Coastal Processes

Coastal Management in NSW 

The coastline of New South Wales is a major element in the geographic, recreational, commercial and ecological fabric of the State. It is an area of unparalleled beauty, characterised by small pocket beaches bounded by rocky headlands in the south, and sweeping beaches of golden sand in the north. Coastal areas are major destinations for local, national and international tourists. More than 80% of the State's population lives and works along the eastern seaboard. 

The coastline is under constant attack from the natural forces of wind and waves. In response to these processes, the coastline is ever changing: beaches and sand dunes erode and are rebuilt in response to wave action; sand dunes can migrate inland in response to wind attack; many sections of the coastline are receding and moving inland at significant rates. Man's activities in the coastal zone can exacerbate these processes. 

The coastline can be a hazardous area to develop. Coastal properties may be at risk from beach erosion, shoreline recession, coastal inundation, sand drift and other hazards. 

There are increasing pressures to develop coastal areas for tourism, residential, commercial, and recreational purposes. Not only is the pressure for coastal developments continuing, but the nature of such developments is changing. Modest weekend cottages are giving way to substantial brick homes and there is a growing trend towards the development of major coastal resorts that provide total holiday packages. 

Coastline hazards are not a minor problem. Beach erosion accompanying storms of the early 1970's caused the loss of 20 houses and other assets valued at many millions of dollars along the central and north coasts. In addition, coastal communities suffered significant social and commercial disruption. 

Studies in 1985 indicated that in New South Wales property worth $500 million could be exposed to coastline hazards during the next 30 to 50 years. The potential damage value, already greatly in excess of the 1985 figure because of property value increases, may escalate rapidly if appropriate planning and development control measures are not instituted. Moreover, the detrimental impacts of coastline hazards could be intensified if the global climate changes postulated to accompany the greenhouse effect eventuate. 

New South Wales can ill afford the social and financial disruption associated with placing this level of development at unnecessary risk. 

Natural Processes affecting Our Coasts 

Beach Erosion - The large waves, elevated water levels and strong winds generated by a storm can cause severe erosion to sandy beaches. Storm wave attack can move significant quantities of sand offshore (up to 250 cubic metres per metre run of beach, as measured above MSL). Storm waves undercut the beach berm and frontal dune to form a pronounced erosion escarpment. The foredune may be cut back by up to 20m during a major storm event. 

Erosion is part of the natural response of a beach to changing wave and water level conditions. Generally, the eroded sand is returned to shore and the beach is rebuilt during calmer periods of swell waves. 

Buildings and facilities located within the "active" beach system, or area subject to erosion, will be undermined, and if not designed for this hazard, may collapse. 

Shoreline recession - is the progressive landward shift of the average long term position of the coastline. Recession is a different phenomena to beach erosion, although they both may be caused by the same processes. The two causes of shoreline recession are sediment loss and an increase in sea levels. 

Coastal Entrance Instability - Both natural and trained coastal entrances can create a variety of hazards. Natural entrances tend to migrate along the beach in response to freshwater flooding and coastal storm effects, so threatening any adjacent developments and the amenity of affected beaches. Training works will stabilise the location of an entrance, but may engender significant changes to the estuary and nearby beaches. Some training works in New South Wales have had detrimental effects of an unforeseen nature. 

Sand Drift - Sand drift is caused by windborne sediment transport and is a seemingly inexorable coastal phenomenon: all sandy beaches experience sand drift to a greater or lesser extent. Moreover, it is a seemingly slow moving and gradual process, but short episodes of strong wind can move surprisingly large volumes of sand (transport is proportional to the cube of the wind's velocity). 

Sand drift is a serious problem along the NSW coastline. A recent study has shown that 5,600 ha, or 11% of the total area of coastal dunes, is completely bare of vegetation and is undergoing active drift (Chapman, et al., 1987). A further 4% of the total dune area is in a state of incipient destabilisation leading to unimpeded drifting. 

