, permeable, low cost. Con: cages rust and break, becoming hazards.

Groyne (pros and cons)

Structure that traps sand moved by longshore drift. Pro: maintains a beach. Con: beaches further along are starved of sand, so the problem is moved, not solved.

Longshore drift

Natural movement of sand and sediment along the coast.