Monitoring wave-induced sediment resuspension/Lainete pohjustatud setete resuspensiooni seire.
Erm, Ants ; Alari, Victor ; Listak, Madis 等
1. INTRODUCTION
During the last decade, wakes from fast ferries in the vicinity of
ship lanes have become a problem of growing concern in many countries
[1-4] including Estonia, and particularly in Tallinn Bay. Large,
high-speed catamarans were operating the Tallinn-Helsinki route between
1999 and 2008. Systematic investigations of the wakes produced by these
vessels and their impact upon coasts and traffic of smaller craft on the
bay started in 2001 [5-7]. Wake parameters in Tallinn Bay are thoroughly
analysed in [8-12]. Intense traffic of even larger and faster ships has
led to a situation where ship wakes may form a key component of the
hydrodynamic activity on some sections of the coast. The
ship-wake-induced resuspension of sediments and the impact of breaking
waves may cause increased transport of bottom sediments in deeper areas
of the nearshore and an alteration of the beach profile. Understanding
these processes is very important in order to protect the coasts, along
with the planning and sustainable management of harbours, wind farms and
other constructions in the sea.
On the Tallinn-Helsinki line, large ships with speeds up to 50 km/h
are sailing close to the shore about 20 times per day (Table 1).
Although some smaller vessels have been taken out of service, the new
generation of large, highly powered ferries (for example, Tallink Star)
that cross the Gulf of Finland in two hours (compared to classical
high-speed catamarans that take 1 3/4 h), have entered into service
since 2007 and made 13 crossings daily in 2008. These ferries are almost
200 m long with a space for up to 2000 line metres for vehicles. The
vessels operating during this time were Star, Superstar and SuperFast
(Tallink), and Viking XPRS. Also, several smaller but faster ferries of
monohull (SuperSeaCat) or catamaran (Nordic Jet and Baltic Jet) type
were operating in 2008.
The reaction of the seabed to the ferry wakes can be established
and roughly estimated using optical measurements [13-15]. In Tallinn
Bay, systematic optical studies, accompanied with wave measurements,
laboratory analysis of water samples and sampling of resuspended
sediments, started in 2003 near the coast of Aegna Island [13], in an
area most susceptible to ship wakes (Fig. 1). Vessels sailing to
Helsinki pass by the coast of Aegna at a distance of only two
kilometres.
The objective of this study is to quantify the resuspension and bed
load transport of bottom sediments, induced by fast ferry wakes and wind
waves near Aegna jetty using an autonomous experimental optical sonde
and experimental sets of sediment traps.
[FIGURE 1 OMITTED]
2. MEASUREMENT METHODS AND DATA
2.1. Optical measurements and quantification of wake impact The
measurement site was located near the SW coast of Aegna, about 100 m
offshore from a small mixed gravel-sand beach and about 60 m west of the
jetty (Fig. lb, 59[degrees]34.259'N, 24[degrees]45.363'E). The
water depth ranged from 3 to 4.5 m and the bottom was covered with sand,
gravel and bigger stones (Fig. 2).
The properties of wind waves and ship wakes were measured
synchronously with the underwater irradiation. The transport of
suspended matter was studied with the use of two types of sediment traps
to quantify the horizontal (both shoreward and seaward) resuspended and
bedload sediment fluxes.
During the period 24-29 July 2008, an autonomous optical sonde,
consisting of two planar PAR (400-700 nm) sensors Li 192SA (Fig. 2a),
were anchored to the sea bottom to measure irradiance data from the
lower sensor [E.sub.d] ([z.sub.l]) and upper sensor [E.sub.d]
([z.sub.u]). The data (mean values over the 5 min periods with the
sampling frequency 1 Hz) were recorded by a data logger (Li 1400).
