Waves (water)

A wave (in liquid) is a ridge or swell on the surface of a body of water that travels in a forward motion. Waves are a periodic disturbance which moves through or over the surface of the medium (eg. water), with speed dependent on the properties of the medium. Waves are characterised by frequency, length, and amplitude.[2].

Waves in water are created by the friction of wind as it interacts with the surface of the water (wind waves). The size of wind waves depends on the wind velocity, duration, direction, fetch (the stretch of water the wind can blow across) and water depth[14]. Wind generated waves provide most of the energy that drives shoreline processes.

Quick facts

Tsunamis are not tidal

waves but are fast moving waves caused by earthquakes or undersea landslides that displace large bodies of water[19].

Wave action is an important influencer of freshwater, estuarine and marine wetland geomorphology. Wave energy magnitude underpins the typology of estuaries, along with tidal range and riverine discharge[4]. Longshore drift is a coastal process driven by waves that transport sediment parallel to the shoreline through the zigzag of wave swash and backwash, such as the longshore drift transport of a ‘sand river’ of sediment from the Victorian border to K’gari. On high wave energy sandy beaches, sand is deposited all the way from offshore to the dunes. Low wave energy beaches are typically muddy to mixed sediment below mean sea level (MSL) and sand above MSL. Both silty and sandy beaches have intertidal sand waves that form ridges, troughs and offshore bars[15].

Wave-dominated estuaries are prominant along the south-west and south-east coasts of Australia, including along the Queensland coast from K’gari (Fraser Island) southward. For estuaries or lakes connected to the shoreline, ocean waves and tides push sand bars from offshore into the entrance, and gradually close the entrance channel and may even accrete to form sand dunes. Accretion of sand bars may periodically form a tidal barrier to cut off tidal influence – forming a type of wave-dominated estuary termed an ‘Intermittently closed and open lagoon lakes’ or ICOLL[9][10][4].

Ocean waves can be used as a renewable energy source and its parameters are mappable – see ARENA the National Wave Energy Atlas.

Larger inland lakes, such as Lake Wyara and Lake Numalla, are also affected by wind induced waves that influence the shoreline geomorphology of the lake[6]. Wave movement in lakes affects water quality by influencing factors such as sediment resuspension, dissolved oxygen concentration and water clarity[8][20][12]. As a result of wind driven waves and mixing, some lakes will have high turbidity levels that lead to a narrow littoral zone for primary production.

Wave, wind and/or tidally-induced water movement promotes circulation, mixing and oxygenation of surface layers of wetland water bodies. The well-mixed surface layer is influenced by wind and wave action, while beneath it there may be a distinct layer with different salinity, temperature, nutrients and oxygen content. Enclosed water bodies may become stratified during low rainfall periods. Wind-forces and wind-driven wave action may modify circulation and turbulent mixing, impacting on the degree of stratification in a waterbody.

Shallow open estuaries receive more wind-waves than narrow, deep estuaries. Other estuarine mixing processes may be driven by the balance between denser, saltier oceanic tidal water and freshwater flows (see Tides). Waves due to storm tides and surges can also interact with freshwater flow events to modify salinity mixing processes in the estuary[4].

As well as sandy beaches, wave energy is a strong shaper of intertidal and shallow subtidal ecosystems and their biota. This includes distinct assemblages on rocky shores and coral reefs, such as intertidal filter-feeders of the surf zone (see Intertidal high energy over consolidated substrate) and corresponding subtidal gravel, mud and sand ecosystems. The growth forms of coral on clear water offshore coral reefs respond to different wave energies. Wave energy magnitude (significant wave height) can be used to predict the distribution of coral types[13], in combination with other factors.

Characteristics of waves

Waves are characterised by their height (vertical distance between the wave crest and trough), period (time for a wave to pass), wavelength (horizontal distance between successive wave crests), speed or celerity (rate of travel) and frequency, (i.e. the number of waves passing through a given point in time). Water wave periods vary with the disturbing energy force causing the wave action (e.g. wind, storm systems and tsunamis) and the energy source restoring the water to equilibrium (see Fig. 1 after Kinsman, 1964, p.22)[7]. Tides also propagate via water waves but differ in their behaviour from wind waves – see tidal range.

Figure 1 - Wave periods vary according to the primary disturbing forces (blue, e.g. wind, storm systems and tsunamis) and those that restore equilibrium (red – e.g. surface tension, gravity and the rotational force of the earth Coriolis force). After Kinsman 1984[7].

