BfR-Wissenschaft 
31 
6 Regions where the application of dispersants can be expected to be bene 
ficial - an assessment based on drift modelling 
Ulrich Callies, Fabian Schwichtenberg 
Helmholtz-Zentrum Geesthacht (HZG), Geesthacht, Germany 
Introduction 
The use of hydrodynamic models for predicting oil slick movements and effects of weathering 
processes is nowadays an essential component of any contingency planning. When the use 
of dispersants is an option, hydrodynamic modelling can also support corresponding decision 
making. 
A key effect of dispersants is that they remove oil from the surface and allow it to be mixed 
into the water column. An essential consequence of this is that oil becomes sheltered from 
the direct influences of wind forcing, which is the most important driver of oil slick movements 
(cf. Figure 6.1). 
Oil slick at the water surface Dispersed oil in the water column 
Wind 
Figure 6.1 : Sketch of dispersed oil becoming sheltered from wind forcing after dispersion. 
Changes in the drift paths of released oil brought about by the application of chemical dis 
persants can be very substantial. Wind forcing lets an oil slick drifting on the water surface 
move faster and possibly also into other directions than the water body underneath. There 
fore, among others a major effect of chemical dispersant application can be a shift to less 
sensitive areas being endangered by an oil spill. 
Flydrodynamic simulations can be used for studying such effects. Figure 6.2 summarizes 
results of simulations that refer to a hypothetical location close to where the PALLAS acci 
dent took place in autumn 1998 (Reineking, 1999). Simulations focus on the drift problem, 
disregarding any oil weathering processes. Linking the particle tracking module PELETS 
(Callies et al., 2011) to hydrodynamic fields from the operational model of Federal Maritime 
and Hydrographic Agency (BSH) (Dick et al., 2001), particles were tracked over a five day 
time span. Such model simulations were set up for each low tide situation within the year 
2008. 
Details of two of these simulations are shown in the left panel of Figure 6.2. For oil assumed 
to be released on Jan 7 (blue dots), all untreated oil drifting at the water surface is simulated 
to end up in the tidal basin. By contrast, the oil-dispersant mixture in the water column stays 
outside the Wadden Sea. For a second simulation, assuming an oil release on Nov 5 (red 
dots), the situation is more complex. In this case chemical dispersion cannot fully prevent the 
pollutant from entering the sensitive Wadden Sea. At the same time, however, the impacts of 
untreated oil on the coast are delayed by a longer drift path which may offer the possibility for 
efficient mechanical cleaning before the oil slick would hit the Danish coast more to the north. 
Already this simplified example, not yet addressing any toxicity issue, illustrates that practical