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NSW Coastline Management Manual

New South Wales Government
September 1990

ISBN 0730575063

Appendix B: Coastline Processes

Appendix B3 - Storms

1 INTRODUCTION

A storm is a major meteorological disturbance generally caused by low atmospheric pressures. Storms are of a temporary nature and are characterised by strong winds, raging seas and possibly heavy rain. An intense storm close to the coast can severely erode the shoreline and damage coastal developments. Even storms located a long distance offshore can generate heavy ocean swells which reach and attack the coast.

Storms can wreak damage along the entire length of the New South Wales coastline. The photograph of Figure B3.1 shows the type and extent of damage associated with major storm events.

Fig 3.1: storm damage to beaches and coastal structures

Figure B3.1 Storm Damage to Beaches and Coastal Structures, New South Wales.

2 THE IMPACT OF STORMS ON THE COAST

Storm winds cause direct damage to coastal developments, e.g. unroofing buildings, uprooting trees, bringing down power lines and even causing the collapse of buildings. This direct wind damage can extend many kilometres inland.

The windborne sediment transport associated with strong storm winds accelerates the migration of coastal sand dunes and the smothering of coastal developments (see Appendix B9).

Storms are responsible for the generation of large and potentially destructive waves (see Appendix B5).

The significant increase in coastal water levels caused by storm induced wind and wave setup intensifies wave damage to beaches and coastal structures and coastal flooding problems (see Appendix B4).

The elevated water levels and large waves associated with storms are a major cause of beach erosion (see Appendix B7).

3 STORM TYPES

Storms along the NSW coast have been classified into the six types listed below. Figure B3.2 shows characteristic synoptic patterns (PWD, 1985) of the first four storm types, which are the more important with regard to coastal processes in NSW:
Fig B3.2: typical synoptic patterns of different storm types Fig B 3.2 Fig B3.2 Fig B3.2

Figure B3.2 Typical Synoptic Patterns of Different Storm Types Affecting the New South Wales Coast

4 COASTAL SECTORS

The coastline of NSW has been divided into four sectors on the basis of storm types generally experienced in each sector. These sectors are shown in Figure B3.3 and correspond approximately to Bureau of Meteorology forecasting districts.

Fig B3.3: coastal sectors, NSW

Figure B3.3 Coastal Sectors, New South Wales

5 STORM LOCATION AND SEASONALITY

The occurrence of the six storm types in each of the four coastal sectors is shown in Figure B3.4. These results are based on an analysis of 50 years of storm data (PWD, 1985). Tropical cyclones are experienced mainly in the North Coast sector. The majority of storm events on the North and Mid-North Coasts are due to locally formed Easterly Trough Lows and Tropical Cyclones. Southern Secondary Lows predominate in Central and South Coast sectors.

The various storm types generally display a distinct seasonality, i.e. certain types of storm are more likely to occur during a particular period of the year. Details are shown in Table B3.1. The seasonal occurrence of all storm types for each of the four coastal sectors is shown in Figure B3.5.

Fig B3.4: occurrence of storms along the NSW coast

Figure B3.4 Occurrence of Storms along the New South Wales Coast.

Table B3.1 Likely Season of Storm Occurrence
Storm Type Likely Season of Occurrence
Tropical Cyclone Summer
Easterly Trough Low Autumn & Winter
Inland Trough Low Summer & Autumn
Continental Low Winter & Spring
Southern Secondery Low Autumn, Winter & Spring
Anti-cyclone Intensification Summer & Autumn
Fib B3.5: seasonal variation in storm occurence, NSW coastal sectors

Figure B3.5 Seasonal Variation in Storm Occurrence, NSW Coastal Sectors.

6 STORM SEVERITY

The central pressure of the storm is a good indicator of its severity and the likely strength of the winds it generates. However, the impact of a storm on the coast depends upon its location, movement and the extent and duration of strong winds. The resulting wind speed, storm surge level and wave height can all be used as measures of storm severity. A classification of storm severity based on significant wave height is given above (PWD, 1985, 1986b).

