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NSW Coastline Management Manual

New South Wales Government
September 1990

ISBN 0730575063

Appendix C: Coastline Hazards

Appendix C9 - The Hazards of Climate Change

1 INTRODUCTION

In coastal areas, the most discussed potential hazard of the postulated warming known as the Greenhouse Effect, is the scenario of rising in sea level. However, climate change may alter wind and wave climates, both of which may produce a realignment of the coast. The impact of any sea level rise would then be exacerbated by the accompanying foreshore erosion.

As yet, little detailed information is available regarding the likely impact of climate change. The situation is further clouded by the highly interrelated nature of impacts on coastal processes. Nevertheless, broad scenarios have been postulated by the scientific community.

2 SEA LEVEL RISE

Figure C9.1a illustrates sea level scenarios to the year 2100 adopted by the U.S. National Research Council after deliberations by a technical committee (NAS, 1987). These scenarios were adopted after review of information available from the scientific community. The three scenarios adopted are for a sea level rise of 0.5m, 1.0m and 1.5m by the year 2100.

Sea level scenarios (NAS, 1987)

Figure C9.1a Sea Level Scenarios (NAS, 1987)

Figure C9.1b illustrates sea level scenarios to the year 2050 based on box diffusion modelling of ocean warming which was undertaken at the University of East Anglia (Commonwealth Group of Experts, 1989). These projections to the year 2050 range from a sea level rise of 7 to 67 cm (best estimate range 24 to 38 cm).

Sea Level Scenarios (Commonwealth Group of Experts, 1989)

Figure C9.1b Sea Level Scenarios (Commonwealth Group of Experts, 1989)

Figure C9.1c illustrates "Scenario A" of sea level scenarios developed by the United Nations Environmental Program Intergovernmental Panel on Climate Change (UNEP-IPCC, 1990). "Scenario A" is based on no limitation of greenhouse gas production which is considered the most realistic option to choose for planning purposes at this time.

Sea Level Scenarios (UNEP/IPCC, 1990)

Figure C9.1c Sea Level Scenarios (UNEP/IPCC, 1990)

Examination of Figures C9.1a, b and c indicates that the sea level scenarios for say 2050 are very similar. As the IPCC Working Group report is the most recent and it accounts for the views of the international scientific community, it is considered that Figure C9.1c illustrates the currently available "best estimate" of sea level scenarios.

Average Annual Tide Level at Fort Denison

Now what of sea level changes along the New South Wales coastline in the immediate past? Figure C9.2 shows the variation in average annual sea level at Fort Denison in Sydney Harbour over the 101 year period, 1887 to 1987.

Variation in average annual tide level, Fort Dension - 1887 to 1987

Figure 9.2 Variation in Average Annual Tide Level, Fort Denison, 1887 to 1987

Over this period of time, the average annual sea level was 0.914m (Fort Denison datum). Mean sea levels have fluctuated around this figure by 70mm in response to the changing position and gravitational influence of the sun and planets and any other factors that influence long term tidal behaviour.

Three distinct sub-periods are apparent in the tidal record: 1887 to 1921, 1921 to 1949 and 1949 to 1987. During the first of these periods, water levels fluctuated around the long term average value. During the second and third periods, water levels were consistently below and above the long term average value. The reason for the "drop" in water levels between 1921 and 1949 and the "rise" between 1949 and 1987 is not known.

Also shown in Figure C9.2 are trend lines for the change in mean sea level over various periods. The trend over the entire 101 year period is a mean sea level rise of about 0.5mm per year. In the first sub-period, the trend is 0.9mm per year, which is about twice the long term value. The trend in the second sub-period is only 0.05mm per year, which is one tenth the long term value. The trend in the third sub-period is 0.4mm per year and is close to the long term value. If the last two sub-periods are considered together, the trend in sea level rise over the period 1921 to 1987 is 1.4mm per year, or about three times greater than the long term value.

The above results demonstrate a number of difficulties in attempting to confirm climate change in the immediate past. First, only a relatively short period of data is generally available. Second, the data may not be consistent and may contain errors (monitoring techniques are constantly changing). Third, the number of factors that affect climate and the resulting "natural" variation make it difficult to identify the effects of a single factor.

On the basis of the above discussion, it can be suggested that mean sea level off the New South Wales coast has been increasing at an average value of about 0.5mm per year over the last 100 years. Before a more definite conclusion can be drawn, the reason for the distinct change in sea level behaviour over the last two sub-periods would need to be determined.

3 METEOROLOGICAL EFFECTS

Storms

Some scenarios suggest that the severity and frequency of storm events may increase. This would be caused by a southward movement of the cyclone belt, i.e. more of the NSW coast would become susceptible to cyclones, and by the increased storm activity associated with greater temperature gradients in the temperate zone.

