The effect of the Mbita Causeway on water currents in the region of Rusinga Channel, Winam Gulf, Lake Victoria: a 3D modelling study with ELCOM

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1 The effect of the Mbta Causeway on water currents n the regon of Rusnga Channel, Wnam Gulf, Lake Vctora: a 3D modellng study wth ELCOM Khsa Patrck 1, Jose R. Romero 2, Jorg Imberger 2, Tom Ewng 2, Jason Antenucc 2, Henry Njuguna 1 and John Okungu 1 1 LVEMP Coordnaton Offce, Re-Insurance Plaza, 2 nd Floor, Ksumu 40100, Kenya Emal: pkhsa@yahoo.com 2 Centre for Water Research, Unv. of Western Australa, Crawley, WA, 6009, Australa Abstract The Mbta Causeway was constructed n the early 1980 s to lnk Rusnga Island to the manland to facltate the transport of people, goods, and servces to the sland. One of the effects of the Causeway was to permanently block a secondary connecton between Rusnga Channel and the offshore waters of Lake Vctora. Ths secondary channel may have served as a substantal pathway of exchange between Wnam Gulf and Lake Vctora pror to the Causeway. There has been consderable scentfc and poltcal debate as to whether decreased flushng wthn the two bays on ether sde of the Causeway has occurred and led to poorer lake water qualty snce the constructon of the barrer. In Apr.-May 2005 an ntensve feld nvestgaton was undertaken to determne the exchange dynamcs between the northeastern offshore waters of Lake Vctora and Wnam Gulf through Rusnga Channel. One of the outcomes of ths scentfc study was the valdaton of ELCOM (Estuary, Lake and Coastal Ocean Model), a three-dmensonal (3D) hydrodynamcs model, over Rusnga Channel. In ths appled modelng study, ELCOM smulatons wth and wthout the Mbta Causeway were run. The smulated exchange through the channel wthout the Causeway was quantfed. Smulated dfferences n the flushng wthn the two bays wth and wthout the Causeway were evaluated through numercal tracer studes to nfer the possble effects on water qualty. Key words: Hydrodynamcs, Modelng, Scenaros Introducton Engneerng works across water bodes often affect water crculaton patterns and thereby water qualty. One example s the constructon of the Mbta Causeway n 1985 that lnked the Mbta pennsula to Rusnga Island. Ths Causeway s located 70 km east of Ksumu, Kenya (Fg. 1). The Causeway essentally blocked a natural channel wth dmensons of 250 m n length and 10 m n depth through earth fllng and constructon of a road to Rusnga Island. Clearly, the constructon of the Causeway resulted n the loss of an alternatve exchange pathway between Rusnga Channel and the offshore waters of Lake Vctora. Numerous hypotheses on the effect of the loss of ths exchange pathway have been suggested by scentsts, polcy makers, local communty members, and specal nterest groups. For example, some speculate that the blockage of the Mbta Channel has affected the ecosystem around Mbta and Rusnga area by blockng a fsh mgraton route. One of the effects we consder n ths paper s the loss of flushng and water exchange n the regons near the Causeway. Untl recently, the water crculaton patterns n the regon of Rusnga Channel were poorly understood, but several papers presented at ths conference of recent feld (Antenucc et al., 2005) and numercal modelng (Romero et al., 2005a) studes have ncreased understandng of the regon s hydrodynamcs. Durng the frst of these studes feld data was collected over 12 days n Apr.-May 2005 wth moored automated nstruments at two statons and free-fallng nstruments that profled many statons throughout Rusnga Channel. Comparson of these hydrodynamc data wth the threedmensonal Estuary, Lake and Coastal Ocean Model (ELCOM) was good (Romero et al., 2005a). In ths paper we extend the applcaton of ELCOM to assess the water crculaton and exchange patterns pror to the constructon of the Causeway. Methods Smulatons were conducted wth the threedmensonal hydrodynamc model, the Estuary and Lake, and Coastal Ocean Model, ELCOM (Hodges et al., 2000). A unform numercal grd was used. The computatonal doman was approxmately 100km long and the smallest feature to be resolved s the 250m wde Mbta Causeway. On removal of the Causeway, the Mbta Pathway was algned wth the computatonal grd and was equvalent to one horzontal grd cell of wdth 250m and 10m adjacent vertcal grd cells. Smulatons were started from rest, wth horzontal free surface and sopycnals. Free slp boundares were used to model sdewall (land) boundares wth a drag parameterzaton at the bottom boundares. Applcaton of the model ELCOM to compare condtons pror to the constructon of the Causeway reled on data from the 12-day feld study of Apr.- May The smulaton to valdate the model over the feld study was the current condtons wth the Causeway. The Causeway was removed and replaced wth a channel for the scenaro pror to Refer to Antenucc et al., (2005) and Romero et al., (2005a) for detals of the feld and model methods. 74

