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

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