Coastal hazards are caused by dynamic processes such as waves, tides and currents and rock weathering along the coast.
Coastal hazards include:
- Beach storm erosion (immediate erosion hazard)
- Coastal recession (long term erosion hazard)
- Coastal inundation - flooding caused by storm waves and storm surge
- Dune instability and sand movement - when sand blows inland across private property and parkland
- Geotechnical hazards (for example, rock fall and landslides).
Beach storm erosion
Over the last century some Shoalhaven beaches have been severely affected by storm wave erosion. Erosion scars from 1974 and 1978 storms can still be seen at some beaches, even though a lot of sand has moved back onto the beach and new dunes have since formed.
Click here for details about beach and dune stabilisation at Collingwood Beach in the 1970s and 1980s. Other images below show Callala Beach and Shoalhaven Heads.
To identify areas affected by coastal erosion, coastal engineers analyse historical aerial photographs, ground level photographs, records of wave height and direction, wind speed, bathymetric information and data about sediment types.
The sand cut from the beach and dunes in a storm is known as the storm bite.
The erosion impact on coastal dunes is more than the sand that is washed away. The dune face may slump and this is known as the zone of slope adjustment. Landward of the slumping area is a zone of reduced foundation capacity where building stability is compromised.
For an example of detailed analysis of the effects of severe storm conditions on coastal processes and coastal landforms, click on this link.
Over time, the alignment of the coast may move landward. This happens when:
- Sand is being permanently lost from the beach. For example, this can happen because sand is lost into deep water offshore, is moved far along the coast (long shore drift), blows inland or is trapped inside the mouth of an estuary.
- Sand moves along the beach for decades or more. Over decades, sand may shift from one end of a long beach to the other and back again. This is known as beach rotation. Long term studies of beaches over periods of decades show from time to time there are shifts in wave direction and height driven by weather patterns
- Climate change changes coastal processes. Climate change causes sea level rise and can also change wave height and wave direction, rainfall, runoff and storminess. All these have a big impact on the shape and stability of beaches. Council has recently updated its coastal hazard assessment to take into account the latest information on sea level rise and climate change.
To view the recently updated coastal erosion hazard assessment for the Shoalhaven coastline, which discusses immediate and long term erosion hazards, click here. (File size: 2 MB)
Coastal inundation (flooding)
Storm waves and rises in water caused mainly by high winds pushing on the ocean's surface (storm surge) can go over the top of coastal dunes. This causes flooding of low lying land behind the dunes. When the entrance of a coastal lake stays closed for long periods, the lake water level may rise to the point where low-lying property or roads are flooded.
Dune instability and sand movement
Periods of storminess and strong winds, combined with loss of vegetation on coastal dunes, mean sand from the beach and dunes can be blown into residential areas behind the dunes. Wind erosion was once common from areas affected by sand mining and can still happen when coastal dunes are disturbed by heavy pedestrian and vehicle traffic. Wind-blown sand is a nuisance for residents, and also means there is less sand on the beach.
Geotechnical hazards - rock falls and landslides
The geology of some coastal cliffs and bluffs means the layers resistant to weathering can be undermined causing rock falls. Weathering of rock layers can also lead to slumping and landslides, particularly in wet weather.
Geological and geomorphic studies, studies of groundwater flows and studies of processes such as wave erosion at the base of the cliff are used to investigate these hazards.
To view the complete geotechnical assessment for cliffs and bluffs along the Shoalhaven coastline, click on the links below.
Scoping Study and Stability Assessment (3.1MB)
- Report Part 1. (2.4 MB)
- Report Part 2. (4.8 MB). This part includes geotechnical hazard maps for headlands at Racecourse Beach, Rennies Beach, Collers Beach, Bannisters Point and Inyadda Point.
- Report Part 3. (4.0 MB). This part includes geotechnical hazard maps for headlands at Berrara Bluff, Hyams Point, Plantation Point and Penguin Head/Culburra Beach.
- Report Part 4. (0.8 MB).
for Surfers Avenue, Tallwood Avenue & Bannister Head Road, Narrawallee.
Which hazards are where along the Shoalhaven coast?