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Sedimentation rates

What are sedimentation rates?

In coastal waterways, sedimentation rates refer to the amount of material (organic and mineral) deposited by the action of water over a given interval of time. Sedimentation is measured in terms of vertical accumulation over time or sediment density per unit area over time.

Vertical accumulation (mm yr^-1)

Changes in the rate at which estuaries have been vertically filling up with sediment can provide useful insights into the functioning and health of an estuary. Marked increases in modern rates of infilling may reflect increased catchment erosion and/or the increased production of organic sediment within the estuary, and indicate that abrupt changes have occurred in estuarine geomorphology and benthic habitats [16].

Mass accumulation (g cm² yr^-1)

Sediment mass accumulation is a more accurate measure of sedimentation where there are significant changes with depth in the density of estuarine sediment that may be related to compaction or changes in the composition of the sediment [4,17].

Figure 1. Enhanced sedimentation rates can lead to the smothering of benthic communities, which can affect how nutrients are recycled.

What causes sedimentation rates to change?

Some natural controls on the sedimentation rates experienced by coastal waterways include climate (rainfall, seasonality), geology, slope (or topography), vegetation and the size of the catchment.

Several studies have shown that sediment loads delivered to estuaries have dramatically increased in response to waterborne erosion (gully, streambank/streambed and sheetwash erosion) in catchments in which large tracts of native vegetation have been cleared and replaced with intensive agriculture [1,2] and urban areas [3,4,22,18]. As a result, modern infilling rates in some Australian coastal waterways are at least double those experienced during the late Holocene (Table 1) [6]. Siltation may be particularly catastrophic following intense rainfall events [2,22,7]. It has also been found that in some estuaries the rate of infilling may have further accelerated during the last few decades compared to earlier in the last century [8,9], highlighting the fact that enhanced sedimentation is an ongoing management issue [6].

Significance of sedimentation rates

Sedimentation rate data can be used to determine whether a waterway has been subject to enhanced sediment loads due to changes in catchment land use practices. Enhanced sedimentation rates can bring about rapid changes in the form and function of coastal waterways. For example, in wave-dominated estuaries the configuration of habitats alters:

• as coarser sediment infills channels and accumulates proximal to the stream outlets, giving rise to rapid growth of fluvial deltas and shoreline progradation [5,10];
• as fine sediment derived from the catchment and also produced within the estuary flocculates and settles in the margins of the estuary, forming mud flats where there may have formerly been relatively clean sand [2,11]; and when
• changes in the hydrological functioning are caused by a marked acceleration in the rate at which channels fill in or tidal flats and fluvial deltas grow into an estuary. Depositional basins in the estuary may be replaced by channel systems that more directly link rivers to the coast, increasing the efficiency of delivery of terrestrial sediment to the marine environment [25,26].

Habitats may be smothered where sediment is deposited more rapidly than tolerated by benthic communities [26]. For example, loss of seagrass areas and macroalgae can destabilise bottom sediments formerly protected from wind and tidal erosion by the sheltering and binding abilities of macrophyte colonies [25,26]. Such changes also constitute pressures on fish assemblages and benthic invertebrate numbers.

Turbidity levels and the amount of sediment-bound nutrients (e.g. Total P, Total N & Total Organic Carbon), trace elements (e.g. Fe, Zn, Pb) and other toxicants entering estuaries from their catchments also tend to increase in association with increased rates of sedimentation [3,13]. Greater nutrient loads can lead to periods of eutrophication which can further enhance sedimentation rates because the amount of organic matter being deposited also increases.

Increased sedimentation rates also allow more organic matter to be degraded by anoxic processes (e.g. sulfate reduction; see also TOC:TS ratios) because the exposure time of organic matter to dissolved oxygen in the water column is shortened. Denitrification efficiencies are lowered under anoxic conditions, and more dissolved nutrients are recycled to the water column. Loss of nitrification and denitrification (and increased ammonium efflux from sediment) in coastal and estuarine systems is also an important cause of hysteresis.

The net result of enhanced sedimentation rates is an increase in the maturity of coastal waterways, and a decrease in their overall lifespans. Reductions in the biodiversity, health and integrity of coastal ecosystems may also occur. In order to make better-informed management decisions there is clearly a need to accurately assess the rate and nature of sedimentation within coastal waterways and any changes in other sedimentological parameters over time [6].

Other sedimentological indicators

Other geochemical analyses of sediment cores can identify pools of nutrients or other pollutants within the estuary fill. This is important information for managers because of the potential for the release of sediment-bound nutrients into the water column, which is also relevant where dredging work is proposed. The identification of microfossils in sediment cores can provide a detailed record of recent changes in estuarine vegetation communities or harmful algal blooms [14,15]. Sedimentological data are especially important where conservation or restoration actions are being planned and there is a lack of historical information to indicate how the estuarine environment has changed over the past two centuries or past few decades. Likewise, these data can aid in the development of models of sediment transportation. The information gained from the analysis of sediment cores, therefore, needs to be viewed as basic environmental data needed for the effective management of estuarine systems.

