Coastal processes (waves, tides, currents, sediment transport) as biophysical interactions producing landforms and hazards

What this dot point is asking

NESA covers coastal processes as a canonical example of biophysical interaction. Waves (atmosphere transmitting energy into the hydrosphere), tides (gravitational), currents and sediment transport (hydrosphere-lithosphere coupling), and the biosphere (dune vegetation, seagrass, mangroves) interact to produce and reshape coasts. Section II stimulus often shows a coastal photograph or topographic map and asks you to identify and explain the processes.

The biophysical processes

Waves

Waves form when wind transfers energy to the sea surface. Wave height depends on wind speed, duration, and fetch. The NSW coast is exposed to long-period swell from the Southern Ocean and to East Coast Low storms. Sydney's offshore wave buoy records a mean significant wave height of 1.6 m and storm peaks above 8 m.

When waves enter shallow water, they slow, steepen, and break. The energy released drives sediment transport, erosion, and longshore drift.

Tides

Gravitational pull of the moon and sun drives the tidal cycle. The NSW coast is semi-diurnal (two highs and two lows daily). Spring tide range is around 2 m; neap range around 1.2 m. Tidal range determines the height to which waves can reach the foreshore.

Currents

Longshore currents move parallel to the coast where waves arrive at an angle. The dominant NSW longshore drift is north-flowing, transporting around 100,000 m3 of sand per year along the Sydney coast. Rip currents return water seaward through gaps in the surf zone, contributing 80-90 percent of beach drownings.

Sediment transport

Coasts are sediment systems with sources, transport pathways, sinks, and budgets. The NSW open coast is a "compartmentalised" coast: each beach cell receives sediment from headland erosion, river input (or historical input now reduced by dams), and offshore deposits. Sediment moves alongshore until it reaches the next headland.

Modern human-induced changes to sediment budgets include reduced river sediment supply (dam construction), beach sand mining (now banned), and structures that trap sand on one side (groynes, breakwaters at port entrances).

Sea level

Sea level is the baseline against which all coastal processes operate. Sea level along the NSW coast has risen around 12 cm since 1914 (Fort Denison gauge, Sydney) and is accelerating. Even small sea-level rise dramatically increases the frequency of overtopping events because storms now start from a higher baseline.

Coastal landforms

The interaction of these processes produces a recognisable suite of landforms:

Erosional

Headlands, cliffs, wave-cut platforms, sea caves, sea arches, sea stacks (the Twelve Apostles), notches.

Depositional

Beaches, spits, tombolos, bars, lagoons, dunes, deltas.

Sediment systems

Estuaries (Sydney Harbour, Port Macquarie), tidal flats, mangroves, salt marshes.

Australian beaches alone constitute around 10,800 individual beaches across 36,000 km of coastline (Geoscience Australia).

Coastal erosion case studies

Collaroy-Narrabeen, NSW

A 14 km open-ocean embayment on Sydney's northern beaches. Houses were built on a low foredune across the 1920s-1960s. Major storms in 1944, 1967, and 1974 had already eroded the dune. The 5-9 June 2016 East Coast Low brought 8 m waves at spring high tide, eroded 50 m of dune in 36 hours, damaged 11 homes, and undermined a private swimming pool. Northern Beaches Council and NSW Government built a $25 million staged seawall (completed 2023) along the most vulnerable section. The seawall has protected property but transferred wave reflection energy to neighbouring beaches, accelerating erosion further north.

Old Bar, NSW (mid-north coast)

A small coastal township 17 km east of Taree. Beach has receded around 80 m since 1980 at an average rate of 1.6 m per year, with peak loss of 7 m in single storms. Around 30 houses are within the active erosion zone. MidCoast Council adopted a managed retreat policy in 2018, including buyback options and demolition orders for properties classified as imminent risk.

Stockton Beach, NSW (Newcastle)

A 32 km sand beach immediately north of Newcastle Harbour. The 1976 Newcastle Harbour breakwater extension cut the natural northward sand supply. Stockton has been sand-starved for nearly 50 years. The 2020 South Stockton seawall and an emergency sand renourishment program (180,000 m3 dredged from offshore) are stabilising the worst-affected section, but the underlying sediment budget remains negative.

Management approaches

Hard engineering

Seawalls, groynes, breakwaters, gabions. Effective at protecting specific assets, expensive, and often shift the problem along the coast. Costs run at $5-30 million per kilometre.

Soft engineering

Beach nourishment (Stockton, Gold Coast), dune restoration (Wallabi Point), revegetation. Restores sediment budget, lower carbon footprint, but requires repeat investment.

Planned retreat

Acquisition of high-risk properties, restrictive land-use planning, building setbacks. Politically difficult but cheaper in the long run.

Statutory framework

NSW Coastal Management Act (2016) requires councils to prepare Coastal Management Programs (CMPs) addressing erosion, recession, climate-change vulnerability, and Aboriginal values. Updated CMPs are required across the next 5-10 years for all coastal LGAs.