Coastal Tide Gauge Observations Dynamic Processes Present in the Fremantle Record

Charitha Pattiaratchi

Abstract Coastal sea level variability occurs over timescales ranging from hours to centuries. Globally, the astronomical forces of the Sun and the Moon are the dominant forcing which results in the tidal variability with periods of 12 and 24 h. In many regions, the effects of the tides dominate the water level variability -however, in regions where the tidal effects are small other processes also become important in determining the local water level. In this paper, sea level data from Fremantle (tidal range ~0.5 m), which has one of the longest time series records in the southern hemisphere, and other sea level recoding stations from Western Australia are presented to highlight the different processes ranging from seiches, tsunamis, tides, storm surges, continental shelf waves, annual and inter-annual variability. As the contribution from each of these processes is of the same order of magnitude - the study of sea level variability in the region is very interesting and reveals both local and remote forcing.

7.1 Introduction

Coastal regions experience rise and fall of sea level which vary at timescales of hours, days, weeks, months, annually and so on, governed by the astronomical tides, meteorological conditions, local bathymetry and a host of other factors. An overview of these processes may be found in Pugh (1987, 2004) and Boon (2004). Globally, the astronomical forces of the Sun and the Moon are the dominant forcing which results in the tidal variability with periods of 12 and 24 h. In many regions, the effects of these tides dominate the water level variability; however, in regions where the tidal effects are small other processes become important in determining the local water level. In this paper, sea level data from Fremantle (Fig. 7.1) which

C. Pattiaratchi (El)

School of Environmental Systems Engineering & UWA Oceans Institute,

The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia e-mail: [email protected]

A. Schiller, G. B. Brassington (eds.), Operational Oceanography in the 21st Century, 185

DOI 10.1007/978-94-007-0332-2_7, © Springer Science+Business Media B.V. 2011

Fig. 7.1 Location of tide gauges used in the present study and the track of the tropical cyclone Frank

has one of the longest time series records in the southern hemisphere, are presented to highlight the different processes ranging from seiches, tsunamis, tides, storm surges, continental shelf waves, annual and inter-annual variability (Table 7.1). It should be noted that there are some processes which are not inherent in the Fre-mantle record but may be present in other tide gauge records are not included in this paper. These include storm surges (due to local changes in atmospheric pressure and winds), sea level changes due to ocean eddy interactions with the coast and wave set-up. In Fremantle, it is difficult to separate the surge effects due to local and remote forcing (Eliot and Pattiaratchi 2010) and thus are included in the section 'continental shelf waves'. The auto spectrum of water levels recorded at Fremantle over three years indicated several peaks, ranging from hours to seasonal timescales

Table 7.1 Decomposition of processes observable at Freemantle tide gauge and their approximate amplitudes

Process

Duration

Scale (m)

Reference

Wave action

2-20 s

~5

Lemm et al. (1999)

Wave set-up

5-30 min

~0.3

Bode and Hardy (1997)

Seiches

30-90 min

~0.2

Ilich (2006)

Pressure surge

1-3 h

~0.2

Reid (1990)

Wind set-up

3-6 h

~0.2

Pugh (1987)

Tidal conditions

12-24 h

~0.8

Easton (1970)

Sea breezes

24 h

*

Pattiaratchi et al. (1997)

Pressure systems (cycle)

1-10 days

~0.8

Hamon (1966)

Continental shelf waves

3-10 days

~0.6

Fandry et al. (1984)

Oceanic currents

Seasonal

~0.3

Pattiaratchi and Buchan (1991)

Nodal tide

18.6 years

~0.15

Pugh (1987)

Climate variability

Decades

~0.2

Pariwono et al. (1986)

Climate change

103+ years

~10

Wyrwoll et al. (1995)

Fig. 7.2 Spectra of water levels at Fremantle showing the different scales of variability

Fig. 7.2 Spectra of water levels at Fremantle showing the different scales of variability reflecting these processes (Fig. 7.2). The contribution from each of these processes, which includes both direct and remote forcing to the total sea level variability is of the same order of magnitude and thus is equally important.

Sea level variability is important for a range of activities including navigation, coastal stability and coastal planning. The significance of coastal sea level change for coastal management has been recognised, effective for both gradual change and intermittent fluctuations (Komar and Enfield 1987; Allan et al. 2003). In order to interpret historic patterns of coastal management and predict possible future needs, it is necessary to document both short and long-term trends and fluctuations of sea level.

7.1.1 The Study Region

Fremantle is located along the western-coast of Australia at latitude 32°S (Fig. 7.1). Weather systems impacting on the region are dominated by anti-cyclonic high-pressure systems with periodic tropic and mid-latitude depressions and local seasonal sea-breezes (Eliot and Clarke 1986). Anticyclones move to the east and pass the coast every 3-10 days (Gentilli 1972). The peak occurrence of mid-latitude depressions is in July and the strongest winds in the system are the north-westerlies (Gentilli 1972; Lemm et al. 1999). Tropical cyclones track down from the Northwest coast infrequently during late summer and can have significant impact on the coastline (Eliot and Clarke 1986). The seasonal movement of the high-pressure systems results in a strong seasonality in the wind regime. During the summer southerly winds prevail whilst in winter there is no dominant wind direction although the strongest winds are north-westerly during the passage of frontal systems.

Sea breezes, which are stronger during the summer dominate the coastal region with offshore (westward) winds in the morning and strong (up to 15 ms-1) shore parallel sea breezes commencing around noon and weakening during the night (Pattiaratchi et al. 1997; Masselink and Pattiaratchi 2001).

During winter, the region is subject to the passage of mid-latitude depressions and associated frontal systems, and ~30 storm wave events are experienced (Lemm et al. 1999). During the passage of a frontal system, the region is subject to strong winds (up to 25-30 ms-1) from the north through west, which rapidly change direction towards west through southwest then progressively more southerly over 12-16 h. South to south-westerly winds gradually weakens over two to three days, and calm, cloud-free conditions prevail for another three to five days prior to the passage of another frontal system.

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