Suspended Transport
Sediments transported in suspension will eventually settle on the bottom, and this process can be detected with the Sedimeter® in real time. The Sedimeter® measures the sediment concentration right through the water-sediment interphase, giving us information about the character of the bottom, as well as the rate of accumulation or re-suspension, and the rate of sediment consolidation.
Sediments accumulate on the bottom and get compacted with a rate that depends on their grain-size and chemistry. Very fine sediments will slowly sink towards the bottom, gradually increasing their concentration in the water. They form a sludge ontop of the bottom, still easily mobile if there is a current. If the environment is tranquil, the water content will continue to decrease until the point where a diver would perceive it as "the bottom".
This experiment was set up to investigate the usefulness of the Sedimeter® for monitoring this very early consolidation phase, by measuring how a suspension of formazin settles.
A 1000 NTU suspension of formazin was created in a 1.5 litre container into which the Sedimeter® was placed. The sampling interval was set to 0.5 hours. The equipment was left for 5 days in darkness. After that time all the formazin was found on the bottom of the container, well below the lowest backscatter detector of the Sedimeter®.
The data were first read into a raster GIS program - MFworks - for treatment. The signal level from each detector was adjusted using the calibration results for this particular sensor, to get rid of the stripes caused by detector variations. The resulting image for the first 8 hours is shown to the right.
means higher concentration
(right=time in 30 min steps,
up=up in 9 mm steps)
Using the calibration values for this particular sensor, the signal level was transferred into NTU values and plotted in the spreadsheet program (using a 3-D graph). The concentration increases initially, from the original value of 1000 NTU to about 1200 NTU in the centre of the water column (one sensor consistently shows a too high value, which must be attributed to dirt or a bubble on the sensor surface).
The most interesting feature of the data set is most evident from the black-and-white illustration: Note how there are two fields with fairly homogenous values, and a sharp - and straight - diagonal line between them. It means that the concentration gradient did not change, instead, the "front" just moved downwards during the settling process. The concentration within the "sludge" is rather constant, both vertically and in time (the precision of the instrument is not good enough to make definite statements based on the small variations observed between the detectors).
(click on the diagram for full resolution)
In shallow seas like the Baltic Sea, there are large areas affected by sedimentation of mud that is re-suspended from time to time. Shallower bottoms may experience re-suspension more than once a month, whereas deeper areas may have re-suspension events once every 10 years. In between, organic matter accumulates on the bottoms. These sediments play a significant role in the nutrient supply to the sea water. The study of the temporal character of accumulation and resuspension of such sediments are therefore essential in order to understand the eutrophication of the sea. The study may be made using Sedimeter® instruments.
