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NSW Coastline Management Manual

New South Wales Government
September 1990

ISBN 0730575063

Appendix B: Coastline Processes

Appendix B2 - Background Information

1 INTRODUCTION

The present coastline of NSW and its features have been shaped by geological processes over the ages. This appendix briefly describes the four major geological units of the coastline, the relief of coastal and continental shelf areas, and the resulting three representative coastline types of the New South Wales coast. The variation of coastal water levels over the last 250,000 years is also presented. For much of this time, the "beaches" of New South Wales were located around the edge of the continental shelf, some 20 to 60 km seawards of their present position. Finally, the coastline terminology adopted in this report is presented.

The overview presented in this section is largely a summary of information presented in Chapman (1982) prepared for the Coastal Council of NSW.

Fig B2.1: main geological units of the NSW seaboard

Figure B2.1 Main Geological Units of the New South Wales Seaboard (after Chapman et. al., 1982)

2 GEOLOGICAL FRAMEWORK

The coastline and continental shelf of NSW were formed during the last 500 million years by three major geological events. First, the underlying rocks of the coastal area were laid down over the period 500 to 350 million years ago. Next, two major sedimentary basins were formed along the central and northern regions of the coast during the period 250 to 120 million years ago. Finally, tectonic events between 120 to 60 million years ago gave rise to the major features of the existing coastal and continental configuration (see Chapman et al., 1982 for further details). Over the last 60 million years, the coastal and continental features have weathered in response to climate and sea level changes.

Figure B2.1 shows the four principal geologic units of the New South Wales coastline. These comprise two fold belt structures and two sedimentary basins:

3 COASTAL FEATURES

Figure B2.1 also indicates how coastal relief, river size and embayment type are related to these four major geologic features. Coastal relief in the fold belt areas is high to moderate; relief in the basin areas is low to moderate. Rivers of the fold belt regions vary in size from very small to medium; rivers of the basin areas vary in size from small to very large. In the south (Lachlan Fold Belt) coastal embayments are generally small and narrow, and consist of "pocket beaches" flanked by rocky headlands that project seawards into deep water. In the north (Clarence-Moreton Basin) coastal embayment are generally large and broad with less prominent headlands. Along the central region of the coast (Sydney Basin and New England Fold Belt) embayments are variable in size and features. Figure B2.2 shows typical north and south coast embayments.

Fig B2.2 coastal embayments, North and South coasts, NSW Fig B2.2 coastal embayments, North and South coasts, NSW

Figure B2.2 Coastal Embayments, North and South Coasts, New South Wales.

4 COASTLINE TYPES

There are a variety of ways in which the coastline can be classified, the appropriate one depending to a large extent on the purpose for which the classification is being made. In this manual a classification has been adopted which allows convenient discussion of the relative importance, and means of management of the various coastline hazards identified. This classification divides the coast into three types:

Fig B2.3 The three coastline types of NSW Fig B2.3 The three coastline types of NSW Fig B2.3 The three coastline types of NSW

Figure B2.3 shows photographs of each type.

Figure B2.3 The Three Coastline Types of New South Wales: Sandy Beaches, Coastal Bluffs and Sea Cliffs.

Sandy beaches are typified by the rapid fluctuation, (erosion and accretion), of the beach in response to changing wave conditions which may be overlaid by longer term recession or progradation. Coastal bluffs are typically relatively steep slopes of cohesive soils, often overlying rock platforms. They may suffer relatively rapid erosion with no corresponding accretion and suffer from stability problems induced by a variety of processes. Rocky sea cliffs are near vertical cliffs of rock characterised by slow erosion but by sudden collapse. The timing and extent of the collapse is determined by the internal structure rather than by the erosive process.

5 THE CONTINENTAL SHELF

The continental shelf is a major physical feature that modifies coastal processes. Figure B2.4 shows the profile of the continental shelf at five locations along the New South Wales coast. At a depth of 100 to 150m, the shelf falls away into the continental abyss, which reaches a depth of some 4,000m. The shelf varies in width from about 20 to 60km, being generally narrower and steeper in the south.

Fig B2.4: profiles of the continental shelf, NSW

Figure B2.4 Profiles of the Continental Shelf, New South Wales.

