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Potential Ballast Water Movement
within the Fal Estuary
Authors:
Mr Mark Symons, (Student) Falmouth Marine School (University of
Plymouth) Fdsc Marine Science, Falmouth, Cornwall, United
Kingdom. 58 Manor Way, Helston, Cornwall, TR13 8LJ
Miss Louise Hockley, (Associate Lecturer) Falmouth Marine School,
Falmouth, Cornwall, United Kingdom. MSc Marine Science.
Abstract
The Fal Estuary a SAC under the E,U’s Habitats Directive but home to
a working docks, and invasive species. Its Hydrodynamic traits are
mainly unknown. Measurements of tidal flow in the dockland area
of the Fal are undertaken over an entire tidal cycle of
February/March. This data plotted in vector graphs shows that the
mean movement of water is towards the docklands, and that
statistically there was no significant difference in direction between a
flooding and ebbing tide. Along with this the velocities were so slow
that the maximum any NIS would travel before the tidal current
reversed is 425.52m, thus negating the need to develop any
management strategies.
Introduction
Transportation of Non Indigenous Species (NIS) through the use of
ballast water in shipping is a well known problem and has been since
the technology was fully established in the 1950’s (Griffiths et al;
2009) (other transportation vectors do also exist such as hull fouling).
Though the use of ballast water has a negative effect on the
environment by introducing NIS, it plays a major role in keeping
vessels stable and improving manoeuvrability when free from cargo
(Packard, 1984; Tsolaki, 2009; Zhang and Dickman, 1999). Tsolaki
(2009) states that “Shipping moves 80% of the world’s commodities
and transfers approximately 3-5 billion tonnes of ballast water
internationally”. This vast movement of ballast water has lead to the
introduction of over 1000 NIS in European coastal waters (Golasch,
2006); though it was the harmful affect to human health and the
economy that attracted the attention of scientists and professionals
to address the issue (Institute for European environmental policy
2008).
In 2004 the International Maritime Organisation organised a
convention for the management of ballast water and sediment in
ships. The convention came up with two strategies to combat the
problem.
1) They must have and implement a ballast water management
plan approved by the administration.
2) To have aboard a ballast water record book, recording when
ballast water is taken on board and when it is discharged, also
any accidental or exceptional discharges must be recorded
(International Maritime Organisation, 2004).
The two most important regulations are D-1, Ballast water exchange
Ref (Matej, 2008), and D-2 which is a ballast water performance
standard which dictates the acceptable levels of organism allowed
within ballast water. A D-2 table of organism’s sizes and quantities
can be found in Tsolaki’s (2009) review of ballast water treatment.
Regulation D-1 is being phased out and after 2016 only ballast water
treatment systems will be utilised to comply with regulation D-2
(International Maritime Organisation, 2004). Therefore the ballast
water related industry is focused mainly on treatment of ballast
water, this maybe port based or ship based. Even with treatment
systems in place there is not a method which can remove 100% of
NIS (Tsolaki, 2009). The treatment systems do have their place and
should continue to evolve, however there maybe alternative
methods to manage NIS.
This study is to investigate whether simple current modelling can
help to manage NIS within the Fal estuary, Falmouth, Cornwall.
When planktonic NIS is deposited with ballast water in the Fal it is at
the mercy of abiotic factors; Transportation will be controlled by the
estuaries hydrodynamic traits (Becker et al., 2010). The region of the
Fal estuary where the docks are located is macrotidal (Pirrie et al.,
2003) with the largest spring tides of 5.7m. The Fal possess a flood
dominant tidal flow, but low tidal currents (Stapleton and Pethick
1996) this means that settlement of sediment in lower half of the
estuary has been minimal which could have similar implications for
NIS. However geochemical data showing the distribution of
contaminates within the Fal suggests that it follows the dominant
tidal flow (Carrick Roads area) (Pirrie et al., 2003), this could also
have similar implications for NIS.
Apart from the work by Stapleton and Pethic, I am only aware of one
physical study of the Fal performed by Matthew Le Maitre who
undertook a hydrographical survey for the Harbour Commission. The
survey was looking at the accuracy of tidal diamonds on admiralty
charts in comparison with real time data collected from in-situ
buoys. The current flow data was only undertaken for surface layers
and modelled in a programme known as PICES for mapping oil spill
distribution.
