Australian Coastal Councils Conference

Australian Coastal Councils Conference Kiama March 2019 Where Has My Beach Gone? (and what can I do about it?) Dr Andrew McCowan Water Technology

Where Has My Beach Gone?

Where Has My Beach Gone? Overview: Geological perspective Sources of sand Physical factors affecting our beaches Coastal processes Causes of erosion Possible remedies

Geological Background Geological temperature changes: 100,000 year Milankowitch Cycles of glaciation and inter-glacial warm periods Temperature variations: -8 to +2 C relative to present

Geological Background 120,000 years ago: Last inter-glacial warm period Sea level comparable to but slightly (3 to 5m) higher than today

Geological Background 25,000 to 12,000 years ago: Peak of last glacial period Sea levels up to 120m below today Mainland Australia connected to Papua-New Guinea and Tasmania

Geological Background 12,000 years ago to present: Holocene, present inter-glacial warm period Sea levels rose to close to present levels 6,000 to 8,000 years ago Sea level approx. 1.0 to 1.5m above present 3,000 years ago

Geological Background Sources of Beach Sand Fluvial supply: weathered rock delivered to the coast by river flows Glacial deposits formed in earlier glacial periods and washed up on the beach by wave action (Gasgoyne River WA, A.D. Short 2012)

Geological Background Sources of Beach Sand Weathering and erosion of sandy cliffs and foreshores Older harder rock areas weather more slowly to form rocky headlands and outcrops

Geological Background Other Sources of Beach Sand Shell washed up from offshore shell beds Sand Composition: Silica sand: most glacial and fluvial sands Calcareous sand: originating from shell (weathers more rapidly) Combinations of silica and calcareous sands

Coastal Processes Factors Wind: Dune build-up: trapping by vegetation Wave generation, Storm surge, Wind-driven currents Waves: Storm waves: generated by local winds shorter and steeper (wave heights function of wind speed, duration and fetch) Swell waves: generated by distant storms longer and lower Currents: Wave-induced currents: generally the main factor Tidal and wind-driven currents: importance depends upon locality (can provide net flow for sand suspended by waves)

Coastal Processes Factors Water Levels: Tides: driven by Earth s rotation and the Moon and the Sun - magnitude and type depends on location - semi-diurnal (2 tides per day), - diurnal (1 tide per day) - semi-diurnal with a diurnal inequality (2 tides per day with 1 tide bigger than the other) Storm surges: typically 0.5 to 1.0m in non-cyclonic areas - can be up to 3.0m or more in tropical cyclones

Coastal Processes Factors Climate Change: More frequent, more intense storms Changes to wind climate and therefore wave climate Sea Level Rise: - 1.8 mm/yr last century - 3.1 mm/yr currently Projections - 0.20m by 2040-0.45m by 2070-0.80m by 2100

Coastal Compartments Coastal Compartments: Compartments along the coast determined by landforms and patterns of sediment transport. Three levels used in Australian mapping Primary: based on large landforms and offshore processes for regional planning and large scale projects Secondary: based on medium landforms and regional sediment processes for local planning Tertiary: based on individual beaches for their management within a broader management plan

Primary Coastal Compartment

Secondary Coastal Compartment

Tertiary Coastal Compartment

Sediment Budget

Coastal Processes Onshore-Offshore Sand Transport: Steep storm waves take sand offshore Milder swell waves build-up the beach Natural cycle of erosion and accretion from Pethick (1984) in Stephenson (2007)

Coastal Processes Onshore-Offshore Sand Transport: Storm surges, wave set-up and wave run-up result in wave attack reaching higher backshore regions and base of cliffs

Coastal Processes Alongshore Sand Transport: Waves breaking at an angle to the coast generate alongshore currents in direction of shore parallel component of waves Breaking waves suspend sand along the coast Suspended sediment is then transported along the coast by waveinduced currents and other wind-driven and tidal currents Rate of transport is function of wave height and wave angle Alongshore and Onshore/Offshore Transport can occur simultaneously

Causes of Erosion Onshore-Offshore Sand Transport: Storm Bite (or sequence of storm bites) exceeds the available beach/dune capacity from Pethick (1984) in Stephenson (2007)