Rainfall

Rainfall intensity may similarly increase in some areas, resulting in higher flood levels and increased scour at river, creek and lagoon entrances and at stormwater outlets.

4 WAVE AND CURRENT EFFECTS

Any increased storminess would generate a more severe wave climate, i.e. larger waves would occur more frequently. Moreover, the principal direction of wave attack on the coast could also alter in response to changed patterns of storm generation. Increased coastal water levels would modify the shoaling, refraction and diffraction behaviour of waves as they move towards the shoreline.

The temperature of ocean and shelf currents is an important factor in storm generation and intensity. Where currents move close inshore they can also affect waterborne sediment transport and water quality. Changes to climatic zones and wind patterns could produce changes in ocean and shelf currents, which in turn could affect storm behaviour and sediment transport.

5 SHORELINE POSITION

If the Greenhouse Effect develops as postulated, the shoreline would move landwards because of two effects: inundation due to increased sea levels; and increased shoreline recession due to greater storminess, higher waves, increased sea levels and any changes to the direction of wave attack . The second effect may be of far more significance than the first.

Bruun (1962) proposed a methodology to estimate coastal recession resulting from sea level rise. Gordon (1988) has suggested a tentative link between coastal recession in New South Wales over the past 40 years and increased sea levels over this period.

6 SEDIMENT TRANSPORT

Longshore Drift

Longshore drift is sensitive to long term changes in wave height and wave direction, both of which would be expected to be altered by climate change.

Onshore/Offshore Transport

The amount of sand removed from a beach and dune system depends upon a number of factors, the most important being the intensity and duration of storms. A possible increase in the severity and frequency of intense storm events associated with the postulated greenhouse effect would accelerate beach erosion.

7 FRESHWATER FLOODING

The severity of freshwater flooding may increase in some areas if greater intensities of rainfall eventuate. Areas may become prone to coastal inundation and freshwater flooding. The increased storm surge and wave setup at estuary and river entrances could also increase flood levels in the lower reaches of rivers.

8 ENTRANCE BEHAVIOUR

Changes in coastline position and alignment would affect the behaviour of both natural and trained entrances. Changes in the hydraulics and sediment transport behaviour of entrances could be expected.

9 COASTAL STRUCTURES

Important factors in the design of coastal structures are wave climate and the depth of scour in front of the structure. Both of these factors may be exacerbated by climate change; the stability of coastal structures could also be reduced.

10 SALINITY LEVELS

The tidal and salinity limits of many coastal waterways would move inland in response to any long term rise in sea level. New areas could be exposed to salt and brackish water. Current low lying areas would become saline wetlands; current wetlands would become shallow lagoons. The increases in salinity and inundation would affect terrestrial flora and fauna and aquatic life. In addition, the changes would interfere with present land use practices, e.g. the use of low lying land for farming purposes.

A long term rise in sea level would also be reflected in changes to coastal groundwater tables. Coastal groundwater levels would rise; the underlying saline lens would move further inland; salinity levels of the existing groundwater regime could change markedly. These changes would be of considerable moment to the use of groundwater by plants and humans.

11 COPING WITH CLIMATE CHANGE

Whilst the available evidence suggests that climate change is occurring, there is as yet little reliable information available on which to base planning and design decisions. "Natural" fluctuations in world climate make it difficult to identify and define long term trends. Both the magnitude and in some cases the nature of various impacts are as yet poorly understood. Even where prediction has been attempted, e.g. sea level rise, the decision maker is faced with a very large range of choices.

In the light of the present uncertainty, an adaptive approach towards planning and design in the coastal zone is necessary. This approach should be sufficiently flexible or "robust" to be able to cater for a range of possible outcomes (see Appendix D7 for details).

12 REFERENCES

Bruun, P., (1962). "Sea Level Rise as a Cause of Shore Erosion". Journal of Waterways and Harbours Division, ASCE WW1 Vol. 88 No. 1 February, 1962.

Commonwealth Group of Experts (1989). "Climatic Change - Meeting the Challenge." Commonwealth Secretariat Report by a Commonwealth Group of Experts.

Gordon, A.D., (1988). "A Tentative but Tantalising Link between Sea Level Rise and Coastal Recession in New South Wales, Australia". Contribution to : "Greenhouse, Planning for Climatic Change", ed. Pearman, G.L., p. 772. (CSIRO, Melbourne, 1988).

Pearman, G.I., (1988). "Greenhouse, Planning for Climatic Change". (CSIRO, Melbourne, 1988)

NAS, (1987). "Responding to Changes in Sea Level: Engineering Implications". (National Academy Press, Washington, 1987).

UNEP/IPCC (1990). "Scientific Assessment of Climatic Change." United Nations Intergovernmental Panel on Climatic Change, Working Group 1.