risk analysis ii, c.a. brebbia (editor) © 2000 wit press ......ecological risks of the...

Ecological risks of the hydrotechnical buildings

in the region of the straits of Estonia,

the Baltic Sea: two case studies

U. Suursaar, M. Otsmann, T. Kullas, R. Tamsalu & P. EnnetDepartment of modelling, Estonian Marine Institute, Estonia

Abstract

Two case studies are analysed in the shallow marine area of West Estonia,consisting of a zone of straits, the islands and the semi-enclosed sub-basins.First, possible environmental risks and ecological developments in relation tothe plans to build a fixed road link (a bridge with sections of the dam) across theSuur Strait from the Estonian mainland to the biggest island of the WestEstonian Archipelago. Second, to make openings into the existing road dam, bywhich the Vaike Strait has been closed for about 100 years; local eutrophicationand sediment displacement is already evident. 2D hydrodynamic and forcedoscillation water exchange models are used for simulations and the futureprospects are discussed. It was found that the Suur Strait link project affects thewater and nutrient exchange in the study area. The ventilation of the Vainamerisub-basin will be decreased by about 10-20% and the acceleration ofeutrophication processes could appear in certain places. As the economicpossibilities of Estonia only enable the cheapest building options, nocompensatory dredging and rehabilitation programs are allowed. The plannedholes in the Vaike Strait dam have a very decent positive ecological effect: it isvery hard to restore the past situation.

1 Introduction

Losses tend to appear when our attitude to threats is inadequate. On the otherhand, being prepared to face them, it is possible to live under serious threats,and, under milder conditions, it is possible to let the problems grow graduallyuntil a catastrophe occurs. In general, building marine hydrotechnicalconstructions, two-way risks should be considered: firstly, the hazards imposedby the extremal meteorological and hydrological conditions on these

Risk Analysis II, C.A. Brebbia (Editor) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-830-9

478

constructions, and secondly, potential environmental changes, which may beprovoked by these artificial objects if ecological criteria are not followed.

Compared, for example, to Florida, the Caribbean Sea or Bangladeshi coastnear the Gulf of Bengal, Estonian coastal waters are locating in a relatively calmand safe part of the world. The maximal height of storm-waves is 11 m, which isas much as for the entire Baltic Sea. However, due to shallowness of the sea, thewaves are steep (the corresponding period is 10s) and potentially dangerous fornavigation: the main cause for the catastrophe of the ferry "Estonia", claiming852 lives in 1994, is said to have been the forces of the storm-waves on thebeaver. Due to the waves and ice activity the Estonian coast and harbours areopen to coastal processes to a large extent. The existence of bays and peninsulasfavours occasional high waters, amplified by storm surges and the so-called longwaves. Extreme floods in the study area are +253 cm (+424 cm nearSt.Petersburg, Russia). Bearing in mind the low lands and the virtual absence oftides (up to 10 cm in the Baltic Sea), people are not prepared enough for highwater, and such infrequent storm-surges may cause considerable losses.

The more typical risks in the study area, however, are related to the overallhigh trophic status of the Baltic Sea supplemented by the local nutrient andpollution load. Due to the restricted water renewal, the eutrophication of thecoastal areas is evident. The depletion of oxygen in the near-bottom layers hasdecreased fish catches and the frequent appearance of poisonous blue-greenalgae are other expressions of the deteriorating quality of the marineenvironment. The case studies described in the paper are related mainly to thataspect: how the results of certain activities may lead to the accumulation ofenvironmental risks, and in which way the environment, chronically damaged byman, will take its toll. The main problem with such cases is that the connectionbetween an action and the results may seem weak and it is hard to present anyfirm quantifications. Emotions may become involved due to activities of heeconomic and political pressure groups. Another example from the same studyarea is related to the plans to make a deep-sea port for cruise ships in the vicinityof the Vilsandi National Park in the NW corner of the Saaremaa Island. Thisresulted in an angry debate between the builders and the environmentalists, thelatter even threatening with an international scandal: in addition to the increasingpossibilities for oil-spills, the ships could disturb the grey eiders, a bird specieshighly protected by international environmental legislative acts.

2 Numerical models and methods for the case studies

2.1 Forced oscillation model

Due to the suitable size and configuration of the study area (Fig. 1) it is possibleto model the currents in the straits and the sea levels inside the sub-basins as acombination of Helmholtz oscillators (on Helmholtz mode see e.g. Maas [1]). Asa result of superposition of 5 individual oscillators the system with two freeoscillation periods (about 24 and 12 h) appears. The basic equations consist offour motion equations, composed into the first eqn in (1), and the volumeconservation equations for the Gulf of Riga (subscript G) and the Vainameri (V):


Risk Analysis II 479

(1)

, _ ^ , , ,dr I. *' //. //.

