IADC: Challenges of Optical Backscatter Monitoring

In the latest edition of their Terra et Aqua magazine, the International Association of Dredging Companies (IADC) put a special spotlight on the challenges of optical backscatter monitoring in mixed sediment environments.

Optical backscatter instruments (OBS) are very popular instruments for measuring suspended sediments in the field. In this IADC article, an analysis is given based on laboratory tests, numerical analysis and field measurements.

OBS instruments are simple and effective devices for measuring turbidity. The signal needs a transfer function to convert turbidity to total suspended matter concentration (TSMC). The transfer function is dependent on several variables, notably the grain size distribution.

There is an almost linear relationship between turbidity (e.g. given in NTU) and TSMC for specific sediment properties. Suspended sediment properties may shift in time either due to flocculation or through erosion and deposition, which may result in erroneous results as calibrations are usually made for stationary conditions.

The present analysis demonstrates that for stationary sediment properties, there is a linear relationship between turbidity and TSMC, but also that the relation is not linear if the sediment properties shift in time.

The conversion factors are shown to tend to an exponential function of median grain size and thus the calibration function between turbidity and TSMC for a mixed sediment environment also becomes exponential.

This is demonstrated through thorough analysis of excessive field measurements of suspended sediment dynamics in a mixed sediment non-tidal lagoon on the Baltic coast in southern Denmark.