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Assessing the impact of the planned approach channel to the seaport Sabetta on salinity changes in the Gulf of Ob

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The assessment is made of the changes in the salt regime near the channel projected in the northern part of the Gulf of Ob that can be caused by topography change due to the construction of the approach channel to the seaport Sabetta. Since the full-scale experiment is not possible in this case, the only way to solve this problem is performing experiments on the salt regime sensitivity to the topography change with using numerical modelling. For this purpose, we use the well-proved Russian universal σ-model of ocean and sea circulation INMOM (Institute of Numerical Mathematics Ocean Model), developed at the Institute of Numerical Mathematics of the Russian Academy of Sciences. This model is implemented to simulate the circulation of the Kara and Pechora Seas. Its global version is used in the framework of international projects CORE (Coordinated Ocean-ice Reference Experiments) and CMIP (Coupled Model Intercomparison Project). The experiments conducted in the Gulf of Ob show us that the strongest annual mean salinity growth due to the presence of the channel occurs near this channel and reaches 0.45 PSU in absolute or 4.5% in relative. While moving away the channel one can see a significant reduction of its impact. The natural interannual salinity variability is revealed in the water area under research (RMS is about 2÷3 PSU, variability is up to 8 PSU), which depends on both river runoff variability and atmospheric forcing. At the same time, using the results of numerical simulations, it was obtained that the natural interannual salinity variability is much higher than its change caused by the presence of the approach channel.
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ASSESSING THE IMPACT OF THE PLANNED APPROACH
CHANNEL TO THE SEAPORT SABETTA ON SALINITY
CHANGES IN THE GULF OF OB
Nikolay Diansky 1,2, Vladimir Fomin 2,3, Ilya Kabatchenko 2,
Gennady Litvinenko 4, Anatoly Gusev 1,5
1Institute of Numerical Mathematics of the Russian Academy of Sciences, Moscow, Russia
2N.N.Zubov State Oceanography Institute of the Roshydromet, Moscow, Russia
3Moscow Institute of Physics and Technology, Moscow, Russia
4Moscow State Academy of Water Transport, Moscow, Russia
5P.P.Shirshov Institute of Oceanology of the Russian Academy of Sciences, Moscow, Russia
ABSTRACT
The assessment is made of the changes in the salt regime near the channel projected in the
northern part of the Gulf of Ob that can be caused by topography change due to the construction
of the approach channel to the seaport Sabetta. Since the full-scale experiment is not possible
in this case, the only way to solve this problem is performing experiments on the salt regime
sensitivity to the topography change with using numerical modelling. For this purpose, we use
the well-proved Russian universal σ-model of ocean and sea circulation INMOM (Institute of
Numerical Mathematics Ocean Model), developed at the Institute of Numerical Mathematics
of the Russian Academy of Sciences. This model is implemented to simulate the circulation of
the Kara and Pechora Seas. Its global version is used in the framework of international projects
CORE (Coordinated Ocean-ice Reference Experiments) and CMIP (Coupled Model
Intercomparison Project). The experiments conducted in the Gulf of Ob show us that the
strongest annual mean salinity growth due to the presence of the channel occurs near this
channel and reaches 0.45 PSU in absolute or 4.5% in relative. While moving away the channel
one can see a significant reduction of its impact. The natural interannual salinity variability is
revealed in the water area under research (RMS is about 2÷3 PSU, variability is up to 8 PSU),
which depends on both river runoff variability and atmospheric forcing. At the same time, using
the results of numerical simulations, it was obtained that the natural interannual salinity
variability is much higher than its change caused by the presence of the approach channel.
INTRODUCTION
The increasing intensity of human impact on the extremely fragile ecosystems of the Arctic
shelf in connection with the production of hydrocarbons, industrial development and Shipping
(Oil Spill 2011) requires physical-geographical and environmental monitoring for the analysis
of harmonious exploitation and sustainable development of the coastal zone of the Arctic seas
(Matishov et al. 1999). Modern numerical methods in some cases make possible such an
analysis at the design stage of economic waterworks, which should lead to substantial savings
by their construction. The present paper is devoted to study of the influence of the projected
approach channel to the seaport Sabetta on salt regime change in the Ob Bay. The salt regime
plays a very important role in the formation of the Ob Bay bio-productivity (Lapin 2011), which
is the water fishery object of the special value.
