Natural versus dredge sediment

How do dredging volumes compare with natural sediment movements?

What do some hard numbers comparing the sediment exposure from natural resuspension and dredging reveal? The Port of Townsville is an informative example. It has fringing reef corals only a few kilometres from the dredged shipping channel and there is considerable information available about this area. For Townsville, annual maintenance dredging is around 400 000 cubic metres (m3) per year and it will be dredged over a few weeks. Of this quantity, most will never be suspended but will be pumped into the dredge and successfully transferred to the dump area or taken onshore for land reclamation.

In an extreme worst-case scenario, assume that 10 percent is suspended and drifts away from the dredge or the dump ground. The dredge contribution is therefore around 40 000 m3. A typical period for dredging would be around 20 days and, thus, on any given day, there is around 2 000 m3 of sediment in suspension over this period if it is assumed that the sediment will take one day to settle out of suspension. This is also a worst-case scenario as most settles within a few hours. There is thus a sediment exposure of around 2 000 m3 for 20 days or 40 000 m3days.[*] This is the number for comparison with the natural resuspension of mud by waves, which will be calculated below.

The natural resuspension of sediment in Cleveland Bay by waves has never been measured with great accuracy. Larcombe and Ridd (2015),[1] however, estimated that during periods of strong trade winds (perhaps 30 days per year), at least 100 000 m3 will be resuspended. This gives a total exposure of 3 000 000 m3days compared with 40 000 m3days for the dredge. This is around 75 times greater than the exposure from the maintenance dredging. 75 more dredges working in Cleveland Bay would be needed simply to equal the naturally suspended sediment.

As mentioned previously, maintenance dredging is only ever required because mud is naturally moving around in the system from waves and currents. A large dredging requirement arises because a very large quantity of sediment is mobile in the first place. Thus, any nearby ecosystems will inevitably be subjected to regular high concentrations of suspended sediment.

Cyclones suspend far more sediment than trade winds because they produce enormous waves. A medium sized cyclone can suspend around 150 millionm3 of sediment by churning up the mud on the seabed to tens of centimetres depth.[2] It is estimated that, in 1971, Cyclone Althea suspended around 40 million m3 in Cleveland Bay alone.[3] This completely dwarfs the quantity of sediment moved by dredging but is totally ignored in all the major reports warning about the dire condition of the GBR.[4] [5]

Capital dredging that occurs every few decades to enlarge ports will generally be of far higher volume than yearly maintenance dredging. Townsville Port is presently undergoing a major expansion that will move around 10 million m3 of material dredging in the next five years. Most of this material will go to land reclamation, not sea dumping, and is likely to be composed of less fine mud so a smaller percentage of the sediment will drift away from the dredge. But, even if the same extreme worst-case assumptions above are used, the exposure to sediment from capital dredging is still minor, a few percent, compared to the natural resuspension.

The conclusion of this is that the fringing reefs of Cleveland Bay are only slightly affected by dredge sediment. The effect is too small to measure. As for the GBR, the nearest point of which is 60 km away, it is inconceivable that there could be any significant influence. Anyone wanting to advocate a contrary view needs to advance some very compelling evidence; argument and conjecture is not enough.

For those wanting to know more of the detail about the movement of sediment in Cleveland Bay – see Appendix 2. It tells the remarkable history of the formation of the Bay over the last few thousand years. Monumental forces have shaped it. These include the multiple movements of the mouth of the mighty Burdekin River, the rise of sea level, and the role of cyclones in joining the mainland to the mountainous island that became Cape Cleveland. These monumental events occurred over just a few thousand years, and put in context the trivial effect of dredging on these environments during the last century.

[*] Note: the unit m3days is the multiplication of the suspended sediment load in m3 by the time it is suspended. It is a similar idea to a man-hour.  A job that requires 10 man-hours could be done by 10 people working for one hour each, or 5 people working for 2 hours each. 40000 m3days was in this case produced by a suspension of 2 000 m3 for 20 days, but the same number could have been produced by a suspension of 4 000 m3 for 10 days.

[1] See Endnote 6

[2] See Endnote 6

[3] See Endnote 6

[4] McCook, L.J., Schaffelke, B., Apte, S.C., Brinkman, R., Brodie, J., Erftemeijer, P., Eyre, B., Hoogerwerf, F., Irvine, I., Jones, R., King, B., Marsh, H., Masini, R., Morton, R., Pitcher, R., Rasheed, M., Sheaves, M., Symonds, A. and St.J. Warne, M. (2015). Synthesis of current knowledge of the biophysical impacts of dredging and disposal on the Great Barrier Reef: Report of an Independent Panel of Experts. Townsville: Great Barrier Reef Marine Park Authority.

[5] Brodie, J. (2014). Dredging the Great Barrier Reef: Use and misuse of science. Estuarine, Coastal and Shelf Science, 142, pp.1–3.

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