The theoretical basis of the light measurements is thoroughly
discussed in [16-18]. The optical parameter, best used to describe the
worsening of light conditions due to sediment resuspension, is the
diffuse attenuation coefficient [K.sub.d] (PAR), calculated from the
underwater irradiation data as
[K.sub.d] = -1/[z.sub.l] - [z.sub.u] ln
[E.sub.d]([z.sub.l])/[E.sub.d]([z.sub.u]), (1)
where [E.sub.d] (z) is downwelling plane irradiance and [z.sub.l]
and [z.sub.u] are the depths of the lower and upper underwater sensors,
respectively.
[FIGURE 2 OMITTED]
To estimate the impact of wakes, a method developed in [19,20] was
used. The method is based on the fact that [K.sub.d] depends on the
concentrations of optically active substances in the water:
[K.sub.d] = [K'.sub.d,S][C.sub.S] +
[K'.sub.d,chl][C.sub.chl] + [K'.sub.d,y,e][C.sub.y,e] +
[K.sub.d,w], (2)
where [C.sub.S], [C.sub.chl] and [C.sub.y,e] are the
concentrations, and [K'.sub.d,S], [K'.sub.d,chl] and
[K'.sub.d,y,e] are the empirical attenuation cross-sections [21] of
suspended matter, chlorophyll and yellow substance, respectively. It can
be assumed that the concentrations of chlorophyll and yellow substance
are not influenced by ship wakes. Assuming that they were approximately
constant during the measurement period, Eq. (2) can be rewritten as
[C.sub.S] = 1/[K'.sub.d,S] ([K.sub.d] - K) (3)
where K is a constant, describing all attenuation components except
suspended matter.
We use here an empirical value of [K'.sub.d,S] = 0.05
[m.sup.2]/g, calculated from the data for Muuga Bay and Tallinn Bay [13]
and used in previous wake studies [13-15,19,20]. Approximately the same
value (0.066 [+ or -] 0.017 [m.sup.2]/g) was obtained for four north
Estonian lakes in [22].
The impact of wakes can be quantitatively described as [14]
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)
where [M.sub.W] [g x s/[m.sup.3]] characterizes the
"total" impact of wakes over the time period
[[t.sub.1],[t.sub.2]], [K.sub.d0] is a background value of the diffuse
attenuation coefficient and [K.sub.d] (t) describes the time-dependence
of [K.sub.d].
2.2. Wave measurements
Wave data sets were collected with a subsurface pressure transducer
(wave gauge) Wave Recorder LM2 (PTR Group Ltd., www.ptr.ee, Estonia,
sampling frequency 4 Hz continuously). In the first case, the gauge was
mounted at a height of 3 m and in the second case at 1 m from the bottom
at water depths of 4.5 m and 3 m, respectively.
In order to monitor the waves, excited by Star, measurements were
carried out in September-October 2007 (case 1, W2007 in Fig. lb). In
summer 2008, optical measurements were conducted at the same location
(case 2, W2008 in Fig. 1b). The deployment in 2008 lasted from 23 June
until the end of September with some gaps.
2.3. Measurement of sediment resuspension and bed-load transport
The seabed composition was analysed in 2007 in an area a short
distance NW of the measurement site [23]. The results showed that 14.2%
of sediments were gravel (typical diameter D > 2 mm), 84.5% sand
(0.063 < D < 2 mm) and only 1.3% silt (D < 0.063 mm). About
76.7% of the sand was very fine (0.063 < D < 0.2 mm).
The intensity of sediment resuspension was quantified using two
types of the experimental sediment trap in the summer of 2008. The first
was used to estimate the horizontal flux of sediments at two different
heights (20 and 60 cm above the sea bed) and directions (shoreward and
seaward). The second set was used to catch the sediments, transported as
bedload onto two plates with slopes of 15[degrees] and 25[degrees]. The
horizontal flux of suspended matter was measured on 2, 22 and 30 July
and also on 11 and 12 September at locations marked with "R"
in Fig. I b. The bed load transport was measured on the same days, with
the exception of 2 July. The amount of sediments was determined by the
dry weight of the samples.