POAC’15
Trondheim, Norway
Proceedings of the 23rd International Conference on
Port and Ocean Engineering under Arctic Conditions
June 14-18, 2015
Trondheim, Norway
THE HYDROLOGICAL REGIME OF THE GULF OF OB IN THE AREA OF THE
APPROACH CHANNEL DESIGN
Gulf of Ob is the bay of the Kara Sea, located between the peninsulas Yamal and Gydansky
consisting the single basin with the eastward adjacent Taz Estuary. With the 3090 km width
and 750 km extent from north to south the Gulf of Ob and Taz Estuary have total area of 62
thousand km2 (Yudanov 1935, Burmakin 1940, Moskalenko 1958). The maximum depth does
not exceed 2830 meters, but the major part of the gulf area is in the range 1015 meters. As
an independent object, Gulf of Ob has sophisticated dynamic regime in the two subsystems,
river and sea, each of which is characterized by its own features, particularly evident in the
open water period. The projected channel area is located in the northern part of the Gulf of Ob
(Fig. 1).
Figure 1. Gulf of Ob area with the proposed construction site of the projected channel.
Gulf of Ob is estuary type, which is microtydal and strongly stratified (Lapin 2011). The total
annual runoff into the Gulf of Ob is 530.5 km3 (Ivanov and Osipova 1972). Seasonal runoff is
distributed as follows: winter 8.4%, spring 14.6%, summer 56%, autumn 21%
(Mikhailov and Gvozdetsky 1978) in accordance with the hydrographer, taken from the site
http://www.r-arcticnet.sr.unh.edu/v4.0/ (Fig. 2), where the daily mean relative Ob runoff is
shown. Winter inflow into the river is mainly due to oxygen-poor ground and bog waters
(Zalogin and Rodionov 1969).
Figure 2. River runoff hydrographer, relative units (1 corresponds to the annual mean).
The total currents in the Gulf of Ob are composed of quasistationary, tidal and wind currents.
The special feature of the Ob runoff is its strong variability, both intra- and interannual.
According to the observations collected over the past 70 years (see. the records compiled by
the analysis of WSAG (Water Systems Analysis Group) at the University of New Hampshire,
USA, the site http://www.r-arcticnet.sr.unh.edu/v4. 0 / index.html), in the most affluent 1979
Ob discharge reached 572.12 km3, that exceeds the one of 267.69 km3 occurred in the least
affluent 1961, more than twice. Thus, we can talk about complex hydrological regime of the
Gulf of Ob, which is rather complicated for simulation.
IMPLEMENTING THE GULF OF OB HYDROTHERMODYNAMICS MODEL AND
THE SCENARIA OF NUMERICAL EXPERIMENTS
The main objective of this paper is to assess the changes in salt regime in the area of the
projected channel in the northern part of the Gulf of Ob (Fig. 2), which may be caused by
topography changes resulting from the construction of the approach channel. Since the full-
scale experiment in this case is impossible, it remains the only way to solve this problem by
investigating the sensitivity salt regime change to topography with using numerical simulation.
For this purpose, we use the well-proved Russian universal σ-model of the oceanic and marine
circulation INMOM (Institute of Numerical Mathematics Ocean Model) (Diansky 2013). This
model has worked well in the framework of the international projects CORE (Coordinated
Ocean-ice Reference Experiments) (Danabasoglu et al. 2014; Gusev and Diansky 2014), and
CMIP (Coupled Model Intercomparison Project) (Volodin et al. 2013) as well as in the
application to practical problems on simulating circulation of the Black and Azov Seas (Zalesny
et al. 2012) and pollution propagation in the waters of the Big Sochi (Diansky et al. 2013), as
well as to estimate the contamination transport from the Fukushima nuclear power plant 1
(Diansky et al. 2012).
The INMOM is based on the complete system of the so-called primitive ocean hydrodynamics
equations in spherical coordinates in hydrostatic and Boussinesq approximations. The
dimensionless σ is used as the vertical coordinate and defined as
σ = (z ζ)/(H ζ)
where z is ordinary vertical coordinate, ζ is sea surface height; H is unperturbed depth of the
sea. The prognostic variables are the velocity vector horizontal components u and v, potential
temperature T, salinity S, sea surface height ζ, sea ice and snow compactness and mass. The
main INMOM feature that distinguishes it from other known ocean models is using method of
splitting into physical processes and spatial coordinates (Marchuk 2009).