In the ocean, waves can be classified as either sea waves (generated by the local winds) or swell (generated over a long distance by significant weather systems). Waves vary in height, and significant wave height is a convention used to measure wave height statistically (an average of the largest one-third of waves). The maximum wave height can be as much as twice the significant wave height. In some cases, waves of different wavelengths can combine as one wave overtakes another, adding their wave heights together.

The motion of water particles in a wave is in a vertical circular orbit, that is, water particles do not move horizontally when in deep water[1][3][18]. With increasing depth, the orbit diameter decreases until there is no wave action when the depth exceeds four times the wavelength (wave base). In nearshore waters, the wave base (i.e. the smallest orbiting water particle) encounters the sea floor (known as the Depth of Closure). As the water shallows, the orbiting particles at the wave base encounter sea floor friction and travel slower than the orbiting particles at the surface. Wave set-up is the process of shoaling that causes the waves to decrease in speed and wavelength and the wave height to increase and tip forward so that it breaks. The breaking wave surges up the beach (wave run-up).

Figure 2 - A wave approaching the shore undergoes significant changes as the water in orbital motion encounters the seafloor.

As waves encounter the substrate/sea floor they create strong currents capable of entraining and transporting sediment. When waves encounter a consolidated object such as a breakwater, they bend around it in a process called diffraction. Breakwaters and rock walls are often installed to reduce wave action around estuary entrances, boat harbours and anchorages to maximise safety for mariners, or to protect shorelines from waves. These hardened shorelines change the way waves move and transport sediment, often with unintended consequences for fish assemblages[5]. For further information on waves and surf zone processes refer to Short (2006)[15] and Woodroffe (2002)[19].

Wave climate describes the seasonal variation in source, size and direction of waves at a location, driven by cyclonic and high-pressure weather systems[16]. Offshore wave climates in Queensland fall into three distinct groups at the regional scale - the eastern Gulf with low, short waves (< 0.2m), Cape York to Hervey Bay (0.5 – 0.8m), and the southeast islands, Sunshine and Gold coasts (1 – 1.2m)[16]. Wave climate is a factor that can be used for determining sediment type as well as ecosystem function[17].

Importance of waves for wetlands

Wave energy associated with storms and cyclones is significant and capable of picking up and breaking coral bommies and depositing boulders across coral reef flats. Wave energy together with tidal range affects beach morphology and beaches can be categorised on this energy spectrum and morphology. This energy spectrum affects the sediment type, distribution on the beaches and ecology of these coasts, separating the high energy coastlines of K’gari (Fraser Island to the border and south to Victoria) from low wave energy coastlines[11] such as Hervey Bay and much of the Great Barrier Reef coastline[9].

Waves are associated with coastal hazards such as storm surge and cyclones. Coastal erosion due to wave action can remove several hundreds of metres of sand at a time, or slowly remove sand from one location and deposit it at another. Those living close to or planning to build close to the coast need to be aware of coastal hazards due to storms and the consequences of changes in wave height and inundation due to climate change – see Coastal hazards and erosion prone area mapping[4].

Climate change will alter wave processes as well as sea levels, with consequences for coastal intertidal and freshwater wetlands.

