Ridd’s background with dredging

Throughout my career I have been closely involved with the dredging issue. When, in 1984, I started as a scientist at the Australian Institute of Marine Science, one of my first duties was to help develop a new instrument for measuring suspended sediment concentrations using an optical backscatter sensor. This used a beam of light sent into the water; the amount of light that scattered back to the sensor, once calibrated, was a crude measure of the concentration of sediment. Eric Wolanski, my supervisor who had conceived the idea, wanted to take measurements of sediment concentrations using logging instrumentation. Until this time, we had been restricted to taking individual water bottle samples – a hopelessly slow and time-consuming method. The first instrument, called the “Mudprobe,” was designed to measure the change in concentration of sediment with water depth and was used mostly for mangrove swamp and estuarine work. The instrument was lowered into the water and took a measurement every second or so.

Downloading a nephelometer on Magnetic Island in 1993

After joining James Cook University in 1989, I began to work on an instrument that could be deployed for many weeks as there was a growing interest in movement of sediment close to coral reefs. The initial problem was that the optical window on the instruments would get covered with a thin film of algal slime within a couple of days which completely destroyed the accuracy of the equipment. It was most severe in coastal waters where the muddy seabed cycled vast quantities of nutrients that fertilised algae on our instruments.

The solution that Peter Smith, Stephen Smith and I devised was perhaps obvious: a windscreen wiper made from a model aircraft servo motor driving a rubber pad that cleaned the optical window every couple of hours. To my knowledge, this was the first use of this system which is now routine practice on such instruments. The improvement in measurements was dramatic. We could now measure sediment concentrations every few minutes for up to six weeks and finally understand the conditions under which sediment was suspended and moved around the coral reefs.

The first major use of these instruments was in 1993, when fourteen were used to monitor the capital deepening of the Townsville Port and shipping channel (Benson et al, 1994).[1] With my co-workers[*] in JCU’s Marine Geophysical Laboratory, our job was to monitor the influence of the dredge on sensitive ecosystems which were in some cases only a few kilometres distant from the dredge. It was easily the biggest monitoring study of dredging on coral reefs to that date, and there was very little idea how far dredge plumes would travel. Instruments were installed on the fringing reefs of Magnetic Island, Middle Reef, and other areas where seagrass beds were known to exist close to the dredge dump ground.

From 1993 until 2018, through James Cook University’s Marine Geophysical Laboratory, I ran a consultancy operation monitoring dredgers. The instrumentation improved in that period and light, waves, sediment concentration, and sediment deposition can now be routinely measured. At its peak during the mining boom up to a dozen people were employed, and we worked in most ports in Northern Australia. James Cook University probably earned more than $10 million from this service.

[*] Bruce Wilson, Arnstein Prytz and Piers Larcombe

[1] Benson, L.J., Goldsworthy, P., Butler, I. and Oliver, J.C. eds., (1994). Townsville Port Authority capital dredging works 1993: environmental monitoring program. Townsville, Qld: Townsville Port Authority.

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