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NSW Coastline Management Manual

New South Wales Government
September 1990

ISBN 0730575063

Appendix B: Coastline Processes

Appendix B5 - Waves

1 INTRODUCTION

Waves are one of the dominant phenomena that shape a coastline: they are largely responsible for beach erosion, longshore drift and elevated water levels. Waves transport energy from remote areas of the ocean to the coastline. When unleashed in the breaking process, this energy erodes beaches and can damage and destroy coastal structures such as seawalls, jetties and breakwaters.

2 WAVE GENERATION

Waves are generated by wind blowing over the ocean's surface. The area in which waves are generated is called the fetch. The size of waves generated in a fetch depends upon fetch dimensions and wind behaviour. For a given wind speed, wave height will increase with fetch length and wind duration.

Waves generated within the fetch area are called "wind waves" or "sea" and have a random appearance, i.e. there are no regular lines of wave crests. As waves travel outside the fetch area, they take on a more ordered appearance and are called "swell".

Swell can travel many hundreds of kilometres across the ocean from a fetch area. During this process, the waves "decay" by losing energy and height. The rate of decay is greatest for the small short period waves. Large waves of longer period can travel great distances with little decay.

Methods are available that allow the size of waves to be predicted from wind and fetch characteristics. Such methods are known as wave hindcasting, and although approximate, are often the only predictive means available in the absence of measured data (CERC, 1984).

3 WAVE CHARACTERISTICS

Figure B5.1 shows a simple waveform consisting of a crest and a trough. The wave train it represents is characterised by its wave height, wave length, wave period and the speed and direction of travel. The wave period is the time for successive crests (or troughs) to pass a given point.

Figure 5.1

Figure B5.1 Characteristics of Waves

Wave energy is an indication of erosion and damage potential, the higher the energy, the greater the potential for beach erosion and damage to coastal structures. Wave energy is proportional to the square of the wave height multiplied by the wavelength. Thus, the higher the waves or the longer their wavelength, the greater their erosive and damage potential. For the same wavelength, doubling the wave height results in a fourfold increase in wave energy and erosive potential.

4 STATISTICAL REPRESENTATION

Wave behaviour in the ocean is far more complex than depicted in Figure B5.1. The area of interest may be subject to swell waves arriving from a number of different fetches and to locally generated wind waves. Waves arriving from different directions can generate a very confused sea surface which takes on a "choppy" appearance.

Observed wave behaviour is the result of interactions between all wave trains arriving at a location, as shown in Figure B5.2. Thus, wave behaviour is inevitably characterised by a range or "spectrum" of wave heights, periods, lengths and directions of travel. Consequently, it is appropriate to treat wave parameters in a statistical fashion.

Figure B5.2

Figure B5.2 The Interaction of Three Simple Wave Trains (After Komar, 1976)

It is often extreme events that are of interest, although periods of calm or average conditions are also important for coastal processes. A common means of presentation of wave climate is in terms of an exceedance plot whereby wave height (or some other parameter) is shown as a function of Annual Exceedance Probability (AEP - see Appendix B3).

Figure B5.3 shows the exceedance plot of significant wave heights for the four sectors of the New South Wales coastline. The "significant wave height" is defined as the average height of the highest one third of waves. In Figure B5.3, significant wave height is plotted against the AEP of storm severity, i.e. the 10% storm event means that there is a 10% chance of a storm of this severity or greater occurring in any one year. The significant wave height associated with the 1% AEP storm event is seen to be considerably greater in the North Coast sector (12.3m) than in the other three sectors (9 to 10m).

Figure B5.3

Figure B5.3 Exceedance Plot of Significant Wave Heights, Hindcast Data, New South Wales Coastline.

The results of Figure B5.3 are based on 47 years of hindcast data (PWD, 1985). Because of their hindcast nature, these results are regarded as approximate only. Nevertheless, these data are the most comprehensive currently available. The amount of measured wave data being collected along the New South Wales coast by the PWD is constantly increasing (see Section 7 of this Appendix).

5 SUB-SURFACE EFFECTS

Although the motion of water waves is most evident as a surface phenomenon, there are also movements below the water surface that decrease with depth. In deep water, the water particles beneath a wave orbit around a circular path and wave motion does not reach the seabed (see Figure B5.4). In shallow water, the water particles have an elliptical orbit and wave motion is felt at the seabed. In these circumstances, wave-induced oscillatory currents at the sea bottom can mobilise bottom sediments and initiate waterborne sediment transport. For some purposes the measurement and analysis of bottom currents would be more relevant than surface characteristics.

