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Coastal Erosion & Sea Level Rise

Coastal Erosion & Sea Level Rise

Advice on legislation or legal policy issues contained in this paper is provided for use in parliamentary debate and for related parliamentary purposes. This paper is not professional legal opinion.
Briefing Paper 6/2010 by Stewart Smith
This paper commences with a discussion of the sea level rise policy of the NSW Government. The link between sea level rise and coastal erosion is discussed, and the first half of the paper concludes with a review of amendments to coastal protection legislation introduced in May 2010 by the NSW Government. The second half of the paper reviews the science behind measuring and projecting global sea levels. It is readily apparent that scientists are grappling with an extraordinarily difficult science, with many unknowns and sparse and often conflicting data.

Global mean sea level is not fixed. It changes in response to such things as ocean temperature and the amount of water trapped as ice on land. The Intergovernmental Panel on Climate Change (IPCC) provides reports on the current state of knowledge of climate change. The most recent report – Assessment Report 4 (IPCC AR4) in 2007 projected sea level rise to be between 18 cm and 59 cm higher in the decade 2090-2099 than compared to 1980–1990. Some scientists are now arguing that this is a conservative estimate, and that a rise of up to 110 cm is not out of the question. Consequences of sea level rise include inundation of land and the erosion of coastlines. [1]

The NSW Sea Level Policy
In October 2009 the NSW Government released a Sea Level Rise Policy. Whilst there are several elements to this policy, the central tenet is that of promoting adaptive risk-based management. The Policy states that planning and investment decisions should consider the sea level rise projections over timeframes that are consistent with the intended timeframes of the decision.

The NSW Government has adopted sea level rise planning benchmarks to support this adaptive risk-based approach. The primary purpose of the benchmarks is to provide guidance supporting consistent considerations of sea level rise impacts, within applicable decision-making frameworks

The NSW sea level rise planning benchmarks are an increase above 1990 mean sea levels of 40 cm by 2050 and 90 cm by 2100. The policy states that the benchmarks were established by considering the most credible national and international projections of sea level rise and by taking into consideration the uncertainty associated with sea level rise projections.

The sea level rise planning benchmarks can be used for purposes such as:
    Incorporating the projected impacts of sea level rise on predicted flood risks and coastal hazards;
    Designing and upgrading of public and private assets in low-lying coastal areas where appropriate, taking into account the design life of the asset and the projected sea level rise over this period;
    Assessing the influence of sea level rise on new development;
    Considering the impact of sea level rise on coastal and estuarine habitats (such as salt marshes) and identifying valuable habitats at most risk from sea level rise; and
    Assessing the impact of changed salinity levels in estuaries, including implications for access to fresh water. [2]

Sea Level Rise and Coastal Erosion
It is considered well established that sea level rise generally leads to erosion and causes the shoreline to retreat landwards. Australia’s coastline has been remarkably stable over the past 200 years or so, given that sea levels have risen by 17 centimetres over that time. Generally in Australia, beaches appear not to be receding on a large scale, except in some localised places where natural recession is occurring.

Generally, the most common experience of acute coastal erosion in Australia has been associated with transient erosion due to storm events involving large waves and abnormally high water levels, especially when storm surges coincide with spring tides.

Coastal scientists have determined that for the most part, property and infrastructure losses associated with extreme storms reflect the inappropriate construction of property within dynamic coastal environments that were mistakenly thought to be stable. Almost all the chronic coastal erosion hotspots around Australia lie in regions where sand is transported along the shore by natural processes. Erosion occurs when this sand transport is interrupted, and in most cases this disturbance is caused by coastal engineering works.

Scientists have not been able to detect an Australian coastal erosion response to sea level rise to date. One of the main reasons for this is the wide availability of sand in the coastal shore face – that area seaward of the foreshore and to a distance beyond where the waves break. Evidence for sand supply to beaches from the lower shore face is common around the Australian coast. For example, off the NSW coast sand is estimated to accumulate onto beaches from the shore face at the rate of 6m3 per metre per year. Off the Victorian coast the estimate is 10m3 per metre per year. Results from geological investigations in south-east Australia and sediment transport modelling from the Netherlands coast indicates that sand supply from the lower shore face fully compensates for the effects of significant relative sea level rise.

Coastal erosion, especially that of private property, is the cause of significant community concern. Understandably, owners of waterfront properties are generally reluctant to stand back and watch their land, and any development on it, be eroded away. For their part, local and state government have difficult decisions to make concerning the extent, if any, to which they should permit or prevent land owners from protecting their property. [3]

The Coastal Protection and Other Legislation Amendment Bill 2010
With the introduction of the Coastal Protection and Other Legislation Amendment Bill 2010, the NSW Government stated that its approach is to provide the tools needed to achieve an appropriate balance between the two competing goals of protecting private property and preventing the transference of erosion onto other sites.

