On the Application of OBS Measurements on Mixed Sediments and the Coupling to Fixed Turbidity Limits
CEDA Dredging Days 2019
U. Lumborg and K. Bundgaard
Abstract: In connection with dredging operations contractors often encounter the so-called turbidity limits. A turbidity limit is a limit the spill from the dredging operation is not allowed to exceed. Optimally these limits are based on site specific EIA's or site-specific investigations on local sensitive receptors. In many cases these limits are given as fixed limits e.g. total suspended matter concentration or similar.
Optical backscatter instruments (OBS) are very popular instruments for measuring suspended sediments in the field. In this paper, an analysis of the sensitivity of optical backscatter (OBS) measurements to sediment grain size and the applicability of OBS measurements in mixed sediment environments, like dredge plumes, is given. The analysis is given based on laboratory tests, numerical analysis and field measurements. There is an almost linear relationship between turbidity and TSMC for specific sediment properties. However, suspended sediment properties may shift in time either due to flocculation or through erosion and deposition, which may result in erroneous results as conversions are usually made for stationary conditions (e.g. Downing, 2006).
The present analysis demonstrates that for stationary sediment properties, there is a linear relationship between turbidity and TSMC. When this analysis is done on several sediment fractions it is seen that conversion factors are correlated to the grain size distribution which in practice makes it a function of time and space. It is also shown that a given turbidity limit in TSMC does not correspond to one light dampening effect but to a range of light dampening effects depending on the grain size distribution. Therefore, if the goal for a given turbidity limit is based on an allowable light dampening a limit based on TSMC will not always be suitable.
Keywords: Mixed sediment, sediment monitoring, optical backscatter, grain size distribution