the construction of an airport, and they are compared for ...€¦ · of an island airport and an...

Ocean environmental change due to an

offshore airport in Ariake Bay

Y. Kyozuka" & K. Yokoyama*

"Deparfmenf o/Eorf/t (em cze/zce

Kyushu University, Japan

Abstract

An assessment of the ocean environment is presented for an airport inAriake Bay, which is well known for its large tides and vast tidal marsh. Anewly developed ADI method including the effects of moving boundaries onthe tidal marsh is applied for calculating the tides and currents in the bay.Moving boundaries on the tidal marsh are treated by a quasi-static approx-imation at every time step in the calculations. Assessment functions for thechanges in tides and currents are defined by the ocean area which would beaffected by the construction of an airport in the bay. Three locations for anisland airport and an approaximated floating airport are compared in theassessment functions. Diffusion of COD from four rivers is also simulatedand the distribution patterns of COD density after 30 tides are compared.

It is found that the location of the airport has great impact on the oceanenvironment, especially near the coast where tidal marsh exists. The resultsare presented graphically.

1 Introduction

An offshore airport is one of the greatest ocean structures. The KansaiInternational Airport which opened this September is a good example. Be-cause of its enormous scale, more than 5 km in length and 1 km in width,it will affect tides, currents and waves all around it. The environmentalchanges in an ocean caused by such a huge structure therefore should begiven the same careful consideration as given the structure itself.

Transactions on Ecology and the Environment vol 6, © 1994 WIT Press, www.witpress.com, ISSN 1743-3541

120 Computer Techniques in Environmental Studies

This is a fundamental study on potential changes to the ocean environmentdue to the construction of an offshore airport in a bay. Tidal simulationswere carried out in Ariake Bay of Kyushu, Japan. Assessment functions intidal level and current are defined as the ocean area which is affected bythe construction of an airport, and they are compared for three locationsof an island airport and an approximated floating airport. The diffusion ofpollutants from rivers is also simulated with/without an offshore airport,and the distribution patterns of pollutant densities are compared.

2 Location of the airport

Ariake Bay*) is well known to have the largest tides in Japan and a vasttidal marsh. There are some good fishery and nursery fields in the bay.Therefore, the physical and biological effects on this environment due tothe construction of an offshore airport should be minimized.The size and location of the airport should be determined by estimatedfuture traffic demand and by its role in the overall traffic network. Theairport assumed is of a size similar to the Kansai airport, 5Akm x \Akrn.

Fig.l shows the three locations for an island airport studied in this report:plan A is an original plan and plans B and C are modifications for compar-ison. Plan A is 5 km offshore to meet airplane sound requirements and thelayout of south to north is determined by year-round prevailing winds.Because the water depth is only about 10m in the planning area, the

major difference between an island airport and a floating airport would bethe permeability of water of the latter. Plan A' shown in Fig. 2 is such anapproximation of a floating airport as same location as plan A.

3 Numerical method of tidal simulation^

We employed the following two-dimensional expression, because the wateris not deep in the northern part of Ariake Bay and layered effects can beneglected due to the large tides there,

(i)

where h : depth of water, £ : surface elevation/ : Coriolis parameter Ah : eddy viscosity


Computer Techniques in Environmental Studies 121

C : bottom friction coefficient (=-(h + ()*/*)n

n : Manning's friction coefficient (=0.026)

Eqs.(l),(2),(3) are discretized by the finite difference and are solved by theADI(Alternative Directional Implicit) method. The boundaries of the tidalmarsh vary depending on whether the points defining the computationalmesh are considered land or ocean. Depth of the point is extrapolated by aquasi-static assumption in a similar way to that by Kato et alAThe density of a conservative material is expressed by the following two-

dimensional diffusion equation:

dt dx dy

where c : density of COD(chemical oxygen demand),KX, Ky : eddy diffusivity

Time evolution of this equation is very slow in general, and the calculationis carried out using solutions of w, v and ( given by eqs.(l), (2) and (3).

4 Assessment functions

Assessment functions of the ocean environment are defined as the ocean areawhich would be affected by the construction of an offshore airport comparedwith that without an airport. Effects on surface elevation are evaluated asfollows:

I f fJ//(±a#) = — I I H(x,y)dxdy (5)

whereIT(„ *,\ — } 1 "• Qrnax 3z(

0 otherwise

S : ocean area, SQ : area of airportCma* : maximum tide after construction of airport

C(o)maar • maximum tide before construction of airportan : excess tide coefficient

Effects on currents are evaluated as follow:

(7)



where* * "mo, 2(1 ±0 otherwise

maximum current after construction of airportmaximum current before construction of airport

ay : excess current coefficient

In the above assessment functions, smaller JH and Jy have less effect onthe ocean environment.

5 Numerical results

Tidal simulations were carried out in the northern part of Ariake Bay (40km x 38 km) shown in Fig.l. The ADI method was applied using Ax =A?/ = 200m. The tidal level of the M? component from the tables of tidalharmonic constants is given at the open boundary, the Nagasu-Ariake line,because this component dominates the tides in this bay.Four rivers were included in the simulation and the volume transported

from each one was taken from mean annual data. COD was input fromthose rivers and the volume was assumed to be proportional to the flowvolume. The eddy diffusivities, K^ and Ky are assumed to be 10m^/s inthe whole domain for simplicity.