3. RESULTS AND DISCUSSION
3.1. Waves and optics
The measurement of wakes from Star in the autumn of 2007 showed
that the typical maximum wave height was 0.9 m and the period about 8 s.
The maximum measured wave height was 1.6 m and maximum periods reached
13 s (Fig. 3). These values are similar to or even higher than those
recorded previously for waves excited by monohulls and catamarans [9].
[FIGURE 3 OMITTED]
The average spectral energy density of ship wakes over 25-29 July
2008 had a similar nature to that described in [12]. While in [12] the
largest spectral peak was at 9.2 s, our data showed a peak of 10.3 s
with somewhat lower peaks at 12.3, 7.8 and 6.6 s (Fig. 4).
The longest continuous measurements of underwater irradiation took
place from 24 to 29 July 2008 (Fig. 5a). The measured underwater
irradiance at both levels ([E.sub.d] ([z.sub.l]) and [E.sub.d]
([z.sub.u])) and [K.sub.d] , calculated from these values, are plotted
in Fig. 5a. As the Sun was the light source, only daytime (from 06:00 to
22:00) data are reliable. The levels of irradiation suggest that clear
skies dominated during the measuring period.
The wind wave background was small on 24-29 July 2008, with wind
wave heights less than 20 cm on 24 July and below 10 cm on other days
(Fig. 5b). The maximum heights (from the trough bottom to the top of the
wave crest) of ship waves reached 1.4 m, but mostly they were below 1 m.
A good match between the amplitude of the wakes and the attenuation
coefficient is evident on some days. For example, the peaks of [K.sub.d]
at 08:10, 10:25, 10:55, 11:25, 16:30, 16:50, 17:55, 18:10, 19:15, 19:25,
and 20:05 on 27 July (Fig. 6) coincide well with the timetable of ships
(Table 1). The match was, however, lower on some other days. For the
sake of compendious presentation of all measured data, the mean values
of the wave amplitude and [K.sub.d] were calculated for each time
instant over all days of measurements. The resulting average daily
variations of the quantities in question are plotted in Fig. 7. These
aggregated graphs show more or less the same peaks as in Fig. 6. The
"still time periods", when no ferries were passing for an hour
or more, are marked by filled circles. The peaks of [K.sub.d] in Fig. 7
are in good qualitative correlation with the mean maximum wave heights
and reflect the schedule of ferries (Table 1).
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
In previous papers we used the value for [K.sub.d] immediately
before the wake [13,14] or the mean value [K.sub.d,mean] [19,20] as the
background value of [K.sub.d0] in Eq. (4). Using the mean value is,
generally speaking, not always adequate because the background changes
during the day. The reasons for that could be inherent (such as changing
water properties) as well as apparent (for example, a variation of
measured [K.sub.d]) values depending on the incoming light conditions.
The data set in question did not allow us to specify the exact reason
for such changes. We do not exclude the possibility that the properties
of the sea water can change during the day due to advection of the water
masses under the impact of wind waves and currents, or primary
production. The changes may also be driven by the ferry wakes. It is,
however, not very likely that these changes occurred synchronously every
day. It is more plausible to adopt the view that the changes are
basically caused by varying light conditions that depend primarily on
the spectral distribution and solar zenith angle. Dependence [K.sub.d]
(t) on a clear day (Kirk's curve) must go through a minimum at the
maximum altitude of the Sun [24-26], but our dataset has a principal
difference from the Kirk's curve in that the minimum values of the
"still period" points were registered between 14:00 and 17:00
(Fig. 7), while the sun time noon in Tallinn was at 13:27
(http://www.aai.ee/?page=oopaev), local summer time.
