Fast Ferry Traffic as a Qualitatively New Forcing Factor of Environmental Processes in Non-Tidal ...

31 Pages·2005·0.61 MB·English

Environmental Fluid Mechanics (2005) 5: 293–323 © Springer 2005
DOI 10.1007/s10652-005-5226-1
Fast Ferry Traffic as a Qualitatively New Forcing
Factor of Environmental Processes in Non-Tidal
Sea Areas: A Case Study in Tallinn Bay,
Baltic Sea
TARMO SOOMERE
Institute of Cybernetics, Tallinn University of Technology, Akadeemia tee 21, 12618
Tallinn, Estonia (E-mail: [email protected])
Received3February2004;acceptedinrevisedform8April2005
Abstract. The impact of wake wash from high-speed ferries on the coastal environment
in non-tidal seas is analysed in terms of wave energy and power, and properties of the
largest waves. Shown is that hydrodynamic loads caused by heavy high-speed traffic may
play a decisive role not only in low-energy coasts but also in certain areas with high
wind wave activity. For example, ship-generated waves form, at least, about 5–8% from the
total wave energy and about 18–35% from the wave power in the coastal areas of Tallinn
Bay exposed to dominating winds. The periods of wake waves from high-speed ships fre-
quently are much larger than dominating periods of wind waves. The leading waves typ-
ically have a height of about 1m and a period of 10–15s. Such waves extremely seldom
occur in natural conditions in many regions of semi-enclosed seas. They cause unusually
high hydrodynamic loads in the deeper part of the nearshore. The fast ferry traffic thus
is a qualitatively new forcing component of vital impact on the local ecosystem. It is
demonstrated that wakes from high-speed ferries may trigger considerable changes of the
existing balance of coastal processes. Owing to their low decay rates combined with their
exceptional compactness after crossing many kilometres of the sea surface, such wakes
may cause considerable remote impact of the ship traffic. This feature has to be addressed
in the analysis of the impact of harbours and associated ship traffic in the neighbourhood
of vulnerable areas.
Keywords: Baltic Sea, high-speed ships, Tallinn Bay, wake wash, wave measurements
1. Introduction
Marine and coastal resources play a major role in sustaining the economic
and social development of society. Today, about 80% of all international
trade is carried by sea. According to some estimates, in the nearest future,
by the year 2020, about 75% of the world’s population will live within
60km of sea coasts and estuaries ([2], pp. 29–30). The continuous increase
294 TARMO SOOMERE
of population in the coastal areas and the ever increasing density of marine
transport combined with a limited number of convenient anchorage places
results in an extreme concentration of both cargo and passenger traffic in
certain sea areas.
The concerns related to heavy ship traffic are traditionally associated
with possible accidents (ship collisions or grounding, technical and naviga-
tion problems caused by severe weather or human errors, etc.) that may
lead to either loss of lives or property, or to environmental pollution.
However, these concerns have been and are being effectively managed by
international shipping and harbour communities. When considering safety
aspects of water surface transport, traditionally it is assumed that the risks
are localised within a small area around the ship. For example, a ship acci-
dent occurs at a certain point and possible oil contamination is transported
relatively slowly owing to winds and currents.
The continuing introduction of evermore faster ship services during the
last two decades has created another set of major worries. They consist in
(i) a massive growth of exhaust emissions per passenger mile (compared,
for example, to passenger cars or buses), (ii) a great increase of external
noise, and, last but the largest, (iii) the waves generated by large high-speed
ships [68]. These by-products of the fast ship traffic are no more located
in small areas. In the contrary, they may travel much faster than the ship
itself (in particular, the ship-generated noise), may become a part of global
troubles (for example, exhaust emissions, Hobbs et al. [20]), or may result
in violent energy concentration in remote sea areas [17].
The importance of the contribution of the ship traffic to the local
hydrodynamic activity in rivers, inland channels and narrow straits has been
recognisedforalongtime.Shipwakescanessentiallycontributetotheshore-
line erosion [3, 14, 43], cause an extensive erosion and resuspension of bot-
tom sediments [33], trigger ecological disturbance and cause harm to the
aquatic wildlife [1, 38], to mention a few studies. Also, damage to structures
and archaeological sites, and safety problems for navigation and for users
of the beach and nearshore may arise (see Parnell and Kofoed-Hansen [45]
and references therein). These aspects are not related to particular types of
ships. They appear in many places all over the world and for a variety of
ships, and only represent a locally important contribution of shipping in the
hydrodynamic activity. Historically, these aspects have been studied perhaps
in the greatest detail for the archipelagos of the Baltic Sea ([50–53]; for an
overview see Madekivi [40] and references therein).
