From WikiLetters
Jump to navigation Jump to search

Target Article

Ridd, P. V., (1992). A sediment level sensor for erosion and siltation detection. Estuarine, Coastal and Shelf Science.35(4), 353-362. Available at: doi:10.1016/S0272-7714(05)80032-0

Significant Claim

In this article an instrument for measuring erosion and deposition of sediment for marine environments is described. The instrument uses the difference in the electrical conductivity of the sediment and overlying water as a basis for detecting the position of the interface. The principle is similar to that used in geophysical “resistivity” surveys. In this case a vertical rod carrying electrodes is driven into the sediment measuring the fields created by passing an AC current between various pairs of electrodes. The measurements of the voltage field can be used to determine the position of the interface.

Possible caveat with this article

Although these instruments can give excellent data under some circumstances, the main problem is that the surface roughness of the seabed can limit application. This roughness may be caused by animal burrows (Aller, 1978[1]), ripples in the sediment, and other factors that likely vary on time scales from seconds to weeks. If a particular application requires the measurement of a small amount of net deposition occurring over long periods, then the varying roughness of the seabed may dominate the long term net accumulation signal. The example given in this article describes an instrument that measured mega-ripple migration on the seabed, and for this application the instrument design is suitable. However, for applications such as measuring deposition of sediment some distance from a dredge operation, where small amounts of sediment are falling on the seabed, it is likely that variations in the bottom roughness will be measured, implying very rapid deposition or erosion, and completely dominating the slow net deposition.

Part of the problem with the instrument is that it measures the position of the seabed very close to the rod upon which the electrodes are mounted. The area of the seabed that it effectively measures is similar in dimension to the electrode spacing which is in the order of a few centimetres. Ideally the area for averaging of the measurement should be considerably larger than the scale of the bottom roughness if the effect of variations in bottom roughness is to be “averaged out”.

One solution to the problem above is to use electrode configurations that effectively average the sample over a wider area. One possibility would be to use multiple horizontal wire electrodes, arranged similarly to an agricultural wire fence. These would average over the length of the wires which could, in various circumstances, be made larger than the scale of the local bottom roughness.


  1. * [1] Aller, A.C., 1978. THE EFFECTS OF ANIMAL-SEDIMENT INTERACTIONS ON GEOCHEMICAL PROCESSES NEAR THE SEDIMENT-WATER INTERFACE. Estuarine Interactions. 157-152., doi:10.1016/B978-0-12-751850-3.50017-0


In WL-Letters you have the freedom to respond to any caveat that may be inaccurate.
-- Andutta (talk) 18:12, 6 July 2022 (UTC)