408
J. Murawski and J. Woge Nielsen
Namely, local wave energy gradients (dE/dx in Eqs. (11.4), (11.5) and (11.6)) due
to the topography and wind shadowing effects may produce significant drift currents
and may have a substantial impact on the drift pattern in the study area.
The performed numerical experiments provide evidence for local differences of
the probability of oil landing of up to ±12 % near certain islands. As mentioned
above, the residence time increases by up to 1.3 days at certain locations when wavedriven transport is accounted for. Although the depth gradients that appear at larger
depths exert a minor impact on waves and currents, the persistency of the waveinduced drift and the longer interaction ranges lead to noticeable results. Here, the
interaction range means the length of a drift path on which the drifter experiences
considerable depth gradients. The impact on the probability of landing and on the
residence time increases with the interaction range as oil experiences more and more
wave impetus on the way. Small wave-induced differences of the drift path after a
short time might grow with distance from the release position and may alter the final
drift path significantly.
In the surf zone and close to the shoreline, the release of momentum carried by
beaching waves often produces coastal jets (Longuet-Higgins and Stewart 1964).
The strongest wave-driven currents occur at incidence angles of about 45 ◦ . Such
coastal jets might prevent onshore oil propagation or carry it further to other coastal
sections (Fig. 11.11 A3, B3). The magnitude of the wave-driven drift depends on
whether the waves can exert a cumulative effect along the pathways of oil propagation. In general, the effect is the strongest in the nearshore where (although featured
by strong interaction forces through large wave energy gradients) it is characterized by a rather small interaction range (which is the distance between the seaward
border of the surf zone and the landing point).
The presence of wind waves may increase the probability for oil landing by about
10–15 %. The residence time decreases moderately, by about 4 hours, in these cases.
The decrease in the residence time is twice as large in the upstream direction of the
coastal jet where the number of landing particles is smaller. The range of probability
changes is about 30 % (from +10 % to −20 %). Some particles reach the coast faster
and are counted in the histogram for travel time (Fig. 11.3), but others are travelling
with the coastal jet for a while and are too much diverted from their original path to
hit the shoreline. It should be remembered that wave-induced currents and coastal
jets not necessarily contribute to the onshore oil propagation. Although wave energy
gradients and contributions of their momentum to the currents are mainly following
the depth gradients, often in a direction towards the coast, nearshore jets are influenced by the land boundaries and more or less follow the coastline. It is unclear
why waves lead to an increase in the residence time by 7–8 hours in some offshore
regions. The reasons may be related to specific pathways of oil transport.
Additionally to the wave breaking and changes in the water depth, there is yet
another process that may modify the impact of waves. At the lee-side of islands,
the wave energy gradients and drift currents are caused by wind shadowing effects.
Considerable fetch length may be required for waves in the island’s shadow to grow
until they match their freely propagating neighbours. For example, the annual mean
J. Murawski and J. Woge Nielsen
Namely, local wave energy gradients (dE/dx in Eqs. (11.4), (11.5) and (11.6)) due
to the topography and wind shadowing effects may produce significant drift currents
and may have a substantial impact on the drift pattern in the study area.
The performed numerical experiments provide evidence for local differences of
the probability of oil landing of up to ±12 % near certain islands. As mentioned
above, the residence time increases by up to 1.3 days at certain locations when wavedriven transport is accounted for. Although the depth gradients that appear at larger
depths exert a minor impact on waves and currents, the persistency of the waveinduced drift and the longer interaction ranges lead to noticeable results. Here, the
interaction range means the length of a drift path on which the drifter experiences
considerable depth gradients. The impact on the probability of landing and on the
residence time increases with the interaction range as oil experiences more and more
wave impetus on the way. Small wave-induced differences of the drift path after a
short time might grow with distance from the release position and may alter the final
drift path significantly.
In the surf zone and close to the shoreline, the release of momentum carried by
beaching waves often produces coastal jets (Longuet-Higgins and Stewart 1964).
The strongest wave-driven currents occur at incidence angles of about 45 ◦ . Such
coastal jets might prevent onshore oil propagation or carry it further to other coastal
sections (Fig. 11.11 A3, B3). The magnitude of the wave-driven drift depends on
whether the waves can exert a cumulative effect along the pathways of oil propagation. In general, the effect is the strongest in the nearshore where (although featured
by strong interaction forces through large wave energy gradients) it is characterized by a rather small interaction range (which is the distance between the seaward
border of the surf zone and the landing point).
The presence of wind waves may increase the probability for oil landing by about
10–15 %. The residence time decreases moderately, by about 4 hours, in these cases.
The decrease in the residence time is twice as large in the upstream direction of the
coastal jet where the number of landing particles is smaller. The range of probability
changes is about 30 % (from +10 % to −20 %). Some particles reach the coast faster
and are counted in the histogram for travel time (Fig. 11.3), but others are travelling
with the coastal jet for a while and are too much diverted from their original path to
hit the shoreline. It should be remembered that wave-induced currents and coastal
jets not necessarily contribute to the onshore oil propagation. Although wave energy
gradients and contributions of their momentum to the currents are mainly following
the depth gradients, often in a direction towards the coast, nearshore jets are influenced by the land boundaries and more or less follow the coastline. It is unclear
why waves lead to an increase in the residence time by 7–8 hours in some offshore
regions. The reasons may be related to specific pathways of oil transport.
Additionally to the wave breaking and changes in the water depth, there is yet
another process that may modify the impact of waves. At the lee-side of islands,
the wave energy gradients and drift currents are caused by wind shadowing effects.
Considerable fetch length may be required for waves in the island’s shadow to grow
until they match their freely propagating neighbours. For example, the annual mean