The introduction of large high-speed vessels extended the above threats
from inland waterways, particularly narrow straits, and archipelagos to
much larger confined sea areas with low natural wave and tide activity
[16, 45]. These vessels have a high ratio of propulsion power to vessel dis-
placement and are able to sail at much larger speeds than conventional
FAST FERRY TRAFFIC IN NON-TIDAL SEA AREAS 295
ships. They are able to cross the two firm ‘barriers’ that restrict the speed
of conventional vessels [66]. First, the hump speed occurs when the half of
the length of ship-generated waves travelling in the sailing direction is close
(cid:1)
to the vessels length. Sailing at this speed means that the ship is continu-
ously moving ‘upwards’ along the slope of her wake. Second, the critical
speed corresponds to the maximum phase speed of surface waves in finite
depths. At this speed the ship generates a wave system that is non-disper-
sive and may remain compact for a long time.
The crossing of either of the ‘barriers’ requires much more energy than
sailing at lower speeds, and is accompanied by a fast increase of wave resis-
tance [66]. It results in much higher wake waves and, accordingly, in much
higher wave loads on the coastal environment near the ship lane. Large
high-speed craft sailing at specific speeds in shallow areas may produce
fundamentally different wave systems than conventional vessels [4, 7, 36,
57]. Ship-generated solitonic disturbances usually have moderate height [70]
and possess no immediate danger to the people or to the beach environ-
ment. However, superposition of solitonic waves (occurring, for example,
when by two systems of ship waves simultaneously reach a shallow area)
may lead to a fourfold amplification of the local surface elevation [46].
Depression areas accompanied with the generation of solitary precursors
may be responsible for a drastic dropdown of water surface in small har-
bours adjacent to narrow waterways [13].
The increase in wave heights at large speeds is accompanied by an
increase of the typical periods and lengths of single waves. High and
long waves from high-speed ships may create explicit danger to the safety
of people [17] and their property, and other vessels [29, 31]. This situa-
tion was recognised as inadmissible in many countries (in particular, in
Denmark, the United States, and New Zealand) and necessary restrictions
were imposed [45, 68]. Today, the experience in managing of wake wash
from high-speed vessels in confined sea areas has been generalised in a sys-
tematic manner. A description of the accompanying adverse effects on the
local ecosystem and possibilities of their mitigation has been presented in
[16, 45].
On the one hand, it has become clear that heavy ship traffic has a great
damaging potential in the vicinity of waterways and the adjacent shore-
line, in particular, in areas that are sheltered from large wind waves (such
as wetlands, low-energy coasts, etc., see Bourne [3], Schoellhamer [57]). On
the other hand, it is generally believed that ship wakes are negligible and
that their effect is sporadic in coastal areas that are open seawards and
where natural waves are frequently much higher than the wakes [38]. This
assumption is true for coasts exposed to high tides or large wind waves
indeed.
296 TARMO SOOMERE
The purpose of the current paper is to demonstrate that wake wash
may be of particular importance in certain parts of coasts that are exposed
to significant natural hydrodynamic loads and that are already subject to
intense beach erosion in natural conditions. The reason is a combination of
specific features of (i) the existing hydrodynamic loads (that are restricted,
for example, to a particular direction or to a certain frequency interval)
and (ii) the coastal environment (that has reached a near-equilibrium stage
of its evolution) with (iii) particularly high anthropogenic wave loads in a
different frequency range.
It is first demonstrated that the share of ship waves in the total wave
activity may be remarkably high in the vicinity of open sea waterways even
in terms of wave energy. The description of properties of ship waves at
different sailing regimes as well as their changes in the far field and in the
coastal areas, and relevant references can be found in [16, 45, 66]. The cur-
rent study is focussed on the occurrence probability and basic properties of
waves of different origin, and on energy density and its flux (wave power)
in the coastal zone owing to wind waves and ship wakes.
The local background of Tallinn Bay (the Baltic Sea) is introduced and
the set-up of field experiments is described in Section 2. The basic prop-
erties of ship wakes and wind waves in the area in question are discussed
in Sections 3 and 4. A comparison of the energy and power of waves of
different origin shows that the annual mean energy of ship-generated waves
is about 10% from the total wave energy at a distance of a few km from the
main ship lane (Section 5). This is not unusual in itself; for example, [57]
have identified much larger portions of ship-generated waves in the total
wave activity. The specific feature of the coasts in question is that they
are open to a part of dominating winds, may experience significant wave
heights 3–4m during strong storms, and undergo extensive coastal erosion
owing to natural wave activity.