Coastal waterways most susceptible to enhanced sedimentation

Wave-dominated estuaries, especially intermittently closed coastal lakes and lagoons, and wave-dominated deltas, have a high sediment trapping efficiency and are susceptible to increases in the magnitude of sediment loads carried by rivers and creeks.

Tide-dominated estuaries are common in northern Australia. In these areas a significant proportion of the catchment-derived sediment may be trapped in the adjacent floodplains, however, monsoonal floods usually export sediment into the open sea. These systems are also characterised by strong tidal currents that can rework sediment deposited around and outside the mouth of the estuary into intertidal areas [5].

Considerations for measurement and interpretation

Sedimentation rates are assessed by accurately dating sediment cores taken from coastal waterways.

Dating methods:

• Lead 210 - this technique works best on mineral and organic-rich sediment and is the main method used for measuring rates of sedimentation that have occurred during the past 150 years (follow link for a more detailed discussion of this technique).
• Radiocarbon (¹⁴C), optically stimulated luminescence (OSL) and thermoluminescence (TL) - methods useful for dating sediments of prehistoric age. ¹⁴C is most useful on organic marine sediment older than ~600 years (due to the marine reservoir effect) and younger than 40,000 yrs. OSL and TL are applied to the mineral component of sediment (quartz, feldspar) and have a greater age range than ¹⁴C. The OSL method can also be used to date sediment deposited during the past few decades.
• aspartic acid racemisation - this technique may be applied to sediments deposited <600 years ago (follow link for a more detailed discussion of this technique). This method is also referred to as Amino Acid Racemisation (AAR) dating;
• Approximate (proxy) dates determined from distinct layers in the sediment horizon related to known events within the catchment. These include major erosion events such as floods, the onset and cessation of mining or pesticide use, the first appearance of radioactive fallout (mainly ¹³⁷Cs) from atmospheric nuclear weapons tests, and the first occurrence of exotic pollen. In addition to providing their own chronology, these horizons are used to validate the above dating methods.

Potential complications:

• reworking of surficial sediments (i.e. the top few centimetres by currents and burrowing animals);
• bioturbation and bioirrigation of sediment;
• advective transport of sediment in tunnels of burrowing animals.
• diffusion of soluble sediment;
• contamination of sediment by radiocarbon older than the sediment;
• incomplete bleaching of the quartz and feldspar (leads to an overestimate of the luminescence age).

Table 1. Sedimentation rates (mm a^-1) for several wave-dominated estuaries in southeastern and southwestern Australia. See references 3, 6, 8, 9, 10 and 17-27 for details. The GA and GA/ANSTO unpublished data represent preliminary results only. NOTE: CB = Central Basin, FD = Fluvial Delta.

Existing information and data

A selection of sedimentation studies are listed in the References below. In addition to the normal scientific publications, sedimentation rate data may be found in the various state agencies responsible for estuary management and research, commonwealth institutions involved in land and coastal management (e.g., CSIRO Land and Water; Geoscience Australia) and university departments involved in coastal geological research.

