In this paper, characteristics of hydraulics and water quality in Kojima Bay are discussed based on field observations. The main results obtained are as follows. (1) The variation of flow rate in Kojima Bay has periods for 30 minutes to 2 hours. This characteristic is represented by a two-dimensional numerical simulation fairly well. (2) The local distributions of salinity and suspended solids show increase in the direction to the mouth of Kojima Bay. On the other hand, the distribution of COD(Mn) is nearly uniform in the whole area of the bay. Salinity appears to be influenced by river discharge sensitively but suspended solids does not. (3) The vertical distributions of salinity and suspended solids are similar. There are similar points between the cross-sectional average of water quality and surface water quality. (4) The surface water quality seems to be influenced by the variation of velocity

Kawara, Osami

Nago, Hiroshi

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Hydraulics and Water Quality in Kojima Bay

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Memoirs of the School of Engineering, Okayama University, Vol. 15·2, March 1981Hydraulics and Water Quality in Kojima BayHiroshi NAGO* and Osami KAWARA*(Received January 28, 198 I)SynopsisIn this paper, characteristics of hydraulics andwater quality in Kojima Bay are discussed based on fieldobservations. The main results obtained are as follows.(1) The variation of flow rate in Kojima Bay has periodsfor 30 minutes to 2 hours. This characteristic isrepresented by a two-dimensional numerical simulationfairly well.(2) The local distributions of salinity and suspendedsolids show increase in the direction to the mouthof Kojima Bay. On the other hand, the distributionof COD(Mn) is nearly uniform in the whole area ofthe bay. Salinity appears to be influenced by riverdischarge sensitively but suspended solids does not.(3) The vertical distributions of salinity and suspendedsolids are similar. There are similar points bet-ween the cross-sectional average of water quality'and surface water quality.(4) The surface water quality seems to be influenced bythe variation of velocity.1. IntroductionHydraulics and behavior of pollutants in estuaries and bays* Department of Civil Engineering147148 Hiroshi NAGO and Osami KAWARAare very complicated because of tidal action, seiche, river discharges,wastewater effluents and so on.In this paper, as the first step to clarify the water pollutionmechanisms in these waters, characteristics of hydraulics and waterquality in Kojima Bay are invetigated based on field observations.The inestigation has been carried out since 1977 and is still con-tinuing. We report the results obtained so far. This paper mainlytreats with the characteristics of unsteady flow, the local distri-bution and temporal change of water quality, correlation among waterquality items and correlation between the variation of water qualityand that of velocity.2. Study AreaStudy area of Kojima Bay is shown in Fig.l, where Asahi River and. No Locat10n of +observation----- Boundary set upfor simUlationHyakkenRiverKojima Baythe Kojima Peninsulathe InlandI Sea of Seto. I0 1 2km 0Fig.l Study areaHydraulics and Water Quality in Kojima Bay 149Yoshii River flow. The width of Kojima Bay is from 1 to 2 km. The southpart of that is a waterway and the depth is about 10 m. But in thenorth part, the waters less than 1 m in depth spread.Fresh water is discharged from Lake Kojima at the inner part ofthe bay and Hyakken River at the north part near a low tide.The tidal portions of Asahi River and Yoshii River are divided byShin Weir at 7.9km from the river mouth and Kamogoshi Weir at 7.3kmrespectively. The upstreams of both weirs are non-tidal portions. Theannual averages of both river discharges are