An energy-based comparison of waves of different origin is equivalent
to a comparison of relevant wave heights. The comparison of wave power
(that characterises the rate of wave energy supply to some area) shows
much larger share of ship waves in the total wave activity. This disparity
reflects the fact that a part of ship waves usually become longer when the
ship’s speed increases. It is shown in Section 6 that the energy of waves
from large fast ferries is mostly concentrated in components with relatively
large periods that greatly exceed the typical periods of natural waves in the
area in question.
It is well known that low-energy coasts are vulnerable with respect to
changes of the wave activity. The presented results extend this threat to a
part of high-energy coasts. In particular, the difference in periods of natu-
ral and anthropogenic waves results in greatly different properties of wave-
bottom interaction and suggests that the heavy high-speed ship traffic may
FAST FERRY TRAFFIC IN NON-TIDAL SEA AREAS 297
add a qualitatively new forcing factor to the local ecosystem in specific
sea areas. This conclusion mainly concerns micro-tidal or non-tidal seas
(such as the Azov Sea or the Baltic Sea) where near-bottom hydrodynamic
activity is governed by the local windseas and currents. Specific wind
regime and complex geometry of some sea areas (for example, Tallinn Bay
in the Baltic Sea) may additionally enhance the influence of high-speed
traffic.
Inspecificseaareas,thenewcomponentofwaveactivity–highlyenergetic
long waves – may considerably change the overall sediment redistribution
pattern.Inunfavourablecases,itmaygreatlyacceleratecoastalerosionowing
toaspecificmulti-stagesedimenttransportmechanismsketchedinSection 7.
Generic implications of the contribution of wake waves from intense high-
speed traffic are discussed in Section 8.
2. Study Area and Set-Up of Field Experiments
The Baltic Sea (Figure1a) is one of the largest brackish water bodies in
the world. Its hydrophysical conditions vary largely both in time and space
[72], thus allowing only the particularly strong species to survive. It is
located at relatively high latitudes with severe climate and each year a large
part of it is covered by ice. Many species here are of the border of their
area of spreading. As a result, the local marine environment is particularly
vulnerable with respect to any changes of the external forcing.
Tallinn Bay is an example of a semi-open sea region (ca. 10×20km)
in the central part of the Gulf of Finland, Baltic Sea. It hosts extremely
Figure1. (a) Location of Tallinn Bay, (b) the mounting scheme of the wave recorder
(not to scale).
298 TARMO SOOMERE
heavy fast ferry traffic since 2000. A variety of different types of high-speed
ships cross the gulf nearly 70 times daily during the high season. The water
depth along the ship lane is such that large high-speed ships frequently sail
in the near-critical1 velocity range. The resulting wave systems frequently
cause violent plunging breakers in certain areas located at a distance of
many kilometres from the ship lane, creating appearance of greatly induced
overall wave load and serving as a potential hazard to moored vessels,
danger to the general public using beaches or small craft, and supposedly
intensifying coastal or bank erosion [60]. A sketch of some aspects of the
influence of ship wakes based on an intense field and theoretical study in
2001–2002 is presented in [65].
A large number of various devices to measure wave characteristics have
been used in different studies of ship wakes [16]. The pressure sensors are
relatively inexpensive, reliable and easy to operate. The pressure data well
represents wave periods and heights but not necessarily the water surface
time series and the wave profile. The sensors may fail to register short
waves (because pressure fluctuations caused by short waves fast decay with
the increase of the depth) unless mounted at small depths not exceeding
2–3m (Figure1b). Since the periods of waves from high-speed ships typi-
cally vary from 3 to 40s [29, 30], pressure sensors can be used for adequate
description of their basic parameters. Three devices were used: pressure
sensor SD204 (SAIV Ltd., Norway), wave recorder SBE26 (Sea-Bird Elec-
tronic, USA) and a domestic water level recorder (PTR Group, Tallinn,
Estonia) with an extended memory capacity.
Althoughseveralwitnesseshaveclaimedthatshipwaveshavebeenashigh
as 4m at specific sites of Tallinn Bay, a series of preliminary experiments in
2001 showed that the ship wave heights at the depths of 5–10m apparently
didnotexceed2mandgenerallywerelessthan1m[60].Yettheactualpossi-
bility of occurring of up to 2m high waves suggests that the wave properties
should be measured at somewhat larger depths than the reference depth 3m
for the wake height criterion in Denmark and New Zealand [32, 45]. The
highest ship-generated waves may already be at the breaking stage at this
depth and their heights may be severely underestimated from the pressure
time series.
