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NSW Coastline Management Manual

New South Wales Government
September 1990

ISBN 0730575063

Appendix B: Coastline Processes

Appendix B4 - Elevated Water Levels

1 INTRODUCTION

Coastal water levels are influenced by a variety of astronomical, meteorological/oceanographical and tectonic factors, the most readily apparent being the tides. At times, these factors interact in a complex way to elevate water levels significantly above normal tide level. Storms, which develop low atmospheric pressure, strong onshore winds and large waves, are the most common cause of elevated water levels.

Elevated water levels are of concern because they intensify damage to the coastline and to coastal developments. Elevated water levels allow larger waves to cross offshore bars and break closer to the beach, which in turn increases beach erosion and the threat to coastal developments. Elevated water levels can inundate low lying areas of the coastline and around estuaries.

2 ASTRONOMICAL TIDES

The astronomical tide is caused by the gravitational effect of the moon, and to a lesser extent, the sun and other planets on the water mass of the oceans. Along the NSW coast, tides are semi-diurnal, i.e. two high tides and two low tides per day.

Tidal ranges vary significantly throughout each lunar month and from month to month. Very high and very low tides occur more frequently around Christmas and in the mid-winter months (King Tides). The tidal range is relatively constant along the open coast of New South Wales. The spring tide range is about 1.8 to 2.2m.

Detailed tidal predictions for the NSW coast are published annually by the Hydrographer of the Royal Australian Navy in the form of Tide Tables. The Maritime Service Board publishes summary tidal information.

3 METEOROLOGICAL/OCEANOGRAPHICAL PROCESSES

Three different meteorological processes can affect coastal water levels:

Storms are local meteorological disturbances. The other two processes are semi-global or global in nature. Climate change, including the Greenhouse Effect, is discussed in some detail in Appendix B11.

Storms

The elevation of water level associated with a storm depends primarily on the following factors:

A storm increases coastal water levels in four distinct ways: by "setup" due to barometric, wind and wave effects and by wave "runup". Figure B4.1 illustrates these components of elevated water levels. Table B4.1 shows typical values for New South Wales. The four components are all additive and their sum represents the superelevation of storm water level above prevailing astronomical tide level.

Table B4.1 Elevated Water Level Components Due to Storms
 Component  Typical range (m)
 Barometric setup  0.2 - 0.4
 Wind setup  0.1 - 0.2
 Wave setup  0.7 - 1.5
 Wave runup  3.0 - 6.0
 Total Setup plus runup  4.0 - 8.0
Figure B4.1

Figure B4.1 Elevated Water Levels During a Storm

Barometric setup

The reduced barometric pressures that generate storm winds also cause a local rise in ocean level (the inverse barometer effect). Providing low pressures persist for a sufficient length of time, the increase in water level amounts to about 0.10m for each 10hPa drop below normal barometric pressure (1,013hPa). In a severe storm with a central pressure of 980hPa, this amounts to about 0.3m.

Wind setup

Wind blowing onshore over the ocean's surface drives the surface waters before it and against the coastline. This results in elevated water levels in coastal areas, the degree of elevation being higher for extensive shallow areas and semi-enclosed bays.

Wind setup is much more severe in north Queensland waters because of the extensive shallow waters associated with the wide continental shelf and the presence of the Great Barrier Reef. Along the Mackay/Rockhampton coast, the continental shelf is 200 to 250km wide; along the New South Wales coast, the shelf is only 30 to 60km wide (see Appendix B2).

Storm Surge

The sum of barometric and wind setup is often referred to as storm surge. Along the New South Wales coast storm surge amounts to 0.3 to 0.6m.

Wave setup

The breaking action of waves results in an increase in water levels in the surf zone known as "wave setup". Wave setup is associated with the conversion of the wave's kinetic energy into potential energy (Battjes, 1974). The degree of setup depends upon the type, size and period of the waves at breaking and the slope of the beach.

On the New South Wales coast wave setup during severe storms can be in the order of 1.5m and often makes the largest contribution to the elevated water level.

Wave runup

Wave runup is an oscillatory phenomenon and refers to the vertical distance the uprush of water from a breaking wave reaches above the combined level of the tide, storm surge and wave setup. A wave runup of more then 6m can occur. The magnitude of runup depends upon a variety of factors, particularly the slope and roughness of the runup surface. Runup on flat beaches is generally less than on steeper beaches; runup on smooth vertical sea walls is generally greater than on protective works with rough sloping faces.

Wave runup can result in the intermittent discharge of seawater into backbeach areas that may appear to be protected by beach barriers, such as sand dunes or seawalls.

Rainfall Runoff

Surface runoff from any rainfall accompanying a storm may cause an increase in water levels within estuaries and tidal inlets. Rainfall and runoff have no significant effect on coastal water levels.

