Australian Coastal Councils Conference

Transcription

1 Australian Coastal Councils Conference 11 March 2015 Estimating Future Coastal Inundation and Erosion Hazards Dr Andrew McCowan Dr Christine Lauchlan-Arrowsmith Warwick Bishop

2 Background Victorian Future Coasts Program 3 rd Pass Local Coastal Hazard Assessments Ninety Mile Beach and Gippsland Lakes Port Fairy Bellarine Peninsula Western Port Phillip Island Nature Parks Phillip Island south coast

3 Study Areas

4 Multi-Disciplinary Approach Geology origins Geomorphology physical characteristics Oceanography waves, tides, sea level rise Vegetation effects, land management Catchment Processes inflows, sediment loads Coastal Processes current, future Threat Assessment representative areas Management/Mitigation Strategies Recommendations

5 Sea Level Rise Recent Geological Perspective 120,000 to 80,000 years ago: (interglacial warm period): Sea level 3-5m higher than present 80,000 to 12,000 years ago: (last glacial period) Sea level up to 120m below present; Western Port and Bass Strait mostly dry 12,000 years ago to present: (Holocene) Sea level rose 120m about 8,000 to 6,000 years ago Modern Bass Strait and Bays formed approx 6,000 years ago Sea level 1.0 to 1.5m above present 3,000 years ago

6 Sea Level Rise

7 Sea Level Rise Sea level rise is happening now Current global rate 3.2 +/- 0.4 mm/year - approximately double that of the 20 th Century Local sea level rise 3.5 mm/year (Stony Point) - consistent with global average Projection 0.8m by 2100 consistent with VCS 2014 SLR lowering of beach relative to MSL - effectively results in a loss of beach of same amount - for 100m wide active littoral zone - approx 350 m 3 loss per km of beach/year - 7,000 m 3 per km of beach over 20 years

8 Coastal Geology Phillip Island Substantial thickness of multiple basalt flows Overlies granite at Pyramid Rock and Cape Woolamai Layer of clay and decomposed basalt (>10m thick) - covered and mixed with wind/water-borne sediments Limited accumulations of sand back from the shore Pyramid Rock

9 Coastal Geomorphology Identification and classification of major shoreline landforms and shoreline classes Western Port Wetland fringed coast saltmarsh, mangroves Low earth cliffs Lang Lang/Jam Jerrup Hard rock cliffs and shore platforms Platform beach and bluff western WP, Phillip Island Soft rock high cliffs Jam Jerrup Sandy spits Sandy Point, Observation Point Estuarine and tidal channels Tooradin, Bass River

10 Conceptual Models - Wetland-fringed Coast Conceptual models developed for each shoreline class Coastal wetland shoreline Mid- Holocene emerged sandy shoreline or chenier Melaleuca/Pasture Saltmarsh Mangroves MHWS MSL Seagrass & Algae Surface elevation increase by sediment accretion and below ground productivity

11 Western Port mid-holocene High Sea Levels

12 Oceangraphic Setting Winds - Dune build-up, wind-driven currents, storm surge Waves - Local wind waves (incl. storm waves, typically erosive) - Swell waves (longer period, typically beach building) - Coastal processes, alongshore currents, wave set-up Tides - Tidal water level variations - Tidal currents Freshwater Inflows (particularly Gippsland Lakes) Sea Level Rise

13 Waves, Tides, Currents, Storm Surge

14 Tide and Storm Surge

15 Coastal Vegetation Salt Marsh, Mangroves Particularly sensitive to sea level - Survive in relatively narrow ranges of inundation regime Flat inter-tidal and marsh areas - Large horizontal changes in inundation frequency for relatively small changes in sea level Salt marsh particularly vulnerable

16 Coastal Vegetation Salt Marsh,Mangroves Conceptual models developed for each shoreline class Coastal wetland shoreline

17 Coastal Vegetation Sandy Beaches, Dunes Determined by wind exposure, salt spray, soil composition, geology, presence of sea bird colonies and past land management practices. Dune vegetation affects dune development and morphology Introduced species (Marram, Sea Wheat) more effective in binding sand. Important function when considering storm impacts on beaches and dunes

18 Summerland Beach 1939

19 Dune Beaches Trends: Recession of dune-beach profile with SLR (up to of order 10m by 2040 and 50m by 2100) On-going changes to dune morphology with vegetation may slow the rate of recession Potential for general lowering of the beach Short term responses to storm events will continue. May be enhanced under on-going SLR conditions, Effects of coastal structures impacting on erosion of beach-dune profile are likely to increase with SLR

20 Barrier Dunes: Lakes Entrance Coastal Outer Barrier Hazards Overwash occurred in 1979

21 Barrier Dunes: Bunga Arm Coastal Outer Barrier Hazard Bunga Arm (Unit 7) +0.2 m (2030) Barrier Erosion Constant rate of hazard zone retreat between present and +0.4m SLR +0.4 m (2070) Total Combine d Coastal Hazard Zone = m Barrier Erosion and Isolated Translation Hazard area significantly increases somewhere around +0.8m SLR due to a switch in response mechanism Total Combined Coastal Hazard Zone = m (270m) +0.8 m (2100) Barrier Translation Total Combined Coastal Hazard Zone = 310m Not to Scale

22 Seaspray: 1% AEP Storm Tide Coastal Outer Barrier Hazard

23 Summary/Conclusions Coastal hazards and their sensitivity to SLR vary significantly across the study areas There are significant coastal hazards already existing across most of the study areas these will be exacerbated with on-going SLR In most areas the extent of inundation does not increase significantly with 0.2m and 0.4m SLR - increased frequency, depth and duration of inundation is likely to have a greater effect Most of the of the coastline is expected to be significantly impacted by 0.8m SLR by 2100