INMOM was adapted to the Gulf of Ob, including Taz Estuary. Schematically, the model area
is shown at the Fig. 1. This region is considerably greater than the region of interest (the
approach channel area) (see Fig. 2) because is necessary to simulate the inflow of saline Kara
waters in the Gulf of Ob. The main factors of this inflow are tidal waves, which, according to
the SOI ( www.oceanography.ru ) data, are propagated to the Ob and Taz mouths. Therefore,
it is needed to move computational domain open boundaries as far as possible to set physically
correct conditions at them and avoid their interaction with currents inside the simulation area.
Thus, the model covers the entire area of the Gulf of Ob (Fig. 3), which is much wider than its
northern part (Fig. 1), where the channel construction is planned. This lets us adequately
perform the simulations of the tidal wave and distribution of fresh water from rivers flowing
into the Gulf of Ob.
Figure 3. Simulation area.
For better simulation, the model of the Gulf of Ob Bay was implemented in the rotated spherical
coordinate system. First, it lets us place one axis along the approach channel position. This
allows one to describe its topology better even with a rather coarse grid. Secondly, it makes it
possible to cover the area with quasi-regular horizontal grid for better spatial resolution, which
is difficult to achieve in the geographic coordinate system due to convergence of the meridians
at the approach to the North Pole, which is close to the rather extended Ob Bay. The
transformation is performed so that the area of interest is placed along the model system
equator. This transformation leads to the quasi-uniform grid step equal to ~ 500 m. Due to small
depth of the Gulf of Ob relative to ocean depths, the number of vertical levels were chosen to
be 5. However, the dimension of the grid area consisting total number of nodes 1354×1399×5
is quite significant even for modern high-performance computers. To simulate the sea ice
characteristics we used sea ice model with elastic-viscous-plastic rheology (Yakovlev 2009;
Briegleb et al. 2004; Hunke and Dukowicz 1997)
The hydrodynamical model is driven by atmospheric forcing, river runoff and tide. At the
boundary between the Gulf of Ob and Kara Sea the conditions for temperature and salinity are
set as the so-called "Padded walls": the climatic annual cycle of temperature and salinity was
set according to the combined data from atlases of Levitus (Locarnini et al. 2010, Antonov et
al. 2010) and the AARI ( http://www.aari.ru/projects/ECIMO /index.php?im=201 ). The
relaxation to the data was performed with a characteristic time scale of about 5 days. To set a
tidal wave on the boundary, the tidal level is prescribed, which is computed by the TPXO
model. It uses version TPXO 7.2 assimilation model of sea surface height data received from
satellite TOPEX and Poseidon (T/P), into the global barotropic tidal model (Egbert and
Erofeeva 2002).
The initial conditions. As the initial conditions we set rest state, ice null and climatic fields of
temperature and salinity. The climatology for these data was compiled through a combination
of open data from electronic climate atlases Levitus (Locarnini et al. 2010, Antonov et al. 2010)
for the oceans and the AARI for the Kara Sea (http://www.aari.ru/projects/ECIMO/ index.php?
im = 201 ). The need for such a combination was because the monthly mean Levitus data have
more detailed seasonal variation compared to seasonal AARI data. However, the latter describe
better the spatial distribution of temperature and salinity in the Gulf of Ob.
Atmospheric forcing. Computation of atmospheric forcing was carried out by well-proven bulk
formulas (see, eg, (Gill 1986)) using the so-called normal year of CORE (Datasets for Common
Ocean-ice Reference Experiments) dataset (Large and Yeager 2009).
River Runoff. We allow for the river runoff from the following main rivers: Ob, Nadym, Pur
and Taz. The data were taken from the site ( http://www.r-arcticnet.sr.unh.edu/v4.0/ ). For all
the rivers, the same hydrograph of seasonal runoff change was adopted as shown at Fig. 3.
River runoff in the model was taken into account by sea surface height change in the mouths
of rivers. The salinity in the mouths was set to zero.