Oceanographic Effects

Distant meteorological disturbances (say in Bass Strait) that are characterised by a sharp pressure gradient can generate a long low wave with a period of up to 10 days and a height of up to 0.2m. As this wave travels along the continental shelf, it becomes a "shelf wave" that is "trapped" by the shelf which acts as a wave guide. Shelf waves also modify coastal water levels.

Other effects which can result in tidal anomalies include variations in sea temperature and salinity and the influence of strong currents such as the Eastern Australian Current. (Davidson et al 1989).

Meteorological Oscillations

Major meteorological phenomena such as the El NiŅo Southern Oscillation (ENSO) affect water levels along the NSW coastline. ENSO results from interactions between the atmosphere and major ocean currents over the Pacific Ocean and appears to occur about every three to seven years. ENSO is thought to have a major impact on climate over the eastern half of Australia, particularly with regard to the sequence of "dry" and "wet" years. The associated water level change along the New South Wales coast attributed to ENSO is ±0.1m. The relationship between ENSO induced rainfall and beach erosion is examined by Bryant (1983 and 1985).

Climate Change

A "eustatic" sea level change refers to a change in the mean water level of the oceans around the globe. A eustatic rise can occur through two mechanisms: the expansion of the surface waters of the ocean caused by a global warming and by the melting of land-based glacier ice that accompanies any such warming. In the initial period of any global warming, i.e. the first 50 to 100 years, the first effect will be the more significant.

The period 17,000 to 6,500 years B.P. saw the demise of the last ice age and the release of vast volumes of frozen water. The eustatic sea level rise associated with this event was approximately 140m. In response to this sea level rise, the shoreline of Australia retreated landwards from around the edge of the continental shelf to its present position (see Appendix B2). Ocean water levels appear to have remained relatively stable over the last 6000 years.

4 TECTONIC PROCESSES

The two tectonic processes that could potentially affect water levels along the NSW coast are subsidence or emergence of the crustal plate on which the coastline of New South Wales rides and the effects of tsunami generated by undersea landslides.

Crustal Movements

Tectonic uplift or sinking is often perceived as a change in MSL (as the land emerges from the sea, MSL is perceived to have fallen). Whilst such changes are occurring in North America and elsewhere, it is believed that they are not taking place along the NSW coast, or if they are occurring they are very small (less than 0.01mm/year - see Appendix B2).

Tsunami

Tsunami, which are caused by sea bed earthquakes, are incorrectly called "tidal waves". Australia is remote from the more active seismic areas of the world. Water level anomalies along the NSW coast due to tsunamis have occurred but are rare.

Studies of Fort Denison tide gauge records from 1867 onwards have identified a number of water level anomalies due to tsunami, the three largest of which occurred in 1868, 1877 and 1960 (PWD, 1985). Water level changes of 1.07m accompanied the 1868 and 1877 events. In 1960 a tsunami resulting from a severe earthquake in Chile caused the water level at Fort Denison to oscillate through a range of 0.84m over a 45 minute period. These rapid water level changes induced strong currents in Sydney Harbour and nearby ports and bays, causing considerable damage to boats and shoreline structures. The damage caused by this tsunami was exacerbated by the semi-enclosed nature of Sydney Harbour. The tsunami probably occurred without notice along the open coastline.

Tsunami occur on a random basis and are independent of all other effects causing elevated water levels. The simultaneous occurrence of elevated water levels due to a major storm event and a tsunami is most unlikely.

5 SURF BEATS

When swell waves from two different storm sources arrive simultaneously at a beach, the resultant waves tend to occur in consecutive groups of large and small waves (leading to the popular belief that every seventh wave is the largest). This has the effect of inducing periodic water level fluctuations in the amount of wave setup at the shoreline. Longer period water level fluctuations (2 to 3 minutes) are often referred to as "surf beat" and may have amplitudes of up to 0.5m.

6 TIDE MEASUREMENT

The Public Works Department and the Maritime Services Board have been measuring tidal data along the NSW coast for the last 100 years or so. Various instruments are used to collect the data which essentially is in the form of water level against time. There exists a substantial data base of tidal information which is presently being integrated into a state database. The recently formed NSW Committee on Tide and Mean Sea Levels coordinates tidal information for NSW (Wylie et al 1990).

7 DESIGN CONSIDERATIONS

Determination of appropriate design water levels for coastal developments requires first, an assessment of each component of elevated water level at the subject site and second, the combining of these components in a realistic and statistically meaningful way. Simple addition of the values for each element is not necessarily appropriate and will usually result in a conservative design value.

In considering the selection of appropriate design levels there is a further issue to be considered. Reference to Figure C9.2 shows not only a long-term trend of increasing sea level, (discussed in more detail in the context of climate change), but a considerable fluctuation in annual mean sea level with an apparent period of the order of a decade and with a magnitude of about 0.1 to 0.2m. This fluctuation is not entirely explained by the various factors discussed in the foregoing sections. The possibility of annual mean sea level varying from the long-term trendline should therefore be considered in any assessment of design levels.

Estimation of Water Level Components

Design values for water level components can be determined from measured values (if available), from analytical formulae or by numerical simulation.