 Storm Category Storm Description Significant Wave Height (m)
 X Extreme >6
 A Severe 5.0-6.0
 B Moderate 3.5-5.0
 C Low 2.5-3.5

7 STORM FREQUENCY AND GROUPING

Minor to moderate storms occur frequently along the NSW coastline. Such storms do not have major impacts on the coast or coastal developments.

Severe storms of categories A and X, in which the significant wave height exceeds 5m, can be expected to occur on average four times per year somewhere along the NSW coast. The occurrence of major storms over the period 1880 to 1985 is shown in Figure B3.6. The past twenty five years have been relatively calm in comparison to the longer term storm record, even allowing for the major storms of 1967, 1974 and 1978.

Fig B3.6

Figure B3.6 Occurrence of Major Storms, New South Wales Coastline, 1880-1985

Storms have a tendency to occur in groups. A useful measure of this grouping is the interval between successive storm events. On the central coast, some 40% of storms occur within 30 days of the previous storm and 60% occur within 60 days of the previous storm event.

There are a number of other factors which affect the impact of storms on the coastline, particularly with respect to their erosive capability. Storm duration, the incidence of high tides during and immediately after a storm are important examples. There is no single satisfactory measure of the damage potential of a storm or series of storms but the above sections provide some indication of the factors involved.

The erosive impact of storms on sandy beaches is discussed in Appendix C. Examples of long-term data sets illustrating beach fluctuations are those collected at Moruya and Narrabeen by members of the Coastal Studies unit, University of Sydney (Thom and Hall (in press)) and Hall, 1988).

8 THE DESIGN STORM EVENT

Typically, coastline hazard is assessed in terms of the likely impact of a design storm event. The selection of an appropriate design event is governed by many factors, including safety aspects, likely damage and social disruption, all of which depend upon the type and nature of the development. It should be noted that longer term coastal changes, such as shoreline recession and sea level rise, also affect the damage potential of the design storm event. Recession exposes additional development to storm hazard; elevated sea levels allow larger storm waves to attack the coast.

Typically, a design storm event is specified in terms of its "Annual Exceedance Probability" (AEP), e.g. the 5% storm event. There is a 5% chance of such a storm occurring in any year. On average five such storms would be expected to occur in a period of 100 years, i.e. the average recurrence interval of a storm of this severity is 20 years.

The coast can experience the design storm event at any time. Long periods of relative calm, as experienced in the past decade, can give a false sense of security.

9 REFERENCES

Kemp, R.L. and Douglas D.A., (1981). "A Coastal Storm Climatology for Engineers." Proc. Fifth Aust. Conf. on Coastal and Ocean Engineering, Perth, W.A., 25-27 November, 1981, I.E.Aust.

McMonagle, C.J. and Fidge, B.L., (1981). "A Study of Extreme Values and Wave Height at Coffs Harbour". Proc. Fifth Aust. Conf. on Coastal and Ocean Engineering, Perth, W.A., 25-27 November, 1981, I.E.Aust.

PWD, (1985). "Elevated Ocean Levels - Storms Affecting the NSW Coast 1880-1980". Report prepared for Coastal Branch, Public Works Department of New South Wales, by Blain Bremner & Williams Pty Ltd and Weatherex Meteorological Services Pty Ltd, Report No. 85041, December 1985.

PWD, (1986a). "Elevated Ocean Levels, Coffs Harbour". Report prepared for Coastal Branch, Public Works Department of New South Wales, by Blain Bremner & Williams Pty Ltd and Weatherex Meteorological Services Pty Ltd, Report No. 86005, 1986.

PWD, (1986b). "Elevated Ocean Levels - Storms Affecting the NSW Coast 1980-1985". Report prepared for Coastal Branch, Public Works Department of New South Wales, by Lawson and Treloar Pty. Ltd. and Weatherex Meteorological Services Pty Ltd, August, 1986.

Hall, W.F. (1988). "Sub-aerial Beach Profile Behaviour". MSc. Thesis, University of Sydney, Department of Geography.

Thom, B.G. and Hall W.F. "Behaviour of beach profiles during erosion and accretion dominated periods". Earth Surface Processes and Landforms (in press).