approximately 60m3 /s.The areas of Kojima Bay and the tidal portions of both rivers areabout l5km 2 and 5km2 respectively. The average of the spring tiderange at Kojima Bay is 1.34m. Along the both rivers and the coast ofthe bay, there are several industrial factories and a outlet of asewage treatment plant.3. Methods of Field ObservationsThe locations of field observations are shown in Fig.l. The brokenlines in this figure represent boundaries which are set up for the nu-merical simulation of hydrodynamics in Kojima Bay.The locations and the durations of observations, the intervals ofsampling and the intervals of measuring tidal level and velocity arenot always the same because the impotance of observation is somewhatdifferent according to observation. The outline of observations is asfollows.The observations are classified into two kinds according to themethod of water sampling; the observations of surface water in thewhole area of Kojima Bay and the observations in which sampling ofwater was carried out at several verticls in the tidal portion ofAsahi River or the waterway of Kojima Bay.In the former, the hourly sampling of surface water and measuringof tidal level at intervals of 10 or 30 minutes were carried out for24 hours. In the latter, sampling of water and measuring velocity werecarried out at 1m vertical interval. In this case the time intervalsof measuring velocity and tidal level were from 15 minutes to an hourand from 10 to 30 minutes respectively.Water samples collected were returned to the laboratory on thesame day. The water quality items analysed are chemical oxygen demandby potassium permanganate cOD(Mn), suspended solids SS and salinity S.COD (Mn) was measured by alkali method, SS by glass fiber filter and150 Hiroshi NAGO and Osami KAWARAS from electric conductance.4. Mathematical Model for Unsteady Flows in Kojima BayThe hydrodynamics in Kojima Bay was obtained from the two di-mensional equations of motion and continuity. The basic equations ofmotion for two dimensional unsteady flow are3(hu)/3t + 3(hu 2 )/3x + 3 (huv)/ay + ghas/ax(1)and3(hv)/3t + 3(huv)/3x + 3(hv2 )/ay + gh3s/3y(2)The equation of continuity can be expressed asas/at + 3(hu)/3x + 3(hv)/ay o , (3)+ +I 0 I+ +I 0 I+ +I1 1-"2 1+1• x+I 0+I 0+1-1 1-tj-ljj- -tj+1j+ i-1yuFig.2 Space-staggered grid+ sOhare as follows. At the mouth ofKojima Bay tidal level is applied.At the mouths of Asahi River andYoshii River, Kojima Bay is assumedto be connected with two reservoirswhere, h; height from the bottom to the reference level,s; water surface elevation with respect to the reference level,u,v; velocity components in x and y directions,f; Coriolis parameter,Ax, Ay; momentum dispersion coefficients in x and y directions,b sT., T.; shear stresses at surface and bottom in i direction.1 1A space-staggered grid shown inFig.2 was selected for the finite-difference approximation of Eqs.(1) through (3) [1,2]. The relativeposition of the variables in themodel is shown in Fig.3.The boundary conditions appli-cable to the numercal simulationHydraulic sand Water Quality in Kojima Bay 151-x(east)Fig.3 Schematic showing coordinate... +----,;' )...---_::=~:..­... ..... ..... .. ..hstresses followingfomulae and values wereselected; T~ of zero,b 1 bT =prb 2ulu L + v 2 Ty=prb2v~ and rb 2 of 0.0026.x 'which have each surfacearea equal to that ofthe tidal portion ofeach river.For the simulationCoriolis parameter of0.00008rad/s and momentumdispersion coefficientsin x and y directionsof 105 cm2/s were se-lected. Besides, withreference to shear5. Reslts and Discussions5.1 Hydraurics in Kojima BayThe tidal level and flow rate obtained by the simulation at themouth of Asahi River are shown in Fig.4 and Fig.5 in cmparison withmeasured values respectively. The simulation result of the tidal levelshows good agreement with the measured values. With reference to theflow rate, the measured values are characterized by the variationE 1000p.. --Observation50 ----- Calculation8,...;(I) a:>(I) /,...; ~ /,...;-50III'"0•.