1Gen(cid:1)er√ally, navigational speeds are distinguished according to the depth Froude number
F =ν gh, that is the ratio of the ship speed ν and the maximum phase speed of sur-
d
face gravity waves, where h is water(cid:1)d√epth and g is acceleration of gravity, or according
to the length Froude number F =ν gL, where L is the length of the ship’s waterline.
l
Operating at speeds resulting F <1 is defined as subcritical, at F >1 as supercritical and
d d
at F =1 as critical. There is a relatively wide transcritical speed range 0.84<F <1.15 in
d d
realistic conditions, where no clear distinction between sub- and supercritical regimes is
possible [21]. The speed resulting F ≈0.5 is called hump speed. Reaching either the criti-
l
cal or the hump speed is commonly accompanied with a drastic increase of wake energy.
The reader is referred to [16, 66] for further information.
FAST FERRY TRAFFIC IN NON-TIDAL SEA AREAS 299
Figure2. Tallinn–Helsinki ship lane and wave measurement sites in the coastal area
of Tallinn Bay. Shown are depth isolines at 2, 5, 10, 20 and 50m.
The wave properties were measured in areas with the depth of 5–7m in
different parts of Tallinn Bay and at different distances (2–8km) from the
ship lane. The measurement sites (Figure2) were chosen near the coastal
areas that apparently were the most vulnerable with respect to the ship
waves according to Kask et al. [27]. Owing to extremely heavy regular and
leisure traffic in the study area the devices were mounted in shallow areas
not used by vessels with draught exceeding 1m. An overview of the mea-
surement procedure and the description of details of both natural waves
and ship wakes can be found in [64].
The analysis of the pressure data follows the classical procedure of
establishing the surface wave properties from the pressure time series
[56, 67]. The data of each record (with duration of 4–34min, separated by
pauses of 30–60s) is first de-meaned and de-trended. The mean is used for
calculating the instrument depth. A Hanning window, followed by a multi-
plication by a proper scale factor, is applied to suppress the spectral leak-
age that occurs when the data contain a periodic signal with a frequency
that does not coincide with any of the exact frequencies of the Fast Fou-
rier Transform (FFT) for the particular record.
300 TARMO SOOMERE
Further, the FFT of the records, with the number of points equal to
the number of pressure snapshots in each record, is performed. To prevent
measurement noise from being mapped into unrealistic short wave heights,
the amplitudes of waves with the frequencies exceeding a certain threshold
are set to zero. The truncated spectral density of the pressure is converted
to the energy spectrum of surface waves with the use of the attenuation
rates of the subsurface pressure for the given sensor position, water depth,
and wavelength. The estimates of water surface time series, obtained by
inverting of the described procedure, not necessarily represent the actual
water surface time series and only a few properties of this series are used
in the further analysis.
Wind wave field is usually characterised by the significant wave height
H (the mean height of one third of the highest waves, IAHR [22]). This
s
measure is not applicable for ship wakes, in particular, for their long-wave
part that consists of transient waves. Their height is estimated using the
classical zero-upcrossing method [22] applied for the smoothed pressure
time series. A de-meaned and de-trended pressure record is filtered with the
use of a low-pass filter. Typically, the cut-off frequency was set to 0.2Hz,
equivalently, waves with periods <5s were excluded. The resulting pressure
time series contains a very small amount of wind waves (see below). The
parameters of the remaining pressure disturbances are estimated individ-
ually for each single pressure wave. The pressure attenuation rate for the
particular wave period and the sensor position, and water depth is applied
to convert the pressure wave amplitude to the surface wave height.
Doing so implicitly presumes that a sinusoidal wave with the period
coinciding with that of the single pressure wave contains all long-wave
energy between the relevant zero-crossings of the pressure disturbance.
However, an error caused by this assumption is relatively small because
the attenuation rate varies insignificantly for the typical period range of
long ship waves in the measurement conditions. The longer the waves, the
smaller is the error, because subsurface pressure in very long waves in shal-
low water simply follows the behaviour of the water surface [41].