Sand drift creates a variety of hazards. At best drifting sand is a nuisance; at worst it represents a permanent loss of sand from the beach system and may completely overwhelm coastal developments. Detrimental effects include the abrasion of motor vehicles, buildings, vegetation and park and garden fittings; the burial of roadways, rail lines, agricultural land and coastal ecosystems; the blockage of street gutters and stormwater drains; and structural damage to buildings caused by forces imposed by the sand. 

Coastal innundation - Coastal inundation is the flooding of coastal lands by ocean waters. Unlike Holland, NSW has no significant areas below normal high tide level that are protected by artificial barriers. However, along the coast there are many low lying areas above normal high tide level that are subject to coastal inundation. These include wetland and other fringe areas of coastal lagoons and rivers, and the areas behind beach and dune systems. The inundation of these areas can be caused by large waves and elevated water levels associated with severe storms. 

Severe coastal inundation is an infrequent event and is normally of short duration (peak flooding usually persists for several hours). Nevertheless, it can cause significant damage to public and private property: crops and livestock can be lost; the contents of flooded buildings can be damaged; problems can be caused by the breakdown of transport and communications. In addition, coastal inundation can be a threat to the life and limb of inhabitants of flood prone areas. 

Slope and Cliff Instability - Slope and cliff instability hazards refer to the possible structural incompetence of these features and associated potential problems with the foundations of buildings, seawalls and other coastal works. Coastal bluffs and the erosion escarpments of sand dunes can slump, sea cliffs can collapse, and foundations can fail, so imperilling coastal developments and structures. 

Slope and cliff instability is a different phenomenon from coastal erosion and recession. Whilst beach erosion and scour can cause stability problems, the collapse of a foreshore slope or the failure of a foundation depends upon the properties of the associated soil and rock constituents. The disciplines of soil and rock mechanics are essential to a stability analysis of coastal slopes, cliffs and foundations built thereon. 

Stormwater erosion - Many backbeach areas are drained by semi-perennial small creeks that cross the beach berm to discharge stormwater into coastal waters. During major runoff events, such creeks can cause significant erosion to the beach berm and the nearshore area around their entrance. This in turn allows larger waves to attack the creek entrance, which if unstable, may migrate along the beach. 

Stormwater disposal hazard is a local problem. Similarly, the entrance behaviour of these minor creeks is essentially a local phenomenon. Although subject to the same coastal processes, the behaviour of these local creeks should not be confused with the behaviour of major coastal entrances.

Management Options 

Options for managing coastline hazards may be conveniently grouped into four categories: 

• Environmental Planning - The object of land use controls is to ensure that the type of development and potential damage is consistent with the hazard. 
• Development Control Conditions - Such conditions are imposed through development and building approvals and are a means of reducing hazard on a site specific basis. Development conditions are especially relevant to limiting the growth in damage associated with new developments and redevelopments. 
• Dune Management - Dune management activities are designed to maintain the integrity of the dune system as nature's last line of defence against wind and wave attack. Dune stability is based primarily on maintaining the dunes' protective mantle of fragile vegetation. 
• Construction of Protective Works - The aim of such works is to eliminate or to reduce hazards to an acceptable level. 

Hazard management generally draws on a mix of all types of options, which are incorporated into a coastline management plan. 

In general, protective works tend to be expensive. However, they often provide the only socially and economically acceptable means of reducing hazards to existing properties at risk. Unless carefully designed and constructed, structural works, by reason of their location within the active beach zone, may have a number of unforeseen detrimental effects on amenity. 

Manly Ocean Beach Seawall

A properly designed and constructed seawall will protect properties and areas of the foreshore from the impacts of beach erosion and coastline recession hazards. However, the recreational use and scenic appeal of the beach may be reduced by seawalls, especially if their presence facilitates the loss of sand in front of the wall and/or delays beach rebuilding after storms. 

Beach Nourishment

Beach nourishment provides coastal protection and increases beach amenity by building a wider beach. However, nourishment does not promote erosion in downdrift locations of the beach. In fact, beach nourishment programs have few if any detrimental effects (this is part of their attraction) provided that an adequate supply of suitable sand is available and that it can be obtained without undue consequences. One potential drawback of beach nourishment is that further nourishments may be needed in the future.