-i8-1009 10 11 12 13 14 15 16 17HourFig.4 Tidal level at the mouth of Asahi River152 Hiroshi NAGO and Osami KAWARA~ Observation200-----Calculation400200~" 100""UlI I""UlI I'""I I S'I IS II'0I II IQ)'0 I +lQ) I I III:> 16; , .--(H ;:3Q) 0 0 ()Ul 10. 12 14 20 .--(.Q I I III0 I I , Hour ()",Q) ~' \., I Q)+l I ,,+lIII I I, IIIH I II H, I:3 ,I ,II I ~0 , I , 0.--( , I I .--(r..-200 I I I -100 r..I I I II , I II II,I I,,\ II VI,IIII-400 I -200II II :I,Fig.S Flow rate at the mouth of Asahi RiverHydraulics and Water Quality in Kojima Bay 153which has periods from a half hour to 2 hours. The result of the simu-lation can explain this tendency fairly well. However, with referenceto the absolute values considerably large differences are found bet-ween the results of the simulation and the observation. The answer tothis cause is considered as follows. The results of simulation repre-sent approximately cross-sectional averages. On the other hand, themeasured values show? in Fig.S are obtained by means of multiplyingthe average velocity at the vertical in the waterway by the cross-sectional area. Since the average velocity at the waterway is con-sidered to be higher than the cross-sectional average velocity, themeasured values may consequantly be larger than the results of simu-lation.As above-mentioned, the results of the simulation are recognizedto explain the hydrodynamics in Kojima Bay farly well. We accordinglydescribe the characteristics of hydrodynamics in Kojima Bay accordingto the results of the simulation in the next place.Fig.6 and Fig.7 show the spatial distributions of velocity com-ponents on the flood tide and the ebb tide seen in Fig.4 respectively.In these figures the flow rates at the cross-sections shown by thebroken lines are noted down.The Change of the flow rate at the mouth of Kojima Bay nearlyranges between -1900m 3/s and 1100m 3 /s. That at the mouth of Asahi Rivernearly ranges between -160m 3 /s and 160m 3 /s. The average inflow ratesfrom both Asahi River and Yoshii River were about lOm 3 /s during theobservation.Fig.8 shows the temporal changes of tidal levels at the mouths ofAsahi River , Yoshii River and Kojima Bay. The changes of the tidallevels at the mouths of Asahi River and Yoshii River show differentshapes from that at the mouth of Kojima Bay near the low tide and hightide.The temporal changes of flow rates are shown in Fig.9. The changesof the flow rates except that at the mouth of Kojima Bay have periodicvariations from lS minutes to 2 hours.The hydrodynamics in Kojima Bay is complicated in the way describedabove.This complication is considered to be caused by topographicalfactors of the bay.So far, the characteristics of the two-dimensional hydrodynamicshave been outlined. However, the actual distribution of velocitychanges in the vertical direction considerably. The details of the ve-locity distribution are described together with that of water quality......c:.n~1I°3=363::c:~.""l0rn::.z>-(l0~:l"""0rn°8=35~e.:;0:::>-::2S>-::0>-°9=1127...__7-.-:"1__~... '\... \""\ " ,\ \'\ , \ ,.:. _'_.L.Ll_''\ .' ,tTlt t t................. '\'" "Nt30cm/sL30cm/s..+ ~ ~ • ~ ~ ~ ~ ~ ~ ~ ~ ~• ~ • + ~ ~ ~ ~ ~ ~ ~ ~to ¥ ... ... .. .. +- +-. . . .. .. ...+ ~ • • ~ r ~ ~ ~ ~ ~ - ~,JI·O~°5=461..",-t-:.-.. I ..•Fig.6 Distribution of velocity components on the flood tideHydraulics and Water Quality in Kojima Bay 155rn.j.Js::QJs::o~otl:>-,.j.J·rltlorlQJ:>4-ios::o·rl.j.J:J.Q·rlH+Jrn·rlQ..(V)00If)IIIIf)0:1 ,1 ,fIf), I ~ If)'<I'1--;-4- III. , (V)0:-~--f00(V)III'<I'0:.. I'" -.. I....L...__---, ~ /' .... /1 __ /-.-+I ~ / ?~_/__r--_............ .//'/~__/---._...... __ ...... .? .//' /"//"'1 / ~-+--.~--}-- - -- /' / /' / ./ /: .__~-_ /' /' / /' /' .-0' I.-1 ........ 1'11 II / 1 ! IL..-_·...,...I..., r f / rT f r t;-,-,1t t t t1111 1 t1 1 tT 1 It i ii , ~.. f t i 'I ".. • ~ , 'I ,# ~ ~ 'I• ~ 'I 'I... ~ 'I'",• • ~ 'i ~• " 't l' .,.., t+L1..J_~• f 1rn156 Hiroshi NAGO and Osami KAWARA100The mouth ofAsahi RiverThe mouth ofYoshii RiverThe mouth of, I Kojima Bay~I50,/sup..E-t a.--tQ):>Q).--t II.--t - ' I-"III,_ ....'0".-IE-t-509 10 11 12 13 14 15 16 17HourFig.8 Tidal levels at the mouths of Asahi River,Yoshii River and Kojima Bay200Hydraulic sand Water Quality in Kojima Bay 157f,I II ,, 'II100IUl,........Im 0SQ)+JIIIH~0r-I~-100-200The mouth of Kojima BayThe mouth of Asahi River-- - The mouth ofYoshii River9 10 11 12 13 14 15 16 17HourFig.9 Flow rates at the mouths of Asahi River,Yoshii River and Kojima Bay158 Hiroshi NACO and Osami KAWARA·in the later section.5.2 Characteristics of Water Quality5.2.1 Local Distribution of Temporal MeanAsahi R.Discharqe fromo 72m 3 /s<D 37m 3 /s~ 15m 3 /s• 7. 5m 3 /s0> Om 3 /so40302010odlOUland Fig.ll show thatSS increase in the di-rection to the mouth of Kojima Bay.Concerning the effect of dichargefrom Asahi River, S appears to beaffected sensitively but SS doesnot so. The discharges from YoshiiRiver were nearly the same as thosefrom Asahi River.As in Fig.12, COD (Mn) seems tofigures.Fig.10both SandFig.10, Fig.ll and Fig.12 show the distributions of S, SS andCOD (Mn) along the waterway from the upper end of Asahi River estuary,Shin WeiT, to the mouth of Kojima Bay. Since the concentration of eachwater quality at each location considerably fluctuated with time,averages for one tidal cycle ortwo tidal cycles are shown in theseo'L-_---.,;I...-_.-L.__-'-_~o 5 10 15 20Distance from Shin Weir (km)Fig.ll Distribution of SS120 Discharge from Asahi R.o 72m 3 /s100 <D 37m 3 /s'i) 15m 3 /so 5 10 15 20Distance from Shin Weir(km)Fig.12 Distribution of COD(Mn)Discharge from Asahi R.8 <D 37m 3 /s'i) 15m 3 /s6 • 7.5m3 /sQ) Om 3 /soo 5 10 15 20Distance from Shin Weir(km)Fig.10 Distribution of S• 7. 5m 3 /sQ) Om 3 /s2080~ 60,..;........tJ1.5 40UlUlHydraulics and Water Quality in Kojima Bay 159be nearly uniform. But COD(Mn) in Asahi River appears to become rela-tively high when the discharge decreases.5.2.2 Cross-Sectional Average of Water Quality and Surface WaterQualityFig.13 shows cross-sectional average concentrations and surfaceconcentrations at typical four locations selected from the observationarea. The data at the locations except Asahi River Bridge were ob-tained at the same observation.Regarding the relationship among each location and the tendencyof the'temporal change, many similar tendencies are found between thecross-sectional average concentrations and the surface concentrations.However, both kinds of concentrations do not always show the similar1208°bss<m8/1)Asahi River 40Bridge ,,_,,I \ _'" -....09 ~3 17 21hour80~The mouth of40 /\I 'Asahi River ,/-,,,-~ - ~08 12 16 20hourThe middle 80of Kojima Bay40 '\~~~­O8 12 16 20hour30b2010 -- /---~----08 12 16 20hour80The mouth ofKoj ima Bay 40hour10--/,,,,/"12 16 20hourFig.13 Cross-sectional average of water qualityand surface water qualityCross-sectional average---- Suface water160changes.Hiroshi NAGO and Osami KAWARA5.2.3 Vertical Distribution of Water QualityFig.14 shows the vertical distributions of velocity, Sand SS atthe mouth of Asahi River. The distributions are complicated. Flowreversals at depth are found near the high tide and low tide. Sand SSshow similar increase from the surface to the bottom however they aredifferent from each other in the smoothness of the increase. SS oftenhas large peaks but S smoothly increases.80 -200 80Fig.14 Vertical distribution of velocity,Sand SS at the mouth of Asahi RiverVelocity (cm/s)S (% 0)SS (mg/l)5.2.4 Variation of Water QualityFig.15 shows the variations of tidal level,SS, Sand COD(Mn) at~EiP< 1.2E-t 008..