Tidal data for New South Wales are available for many ports and extend over a considerable period of time, (PWD 1989). These data can be used to estimate tidal behaviour at unreferenced locations. At sites where tidal effects may be significantly modified by the local bathymetry, a "harmonic analysis" of measured tidal data may be required to better define likely tidal behaviour. This requires water level data collected at the site over a period of time, the length of which depends on the complexity of the tidal system and the accuracy sought.

Mathematical modelling is necessary to derive long-term storm statistics at specific sites. Computer simulation has been used for wind field modelling (Graham and Nunn, 1959) and for storm surge modelling (Sobey, Harper & Stark, 1977).

Wave setup may be calculated using simplified methods found in the Shore Protection Manual (CERC, 1984) or by using computer models where the offshore bathymetry is complex and natural wave spectra are being considered (Goda, 1975). Field measurements of wave setup are available at a limited number of locations (PWD, 1988; Nielsen, 1988; Davis and Nielsen, 1988).

Wave runup is a function of the beach profile, surface roughness and other shoreline features affecting breaking waves at the particular site. Physical model results are available in the Shore Protection Manual for simple beach profiles and wave conditions (CERC, 1984). Where runup levels are of significance, it may be necessary to undertake physical model studies.

Extreme Value Analysis

There is some difficulty in meaningfully combining storm surge statistics with tide height statistics to determine the extreme values of elevated water levels. Methods based on the application of conditional probabilities have been applied (Dexter, 1975; Haradasa et al, 1989), but inconsistencies remain. The mathematical simulation of the occurrence of a large number of random storms with coincident tides is another method of determining the likelihood of extreme water levels (McMonagle and Fidge, 1981).

8 REFERENCES

Battjes, J.A., (1974). "Computation of setup, longshore currents, runup and overtopping due to wind generated waves". Report 74-2, Dept. of Civil Eng., Delft Univ. of Technology.

Bryant, E.A., (1983). "Regional Sea Level, Southern Oscillation and Beach Change, NSW, Aust.", Nature, 305, pp 213-216.

Bryant, E.A., (1985). "Rainfall and Beach Erosion Relationships, Stanwell Park, Aust. 1895-1980: Worldwide Implications for Coastal Erosion." Zeitschrift fur Geomorphology, Sup. 57, pp 51-65.

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.

Davidson, P.J., Wyllie, S.J. and Gordon, A.D. "Port Kembla Harbour Tidal Investigations" AWACS Rpt 89/15.

Davis, G.A. and Nielsen, P., (1988). "Field Measurements of Wave Setup". Proc. 21st International Conference on Coastal Engineering.

Dexter, P.E., (1975). "Computing Extreme-Value Statistics for Cyclone-Generated Surges". Proc. Second Aust. Conf. on Coastal and Ocean Engineering, I.E.Aust., Gold Coast, Qld. 1975.

Geary, M.G. and Griffin, A.G., (1985). "Significance of Oceanographic Effects on Coastal Flooding in NSW" Proc. 7th Australasian Conference on Coastal and Ocean Engineering, I.E.Aust., Christchurch, New Zealand 1985.

Graham, H.W. and Nunn, D.E., (1959). "Meteorological Considerations Pertinent to Standard Project Hurricane, Atlantic and Gulf Coasts of the United States". U.S. Weather Bureau Report 33, 1959.

Goda, Y., (1975). "Irregular Wave Deformation in the Surf Zone". Coastal Engineering in Japan, 1975.

Haradasa, D., Wyllie, S.J. and Couriele, E., (1989). "Design Guidelines for Water Level and Wave Climate at Pittwater" Australian Water and Coastal Studies Pty. Ltd., Rpt 89/23.

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

Nielsen, P., (1988). "Wave setup: a field study". J. Geophysical Research, Vol. 93.

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

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

PWD, (1986b). "Elevated Ocean Levels Storms affecting the NSW Coast 1980-1985". Report prepared for Coastal Engineering Branch, Public Works Department of New South Wales, by Lawson and Treloar Pty Ltd in conjunction with Weatherex Meteorological Services Pty Ltd. PWD Coastal Branch Report No. 86026, August 1986.

PWD, (1988). "Wave Setup and the Water Table in a Sandy Beach". Coast and Rivers Branch, Public Works Department of New South Wales. Report No. TM 88/1

PWD, (1989). "New South Wales Ocean Tide Levels - Annual Summary" Manly Hydraulics Report MHL 544.

Sobey, R.J., Harper, B.A. and Stark, K.P., (1977). "Numerical Simulation of Tropical Cyclone Storm Surge". James Cook Univ. of North Qld., Dept. of Civil & Systems Engineering, Bulletin CS14, 1977.

Wyllie, S.J., Jones, C. and Blume, P. (1990) "The Role of the NSW Committee on Tides and Mean Sea Level." Proc. Local Government Engineers Conf., Sydney, 1990.