Figure B5.4

Figure B5.4 Motion of Water Particles Beneath Deepwater and Shallow Water Waves.

6 WAVE MODIFICATION NEAR SHORE

The presence of the seabed begins to significantly modify deepwater wave behaviour at a depth of approximately one half the deepwater wave length (about 60m depth for ocean swell). This interaction between wave motion and the sea bottom results in several changes to wave behaviour: wave speed and wave length are progressively reduced; wave height, after a small initial reduction, is progressively increased. The increase in wave height culminates in the wave breaking in the "breaker zone", which is typically located around an offshore bar. The broken wave then moves shorewards as a bore in the "surf zone" and ultimately runs up the beach in the "swash zone" (see Figure B5.5).

Figure B5.5

Figure B5.5 Wave Behavior in the Nearshore and Inshore Zone.

The morphodynamic coupling of waves and currents with nearshore morphology in the surf zone is analysed further by Wright and Short (1983) and Wright, Short and Green (1985).

Shoaling

Shoaling is the general term for waves moving into shallow water and "feeling bottom". The slower speed of waves in shallow water causes a "bunching up" of wave crests, i.e. wave lengths become shorter.

Refraction

Refraction, refers to the tendency of wave crests, as they move into shallow waters, to come into alignment with bathymetric contours, irrespective of the angle of incidence of the waves to these contours. Refraction can also occur as a result of current action. Refraction results in wave attack being concentrated on headlands. Figure B5.6 shows refracting waves at Cape Byron on the New South Wales North Coast.

Figure B5.6

Figure B5.6 Wave Refraction, Cape Byron, NSW North Coast.

Diffraction

Diffraction refers to the transmission of wave energy along wave crests into "shadow" areas created by breakwaters or headlands. Diffracted wave crests are lower and contain less energy than the incident waves. Figure B5.7 shows diffracted waves moving into areas sheltered by breakwaters in a physical model of Coffs Harbour built by the PWD.

Figure 5.7

Figure B5.7 Wave Diffraction, Physical Model, Coffs Harbour.

These three processes together with reflection in some circumstances, combine to produce a near shore wave climate with wave length, wave height and particularly wave direction being different to those offshore.

Breaking

Breaking occurs when the increase in wave steepness due to shoaling and refraction exceeds a limiting value. Typically, breaking occurs when the water depth is about equal to the wave height. Breaking is one of the main mechanisms for the dissipation of wave energy and is responsible for much of the sand movement within the surf zone. Breaking waves impose much higher forces on structures than equivalent non-breaking waves. Depending upon wave characteristics and the slope of the nearshore seabed, waves break in either a "surging", "spilling" or "plunging" mode, the difference in these types being visually quite obvious.

Reflection

Reflection refers to the re-direction by the shoreline of non-dissipated wave energy back to sea. Reflection is most apparent at solid seawalls where reflected waves can be seen moving seawards, virtually unaffected by incoming waves. Wave motion can be amplified by factors of 1.5 to 1.8 at the point of reflection, which can exacerbate erosion and damage to coastal structures.

7 WAVE MEASUREMENT AND DATA

The Public Works Department, the Maritime Services Board and the Commonwealth Department of Administrative Services all collect basic deepwater wave data along the NSW coastline. The standard instrument used to measure deepwater wave data is the "Waverider Buoy" manufactured by the Dutch firm Datawell. It consists of a stainless steel buoy, 0.7m in diameter, fixed to the seabed by a flexible mooring that allows it to follow movements of the sea surface. An instrument inside the buoy detects and measures these motions, which are transmitted to shore via radio for analysis by computer. Table B5.1 shows details of the location, operating authority, and periods of record of deepwater wave data along the NSW coast. The locations of the recording stations are shown in Figure B5.8.


