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Geo-Mar Lett (2017) 37:163-170 
*£) Springer 
algorithm for natural oil dispersion in PADM was modified as 
suming that at each time step 100% of surfaced oil becomes 
dispersed, regardless of its viscosity. 
For each element of the ensemble of simulations (both with 
and without the assumed use of dispersants), the percentage of 
the released pollutant that crossed sensitive tidal flats (blue areas 
in Fig. 2) was evaluated at hourly time steps for the 7 day drift 
period. Additionally, times until the first 10% of oil polluting 
tidal flats arrived were monitored. Given the large ensemble of 
pair-wise simulations with and without dispersant application, 
only those hypothetical releases were counted as being success 
fully combated using chemical dispersants that fulfil two 
conditions; 
1. Mean significant wave heights prevailing at the respective 
release position were within the range of 0.5-3.0 m during 
the first 6 h after the assumed accident. 
2. The application of dispersants decreased the amount of oil 
entering sensitive areas by at least 95%. 
The first condition refers to Allen (1988, cited in Fingas 
2004), who specified wave heights between about 0.5 and 3 m 
as a prerequisite for successful dispersant application. Although 
chemical dispersion needs a certain level of wave energy for 
mixing, the application of dispersants is unnecessary and ineffec 
tive if the wave energy is high enough for sufficient natural 
dispersion. The ocean wave information was taken from hindcast 
simulations (Groll and Weisse 2016) for the North Sea within the 
framework of coastDat (Weisse et al. 2015). The underlying 
model was version 4.5.4 of the third-generation spectral wave 
model WAM (WAMDI-Group 1988). The model calculated di 
rectional wave spectra on a 0.050x0.075° latitude-longitude grid 
(approx. 3x3 NM) with 35 frequencies and 24 directions. Wave 
parameters, such as the significant wave height, were stored on 
an hourly basis. At the lateral boundaries, wave spectra from a 
North Atlantic simulation were used to take into account wave 
fields generated outside the North Sea. Wind forcing for the wave 
data originated from a regional atmosphere hindcast simulation 
using the NCEP/NCAR reanalysis (Kistler et al. 2001) with a 
spectral nudging scheme (Geyer 2014). 
The second of the above conditions seems to be very restric 
tive, but it must be seen in the context of the assumption of a 
100% effective dispersant, ff efficacies depending on oil type, 
weathering processes and environmental conditions were taken 
into account, then the threshold of pollution reduction would 
probably have to be adjusted differently. 
Fig. 2 Probabilities that 
application of a 100% effective 
chemical dispersant would 
prevent at least 95% of the 
released oil from entering 
sensitive areas (blue), considering 
a maximum drift time of 7 days. 
Lines are the 10 m (grey) and 20 
m (blue) depth contours 
7°0'0"E 
7°30'0"E 
8°0'0"E 
8°30'0"E 
9°0'0"E 
Probability 
successful 
dispersant 
application 
65% 
55% 
45% 
35% 
25% 
15% 
5%