This field application shows how the SediMeter™ can be useful for ecologists and sedimentologists alike. The instrument was deployed off a mangrove shore in Biscayne Bay, south of Miami, about 0.5 m under the low tide level. It was deployed by wading, and the holder was pushed down rather than screwing it down, so as to minimize the disturbance of the sedimentary structure.
The SediMeter™ was placed in a small field of sand within the seagrass-covered bottom.
The instrument was deployed at low tide and retrieved at low tide 4 days later. Unfortunately a local fisherman had seen the instrument and turned it in the holder after two days, so we will only show the first two days before the disturbance.
First we note that the bottom level was not stable (bottom graph). It varied by several millimeters up and down. A look at local wind data showed that it was caused by the waves. When the wind died down, the level stabilized (after about 6 AM UTC on the 21st).
Secondly, take a look at the intensity chart in the top graph. The top of the sediment pack has more reflectivity that the interior of it. This is likely a reflection of the poor oxygenation level in the sediments, and shows another possible application of the SediMeter™: To measure when anoxic conditions appear, and at what depth. The instrument can detect this since anoxic sediments turn increasingly black.
In this case the wind shift may have brought in less well oxygenated water, or the lower wind speed can have decreased the gas exchange with the atmosphere, which in turn seems to have led to a decrease of oxygenation of the sediments, as evidenced by them turning darker on February 23rd.
The differences between the dotted and straight lines is what a 2013 US government report calls “random errors”. In reality it is a non-linearity error, a systematic error that is repeated cyclically every centimeter. By labeling it a “random” error the accuracy estimate of the instrument was downgraded by more than on order of magnitude, which was then used to justify not to use it to monitor sediment spill at the dredging of the Port of Miami—an operation that killed some 200 acres of coral reef due to siltation that was not monitored in real time.
Of course, this non-linearity error does not in any way prevent the instrument from measuring siltation with sub-mm accuracy, since the error is proportional to the distance measured: The shorter the distance, the smaller the error. The report also, for good measure, doubled the error in the conclusions, without any valid justification whatsoever (they claimed that difference data were level data and therefore doubled the error to account for difference data—which it already was from the start).
Furthermore, what matters in siltation monitoring is actually not the accuracy in determining the bottom level, but rather if it’s possible to detect siltation. This self-evident truth is easy to forget. The SediMeter measures the turbidity at 37 different levels. It’s like a scanner that delivers a vertical profile through the bottom, like a “photo” with elevation up and time to the right. It’s much more information than just a bottom level, all of which makes the SediMeter a tremendous asset for monitoring siltation in real time.
If you have something to hide don’t deploy a SediMeter, since it was designed specifically for siltation monitoring with utmost transparency.
When Ulf Erlingsson invented the SediMeter™ for his doctoral dissertation in 1985, the goal was to detect incipient sediment motion on the bottom of the sea, so as to compare that with wave and current data to see what combination of processes led to the initiation of sediment transport for different grainsizes, waves, and currents. The question was how to define sediment transport, but once the SediMeter was invented, it became a non-issue: The instrument is capable of detecting the difference that a single grain of sand makes in front of the sensor, and it is stable enough to give the same value when nothing changes. Thus, the definition became “what the instrument can detect,” and that was pretty much anything that happened to the sediments.
Fast forward to the 1990’s, and now Dr. Erlingsson was hired as an expert in sediment spill monitoring by the Swedish government, during the building of the Öresund bridge and tunnel between Sweden and Denmark, and the dredging of a new navigation channel to the Baltic Sea. Seeing this ambitious project from the front seat, from the regulator’s perspective with full insight into the executor’s monitoring and analysis, he became convinced that it would be more cost-effective, and wise, to use a monitoring system of stationary SediMeters™ in a real-time network, monitoring the sediment accumulation and near-bed turbidity directly, and to connect permit conditions to the sensitivity of each biotope.
When Erlingsson in 2006 got an opportunity to manufacture the SediMeter™ instrument himself, he decided to create “the best siltation monitoring system in the world,” based on his experience from the Öresund project. Since he by then lived in Miami, he designed it with the purpose of monitoring hard bottoms—including coral reefs—when there were dredging operations going on nearby. His new version of the SediMeter™ that came out in 2007 was designed specifically for the requirements identified in the siltation monitoring white-paper.
Since the only transparent anti-fouling paint on the market was banned a few years back, he next had to develop a new method for keeping the sensor clean from biofouling. In 2013 he released the third generation SediMeter™, with exactly the same proven sensor, but with a mechanical cleaner integrated in the instrument from the outset (it is also offered without cleaner). It has no logger house at all, since everything has been made to fit on the half inch wide sensor PCB.
Next Dr. Erlingsson turned his attention to wireless networking. All SediMeters™ made in Miami can be networked using RS485, which allows for mile long cables, but cables cost money. After several semi-custom solutions, in 2015 he developed the SediLink™ radio modem with a built-in small solar panel that can sit on a buoy over a single or a few SediMeters™. This allows for mixed networks with radio links and cables. The radio modem has a socket for a radio that the customer himself can mount, meaning that wherever in the world the client is, there is a license-free radio available.
The SediMeter™, its program and network abilities were developed to fit the role of a siltation monitoring system, which was formulated based on experiences from the most ambitious sedimentation monitoring project in the world. That is why Dr. Erlingsson does not hesitate to say that in his opinion, his design is the best siltation monitoring system in the world.
On February 1st, 2016, a public demonstration of the SediMeter™ precision was held in Miami, Florida. During the course of the day, people were given a chance to put sand in the tank and see in real time on a connected computer the sediment level the instrument was measuring with a resolution of 0.001 cm (10 µm, equivalent to the thickness of a typical human hair). The instrument never failed to detect the addition of the 2.2 g of sediment, which corresponds to 0.1 mm in the 162 cm2 tank (given a bulk density of the top few centimeters of 1360 kg·m-3). The event was summed up in this video.