Thus, another approximation must be used for [K.sub.d]o. A high
correlation ([R.sup.2] = 0.83) with the values at "still
periods" was achieved using a second order polynomial (thin line in
Fig. 7). We used this curve for [K.sub.d0] (t) as the background
attenuation level in Eq. (4). The standard deviation [MATHEMATICAL
EXPRESSION NOT REPRODUCIBLE IN ASCII] 1/m for [K.sub.d0](t) was
calculated using the values of the above-mentioned "still
periods".
The next step was to calculate the daily impact of ferry wakes. It
seems that no large "ventilation" event (that is, rising of
sediments above both sensors) took place during measurements. Therefore
the condition [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] was
always satisfied and one can perform the integration only over values
above the background value. A value [M.sub.W] =14 430 g.s/[m.sup.3] was
obtained for the daily total impact. The mean concentration of
(re)suspended sediments can be calculated for a navigation day from Eq.
(4) as
[C.sub.S,m] = [M.sub.W]/[t.sub.2][t.sub.1] = 14 43-/14 x 3 600
[approximately not equal to] 0.3 g/[m.sup.3] (5)
In previous studies, the impact was calculated for an approximately
5h interval either by means of summarizing the impacts of several
ferries [14] ([M.sub.W] =14 000 g x s/[m.sup.3] for 2003-2004) or
integrating [DELTA][K.sub.d] (t) both above and below the initial value
of [K.sub.d] [20] ([M.sub.W] =13 000 g x s/[m.sup.3]). In this work we
found an almost equal value for the impact over a whole navigation day.
Therefore the impact of ship wakes in terms of resuspension of finer
bottom sediments in 2008 was about three times lower than estimated
earlier. Owing to different integration methods in these studies, the
earlier data are apparently overestimated.
3.2. Transport and resuspension of sediments
The rates of horizontal sediment fluxes (Table 2) are highly
variable ranging from 0.3 to 740 g/([m.sup.2] x h) and depending on the
elevation from the sea bed, the direction of the flux (shoreward or
seaward), time in the day and the water depth. As expected, resuspension
is generally greater in the lower layer than in the upper one. While in
the lower layer in most cases the seaward sediment fluxes are larger (up
to 2.5 times, with a mean of 1.6 times), in the upper layer the mean
shoreward and seaward fluxes are the same (Table 2). Daytime fluxes are
many times greater than night-time fluxes (Table 3), and this can be
interpreted as the effect of fast ferry wakes, but not exclusively.
A WSW-SW wind with a speed up to 10 m/s occurred on 22 July. During
that day, three measurements of resuspension were undertaken at a water
depth of 3 m. It seems that at such small depths, not only the ferry
wakes but also moderate wind waves cause significant resuspension of
bottom sediments. The significant height of wind waves was 0.3-0.5 m
during the morning, with wave periods of 2.8-3 s. In the daytime, the
significant wave height was about 0.5 m and the wave period 3-3.2 s. In
the evening of 22 July and on the night of 23 July the significant wave
height varied between 0.3-0.5 m with periods of 2.6-3.1 s. Under these
conditions, the near-bottom shear velocity exceeded the resuspension
value by almost twice. The greatest bed load transport, with over 25
g/([m.sup.2] x h]), was recorded on 22 July (Table 4).
On 30 July, wind waves were below 10 cm during the day, and the
induced resuspension was negligible. Between 9:57 and 15:10, the total
flux of resuspended sediments was 2.1 g/([m.sup.2] x h) at a height of
20 cm and 1.9 g/([m.sup.2] x h) at a height of 60 cm above the bottom.
This is hundreds of times less than the resuspension induced by wind
waves at a depth of 3 m.
On 12 September, sediment resuspension was measured all day.
However, the natural wind wave background had a height of 0.3 m and a
period of 2.25 s, and only 13 fast ferry wakes were measured. The shear
velocities at a water depth of 4.5 m did not exceed the critical limit
with respect to the above-mentioned parameters. The maximum flux was
only 1.4 g/([m.sup.2] x h) (Table 2). The maximum bed load transport was
only 0.3 g/([m.sup.2] x h]) (Table 4).
