Geo-Mar Lett (2017) 37:163-170
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Ô Springer
French-McCay and Graham (2014) concluded that using
dispersants may generate a net environmental benefit if the
region within which water column biota would be exposed
to dispersed oil is smaller than the region within which wild
life would be exposed to oil floating on the surface. Flowever,
for the German North Sea coast and Wadden Sea, decision
making should also consider that in shallow intertidal areas
interactions of dispersed oil with the sediment could be par
ticularly relevant. Several biological habitats and communities
are highly sensitive to untreated oil slicks, including mussel
beds, shell mounds, sea grass meadows, salt marshes and
stocks of resting and moulting birds. During 1 to 2 months
in summer moulting birds are just swimming or drifting on the
water, unable to fly. During this time they are much more
vulnerable to untreated oil slicks than to chemically dispersed
oil. Stock sizes and population dynamics are well known and
monitored in the region (e.g. Koffijberg et al. 2003; Garthe
et al. 2012). In some areas, stocks can reach far more than
100,000 individuals.
One aspect that often attracts less attention than it deserves
is the simple fact that an oil slick’s drift path would be altered
by the application of chemical dispersants (API 1999). Oil
dispersed in the water column remains sheltered from extra
wind forcing as the most important driver for oil slicks drifting
on the surface. Such changes of drift paths may prevent sen
sitive areas from being polluted, depending on prevailing
wind and marine transport conditions. The present study fo
cuses on this purely physical effect. Based on large ensemble
simulations, chances of a successful reduction of pollution in
sensitive areas were quantified probabilistically as a function
of the location where an accident takes place. The criterion
“successful” is defined as a reduction of at least 95% of oil
entering sensitive areas. To enable a realistic representation of
a large variety of different meteo-marine conditions, the sim
ulations were initiated every 28 h within the 7 year period
2008-2014. The study postpones the dispersant-specific ques
tion of how effectively a particular oil type would be dispersed
in a given environment. Of course, that is not to say that
decision making in real hazardous situations would not re
quire the inclusion of case-specific information regarding var
ious aspects of the specific problem concerned.
The study is part of a German scientific project carried out
by an interdisciplinary research consortium addressing vari
ous European and German regulations to assess the state ofthe
marine environment in the German Bight, SE North Sea (for
overviews, see Winter et al. 2014; Winter et al.. Introduction
article for this special issue). Until now, the capability of dis
persants to protect coastal areas along the German part of the
Wadden Sea is unknown, especially in the case of inshore oil
spills. Therefore, the purpose of this work is to delineate the
maximum range of drift path changes brought about by chem
ical dispersants simply by decreasing wind forcing. This in
formation provides a key ingredient for a follow-up net
environmental benefit analysis (NEBA) focusing on the eco
logical consequences of dispersant use.
Methods
The study analyses ensembles of hypothetical oil releases
from 636 cells of a regular grid with about 5 km resolution.
Disregarding the initial spreading phase, 2,190 hypothetical
oil spills (every 28 h in the years 2008-2014) were initiated
from each grid cell. For all accidents, 7 day drift paths were
calculated both with and without the assumed application of
chemical dispersants. In each of these about 2x1.4 million
simulations, 1,000 tracer particles represent either pure oil or
an oil-dispersant mixture.
Ensembles of simulations started according to a regular
time schedule represent both frequencies and successions of
different weather conditions in a realistic way. In this study, oil
drifts are calculated using the offline Lagrangian transport
toolbox PELETS-2D (Program for the Evaluation of
Lagrangian Ensemble Transport Simulations, (Tallies et al.
2011), designed for conducting and evaluating comprehensive
ensemble simulations based on 2D hydrodynamic fields.
Marine currents were specified based on the 3D operational
circulation model BSFtcmod of the Federal Maritime and
Hydrographic Agency (BSH) with 900 m resolution in the
German Bight (Dick et al. 2001). BSHcmod is forced by
winds from the regional model COSMO-EU (Consortium
for Small-Scale Modelling) of the German Meteorological
Service (DWD) with a spatial resolution of about 7 km over
Europe. Both atmospheric and marine fields are available on a
15 minute basis. For drifting oil slicks, currents from the top
layer of BSHcmod were selected, plus an extra wind drift
parameterized as 0.018 times the wind velocity in 10 m height
(Huber et al. 1987). By contrast, oil-dispersant mixtures sub
merged into the water column were assumed to be transported
with vertically averaged BSHcmod currents. All particle tra
jectories were calculated including the effects of random
movements due to subscale turbulence effects.
Using a 2D approach obviously means a substantial simplifi
cation. Further simplifications stem from the complete neglect of
any oil weathering processes. Implications of these assumptions
were exemplified by comparing results of the simplified ap
proach used in this study with corresponding detailed simulations
based on the fully fledged Lagrangian transport model PADM
(PArticle Dispersion Model). PADM is the numerical core of the
drift forecasting and backtracking system SeatrackWeb (STW;
Ambjom et al. 2013; Matknann et al. 2014) operated by the BSH
to provide advice to the Central Command for Maritime
Emergencies Germany (CCME, Havariekommando). PADM is
forced by full 3D current fields from BSHcmod and accounts for
all relevant weathering processes. To mimic the influence of
100% effective chemical dispersants evaluated in this study, the