The Fal estuary is a special area of conservation under European
Habitats Directive, which aims to protect the site and stop any
Figure 1. Map showing dominant tidal flow in the Fal. Map A- 3 hours before high water.
Map B – 3 hours after high water. Courtesy of Stapleton and Pethic Institute of Estuarine
and Coastal Studies.
degradation and obtain favourable conservation status of the
interest features across their bio- geographical ranges (Langston et al
2006) NIS would conflict with this. The Fal estuary is also a ria
making it one of the deepest natural harbours in the world. This
allows there to be a commercial docks run by A&P Ltd, again
conflicting with the special area of conservation principals. Although
export is low from Falmouth docks ,it still exists. Therefore ballast
water will be released into the estuary; the majority of ballast water
released in the docks is from vessels entering dry docks for repair
(Mike Pereir, A & P Ltd, Pers. Comm., January 14, 2011).
Species that have been recorded in the Fal are:
Crepidula fornicate,
Caprella mutica,
Styelea cava,
Crassostrea gigas,
Sargassum muticum,
Watersipora subtorquata,
There is also no management in place by Cornwall council to deal
with invasive species. (Jenny Christie, Maritime Environment officer,
Cornwall Council. Pers. Comm., November 25th 2010). However both
the Environment Agency and Natural England “would be responsible
should an outbreak occur that could be contained or eradicated”
(Lisa Rennocks, Cornwall Wildlife Trust, Pers. Comm., May 04 2011)
There is a hotline for reporting NIS and each cased is judged on
whether action is needed.
Sampling area
The sampling area is shown in fig 2, the site was not the preferred
location but had to be used to coincide with the working
functionality of the docks. This site is however a good representation
for ballast water transport, as Duchy Wharf is one of the primary
wharfs used by A&P Ltd, thus NIS could potentially be released into
the recorded currents.
The Hydrodynamic traits of this area are unknown, being affected by
natural fluxes and anthropogenic structures. These structures may
be permanent such as the wharfs, or mobile structures such as the
access barge and other vessels using the docks.
Wharf Destroyed in
Fire 2003.
Figure 2. This GiS Map shows the location of the sampling site, note anthropogenic structures that may
influence hydrodynamic traits. (a) Location was an access barge used for smaller vessels; this was constantly
in-situ.
(a)
Methodology
The sampling was conducted using a Valeport 106 Current Meter. It
was used in self recording mode with the 10-way subconn connector.
Direct operation was not possible due to location. The current meter
was then lowered through the water column stopping at each 1m
interval for a period of two minutes allowing data collection. This
would be done each day over one entire tidal cycle, alternating daily
between flooding and ebbing tides. During spring and neap tides,
both the flooding and ebbing tide would be measured to give a tidal
flow for the extremes of the cycle.
As the 106 current-meter was used in self recording mode, the data
needed to be removed after each daily sample obtained. This is
done using Datalog (software provided by Valeport) and the Y lead to
connect the fish to the computer. Once the data had been removed
it is to be converted into an excel format and filtered (shading
alternative depths for clarity). Each depth’s flow and heading data
would be copied into a new spreadsheet (Flooding, Ebbing, Spring
Flood, Spring Ebbing, Neap Flooding or Neap Ebbing): Sheets are
then separated into individual depths. Vectors would then be
converted into Cartesian coordinates and then averaged.
X= 𝑎 cos 𝑡ℎ𝑒𝑡𝑎
Y= 𝑎 sin 𝑡ℎ𝑒𝑡𝑎
(𝑎 = 𝑚𝑎𝑔𝑛𝑖𝑡𝑢𝑑𝑒 𝑜𝑓 𝑣𝑒𝑐𝑡𝑜𝑟)
(𝑡ℎ𝑒𝑡𝑎 = 𝑎𝑛𝑔𝑙𝑒 𝑜𝑓 𝑣𝑒𝑐𝑡𝑜𝑟)
Once the data is averaged it is changed back to vector (polar)
coordinates using Atan2:
Atan2 = 2𝑎𝑟𝑐𝑡𝑎𝑛 (𝑦 ÷ √𝑥² + 𝑦² + 𝑥)
Hypothesis
H0–Mean directional flow of an ebbing tide is = mean directional flow
of a flooding tide.