Scenario of experiments. To investigate the impact of the projected approach channel to the salt
regime change in the northern part of the Gulf of Ob we developed three scenario. They are
differentiated upon the river runoff (minimum, maximum and climatological mean) with using
the same atmospheric forcing. In each of these scenarios, two experiments are conducted with
different bottom topography: the natural and with the presence of the approach channel. Thus,
there was six numerical experiments, which results are presented below. The duration of each
experiment is 1 year and 1 month. The experiments started in July and continued until August
of the next year. During the first month the spin-up occured, so it was excluded from the
analysis and the July for analysis was taken from the end of the runs.
REPRODUCING THE HYDROLOGICAL REGIME OF THE GULF OF OB
The most representative climatic characteristics of the observational data for the Gulf of Ob, in
our opinion, are the AARI data ( http://www.aari.ru/projects/ECIMO/index.php?im=201 ).
However, they are available for the winter and summer seasons only. Therefore, the comparison
of the simulated and observed fields of temperature and salinity was conducted using the data.
Since the main interest of this paper is the salt regime simulation, we present here the
comparative maps of salinity fields only. Fig. 4 shows the observed and simulated climatic
near-surface salinity distribution at the depth 5 m for the summer season for the climatic mean
runoff. This figure shows us that the INMOM adequately reproduces salt mode in the Ob Bay
and is quite suitable for the task.
Figure 4. Left: salinity [PSU] of Kara Sea and the Gulf of Ob in summer, climatic atlas AARI
[http://www.aari.ru/resources/a0013_17/kara/Atlas_Kara_Sea_Summer/Fields/SAL/S_0000_
MEA1.htm]. Right: near-surface salinity [PSU] in the Gulf of Ob in summer, model results.
Consider the features of the surface and bottom circulation mode in the Gulf of Ob in the
approach channel area. Fig. 5 shows us the April and June mean climatic velocity along the
Gulf of Ob in cross-section, passing approximately through the middle of the approach channel.
These months are characterized by relatively high (June) and low (April) Ob discharge
according to runoff hydrographer shown in Fig. 2. At the figures, one can see the complexity
of the vertical and horizontal distribution of velocity in the Gulf of Ob. At the same, in April,
when discharge is low, the velocity stratification is caused mainly by salinity: freshened, lighter
water flowing northward into the Kara Sea in the surface layer and the denser seawater spread
southward in the bottom layers. The values of bottom velocities are comparable to surface ones.
However, since the area of the cross sections for the surface speed is higher, then, in general,
the outflow occurs from the Gulf of Ob to the Kara Sea in accordance with the river runoff.
In the periods of maximum runoff, northern velocities significantly prevail, with the character
of stratification significantly changing in comparison with winter. We can note the pattern of
section along course current in June. Here, the current has a well-pronounced rod with the
center at a depth of 8 m. Near-surface water have lower northern velocity, while the coastal
currents are directed to the south. This character of stratification is caused by northerly winds,
which, as mentioned above, typical for the summer months, which "blow away" the surface
waters to the south.
Figure 5. Passageway cross-section of the meridional velocity component [cm/s], averaged
for April (left) and June (right). Positive values correspond to the northern direction, negative
to the southern one.
REPRODUCING SALT MODE IN THE GULF OF OB FOR NATURAL COURSE AND
WITH THE APPROACH CHANNEL IN PERIODS OF CLIMATIC MEAN,
MINIMUM AND MAXIMUM RIVER RUNOFF
The presence of the approach channel is accounted in the model by topography change. It
should be noted that the model spatial resolution ~ 500 m allows us to describe the large-scale
features only. Therefore, apparently, in the performed simulations its impact is somewhat
overestimated, which, however, even good because its possible impact on the hydrological
regime is overestimated as well. The impact of the approach channel to salt regime change in
the Gulf of Ob was investigated by comparative analysis of the experimental results, which
showed us that the strongest approach channel influence on the salinity distribution occurs with
minimum runoff in the bottom layer.