3. Typical Properties of Ship Wakes
The majority of conventional passenger and cargo ships operating in
Tallinn Bay excite wakes that are practically indistinguishable from the nat-
ural background in the coastal zone even in calm days. They sail at mod-
erate length and depth Froude numbers, and excite waves that have typical
heights of 20–30cm (a maximum height of 40cm) and periods of 3–4.5s at
a distance of about 2km from the ship lane. Hydrofoils are capable to sail
at near-critical speeds in a large part of the bay [64] but their wakes are
FAST FERRY TRAFFIC IN NON-TIDAL SEA AREAS 301
even smaller. Generally, their height does not exceed 10cm in the coastal
area of the bay whereas the wave periods are 3–4s.
The fast ferries (four high-speed catamarans and one monohull vessel
capable of carrying cars and operating already in 2001–2002, and another
monohull operating from 2003) frequently create wakes that are clearly dis-
tinguishable from the wind wave field even in moderate and strong wind
conditions [64]. Below we consider mostly waves generated by these ships
and call them ship waves. Technically, the listed vessels may sail in the
transcritical velocity range between Tallinn passenger harbour and Aegna
jetty. A part of the resulting wave system disperses slowly and may remain
more or less compact for a long time [70].
A wake of a fast ferry operating at high speeds (both in sub- and
supercritical regime) typically consists of two or three wave groups (Figure3;
cf. [19, 29–32, 70]). It arrives into the measurement sites of the eastern
coast of Tallinn Bay (located at a distance of 2–3.5km from the ship route,
Figure2) about 5–10min after the ship has passed, and lasts about 10min.
The first group contains the longest waves (periods 15–9s; the longer waves
arriving first). The second group arrives a few minutes later and usually
consists of waves with periods 9–7s. An underlying swell wave with periods
exceeding 10s and probably representing either transverse waves or open
sea swell (cf. Kirk McClure Morton [29]) was not identified in Tallinn Bay.
The third wave group of highly monochromatic relatively short waves
with periods of 3–4s and with a typical duration of less than one minute
was recognised in a small number of recordings. These groups usually are
smaller than other ship waves in the eastern part of the bay. However, they
are easily identifiable in the spectral representation as a sharp and narrow
Figure3. Pressure fluctuations caused by the wake of SuperSeaCat IV near the
western coast of Aegna. The first wave group has the maximum height of 45cm
and the second group of 25cm. The third group is the highest (52cm). The signifi-
cant height of the natural wave background is about 30cm (From [64]).
302 TARMO SOOMERE
peak provided a proper part of a record is selected. Their components are
much shorter than the leading waves, thus their interaction with the seabed
is also much weaker. Owing to a fixed celerity of its components, the group
remains compact during a long time also in deep water where other parts
of the wake experience strong dispersion. As a result, the third group well
maintains its height and frequently contains the highest waves of the wake
in remote areas. It was occasionally found severely high (about 70cm) as
far as 8–10km from the ship lane in the remote coastal zone of Naissaar
where it arrived more than one hour after the leading waves [64].
It is well known that ships sailing at transcritical speeds frequently
excite solitonic wake components that propagate ahead of the ship and are
called precursors (see [36] and bibliography therein). They resemble sin-
gle moving water elevations and are usually interpreted as solutions to the
Korteweg-de Vries equation for one-dimensional waves ([35], chapter 12)
or the Kadomtsev–Petviashvili equation for the weakly two-dimensional
case [18]. The third group also exhibits solitonic properties. However, it
is basically different from precursors, because it is highly oscillatory and
resembles an envelope soliton [4]. The non-linear Schrödinger equation [15]
might be a proper tool for its analysis.
The highest individual waves of wake patterns occur near Aegna jetty.
This area may be hit by large bow waves generated by ships sailing at
transcritical speeds [36]. The number of wave crests with the heights close
1
m)
ht (
g 0.8
ei
h
e
av 0.6
w
m
mu 0.4
xi
a
M
0.2
0
08:00 10:00 12:00 14:00 16:00 18:00
Time on 14 April 2002 (hours:minutes)
Figure4. The maximum height of the long wave components (periods >5s, bold
line, restored from the filtered pressure record with the use of the zero-upcrossing
method) and of the whole wake of fast ferries (dotted line, estimated from the water
surface time series) near Aegna jetty in a relatively deep water (6.7m). The tempo-
ral resolution is 5 min. The significant height of wind waves was <10cm and the
height of long-period (>5s) natural wave components was a few centimeter. The
data represent the beginning of the navigation season when the number of depar-
tures of high speed ships was less than in the high season. The peaks at 10:20,
13:10 and 15:10 are caused by simultaneous passing of two or three vessels [64].