Causes of Erosion Onshore-Offshore Sand Transport: Storm Bite (or sequence of storm bites) exceeds the available beach/dune capacity Can lead to catastrophic erosion (e.g., NSW coastline June 2016)

Causes of Erosion Net Sediment Budget Deficit: Reduced supply of sand into a compartment - Structure updrift of beach (e.g., groyne or breakwater)

Causes of Erosion Net Sediment Budget Deficit: Reduced supply of sand into a compartment - Structure updrift of beach (e.g., groyne or breakwater) - Protection of eroding beaches and cliffs supplying sand

Causes of Erosion Net Sediment Budget Deficit: Increased loss of sand due to changes in wave climate

Climate Change More frequent, more intense storms: More frequent, more destructive storm erosion events Less time for beaches to recover naturally? Changes to wind and wave climate: Changes to stable equilibrium beach alignment Changes to net supply or loss of longshore sediment transport

Climate Change Sea level rise: Storm surges, wave set-up and wave run-up result in wave attack reaching higher backshore regions Effective loss of beach due to increased water levels Sea level is rising NOW!!! (not 2040, 2070 or 2100) Currently 3.1 mm/year

Climate Change Sea level rise: Storm surges, wave set-up and wave run-up result in wave attack reaching higher backshore regions and base of cliffs Effective loss of beach due to increased water levels Sea level is rising NOW!!! (not 2040, 2070 or 2100) Currently 3.1 mm/year For a 100m active nearshore zone, this means: - an effective loss of 300 m 3 of sand per 1km of beach per year - an effective loss of 3,000 m 3 of sand per 1km of beach per decade Many beaches already have a net sediment deficit due to SLR We already have a problem!!!

Shore Protection Options Hard engineering options: Sea walls Groynes Offshore breakwaters and artificial reefs Soft options: Planning and planned retreat Beach nourishment

Shore Protection Options Sea Walls Hard (Old School) engineering option - Last line of defence Protects and prevents on-going erosion of backshore areas Good for protecting valuable infrastructure, but. - reflections result in increased erosion in-front of the wall - causes erosion down-drift - beach will not recover

Shore Protection Options Sea Walls Hard (Old School) engineering option - Last line of defence Protects and prevents on-going erosion of backshore areas Good for protecting valuable infrastructure, but - reflections result in increased erosion in-front of the wall - causes erosion down-drift - beach will not recover

Shore Protection Options Groynes Act as artificial headlands: - trap alongshore sand transport - cause accretion (build-up) of beach up-drift of groyne - effectively slows the alongshore transport of sand by locally realigning the beach Cause erosion of beach down-drift of groyne - tendency to need continuous groyne field

Shore Protection Options Offshore Breakwaters and Artificial Reefs Can be constructed from rock (conventional) or by sand-filled geotextile bags Reduce wave action on local area of beach which reduces local offshore and alongshore transport, allowing sand to build-up Can still allow alongshore transport of sand but has potential for similar problems to groynes Artificial reefs are an emerging technology and still need careful design and planning

Shore Protection Options Offshore Breakwaters and Artificial Reefs Can be constructed from rock (conventional) or by sand-filled geotextile bags Reduces wave action on local area of beach which reduces local offshore and alongshore transport, allowing sand to build-up Can still allow alongshore transport of sand but has potential for similar problems to groynes Artificial reefs are an emerging technology and still need careful design and planning

Shore Protection Options Beach Nourishment Beach is artificially nourished with sand: - needs a cheap source of suitable sand - fine sand is readily eroded - typically aim for medium to coarse sand - available from offshore deposits, dredging, quarries Needs on-going maintenance Effectiveness can be enhanced by use of a groyne to anchor the down-drift end of the beach Can be used to provide a beach in areas where sea walls are used as a last line of defence

Ultimate Nourishment The Sand Motor The Hague, Netherlands 21,000,000 m3 of sand pumped up onto the coast in 2011 - protects extensive length of coast - provides amenity and valuable inter-tidal wetland habitat - is continually evolving 3km