Table B5.1 Deepwater Wave Data Locations, New South Wales

 Period of Record

Location

Operator

Water Depth

 Start

 Finish

Byron Bay

PWD

80m

14/10/76

present

Coffs Harbour 

PWD

80m

26/05/76

present

Crowdy Head 

PWD

80m

10/10/85

 present

Newcastle

MSB

71m

12/02/75

 20/10/82

Long Reef

PWD

80m

17/07/87

 present

Botany Bay

MSB

73m

18/02/71

 present

Port Kembla

PWD

80m

07/02/74

 present

Jervis Bay

DAS 

50m 

01/09/81

 present

Batemans Bay

PWD

80m

27/05/86

 present

Eden

PWD

80m

08/02/78

present

In shallow water, other instruments such as wave staffs, pressure sensors and current meters are used to gather wave and current data. These measurements are often used in conjunction with waverider data to infer long-term wave conditions.

Figure B5.8

Figure B5.8 Deepwater Wave Recording Stations, New South Wales.

8 WAVE DIRECTION

With the exception of current meters, none of the techniques described above provide information on wave direction. Limited directional data are available for the Sydney area from a radar system, but most directional information comes from visual observation or from deductions based on wind patterns supplied by the Bureau of Meteorology.

Because of prevailing weather patterns, most deepwater waves approach the New South Wales coastline from a south-easterly direction. As these waves move into shallower water, refraction and diffraction modify their final direction of approach to the coast.

Figure B5.9 shows the "wave energy rose" at the seabed of the entrance to Broken Bay in 24m of water. These data were obtained with an electromagnetic current meter and depict the directions from which incident wave energy, i.e. the erosion and damage potential of the waves, arrives at this location. Over the period of record, most of the incident wave energy arriving at Broken Bay originated from the ESE direction (55%). Being inferred from near bed current data this method of analysis represents a combination of various incident wave trains.

Figure B5.9

Figure B5.9 Wave Energy Rose, Broken Bay, New South Wales.

9 EFFECTS OF WAVES ON THE COASTLINE

Breaking waves, and to a lesser extent non-breaking waves, have great erosive potential. In particular, headlands are subject to concentrated attack as a result of refraction, focusing wave energy onto headlands. During storm conditions, storm waves can cause massive erosion to sandy beaches and foredunes with vast quantities of beach sediments being moved offshore (see Appendix B7). During calm conditions long period ocean swell rebuilds beaches by moving sediment back onshore. Windborne sand is trapped by dune vegetation to rebuild the dune system.

Within the surf zone, waves are the major mechanism of waterborne sediment transport. The rates of erosion, transport and deposition depend, amongst other things, on wave energy, the angle of wave approach to the coastline, and the strength of wave generated currents (see Appendix B7).

10 EFFECTS OF WAVES ON COASTAL STRUCTURES

The response of a coastal structure to wave attack depends not only on the local wave climate but also on the nature of the structure itself.

"Flexible" structures, such as rubble mound breakwaters (see Appendix D6), can suffer considerable and progressive damage before complete structural failure occurs. In contrast, "rigid" structures, such as some masonry seawalls, can fail catastrophically as soon as a critical force is exceeded.

The periodic nature of waves can also affect structural damage. If the dominant wave frequency matches a natural resonant frequency of the structure, then wave induced motion may be amplified so increasing damage.

The design of coastal structures to adequately withstand wave attack usually requires simulation studies, using either mathematical techniques or physical scale models.

11 REFERENCES

C.E.R.C., (1984). "Shore Protection Manual." Fourth Edition. Coastal Engineering Research Centre. Waterways Experiment Station, US Army Corps of Engineers, US Govt. Printing Office, Washington, D.C., 20404. Two Volumes.

Komar, P. D., (1976). "Beach Processes and Sedimentation." (Prentice-Hall, New York: 1976). ISBN 0-13-072595-1

PWD, (1985). "Storms Affecting 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.

Manly Hydraulics Laboratory, "New South Wales Wave Climate Annual Summary", Public Works Department, NSW, MHL Report Nos. 465, 520, 547 and 560, September 1986, October 1987, October 1988 and November 1989.

Trenaman, N.L. and Short, A.D., 1987, "Deepwater and Breaker Wave Climate of the Sydney Region." Coastal Studies Unit Technical Report 87/1, Coastal Studies Unit, University of Sydney.

Wright, L.D. and Short, A.D. (1983). "Morphodynamics of Beaches and Surf Zones in Australia". In P.D. Komar (Ed.) CRC Handbook of Coastal Processes and Erosion. CRC Press. London pp 35-64.