To address coastal erosion and projected sea level rise, the Bill makes amendments to the Coastal Protection Act 1979 and other legislation. The Bill has two main components:
    It allows landowners to place emergency coastal protection works such as sandbags on beaches and sand dunes to mitigate erosion in specified circumstances without obtaining development consent or other specified permission;
    It allows local councils to make and levy an annual charge for the provision of coastal protection services (such as the building of a rock wall) on rateable land that benefits from such services. [4]

The Science of Sea Level Rise
Over decadal and longer time scales, global mean sea level change results from two major processes that alter the volume of water in the global ocean: thermal expansion; and the exchange of water between oceans and other reservoirs of water such as the ice caps, glaciers and ice sheets. The measurement of global sea level is reliant on two techniques: tide gauges and satellite altimetry.

On the available evidence, the IPCC AR4 concluded that global sea level had risen during the 20th century at 1.7 ±0.5mm per year, and from 1961 to 2003 at 1.8 ±0.5mm per year. Since 1992, global mean sea level can be computed by satellites. Using this method, the IPCC AR4 reported that sea level over the period 1993 to 2003 showed a rise of 3.1 ±0.7mm per year.

Global sea level rise is subject to considerable decadal variability. The rate of sea level change was found to be larger in the early part of last century (2.03 ±0.35mm per year 1904-1953) in comparison to the latter part (1.45 ±0.34mm per year 1954-2003). The highest decadal rate of rise occurred in the decade centred on 1980 (5.31 mm per year), whilst the lowest rate of rise occurred in the decade centred on 1964 (-1.49 mm per year). Whilst satellite measurements showed global mean sea level rising at a rate of 3.1 ±0.7mm in the decade to 2003, for the period 2003 to 2008 it has since slowed to about 2.5mm per year.

Much of the uncertainty about the rate of sea level rise in the future centres on the behaviour of the large polar ice sheets. In recent years, the velocities of outlet glaciers in coastal regions of Greenland and Antarctica have accelerated, showing that a large fraction of ice-mass loss occurs through dynamic processes rather than surface melting. The dynamic response of the ice sheets to present-day climate may thus play a much larger role than previously assumed. [5.2] – [5.4]

Projections of Future Sea Level
Climate models are used to project sea levels into the future. The IPCC has projected sea level to rise 18 cm by 2090-2099 at the lower end of the scale, and at the upper end 59 cm. On a per year basis, the rate of sea level rise by 2090-2099 is calculated to be 1.5mm at the lower end, and 9.7mm per year at the other extreme. In all scenarios, the IPCC stated that average rate of rise during the 21st century is very likely to exceed the 1961 to 2003 average rate of 1.8 ±0.5mm per year.

The IPCC noted that thermal expansion of the ocean is the largest component of the projected sea level rise, contributing 70 to 75% of the central estimate in these projections for all scenarios. The dynamical response of ice sheets was not factored into the IPCC projections due to the fact that they cannot be modelled quantitatively with confidence.

Since the publication of the IPCC AR4 report in 2007, scientists have projected sea level rise from 75 to 190cm, approximately three times as much as the IPCC AR4 projections. There is still considerable uncertainty surrounding estimates of future sea level rise. Steffen observes that nearly all of these uncertainties operate in one direction – towards higher rather than lower estimates.

Sea level rise will impact on extreme sea level events. With a mid-range sea-level rise of 0.5 metres in the 21st century, sea level events that now happen every 10 years would happen about every 10 days in 2100. [6]

Susceptibility of the Australian Coast to Sea Level Rise
Australia wide, a study found that a sea level rise of 1.1 metres combined with a 1 in 100 year storm surge would result in the flooding of between 157,000 and 247,600 properties. Nearly 39,000 buildings are located within 110 metres of ‘soft’ shorelines and at risk from accelerated erosion due to sea-level rise and changing climate conditions. For NSW the analysis suggested that between 40,800 and 62,400 residential buildings may be at risk of inundation.

Local government areas in NSW that have the greatest level of risk are Lake Macquarie, Wyong, Gosford, Wollongong, Shoalhaven and Rockdale, which collectively represent over 50 per cent of residential buildings at risk in New South Wales. The inundation analysis also indicated that the local government areas of Great Lakes, Rockdale and Shellharbour have a high proportion of existing residences at risk within their boundaries, with a substantial 18–20 per cent of existing buildings potentially affected by 2100.

Erosion due to higher sea levels is also a key risk for coastal areas. In New South Wales there are approximately 3,600 residential buildings located within 110 metres of ‘soft’ erodible shorelines, of which approximately 700 are located within 55 metres of ‘soft’ coast. Of the coastal local government areas, Sutherland and Port Stephens have the highest number of potentially affected properties. [7]