Therefore, we present the figures for this scenario. Minimum annual runoff (see above) was
taken in accordance with the available data for rivers Ob, Nadym, Pur and Taz. According to
the hydrographer, the daily mean runoff was set for each river during simulations. Fig. 6 (top)
shows us the annual mean simulated salinity distribution in the bottom layer without the
approach channel (Fig. 6a) and with the one (Fig. 6b). The influence of the approach channel
to the surface salinity distribution is shown in the bottom figure as an absolute difference of
annual mean salinity distribution (Fig. 6c) and relative one(Fig. 6d). The relative difference
was calculated in relation to the situation without the planned approach channel. Fig. 6 shows
us that in absolute terms the difference between the salinity distributions with and without
channel is almost negligible. This is because the absolute salinity differences are only -
0.1÷0.45 PSU, which, in turn, is not more than 4% (see. Fig. 6c,d).
The spatial distribution of absolute and relative deviations, showing the impact of the approach
channel on the surface and bottom are in good match in their shapes. However, the bottom
values directly in the approach channel, as one would expect, are higher. The main feature is
noted that the difference is positive to the left of the channel, and negative to the right of it.
This is easily explained by the fact that the presence of the channel increased inflow of more
saline waters of the Kara, which, according to geostrophic relations, approaches to the left bank
of the Gulf of Ob, as directed to the south. Naturally, in order to compensate for the excessive
Kara Sea water inflow, the Ob freshened water move northward, and, therefore, should
approach to the right bank of the Gulf of Ob.
Fig. 6c,d also show us that the spatial distribution of absolute and relative deviations do not
coincide with each other. The main difference is expressed in the fact that the relative difference
takes over a wider area, extending to the south of the Gulf of Ob. This is easily explained by
the fact that absolute salinity in the Gulf of Ob drops while moving southward, which leads to
the increase of the relative deviations, since it is computed with division to smaller salinity
values.
Figure 6. Bottom annual mean salinity fields [PSU] in the northern part of the Gulf of Ob in
the simulations without (a) and with (b) the projected approach channel. The absolute [PSU]
(c) and relative [%] (d) salinity difference in the simulations with the projected approach
channel and without. Сolorbars аre shown to the right of each figure.
Figure 7. Characteristics of salt-mode at the Gulf of Ob section through the passageway. The
annual mean salinity [PSU] in simulations without the planned approach channel (a), with it
(b). The absolute [PSU] (c) and relative [%] (d) salinity difference in the simulations with and
without approach channel. Colorbars are shown to the right of each figure.
Fig. 7 shows us the vertical section, passing through the approach channel center, of the salinity
and its deviations due to the approach channel presence, computed for minimum annual Ob
runoff. The explanation of the results at these figures is the same as for Fig. 6. For all of these
salinity sections one can state that in absolute terms the effect of the channel is negligible to be
revealed. Through the deviations, one can see the overall pattern of the channel influence that
is the fact that the increase in salinity is observed from the channel to the left bank, and a
decrease is to the right one. It has already been explained above. Another common pattern of
salinity variations due to the presence of the channel is shown in the fact that these deviations
are reduced from the bottom to the surface, which is easily explained by the fact that more salty
and therefore denser waters of the Kara Sea are distributed to the Gulf of Ob in the bottom
layers. Deviations figures Fig. 6 and 7 show us that the largest salinity deviation, as one would
expect, are situated in the channel and its immediate vicinity.
CONCLUSIONS
The simulation results showed us that the change in salinity, both in absolute and in relative
terms occur mainly in the area nearby the channel. Maximal changes occur in the channel and
reach 0.45-0.5 PSU in absolute values and 4.5-5% in relative. The relative changes were
calculated in relation to the calculations with the natural bottom topography. According to the
observations, the interannual salinity variability (Lapin, 2012) in the flow channel can be up to
10 PSU. Thus, we can conclude that the influence of the channel is mainly local in nature and
will not lead to increase of the zone of salt water. Naturally, both the interannual and intraannual
salt regime variability in the waters of the Gulf of Ob are significantly higher.
This work was supported by RFBR (grant 15-55-20003) and the Council for Grants of the
President of the Russian Federation (grant MK-3241.2015.5)
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... First, marine environments are considerable distances from Ust'-Polui. The southern half of the Gulf of Ob is heavily dominated by the outflow of the Ob River, meaning that freshwater environments continue to be found for several hundred kilometers to the northeast within this body of water (Diansky et al. 2015). The nearest high salinity marine environments to Ust'-Polui are at Baydaratskaya Bay on the west side of the Yamal peninsula, about 200 km directly overland to the north; this bay is an extension of the Kara Sea. ...
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