— r- = ~"i4 + "3 3 + "4 4 +

where z =1,2,3,4 denote the different channels (straits, Fig. Ib), g is accelerationof gravity, t is time, A is the surface area of the sub-basin or a strait, /,/ and //,- arethe generalized lengths and depths of the straits, respectively. The driving forcesare the wind stresses % above the channels calculated from HIRLAM (HighResolution Atmospheric Model) winds with the time step of 3 hours, the sealevel differences (A£/) and the constant river inflows to the sub-basins (Qc andQv). Model outputs are the space-averaged flows in the straits (%,) and the sealevels inside both of the two sub-basins ( and %y). The last term of the first eqnin (1) describes the bottom friction of the straits. The generalized measures ofthe basins and channels, the directions of the channels and the frictioncoefficient serve as model parameters. For the parameter values, verification anda more detailed description of the model behaviour see: Otsmann et al. [2,3].

(b)

(C) KessulaidIslet

I

Viirelaid Islet

Figure 1: The study area (a). The straits: 1, Suur; 2, Irbe; 3, Soela; 4, Hari; 5,Vaike (b). The Suur Strait area with four preliminary road link versions(c). Double lines denote dams, single lines are bridges.


480

2.2 Two-dimensional hydrodynamic model

The 2D model is based on the Baltic Sea hydrodynamic model adapted for theVainameri and the Gulf of Riga. (The Baltic Sea model, grid step 2.5 miles, isalso used by us for calculating the sea-level input data for the water exchangemodel described above.) Its grid step is 1 km, adopted on the basis of Latvianbathymetric database (Berzinsh et al. [4]). The model domain is 150 x 241points, including 16405 marine points in the Gulf of Riga and 2510 marinepoints in the Vainameri sub-basin. 2D HN models are based on hydrodynamicequations for shallow sea (vertically integrated barotropic Reynolds equations,see also Hansen [5]). The model inputs are the HIRLAM winds and the BalticSea levels (or Kattegat levels at Goteborg for calculations of the Baltic Sealevels). The outputs are the horizontal distributions of the sea levels and thevertically integrated velocities in each marine grid step. Simulations werecarried out for the year of 1995. Using empirical relationships between thebottom stresses and the concentrations of the suspended solids (taken from [6]),fluxes of the suspended matter in the designed holes of the dam were calculated.

3 Case study I: Bridge over the Suur Strait

About 1 mln passengers and 0.25 mln vehicles per year have been ferried acrossthe Suur Strait, separating the Estonian mainland from the Saaremaa Island (Fig.1). In relation to the preliminary plans to build a road link across the Suur Straitsome environmental problems arise: what will happen to the currents and to thewater exchange processes in the straits, and, will the (possible) decrease in thewater exchange affect the trophic status of the Vainameri ecosystem? TheVainameri is a beautiful and vulnerable marine area, a buffer zone, where thewater masses, originated from the Gulf of Riga (salinity 5-6 psu, high trophiclevel) and the Baltic Proper (7-8 psu, relatively lower trophic level) counteract.We can take an example from the Belt-0resund Links (1988-2000) betweenScandinavia, Danish major islands and the European mainland. The plansinvolved strong environmental design criteria, for example, that the link shouldnot alter the exchange of water and salt through the Danish straits. The blockingof the water transport was compensated there by dredging the bottom in the sillarea following the so-called zero solution link concept (e.g. [7,8]).

Four main link projects exist in the Suur Strait, each consisting of a bridgeand sections of dams in the shallow areas (Fig. Ic). Depending on the versionthe dams close 3-10% of the cross-section area, in addition, the bridge pillars(piers) close 10-16%. As the different project versions are located in thedifferent parts of the cross-section, the actual closure varies between 5%(version I, Fig. Ic) and 25% (version III) from the minimum (or limiting) cross-section area (0.04 km*). Simulations carried out by the water exchange modelshow that such closure produces an increase in velocities of about 2-12% in theSuur Strait and a decrease in other straits. The sums of annual in- and outflowsdecreased by 5-24% in all the straits and integral flows decreased by 5-20%,depending on the link version (Fig. 2).