The spectrum of wind waves for 22 July and the spectrum of fast
ferry wakes for 30 July are plotted in Fig. 8. It is evident that wind
waves were tens of times more powerful than the fast ferry waves at
these low depths and, therefore, induced greater resuspension. To more
clearly capture the resuspension, induced by fast ferries, the effect of
wind waves must be eliminated. Further measurements, therefore, should
be undertaken in deeper water, preferably at depths of 10-20 m. At a 20
m depth, for example, a wind wave with a period of 6 s has to be more
than 1.7 m high in order to resuspend fine sand. In the study area, the
probability of occurrence of such waves is less than 1% [27]. On the
other hand, an only 0.8 m high fast ferry wave with a 10 s period will
resuspend sand at this depth, and as shown by this and previous studies,
ferry wakes are frequently higher than 0.8 m.
[FIGURE 8 OMITTED]
The dataset of bedload sediment fluxes (Table 4) is quite limited.
However, the above experiments have given us some guidance for further
studies. First, contrary to projections, no systematic dependence of the
magnitude of the bedload flux on the slope of the rolling plane was
registered during the experiments. Second, no significant bedload
transport was recorded at a depth of 4.5 m. This is somewhat unexpected,
because the long-period ferry wakes should bring a substantial amount of
energy to near-bottom motions at these depths.
4. CONCLUSIONS
A new autonomous experimental device for underwater optical
measurements was tested over a week to monitor the impact of ship wakes.
Measurements on 2429 July 2008 showed that the average daily impact of
ship wakes in terms of resuspension of bottom sediment near the coast of
Aegna in Tallinn Bay is 14 400 g x s/[m.sup.3], which is approximately
three times lower than estimated previously. Moreover, ship traffic in
Tallinn Bay causes a daily increase in the suspended matter
concentration of about 0.3 g/[m.sup.3] in the near-bottom coastal
waters.
Measurement of wakes from a big fast ferry (Tallink Star) in autumn
2007 showed that the typical height of the largest wake waves is 0.9 m
and their period is 8 s. These values are comparable to earlier
measurements of wakes from classical high-speed monohulls and
catamarans. The average spectral energy density of ship wakes on 25-29
July shows the largest spectral peak at 10.3 s and somewhat lower peaks
at 12.3, 7.8 and 6.6 s.
At water depths less than 3 m, even relatively small wind waves
(about 0.5 m in height and period about 3 s) induce much more intense
resuspension than the fast ferry waves.
Two types of sediment traps were used for pilot studies to directly
measure resuspended and bedload sediment fluxes. The seaward flux of
sediments at a height of 20 cm above the bottom was in most cases much
greater than the shoreward flux. At a height of 60 cm above the bottom,
the seaward flux is, on average, the same as the shoreward flux. The
data, concerning the magnitude of resuspension and bedload transport,
are indicative and call for further research.
doi: 10.3176/eng.2009.3.04
ACKNOWLEDGEMENTS
This work was financially supported by the Estonian Science
Foundation (grants No. 7000 and EMP53). We are grateful to Tarmo Soomere
and his staff for cooperation at the measurement site.
Received 31 March 2009, in revised form 12 July 2009
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Ants Erm (a), Victor Alari (a) and Madis Listak (b)
(a) Marine Systems Institute, Tallinn University of Technology,
Akadeemia tee 21, 12618 Tallinn, Estonia; ants@bphys.sea.ee
(b) Centre for Biorobotics, Faculty of Information Technology,
Tallinn University of Technology, Akadeemia tee 15A, 12618 Tallinn,
Estonia; madis.listak@biorobotics.ttu.ee
Table 1. Time schedule of passenger ships, travelling from Tallinn
to Helsinki on 23 July 2008. Ferries that produce optically
detectable wakes are shown in bold
Ferry Dep.