H1 – Mean directional flow of an ebbing tide is ≠ directional flow of a
flooding tide.
Observations
If vectors are
broken down
in ratio’s of
45 degrees Fig 3 shows 4:5:3:1 (0-90 : 90-180 : 180-270 : 270-360)
and that the highest velocities were recorded at 12 and 8 metres in
Ebbing Vector Graph
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0.00.20.40.60.81.01.21.41.6
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0
30
60
90
120
150
180
210
240
270
300
330
1M
2M
3M
4M
5M
6M
7M
8M
9M
10M
11M
12M
13M
Figure 3. Averaged Ebbing vector plot, taken from samples
between 04/02/2011- 22/03/2011 from location shown in
Fig 2. Each plot is representing metre depths see legend,
axis are cm/S.
depth. 0-180 degrees holds the majority of the vectors from North
East to South South East in direction.
Flooding Vector Plot
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0.00.20.40.60.81.01.21.41.61.8
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0
30
60
90
120
150
180
210
240
270
300
330
1M
2M
3M
4M
5M
6M
7M
8M
9M
10MVectors are
broken down into the same ratios as above, 1:5:4:0. 0-180 degrees
holds the majority of the vectors between North East and South East
Figure 4. Averaged Flooding vector plot, taken from
samples between 04/02/2011 – 16/03/2011 from
location shown in Fig 2. Each plot is representing
metre depths see legend, axis are cm/S.
in direction. The highest velocities are found at 5 and 8 metres in
depth.
Vectors of ebbing and flooding tide, depths ignored to show an
easier graphical comparison of hydrodynamic traits of the two tidal
Ebbing vs Flooding
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0.00.20.40.60.81.01.21.41.61.8
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0
30
60
90
120
150
180
210
240
270
300
330
Flooding
Ebbing Figure 5. Averaged vector plots, taken from samples between
04/02/2011 – 16/03/2011 from location shown in Fig 2. Axes are cm/S.
flows. The graph indicates that the main vector headings are south
easterly and that flooding velocities are higher than ebbing.
With the coding for mean directional flow, anything above 0.5 would
be travelling in a Northward direction (>270-<90 degrees). Any value
below 0.5, would be travelling in a southward direction (>90-2<70).
Both Mean values for both tidal flows are below 0.5 so mean
directional flow in both cases, is more southerly than northerly. The
Depth (M) Flooding Ebbing Flooding Code Ebbing Code
1 201.6162306 106.4414867 0 0
2 38.84300014 181.0517796 1 0
3 183.2028478 224.6783325 0 0
4 183.2958067 190.2172458 0 0
5 138.8962462 98.70066586 0 0
6 266.7919884 163.2364974 0 0
7 116.4006659 23.34776993 0 1
8 116.9323133 50.28114248 0 1
9 131.320674 342.3382264 0 1
10 124.8657787 110.1098376 0 0
11 74.3705397 1
12 173.1563756 0
13 69.70374148 1
Mean Values 0.1 0.384615385
Critical Value 2.08
Test Value 0.134912594
Table 1. Coding of vector heading data for statistical unpaired t-test,
Table 1. Coding of the data: Any vector with a northward heading (270-90 degrees) was given
a score of 1. Any vector with a southward heading was given a score of 0.
Test value is also far below the critical value for a two tailed test with
21 degrees of freedom. This suggests that there is no statistical
difference between the two input groups and so the Null Hypothesis
should be accepted. The difference could be a product of random
sampling variability.
Depth (M) Ebbing Velocities Flooding Velocities
1 0.81 1.21
2 0.60 0.92
3 0.46 0.84
4 0.75 0.10
5 1.11 1.97
6 0.85 0.42
7 0.23 0.55
8 1.62 1.42
9 0.36 1.02
10 0.77 0.79
11 0.59
12 1.49
13 0.86
Mean Values 0.81 0.92
Standard Deviation 0.41 0.53049043
Critical Value 2.08
Test Value 0.57
Table two shows that the flooding tide has a slightly higher mean
over all, however this is not statistically significant and could have
occurred through sampling variability.