Risk Analvsis II 481

IQ, kirf (a) Integral flows in the Suur Strait-10

-30All *0.5Suur *0.5Hah *0.5Suur *0.7

Suur *0.9Control =Soela *0.5

0 30 60 90 120 150 180 210 240 270 300 330 360IQ, km* (b) Integral flows in the Hari Strait-10

-50

-70

All *0.5Hari *0.5

Suur *0.5

Suur *0.7Suur *0.9~Soela*0.5-Control

0 30 60 90 120 150 180 210 240 270 300 330 360"™Y (c) Integral flows in the Soela Strait

Suur *0.5Suur *0.7Suur *0.9Control

Soela *0.5All *0.5Hari*0.5

30 60 90 120 150 180 210 240 270 300 330 360

Days (Year 1995)

Figure 2: Modelled curves of integral (cumulative) flows (IQ, km*) during (360days in the Suur (a), the Hari (b) and the Soela (c) straits in the case ofdifferent reductions of the cross-section areas: "control" (no decrease),"Suur Strait*0.9" (decrease of 10%), "Suur Strait *0.7" (30%), etc.


482 Risk Analysis II

The free oscillation periods of the whole oscillatory system would alsochange somewhat, however, with no considerable ecological impact. The localinfluence, connected with the shadow created by the dams and the bridge rampsshould be considered as well. The dams change the already establishedequilibrium between erosion and accumulation, resulting in the displacement ofthe bottom sediments. A zone of low velocities and turbulent backflows appears,favouring the siltation of the bottom sediments and local eurrophication events.In the case of northern winds the dam in the eastern section of the link may trapthe more polluted Kasari River jet, the dilution conditions of the pollution willdeteriorate. Further quantification of the environmental effects is planned to becarried out using 2D hydrodynamic (Fig. 3) and 3D ecosystem models (Tamsalu[9]). Unfortunately, precisely the cheapest version (III, Fig. Ic) has the highestenvironmental risks in relation to the water exchange. As economic calculationsand Estonian possibilities allow only the cheapest options, no compensatorydredging is probably allowed. The idea of building a tunnel seems to have beendiscarded as well due to emotional reasons.

(a) 20 m hole in the dam, A<jf = 10 cm

(b) 24.01.1995, 23.00 GMT, wind 6 m/sW

Figure 3: An example of 2D flow simulation through the hole (a) and themodelled situation of the flows in the Vainameri, a snapshot (b).


Risk Analysis II 483

4 Case study II: Holes in the Vaike Strait dam

The Vaike Strait is a minor channel (length 30 km, width 2-4 km, typical depths1-2 m) roughly parallel to the Suur Strait. It has been closed by a 3 km-longroad-dam since 1896 and no water exchange occurs through it. In recent yearsthe ecological situation is said to be deteriorating in the area, sedimentation andaccumulation processes prevail and eutrophication has been observed [10]. Inorder to restore the past situation two openings (provisionally 18m wide each)has been designed into the dam. We tried to estimate the flow characteristicsthrough the holes and predict the environmental changes in the area. Using thewater exchange model and taking into account the term describing the localwind-generated pile-up effect due to the existence of the dam, temporalvariations of sea levels were calculated on both sides of the dam (Fig. 4a). Then,using Chezhy formula, corresponding velocities through the holes (Fig. 4b) andfinally the integral flow curve (Fig. 4c) during the year 1995 were calculated.

Due to the local wind effect the sea level differences can reach 2 m. Suchsituations are short-term, the difference usually levels within a day due to naturalcauses, otherwise it could take about 5 days for the flows (average velocity 1.5m/s) in the holes. Thus, the holes are not appropriately large for the quickequalization of the extreme sea levels. The flow with the average velocity of 50cm/s and maximal values up to 1.9 m/s will appear in the holes. The flow willrapidly change its direction, but statistically the flows directed from the Gulf tothe Vainameri will prevail (about 0.1 knf per year). Receding the dam thisinfluence diminishes quickly, roughly proportionally with the second power ofthe distance. This could be illustrated by the idealistic 2D model of the hole,which was created on the basis of the Vainameri 2D model (Fig. 3a). The gridstep is 2 m and the flow pattern is simulated with the sea level difference of 10cm as the input. Off the dam the increase due to the holes would be 10%, thegain is too low and the new velocities are still up to 3-4 times lower whencompared to the "normal" currents in such a strait. If we wish to get velocitiestwice as high as now, we should have the holes 5-10 times larger than designed.