Links and References

OzCoasts estuarine typology

OzCoasts - Beaches

Wave dominated beaches

Wave dominated estuaries

Storm tide monitoring sites

References

1. ^ Australian Bureau of Meteorology, Marine weather knowledge centre. [online] Available at: http://www.bom.gov.au/marine/knowledge-centre/index.shtml [Accessed 12 October 2023].
2. ^ Australian Hydrographic Office, DD, Australian Hydrographic Office - Tidal Glossary. [online] Available at: https://www.hydro.gov.au/prodserv/data/tides/tidal-glossary.htm [Accessed 6 September 2023].
3. ^ 'Water Waves, Concept, its Types and Explanation.'. [online] Available at: https://byjus.com/physics/water-waves/ [Accessed 12 October 2023].
4. ^ a b c d Glamore, WC, Rayner, DS & Rahman, PF (2016), Estuaries and climate change. Technical Monograph prepared for the National Climate Change Adaptation Research Facility.. [online], Water Research Laboratory of the School of Civil and Environmental Engineering, UNSW. Available at: https://coastadapt.com.au/sites/default/files/factsheets/T3I6_Estuaries_and_climate_change_0.pdf.
5. ^ Henderson, CJ, Gilby, BL, Schlacher, TA, Connolly, RM, Sheaves, M, Flint, N, Borland, HP, Olds, AD & Handling editor: Henn Ojaveer (4 February 2019), 'Contrasting effects of mangroves and armoured shorelines on fish assemblages in tropical estuarine seascapes', ICES Journal of Marine Science. [online] Available at: https://academic.oup.com/icesjms/advance-article/doi/10.1093/icesjms/fsz007/5306605 [Accessed 13 June 2019].
6. ^ Information Sheet on Ramsar Wetlands, vol. Currawinya Lakes (Currawinya National Park).
7. ^ a b Kinsman, B (1984), Wind waves: their generation and propagation on the ocean surface, p. 676, Dover, New York.
8. ^ Lewis, S, Petus, C, Tracey, D, Bainbridge, Z, James, C, Langlois, L, Waterhouse, J, Brodie, J, Stevens, T, Mellors, J, Smithers, S, Devlin, M & Gruber, R (2021), The dissipation of suspended particulate matter in river flood plumes and implications for marine ecosystems: case studies from the Great Barrier Reef. [online], vol. Chapter 5, p. 148, Tropical Water Quality Hub, James Cook University. Available at: https://nla.gov.au/nla.obj-3203168341/view.
9. ^ a b Masselink, G, Kroon, A & Davidson-Arnott, RGD (January 2006), 'Morphodynamics of intertidal bars in wave-dominated coastal settings — A review', Geomorphology. [online], vol. 73, no. 1-2, pp. 33-49. Available at: https://linkinghub.elsevier.com/retrieve/pii/S0169555X05001984 [Accessed 10 June 2020].
10. ^ McSweeney, SL, Kennedy, DM, Rutherfurd, ID & Stout, JC (September 2017), 'Intermittently Closed/Open Lakes and Lagoons: Their global distribution and boundary conditions', Geomorphology. [online], vol. 292, pp. 142-152. Available at: https://linkinghub.elsevier.com/retrieve/pii/S0169555X16312405 [Accessed 12 October 2023].
11. ^ Poloczanska, ES, Smith, S, Fauconnet, L, Healy, J, Tibbetts, IR, Burrows, MT & Richardson, AJ (2011), 'Little change in the distribution of rocky shore faunal communities on the Australian east coast after 50years of rapid warming', Journal of experimental marine biology and ecology, vol. 400, no. 1, pp. 145-154, Elsevier.
12. ^ Roberts, DC, Moreno‐Casas, P, Bombardelli, FA, Hook, SJ, Hargreaves, BR & Schladow, SG (February 2019), 'Predicting Wave‐Induced Sediment Resuspension at the Perimeter of Lakes Using a Steady‐State Spectral Wave Model', Water Resources Research. [online], vol. 55, no. 2, pp. 1279-1295. Available at: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018WR023742 [Accessed 20 June 2024].
13. ^ Roelfsema, CM, Kovacs, EM, Ortiz, JC, Callaghan, DP, Hock, K, Mongin, M, Johansen, K, Mumby, PJ, Wettle, M, Ronan, M, Lundgren, P, Kennedy, EV & Phinn, SR (August 2020), 'Habitat maps to enhance monitoring and management of the Great Barrier Reef', Coral Reefs. [online], vol. 39, no. 4, pp. 1039-1054. Available at: http://link.springer.com/10.1007/s00338-020-01929-3 [Accessed 12 June 2024].
14. ^ Short, AD (2000), Beaches of the Queensland Coast, Cooktown to Coolangatta: A Guide to Their Nature, Characteristics, Surf and Safety, Sydney University Press.
15. ^ a b Short, AD (2006), 'Australian beach systems-nature and distribution', Journal of Coastal Research, pp. 11-27.
16. ^ a b Tannock, S (2018), Presentation to the Treatment Systems for water quality improvement - Regional Forum. [online] Available at: https://wetlandinfo.des.qld.gov.au/resources/static/pdf/management/regional-forum/6-tannock.pdf.
17. ^ Thom, BG, Eliot, I, Eliot, M, Harvey, N, Rissik, D, Sharples, C, Short, AD & Woodroffe, CD (March 2018), 'National sediment compartment framework for Australian coastal management', Ocean & Coastal Management. [online], vol. 154, pp. 103-120. Available at: https://linkinghub.elsevier.com/retrieve/pii/S0964569117306129 [Accessed 12 June 2024].
18. ^ US Department of Commerce, NOAA, Why does the ocean have waves?. [online] Available at: https://oceanservice.noaa.gov/facts/wavesinocean.html [Accessed 12 October 2023].
19. ^ a b Woodroffe, CD (2002), Coasts: form, process and evolution, Cambridge University Press.
20. ^ Zhang, C & Chen, L (2023), 'A review of wind-driven hydrodynamics in large shallow lakes: Importance, process-based modeling and perspectives', Cambridge Prisms: Water. [online], vol. 1, p. e16. Available at: https://www.cambridge.org/core/product/identifier/S275517762300014X/type/journal_article [Accessed 20 June 2024].

Last updated: 12 June 2024