Table 2. Unpaired t-test on ebbing and flooding tidal velocities, original and mean values
are in cm/S.
Discussion
Vector heading analysis
Vector headings are not as expected with no statistical significant
difference in tidal flow direction (table 1) between the two tidal
states. This is very surprising as the sample location is macrotidal.
The majority of vector headings were in a mean southward direction
(Figure 5, Table 1) this would be characteristic of a flooding tide. A
more equal split between south/north directions was expected. The
ebbing tide is very variable over depth (Figure 3), and with only 5
vectors considered to be flowing outwardly in direction. These
depths are 7,8,9,11,13 (Figure 3). This could potentially be one of
following variables, anthropogenic obstructions or wind driven
mixing creating turbulent flow. Boats that docked at the wharfs
were recorded as was was wind speed and direction. However, as
the Flooding tide only had one anomaly in directional flow, 2M depth
(suggesting that this is wind interference), if wind was the variable
effecting the ebbing tide, you would expect to see more anomalies in
the flooding tide. The vessel, Mounts Bay, was docked alongside
duchy wharf for 12 out of the 29 days so would influence the data a
great deal. It also had a depth of 5.2m, so would be affecting
movement of water to this depth. However this is not conclusive
and greater analysis would have to be undertaken comparing wind
vectors, obstructions and current vectors to come to a more
substantial conclusion.
Velocity analysis
As the Fal is flood tidal dominant in its hydrodynamic traits, you
would expect to see a stronger flooding current than ebbing. Whilst
this is the case for one off depths such as 5m (Figure 5), as a whole it
isn’t (Table 2) statistically significantly different. This may be due to
the sample location (Figure 2). If the data was collected from the
flooding channel, a difference in velocities may occur. The current
flow is slow as suggested. Again this is partly due to location with
maximum averaged velocity of 1.97cm/s. The velocities also don’t
have any correlation with depth, obstructions or wind vectors.
Potential movement of NIS
NIS deposited within the ebbing tide mixing to the following depths
1,2,3,4,5,6,10,12 would stay within the area they are deposited
(Figure 2). The data suggests that if they were to colonise, it would
be within this area as they would be constricted by the solid wharf
structures which are rocky (potentially good substrates species
dependant). Whilst sampling, a vast number of anemones were
growing on the sides of the access barges. This may suggest settling
of planktonic larvae before the sessile stage of the life cycle would
begin. The depths 7,8,9,11,13 would carry planktonic larvae away
from the area and potentially stop colonisation within the sample
location, however, even travelling at the highest velocity, the
furthest the plankton would travel before the tide turns and it starts
travelling in a different direction is 394.92m. This slow velocity
means that plankton would not travel any further than the most
northern jetty (Figure 2), so it would be maintained within the
dockland area unless the velocity was to increase as moving away
from the sampling location.
This is all similar for NIS deposited in a flooding tide; it possess a
higher chance of staying within the sampling area as only one vector
out of ten is travelling in a northward direction. Again the velocity
that the plankton would be travelling is so slow that it would only
reach a maximum of 425.52m. This would again keep it within the
dockland area unless affected by a change in the velocity or direction
of the vector.
Conclusion
Data of average vectors over a monthly tidal cycle cannot be
conclusive for NIS distribution. However Duchy wharf, south of the
sampling location, is on “stilts” and may provide suitable substrate
for colonisation of NIS; further biological investigation of this area
would help to prove or disprove the theory. The vector velocities
were so slow that any plankton deposited at this site would not
travel out of the dockland area, so colonisation (if necessary), would
occur within the dockland area for the species to progress through
its life cycle. This suggests that the evolution of management
strategies would not be necessary as this would be impractical and
conflict with the docks.
Acknowledgments
Thanks to Harriet Knowles at Falmouth Harbour Commission for
support and helping networking with A&P Falmouth to gain access to
the docks. Mike Pereir for advice on potential sample locations
within the docks. Danielle Perrin & Amber Thornton for helping with
the physical act of sampling. Matt Le Maitre for help with vector
averaging and initial direction in methodology. Claire Eatock for
intial networking with Falmouth Habour Commission, to make
projects possible.
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