The resuspension of the bottom sediments was calculated on the basis ofthe velocities on both sides of the dam, obtained using 2D model simulations(Fig. 5a). A snapshot for the area could be seen from Fig. 3b. Velocitycomponents seldom reached 20 cm/s in the two selected points, about 2 km offthe dam. Bottom stresses (Fig. 5b) calculated from the velocity simulations quiteoften exceeded the critical value of 0.5 dyn/cnf (by the criteria taken from [6]).Using the time series of bottom stresses, concentrations of suspended matter andsediment transport through the holes were calculated (Fig. 5c). The sedimentflux pulsates in accordance with the velocity, but the cumulative flux shows thatonly about 1000 tons of suspended matter left the Gulf of Riga and movedtowards the Vainameri during the year. As the trophic status of the Gulf of Rigawater is higher, nutrient fluxes appear as well. However, they are still relativelyvery small, the desired positive ecological effect will remain modest and will belimited to the vicinity of the holes. Only if the holes cover about half of the damsextension, exchange processes and resuspension will notably increase. Thesecondary pollution may even cause a small-scale environmental disaster then.


484 Risk Analvsis II

-50

:: NW sea level-100 J-,

SE sea level

60 90 120 150w, cm/s

150

0.05

(c

/\f

)

/

y

/v>

v--AA^\

1\fJV ^

VA,

-0.050 30 60 90 120 150 180 210 240 270 300 330 360

Days (year 1995)

Figure 4: Modelled sea levels at the NW and SE sides of the dam with the localwind-influenced pile-up (a), calculated velocities in the openings of thedam (b), and variations in integral flows (knf) through the openings ofthe Vaike Strait dam as modelled for 1995 (c). (a and b): excerpts from1995, days 60-150.


Risk Analysis 11

%, cm/s

20 -

1C

485

1

0,8

0,6

0,4

0,2

0

2* 2

1.5

1

0.5

0

-0.5

-1

(a) N-S velocity component

2 km NW from the dam

(b) — Bottom stress, dyn/cnf

60 120 180 240

Sediment transport in

~~ the openings, kg/s

60 120 180 240

Days (year 1995)

360

300 360

300 360

Figure 5: Modelled velocity component (a), calculated bottom-stresses (b) andtransport of suspended solids (Q$) through the holes of the Vaike Straitdam (c) in 1995.


486

5 Concluding remarks

The analysis of the case studies showed that the hydrotechnical buildings have aspecific environmental impact. While it is relatively easy to quantify thedecrease in the water exchange in the case of the bridge or the sedimentdisplacement in the case of the holes in the dam, it is hard to assess the wholecomplex of possible ecological changes. It is also hard to decide how theenvironmental and economic aspects will balance each other: the cheapestoptions tend to be more risky and vice versa. Further analyses based on 2D and3D ecosystem models are needed, and consideration of the specific economicand social issues is always important in such cases.

Acknowledgements

The study was financially supported by the ESF Grant Project No 4057.

References

[1] Maas, L.R.M. On the nonlinear Helmholtz response of almost-enclosed tidalbasins with sloping bottoms. Journal of Fluid Mech., 349, pp. 361-380, 1997.

[2] Otsmann, M., Astok, V. & Suursaar, U. A model for water exchangebetween the Baltic Sea and the Gulf of Riga. Nordic Hydrology, 28, pp.351-364, 1997.

[3] Otsmann, M., Suursaar, U. & Kullas, T. Modelling currents along thewestern coast of Estonia as superposition of Helmholtz oscillators. CoastalEngineering and Marina Developments, eds. C.A. Brebbia & P.Anagnostopoulos, WIT Press: Southampton and Boston, pp. 99-108, 1999.

[4] Berzinsh, V., Bethers, U., & Sennikovs, J. Gulf of Riga: Bathymetric,hydrological and meteorological databases, and calculation of waterexchange. Proc. Latvian Acad. ScL, Section B, 7/8, pp. 107-117, 1994.

[5] Hansen, W. Theorie und Errechnung des Wasserstandes und der Stromungenin Randmeeren nebst Anwendung. Tellus, 8(3), pp. 287-300, 1956.

[6] Hutrula, T. Modelling the transport of suspended sediment in shallow lakes.Academic Disseration, University of Helsinki, 1994.

[7] Touborg, P.P. Consequences of the Great Belt bridge for the salt waterexchange of Langelandssund. Proc. of the 17th Conference of the BalticOceanographers, Norrkoping, pp. 599-610, 1991.

[8] Poulsen, K.M. Environmental optimizing and impact of the constructions ofthe Great Belt and 0resund links - the Sound. Proc. of the 19th Conferenceof the Baltic Oceanographers, Sopot, pp. 233-246, 1994.

[9] Tarnsalu, R. (ed.) The coupled 3D hydrodynamic and ecosystem model FinEst.MERI — Report series of the Finnish Institute of Marine Research, 35, 1998.

[10] Woitsch, E. (ed.) Ecological studies in the aquatic environment of the VaikeVain Strait in West Estonia. Estonian-Finnish cowork during summer 1993,Report: Helsinki and Kuressaare, 1994.


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