time
Star/SuperStar# 07:30
SuperSeaCat# 07:45
Viking XPRS# 08:00
Nordic/Baltic Jet# 08:00
Jaanika/Merilin * 10:00
Nordic/Baltic Jet# 10:15
SuperSeaCat# 10:30
Star/SuperStar# 11:00
Nordic/Baltic Jet# 12:55
Galaxy ** 13:30
SuperSeaCat# 14:00
Star/SuperStar# 14:00
Nordic/Baltic Jet# 15:00
SuperFast 1/2# 15:30
Jaanika/Merilin * 16:00
SuperSeaCat# 16:15
Nordlandia ** 17:00
Nordic/Baltic Jet# 17:25
Viking XPRS# 18:00
Star/SuperStar# 18:30
Nordic/Baltic Jet# 19:30
SuperSeaCat# 19:30
Jaanika/Merilin * 20:00
Star/SuperStar# 21:00
SuperSeaCat# 21:40
* Hydrofoils.
** Classical low speed ferries.
Note: Ferries that produce optically detectable wakes are shown
in bold indicated by #.
Table 2. Measured horizontal fluxes and relations between seaward
and shoreward fluxes of sediments. Four traps (94 x 94 [mm.sup.2])
on the same frame were used to trap the sediments
Date, 2008 Depth, Time Level,
m cm
From To
02 July 4 15:00 22:40 20
15:00 22:40 60
22:50 07:40 20
22:50 07:40 60
07:50 15:56 20
07:50 15:56 60
22 July 3 07:30 15:05 20
07:30 15:05 60
17:35 20:49 20
17:35 20:49 60
23 July 3 20:58 08:00 20
20:58 08:00 60
30 July 3.5 09:57 15:10 20
09:57 15:10 60
20:25 09:48 20
20:25 09:48 60
11 September 4.5 19:45 10:45 20
19:45 10:45 60
12 September 4.5 10:55 18:35 20
10:55 18:35 60
Average 20
60
Date, 2008 Flux, g/([m.sup.2] x h) Seawards flux/
shorewards flux
Seawards Shorewards
02 July 63.7 42.4 1.5
28.3
6.0 6.0 1.0
6.0
8.0 6.7 1.2
4.0 4.0 1.0
22 July 71.5 28.6 2.5
6.4 14.3 0.4
741.9 292.7 2.5
33.6 50.3 0.7
23 July 220.1 156.1 1.4
0.9 0.9 1.0
30 July 2.1
2.6 1.9 1.4
3.1 5.4 0.6
2.1 2.1 1.0
11 September 1.1 0.5 2.2
0.4 0.3 1.3
12 September 1.4 1.0 1.4
1.4 0.8 1.7
Average 124 54 1.6
26 24 1.1
Table 3. Mean horizontal seaward and shoreward fluxes of sediments
and their average day/night ratios
Mean sediment flux, g/([m.sup.2] x h)
All Seaward Shoreward
All 49 65 34
20 cm 87 124 54
60 cm 8.9 6.4 11.4
Day 54 85 36
Night 27 30 24
Relation day/night of mean fluxes
All Seaward Shoreward
All 6.5 6.1 6.9
20 cm 3.5 4.1 2.9
60 cm 11.6 9.5 13.7
Day
Night
Table 4. Measured shoreward fluxes of bedload sediments. A frame
consisting of two plates (with slopes of 15[degrees] and
25[degrees]) with the traps in them (196 mm of diameter) was used
to trap the sediments
Date, 2008 Depth, Time Flux, g/([m.sup.2] x h)
m
From To 15[degrees] 25[degrees]
22 July 3 23:10 07:50 10.9 1.7
15:00 22:55 4.2 25.1
23 July 3 08:00 15:56 2.0 4.2
30 July 3.4 20:28 09:40 0.8 4.9
09:45 15:10 3.9 2.4
11 September 4.5 19:45 10:30 0.3 0.1
12 September 4.5 10:35 18:25 0.1 0.3