In discussing the processes that shape and affect the NSW coastline, it is the inner shelf areas that are of most significance. Storm waves generated hundreds or thousands of kilometres out to sea travel across the ocean and the outer shelf region without "feeling bottom". Bottom effects only begin to significantly modify wave behaviour when waves enter inner shelf areas about 60m deep (see Appendix B5). Large scale and complex currents are generated over the continental shelf ("Shelf Currents"). These are described in Appendix B6 and represent the only coastal process of relevance to this manual in which the whole shelf area is of significance.

6 PAST SEA LEVEL CHANGES

Figure B2.5 shows the estimated variation in coastal water levels along the New South Wales over the last 250,000 years (Chapman et al., 1982). For some two thirds of this time, sea level has been in the range 20m to 70m below present level. The highest it has risen is 5m above present level; the lowest it has fallen is 140m below it's present level.

Fig B2.5: variation in mean sea level over the last 250,000 years

Figure B2.5 Variation in Mean Sea Level over the last 250,000 Years.

The above sea level variations are attributed solely to the effects of temperature changes in the earth's atmosphere, which are reflected in the expansion and contraction of water in the oceans and the formation and melting of glaciers. The coastline of New South Wales has been tectonically stable over this period of time. Subsidence of the coastline has been estimated at only 0.01 mm/year or less (Chapman, et al., 1982).

The most recent rise in sea level has been both rapid and extreme. Over the period 17,000 to 6,000 years BP, ocean levels rose by some 140m to the present level, i.e. 17,000 years ago waves were breaking around the edge of the continental shelf.

7 COASTAL ZONE TERMINOLOGY

Figure B2.6 shows a typical cross-section of a "sandy beach" coastline. It consists of four major zones: the offshore zone, the nearshore zone, the inshore zone and the onshore zone. Taken together, the last three zones constitute the Coastal Zone of this manual.

Fig B2.6: coastal zone terminology

Figure B2.6 Coastal Zone Terminology.

Offshore Zone

The Offshore Zone is sufficiently deep that wave behaviour is not modified by the presence of the sea bottom. The landward limit of the Offshore Zone lies on the continental shelf around the 60m depth contour. The behaviour of large swell waves begins to be significantly influenced by bottom effects at about this depth.

Nearshore Zone

Within the Nearshore Zone, the sea bottom influences wave behaviour through shoaling and refraction effects, but large swell waves remain unbroken. The seaward limit of the Nearshore Zone is marked by the 60m depth contour; the landward limit is marked by the Offshore Bar on which waves break.

Inshore Zone

The Inshore Zone extends from the Offshore Bar to the upper limit of the swash zone. It is characterized by breaking waves and broken wave behaviour.

The Inshore Zone is divided into three further zones: the Breaker Zone, the Surf Zone and the Swash Zone. Waves break in the Breaker Zone; they move towards the beach as a "surge" or a "bore" in the Surf Zone; they wash up and down the beach in the Swash Zone.

Onshore Zone

The onshore zone is the land portion of the coastline. It runs inland from the swash zone and may consist of a beach berm and dunefield (Figure B2.6), a coastal bluff or sea cliffs.

A dunefield can be sub-divided into incipient dunes, foredunes and hinddunes. During storm wave attack, sand from the beach berm, the incipient dune and the foredune is often carried offshore. This sand is returned to the shore and the beach is rebuilt during calm weather conditions (Short and Hesp, 1982 and Hesp, 1984).

8 REFERENCES

Chapman, D.M., Geary, M., Roy, P.S. and Thom, B.G., (1982). "Coastal Evolution and Coastal Erosion in New South Wales: A report prepared for the Coastal Council of New South Wales." Coastal Council of New South Wales,1982.

Thom, B.G., (1974). "Coastal Erosion in Eastern Australia", Search 5 pp 198-209.

Short, A.D. and Hesp, P.A., (1982). "Wave, Beach and Dune Interactions in Southeastern Australia". Mar. Geol., V.48, pp 259-284.

Hesp, P.A., (1984). "Foredune Formation in Southeast Australia". In B.G. Thom (Ed.) Coastal Geomorphology in Australia. Academic Press, pp 69-97.