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.
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.