The year 2017 promises to be the most interesting year ever for the SediMeter, with many new features and even new models in the works. Lindorm has today announced a model with a built-in vibrator that shakes off particles and bubbles that are loosely attached to the sensor. Such particles offset the zero on the turbidity measurements, so getting rid of them is important for accuracy. They have had a model with a mechanical cleaner, but it is pricey, and anything mechanical in a liquid full of suspended sand is an invitation for problem. So the new vibrator model, costing no more than the base model of 2016, might be of great interest for many applications.
Another novelty is that it measures not just straight backscatter but also oblique backscatter, thus effectively doubling the vertical resolution of the turbidity profile.
This double resolution will be offered in all SediMeter versions from 2017 according to underhand information.
The SediMeter is probably one of the best instrument in the world for measuring sediment transport, including siltation resulting from dredging, but there is one little hitch. To get data in real time the instruments must be connected, and cables on the sea floor can get in the way of works — plus they can pose dangers to corals. Although buoys have their disadvantage (the potential of interfering with boat operations) the combination of buoys and cables should be able to solve most if not all challenges.
The radio modem has an UW connector that fits directly into the SediMeter, although for obvious reasons an extension cable will be used offshore. The direct connection is useful in a lab setting, though. Five years ago Lindorm delivered a custom solution of that kind to Taiwan. The requirements of the customer to be able to use the instruments with telemetry both in the lab and in the field was carried over to this design, which works in both settings with no other change than possibly adjusting the transmitting effect on the radios.
Once the monitoring has been initiated, the user interface is the same as when using cables. The window allows watching a single instrument over time, or the present situation of all instruments in the tab Network Real-Time Data. It also allows for alarms by e-mail for excessive erosion, sediment accumulation, or turbidity. It is a complete monitoring system that can handle many sensors in real time. It’s available from Lindorm, Inc. for purchase, leasing, rental, and they also offer consulting services to get the user up and running.
A hundred years ago Physical Geography concerned itself with the description of landforms and processes, and deductions about how these processes had led to those landscapes. Then in the 1930’s a research student in Uppsala called Filip Hjulström crossed the river called Fyrisån every day on his way to the department. He stopped, took a water sample and measured the water level. He then analyzed the sediment concentration and made a quantitative estimation of soil loss through river runoff. Years later he became the professor of the department, and a series of research students dedicated themselves to the quantification of the geomorphological processes: Åke Sundborg (who would succeed him as professor, studied fluvial processes in the river Klarälven), Anders Rapp (who would become professor in Lund, quantified mass transport in the Swedish mountains), John O Norrman (who succeeded Sundborg, studied coastal processes in the lake Vättern), Valter Axelsson (whose homepage is on a “museum domain”, studied delta deposition), and others.
To carry out quantitative geomorphological studies frequently requires inventing new instruments and methods. The department got a world-class Geomorphological Laboratory with flumes and a professionally staffed workshop. Valter Axelsson developed a method for quantification of recently deposited sediments using X-ray and the rectangular Axelsson corer. Bengt Nilsson developed a suspended sediment sampler for vertical integrated suspended sediment sampling, during the International Hydrological Decade. The sampler was widely used especially in remote parts of the world, and it is still available for purchase – even though it will soon turn 50 years!
I was lucky enough to have Rapp as professor during my undergraduate years in Lund University, and to then come to Uppsala University for my PhD studies. Having access to the Geomorphological Laboratory and the workshop I was able to develop the SediMeter. The purpose of the instrument in my thesis was to determine the onset of bedload transport on nearshore bottoms, and to find out what happens off the “closing depth”. However, already during the initial field trials in 1986 (under the ice of a frozen lake; working near the Arctic Circle does tend to limit the time available for field testing) I found that the instrument had potential applications that went far beyond those initially contemplated.
Since my career took a different path I didn’t continue using the instrument until I decided in 2007 to develop a new, better version. That second generation was again replaced by a third generation in 2013. Electronics have developed tremendously, but the basic design of the sensor has stayed the same, because it works so well.
We now write 2016 and 30 years has passed since the first field deployment of the SediMeter. It has developed into the world’s arguably best system for monitoring siltation caused by sediment spill and pollution from dredging and other works. It is also used to monitoring sedimentation in reservoirs, harbors, and navigation channels, and in laboratory experiments, as well as for monitoring resuspension and erosion.
The Geomorphological Laboratory is, alas, gone, and the Department of Physical Geography has been merged and reorganized, but a number of instruments and samplers developed in the Uppsala School of Physical Geography live on as commercial products – and the SediMeter is one of them.
In this a lab demonstration with computer screen and sedimentation tank side by side, you can see how two different sedimentation events are reflected in the SediMeter™ data. The first consists of soil, so it has a lot of dark and fine matter (humus). The second is washed white sand. Pay attention to how the turbidity (blue line) varies, and how the bottom changes (red line) when sediment is added to the tank and settles out of suspension.
The first ever deployment of a SediMeter™ in Africa was recently made in a shallow reservoir in Zambia, at the start of the rainy season. The first results show great promise. Not only did the SediMeter™ measure sedimentation, but the turbidity profile also suggests that a gas bubble formed below the bottom, and that this gas bubble lifted the bottom by about 2 cm, in two steps. Any other instrument for measuring the bottom level would have recorded this as sedimentation, but the SediMeter™ profile provides the analyst with the necessary information with which to interpret the lithological and sedimentological processes, and thus avoid an erroneous conclusion.
The top, unconsolidated, layer of the sediment pack is dynamic, why it may be essential to monitor not just the bottom level, but the entire interface from several centimeters below to several centimeters above the actual bottom. In this case the complication happened below the bottom, but in other cases there may be a fluid mud layer on top of the “solid” bottom. The definition of bottom may vary depending on the situation. For navigation, the fluid mud is part of the water column, but for using the water in a water intake, the fluid mud is part of the bottom and must be avoided. For this reason, the SediMeter™ vertical turbidity profile gives a much more valuable dataset than a simple bottom level value.
The sediments in the reservoir are very soft. The backscatter values below the bottom (which rose from 21 cm to 23 cm during this period according to the data) reveal that the sediments are stratified, with three lighter layers (more solid) separated by two darker layers (suggesting that they are darker in color, less consolidated, or most likely both; the darker color indicates organic matter, and if it darkens, the onset of anoxic conditions or in extreme cases, the creation of methane gas bubbles).
Around midnight to March 5th (indicated by cursors) a dark line appeared at level 15 cm. At the same time the bottom seems to rise by about one centimeter. The line gets darker about a day later, and the bottom seems to rise another centimeter. On March 7, the bottom sinks about a centimeter, and the dark area within the bottom simultaneously sinks a centimeter before disappearing. This suggests the creation of a gas bubble (sump gas due to anaerobic decomposition of organic material), which lifted the bottom. Then, on March 7, the temperature suddenly dropped from 25º to 22º. The temperature drop increased the solubility of the gas in the water, which would seem to explain why the bubble disappeared and the bottom sank.
The temperature then stayed low for two days, suggesting overcast weather. Thus sudden rise in turbidity suggests that the temperature drop started with a heavy rain shower locally. However, the bottom level does not rise appreciably from sedimentation until one day after the sun seems to have returned, based on the daily temperature fluctuations. This could be taken as a hint that the sediment that is reaching the reservoir is not local, but comes from up river, taking several days to reach this reservoir.
Finally, note that all of this is speculation based on the SediMeter™ data alone, without knowing the local are or conditions. It is offered only as an example of how the data can be used in a study, and that it provides much more information that just the bottom level.
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.