References

1. Hodgkin, E. P. and Hesp, P. 1998. Estuaries to salt lakes : Holocene transformation of the estuarine ecosystems of south-western Australia. Australian Journal of Marine and Freshwater Research 49, 183-201.
2. Neil, D.T. 1998. Moreton Bay and its catchment: seascape and landscape, development and degradation. In Tibbetts et al. (Eds), Moreton Bay and Catchment. School of Marine Science, University of Queensland, p. 3-54.
3. Chenhall, B.E. et al. 1995. Anthropogenic marker evidence for accelerated sedimentation in Lake Illawarra, New South Wales, Australia. Environmental Geology 26, 124-135,
4. Hancock, G. J. and Hunter, J. R. 1999. Use of excess 210Pb and 228Th to estimate rates of sediment accumulation and bioturbation in Port Phillip Bay, Australia. Australian Journal of Marine and Freshwater Research 50, 533-545.
5. Chappell, J. and Woodroffe, C.D., 1994. Macrotidal Estuaries. In: Coastal Evolution : Late Quaternary shoreline morphodynamics. Cambridge University Press, Cambridge, pp. 187-218.
6. Brooke, B. 2002. The role of sedimentological information in estuary management. Proceedings of Coast to Coast 2002 - "Source to Sea", Tweed Heads, pp. 31-34.
7. Hodgkin, E. P. and Clarke, R. 1989. Estuaries and coastal lagoons of south Western Australia. Environmental Protection Authority, Estuarine Studies Series 5. Stokes Inlet and other estuaries of the Shire of Esperance.
8. Hancock, G.J. 2001. Sediment accumulation in central Moreton Bay as determined from sediment core profiles. Report on Sediment Source Project Phase 3, Part A. CSIRO Land & Water, Canberra, .
9. Jones, B.G. and Chenhall, B.E. 2001. Lagoonal and estuarine sedimentation during the past 200 years. In Archives of human impact of the last 200 years, Environment Workshop, Proceedings. Australian Institute of Nuclear Science and Engineering, Sydney, p. 42-46.
10. Sloss, C. 2001. Holocene stratigraphic evolution and recent sedimentological trends, Lake Illawarra, NSW. Unpublished BSc Honours Thesis, School of Geoscience, University of Wollongong.
11. Thornton, J.A., McComb, A.J., Ryding, S. O. 1995. The role of sediments. In McComb A.J. (Ed.), Eutrophic shallow estuaries and lagoons. CRC Press; Boca Raton, Chapter 13, 205-223.
12. Chenhall, B.E. et al. 1994. Ash distribution and metal contents of Lake Illawarra bottom sediments. Australian Journal of Marine and Freshwater Research, 45, 977-992.
13. McComb, A. J. & Lukatelich, R. J. 1995. The Peel-Harvey estuarine system, Western Australia. In McComb A.J. (Ed.), Eutrophic shallow estuaries and lagoons. CRC Press, Boca Raton, p. 5-17.
14. McMinn, A., et al. 1997. Cyst and radionucleotide evidence for the recent introduction of the toxic dinoflagellate Gymnodinium catenatum into Tasmanian waters. Marine Ecology Progress Series 161, 165-172.
15. Harle, K.J., Heijnis, J.,F., and Zawadzki, A. 2002. Reconstructing sedment and pollution histories of coastal lakes and estuaries using isotopic, geochemical and microfossil techniques: An important management toolbox. Proceedings of Coast to Coast 2002 - "Source to Sea", Tweed Heads, pp. 141-144.
16. Appleby, P.G. and Oldfield, F. 1992. Application of lead-210 to sedimentation studies. In Ivanovich, M. and Harmon, R.S. (Eds), Uranium-series Disequilibrium: Applications to the Earth, Marine and Environmental Sciences. Clarendon Press, Oxford, pp. 731-778.
17. Hancock, G.J. 2000. Identifying the source of resuspended sediment in an estuary using the 228Th/232Th activity ratio: The fate of lagoon sediment in the Bega River estuary, Australia. Marine Freshwater Research 51, 659-667.
18. Marston, F. Prosser, I., Hughes, A., Lu, H., and Stevenson, J. 2001. Waterborne erosion - an Australian Story, CSIRO Land and Water, Canberra, Technical Report 17/01. http://www.clw.csiro.au/publications/technical2001/tr17-01.pdf
19. Baumber, A. 2001. Holocene infill and evolution of Lake Wollumboola, a saline coastal lake on the NSW south coast. Research Report, Environmental Science Program University of Wollongong.
20. King, R.J. and Hodgson, B.R. 1995. Tuggerah lakes system, NSW, Australia. In A.J. McComb (Ed), Eutrophic shallow, estuaries and lagoons. CRC Press; Boca Raton, p. 19-29.
21. Logan, G.A., Fredericks, D.J., Smith, C. and Heggie, D.T. 2001. Sources of organic matter in Wallis Lake. AGSO Research News Letter 34, 15-20.
22. McLaughlin, L.C. 2000. Shaping Sydney Harbour: sedimentation, dredging and reclamation 1788 - 1990s. Australian Geographer, 31(2), 183-208.
23. Barnett, E.J. 1994. A Holocene paleoenvironmental history of Lake Alexandrina. Journal of Paleolimnology, 12, 259-268.
24. Hodgkin, E.P. and Hesp, P. 1998. Estuaries to salt lakes : Holocene transformation of the estuarine ecosystems of south-western Australia. Australian Journal of Marine and Freshwater Research, 49, 183-201.
25. May, D. and Stephens, A. (1996). The Western Port Marine Environment. Publication 493, Environment Protection Authority, Victoria.
26. Hancock, G.J., Olley, J.M. and Wallbrink, P.J. 2001. Sediment transport and accumulation in Western Port, Report on Phase 1 of a study determining the sources of sediment to western Port, CSIRO Land and Water, Environmental Hydrology, Canberra Technical Report 47/01, November 2001. (http://www.clw.csiro.au/publications/technical2001/tr47-01.pdf)
27. Sloss,C.R., Murray-Wallace, C.V., Jones, B.G., Wallin, T., 2004. Aspartic acid racemisation dating of mid-Holocene to recent estuarine sedimentation in New South Wales, Australia: a pilot study. Marine Geology, In Press.

Author

Brendan Brooke, Geoscience Australia

Contributors

Gary Hancock, CSIRO Land and Water