5.1 Tidal level and current

Fig.3(a) shows equi-contours of tidal level at high tide in the case withoutan airport. A contour is regarded as being proportional to the amplificationfactor of MI in the bay; it becomes large in the interior of the bay, whichagrees with other results. Fig.3(b) shows the results of plan A. The patternsof contours are complicated, which may be interpreted as the interaction ofthe waves between the airport and coastline. Fig.3(c) is the case of planA'. The contours are much simpler than those in Fig.3(b), reflecting thepermeable effects of water of a floating airport.Fig.4(a) shows the change in tidal level around the airport of plan A,

which is obtained relative to the values without an airport. The contoursare not simple but their values are almost unity around the airport. Thegreatest effect appears on the tidal marsh near the mouth of the ChikugoRiver, 10 km from the airport. Fig.4(b) shows the change in tidal levelof plan A'. The change in contours near the river mouth is small and thepatterns are more simple than in Fig.4(a), so that the difference betweenplans A and A' is clear.Fig.5(a) and 5(b) show the changes in tidal currents after construction

of an airport similarly to the tidal level. Compared with Figs.4(a) and



4(b), the contour patterns are very complicated in the shallow water nearthe coastline, suggesting that currents may be sensitive to changes in theflow field. The difference between plans A and A' is not clear in this case.In both plans, currents around the airport are reduced, but they becomestronger in the channel between the airport and Miike port, because of adam-like effect.

Fig.6 shows a comparison of assessment functions for the four plans.J#(±0.05) denotes the ocean area where the tidal level becomes 5 % largeror smaller after the construction of an airport. J (±0.1)/5 denotes 1/5 ofthe ocean area where the currents become 10 % larger/smaller after con-struction. We assume that the smaller the assessment functions are, thelower is the effect on the ocean environment. Plans A' and B are superiorto plans A and C in J#, whereas there is no significant difference in Jysamong the four plans.

5.2 Diffusion of COD

Fig.7(a) is a result of diffusion of COD in the case without an airport. Itshows the distribution of COD 30 tides following the initial zero condition,although the distribution is not steady at this stage. A high density areaappears near Miike port, which happens to correspond to a measured distri-bution of the total amount of mercury in the bottom mud. At such a highdensity area, the tidal residual flow tends to converge due to the topographyof the sea bottom.Fig-7(b) is the result for plan A. High density areas appear at two places

north and south of the airport. As the northern area is near the tidal marsh,the diffusion could be accelerated if the vertical mixing effects were takeninto account. Fig.7(c) is the result for plan A\ The high density area northof the airport is fairly less as a consequence of the permeability of water ofa floating airport.

6 Conclusions

Ocean environmental changes caused by a hypothetical offshore airport inAriake Bay are studied numerically. Tidal levels, currents and diffusions ofCOD are calculated and the results are presented graphically. The effects ofthe moving boundary on tidal marsh is taken into account by a quasi-staticapproximation in the tidal calculations.An assessment function is defined by the ocean area which is affected by

the construction of the airport and the functions in tidal level and currentsare calculated for a few island airports and an approximated floating airport.The diffusion of COD from rivers is simulated with/without an offshoreairport, and the distribution patterns of COD density are compared.



These results show that the effect of an offshore airport on the problemis fairly large that positioning of the airport should take into considerationthe environmental changes impacting on ocean environment.

References

[1] The Oceanographycal Society of Japan : Coastal Oceanography ofJapanese Islands, Tohkai University Press, 1985.

[2] Horie,T. : Hydraulic Research on Currents and Diffusion near theCoast, Technical Note of the Port and Harbour Res. Inst., Ministryof Transport, Japan, No.360, 1980.

[3] Katoh,K., Tanaka,N. fe Nadaoka,K. : Tidal Simulation on Tidal Marshand Numerical Forecasting of Its Topographic Deformation, Report ofthe Port and Harbour Res. Inst., Ministry of Transport, Japan, Vol.18,No.4, 1979.

Fig.l Three locations of an offshore airport



Fig.2 Schematic of a floating airport, plan A'

Fig.3(a) Tidal level at high tide without airport



Fig.3(b) Tidal level at high tide with airport in plan A

Fig.3(c) Tidal level at high tide with airport in plan A'



Fig.4(a) Change in tidal level around airport in plan A

Location A'

0.99

D D D D D D D D D D D a a DD D D D D D D D a D D D D Da D D D D D D D D D D a n Do a c

Fig.4(b) Change in tidal level around airport in plan A'



Fig.5(a) Change in tidal current around airport in plan A

Fig.5(b) Change in tidal current around airport in plan A'



Assessment Functions 0.05)JH(-0.05)JV(JV(-0.1)/5

A A' B C

Fig.6 Assessment functions of four plans

30 TIDE360hr.

Fig.7(a) Distribution of COD without airport



30 TIDE360hr.

Fig.7(b) Distribution of COD with airport in plan A

30 TIDE360hr.

Fig.7(c) Distribution of COD with airport in plan A'


the construction of an airport, and they are compared for ...€¦ · of an island airport and an...

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