-IQ) 004:>Q)0..-I';d-004'0 9 12 15 18 21 0 3 6 9 12 15 18 21 0 3 6 9•.-1E-t '''1"\1"\ r. J A ~ ,~ 8/6 Hour$"'-::.."!:..;:;.0'"riP"~ :s"'-Ul~"o 9 12 15 18 21 0 3 6 9 12 15 18 21 0 3 6 9 ~..50 Hour P:::JO:!40 t:~..-I 30 ;;."- ~t1' 20Ei ~:Ul 10 "Ul tll0 "'"10~8.-i 6"-t1'Ei 4CI0 2()() I I I12 15 18 21 0 3 6 9 12 15 18 21 0 3 6 9Figo15 Variations of tidal level, S, SS and COD(Mn) Hour ......0>......162 Hiroshi NAGOand Osami KAWARAthe mouth of Asahi River. The intervals of sampling and measurementare 10 minutes. The variations of the water quality seem to have someperiods.The power spectra calculated from these variations are shown inFig.16, Fig.l? and Fig.18. 5 is found to have peaks which representperiods for about 2 hours, 80 minutes and 50 minutes, 55 for about 4hours, 2 hours, 80 minutes, 60 minutes and 30 minutes. COD (Mn) hasmany peaks besides those described above. These peaks are not always150100C·rlSN0 50ciP~4-lP<00co0.011000.02500.03100/30.04 0.05f(cycle/min)25 20T(min)1500Fig.16 Power spectrum of 5500ooco 1000.02500.03100/30.04 0.05f (cycle/min)25 20T(rnin)Fig.l? Power spectrum of 55150.~ 1008'"50Hydraulics and Water Quality in Kojima Bay 163o 0100 50 100/3Fig.18 Power spectrum of COD(Mn)Ul-.......8~ 10...Io~x5oom(l. 011000.02500.03 0.04f(cycle/min)100/3 25T(min)notable, but the periods for about 2 hours and 80 minutes are incommon with three water quality items.Fig.19 shows the power spectrum of velocity calculated from thedata shown in Fig.5. The peaks of periods for about 2 hours, 80 minutesand 30 minutes are found. The peaks for 2 hours and 80 minutes arefound in all water quality items,too. Besides, the peak for 30 minutesis found in 88. The variation of the flow rate or velocity seems to164 Hiroshi NAGOand Osami KAWARAhave an effect on the variations of surface water quality.6. ConclusionsIn this paper, as the first step to clarify water pollution mecha-nisms in estuaries and bays, the characteristi~s of hydrodynamics andwater quality in Kojima Bay are investigated based on field obser-vations. The summary of the main results obtained is as follows.(l) The variation of flow rate or velocity in Kojima Bay has theperiods from 30 minutes to 2 hours besides a tidal cycle. Thischaracteristic of flow is represented by a two-dimensionalnumerical simulation fairly well.(2) The local distributions of Sand SS show the increase in the di-rection to the mouth of Kojima Bay. On the other hand, the distri-bution of COD(Mn} is rather uniform in the whole area of the bay.S appears to be affected by river discharge sensitively but SSdoes not.(3) The vertical distributions of Sand S8 are similar, and there aresimilarities between the cross-sectional averages of water qualityand the surface water quality concerning the relationship bet-ween each location and the tendency of the temporal change.(4) The variations of the surface water quality at the mouth of AsahiRiver show common periods for about 2 hours and 80 minutes. Thevelocity has the same periods, too. Therefore, it will be concludedthat the surface water quality is influenced by the variation ofvelocity.Thanks are due to our colleagues who contributed their sustainedefforts and grasp of objectives to the accomplishment of the obser-vation programs. We should like to express our grateful thanks to thepublic officials who extended us their kind assistance.References[1] Leendertse,Jan J., Alexandar, Richard C., and Liu, Shiao-Kung: TheRand Corporation, R-l417-0WRR, (1973).[2] Leendertse, Jan J., and Lui, Shiao-Kung: The Rand Corporation,R-1764-0WRT, (l975).

Hydraulics and Water Quality in Kojima Bay

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