In 2018 the SediMeter™ SM4 model started being used in production monitoring projects. Among the experiences learned was the need for knowledge transfer in order to get the projects on track quickly and efficiently. The SediMeter™ system for sediment spill monitoring is advanced and capable, but this also means that operators need to be trained properly.
Lindorm Inc is therefore offering an introductory course over two days on a beach resort, where the attendees will carry out all parts of a practice deployment, including actually collecting data from a deployment over night.
The fourth SediMeter™, released in 2018, has become an immediate success for sedimentation monitoring. In two projects, on both sides of “The Pond,” SediMeters are now used to monitor sedimentation on sensitive bottoms in real time (article in IDR). The decisive feature in SM4 is that it includes a turbidimeter, apart from the SediMeter sensor itself. This means that for dredging spill monitoring, it handles both the key tasks required: It monitors near-bed turbidity, and it monitors sedimentation on the bottom.
Among the most interesting results so far is the realisation that near bed turbidity and sedimentation are not nearly as closely correlated as one might expect. There can be high near bed turbidity without sediment accumulation, and vice versa, says Dr. Ulf Erlingsson, who has helped the client in interpreting the first results.
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.