Measuring Dredging Spill Without Cables on the Seafloor

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 SediLink radio modem with an XBee Pro 900 MHz radio installed, mounted directly on a SediMeter SM3A
The SediLink radio modem with an XBee Pro 900 MHz radio installed, mounted directly on a SediMeter SM3A

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.

Real-time monitoring in action using a SediLink radio modem for telemetry to the SediMeter.
Real-time monitoring in action using a SediLink radio modem for telemetry to the SediMeter.

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.

SediMeter and the Uppsala School of Physical Geography

Eighty-five years ago a student crossed a bridge in Sweden and planted the seed of a new paradigm in Geography. A paradigm that eventually led to the invention of the SediMeter.

The article below originally appeared in the Lindorm Blog.


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.

Filip Hjulström (sitting) and Åke Sundborg in 1957.
Filip Hjulström (sitting) and Åke Sundborg in 1957.

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!

Valter Axelsson in the Cachí Reservoir, Costa Rica, putting his quantitative X-ray based method of estimating sediment bulk density to good use.
Valter Axelsson in the Cachí Reservoir, Costa Rica, putting his quantitative X-ray based method of estimating sediment bulk density to good use (1989).

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.

Bengt Nilsson (left) and Ulf Erlingsson (author) in Örserumsviken, Västervik, Sweden, 1999.
Bengt Nilsson (left) and Ulf Erlingsson (author) in Örserumsviken, Västervik, Sweden, 1999.

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.

Siltation measured with a SediMeter™

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.

Sedimentary processes measured in African reservoir

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 reservoir just before deploying the SediMeter™
The reservoir just before deploying the SediMeter™

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.

SediMeter™ data from the reservoir in Zambia. The intensity chart (top) shows the turbidity values in shades from dark blue through beige to white (low to high turbidity). The bottom chart shows level (black line, left y-scale), temperature (green line, left y-scale), and turbidity average of the top 6 detectors (magenta line, right y-scale).
SediMeter™ data from the reservoir in Zambia. The intensity chart (top) shows the turbidity values in shades from dark blue through beige to white (low to high turbidity). The bottom chart shows level (black line, left y-scale), temperature (green line, left y-scale), and turbidity average of the top 6 detectors (magenta line, right y-scale).

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.

More information available from lindorm.com

SediMeter™ deployment in seagrass off mangroves

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.

This photo was taken at low tide from the position where the SediMeter™ was deployed.
This photo was taken at low tide from the position where the SediMeter™ was deployed.

The SediMeter™ was placed in a small field of sand within the seagrass-covered bottom.

The SediMeter™ was placed out of reach of seagrass leaves, to avoid having them impact the measurements as they move in the waves.
The SediMeter™ was placed out of reach of seagrass leaves, to avoid having them impact the measurements as they move in the waves.

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.

Two days of recordings of turbidity (top) and the interpreted level (bottom).
Two days of recordings of turbidity (top) and the interpreted level (bottom).

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.

Note how the sediments turn darker the last day, from about 3 to 14 cm below the bottom surface. This coincided with a turn of wind direction from NE to SE.
Note how the sediments turn darker the last day, from about 3 to 14 cm below the bottom surface. This coincided with a turn of wind direction from NE to SE. Someone turned the instrument in the holder at the time marked by the red cursor, which explains the momentary change in values on most of the optical backscatter detectors.

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.

More information available from lindorm.com

On “random errors”

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.

The data in the USACE report clearly shows that the errors are not random, yet they assumed that they were and calculated statistics as if they were.
The data in the government report clearly shows that the errors are not random, yet they assumed that they were and used statistics completely wrong. Data from Appendix B in the report  DOERT11.

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.

Sedimentation from suspension measured with a SediMeter in a 40 cm deep tank. And the USACE claim it can't be used to detect siltation. Hard to imagine a more absurd claim.
Sedimentation from suspension measured with a SediMeter in a 40 cm deep tank. And the government claims it can’t be used to detect siltation! Hard to imagine a more absurd claim. The red curve shows the level with cm scale on the left, while the right side shows relative turbidity.

If you have something to hide don’t deploy a SediMeter, since it was designed specifically for siltation monitoring with utmost transparency.

The best siltation monitoring system in the world

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.

Dr. Ulf Erlingsson
Dr. Ulf Erlingsson, CEO of Lindorm, Inc.