2 Causeway Fgure 1. Map of general regon (left panel) llustratng northeastern offshore waters of Lake Vctora, Rusnga Channel and Wnam Gulf along wth the major ctes (crcles) and the focus regon of ths study (dotted rectangle). The dotted lne from the eastern tp of Rusnga Channel drectly to the eastern shorelne at the constrcton of Rusnga Channel marks the ntalzaton boundary of conservatve Tracer 1. The rght panel shows the focus regon of ths numercal study wth the locaton of the Causeway. The hourly water volume flux through the former Mbta Channel (now the Causeway) was calculated by applyng the method used by Laval et al., (2003) for Lake Maracabo and the Carbbean Sea. The water volume flux (Q v ) was defned as; Q v = A VdA (1) where A s the cross-sectonal area and V s the velocty along the cross-secton. Q v s computed dscretely for each hour from smulated data as; Q = v A (2) v the valdaton of water temperatures and currents at T2 (see Romero et al., 2005a) provded confdence that ELCOM smulates the Rusnga Channel dynamcs reasonably well. Next a comparson of smulatons wth and wthout the Causeway s consdered SIM T Julan day Water level (m) where s the th depth along the cross secton, A s the north-south unt area of the face and V s the eastward component of velocty along Mbta Pathway. The eastward velocty (nflow) was consdered postve whle the westward velocty (outflow) was taken as negatve. The total crosssectonal area of Mbta Channel was taken as 9 m depth by 250 m wdth. The cumulatve water volume flux from hourly model output was computed over the 12 day smulaton. Results The smulated lake level at staton T2 reproduced reasonably well the observatons recorded by the ADCP (Fg. 2). Ths lake level valdaton along wth Fgure 2. Comparson of ADCP observatons (lne) and smulaton (dashed) of lake level at statons T2. The smulated currents wth and wthout the Mbta Causeway were smlar (Fg. 3). Ths suggests that the effect of the constructon of the Causeway had a mnmal affect on the regonal hydrodynamcs, exchange, and transport of water masses between the offshore and Wnam Gulf through Rusnga Channel. 75

3 Fgure 3. Water current drectons (vectors) and speed (shadng) n the regon of Mbta on May 2 durng the afternoon at 1300 when currents are nto Wnam Gulf (top panels) and n the evenng at 2100 when currents are towards the open waters. Left panels are wth the Causeway and rght panels are wthout the Causeway. Vectors represent current drecton and not the speed. Conservatve tracers were used n the smulatons to vsualze the dfferences n transport between the Causeway and Channel scenaros (Fg. 4). Tracer 3 was added to nputs from the open boundary on the western margn of the model doman. Tracer 1 was ntalzed throughout Rusnga Channel and Wnam Gulf as llustrated n Fg. 1. Lower Tracer 3 concentratons were smulated n the regon of the small sland at the nterface of the bay to the west of Mbta and the open waters of Lake Vctora wthout the Causeway. Ths suggests that greater flushng of ths bay may have occurred wthout the Causeway. In contrast, Tracer 1 patterns n Rusnga Channel were nearly the same for the two smulatons throughout Rusnga Channel and the offshore waters to the north. Ths agan suggests that ether condton (.e. Causeway or Channel) dd not have much nfluence on the bulk exchange and transport patterns between Rusnga Channel and the offshore waters to the north. 76

4 Fgure 4. Tracer concentratons n the Mbta regon on the late mornng of May 3 for the smulatons wth (left panel) and wthout (rght panel) the Causeway. The smulaton wth the Mbta Channel resulted n net transport of Tracer1 through the channel and nto the bay to the west of Mbta (Fg. 4, lower rght panel). The cumulatve volumetrc flux of water through the channel ndcated net westward transport through the channel (Fg. 5). The net westward flux through the Mbta Channel occurred prmarly after the 4 th day of the smulaton. Over the fnal 8 days of the smulaton ca. 5 x 10 7 m 3 of water flowed westward. Ths s equvalent to a daly flux of ca km 3 day. Applcaton of a smlar methodology across Rusnga Channel yelds a net volumetrc flux nto Rusnga Channel of 0.17 km 3 day over the smulaton (Romero et al., 2005b), approxmately a factor of 30 greater than the Mbta Channel flux estmate Tme (hrs) Fgure 5. Cumulatve volume flux over the 12 day smulaton through Mbta Channel. 77 Dscusson Smulated crculaton patterns were only nfluenced n the mmedate vcnty of the Causeway when t was removed. Closer nspecton of the smulatons revealed hgh easterly currents n the evenngs through the Mbta Channel that perssted approxmately 2 km nto Rusnga Channel (not shown). Anecdotal evdence from long-tme fsherman n the regon supports these smulaton results (P. Khsa, pers. comm.). For example, fshng accdents n Mbta Channel pror to the Causeway often led to rapd easterly transport of boats and people nto Rusnga Channel. Smulated veloctes through the Mbta Channel were approxmately 50 cm s -1 wth contnued currents nto Rusnga Channel of nearly 30 cm s -1 (not shown). However, the removal of the Causeway lkely dd not have a large nfluence on the overall Rusnga Channel exchange dynamcs. The constructon of the Causeway lkely had a substantve localzed effect on the crculaton patterns n the near-regon of Mbta and hence water qualty n the locale as well. Greater flushng of the bay to the west of Mbta lkely occurred wth mproved water qualty relatve to the current condton. One of the results of less flushng n the locale of Mbta has been ncreased sedment deposton n the two bays on ether sde of the

5 Causeway, where depths have been reduced by 5 m (LVEMP, unpubl. data). Further, because of the large populaton densty n ths regon and the assocated pollutant loadng, the greatest beneft derved from the former Mbta Channel was lkely enhanced transport and dluton of these pollutants. Ths modelng study represents a frst quanttatve evaluaton of the effect of the constructon of the Causeway on the crculaton patterns of Rusnga Channel. Agan, t s unlkely that the Causeway nfluenced the general crculaton and exchange of Wnam Gulf wth the open waters of Lake Vctora. However, localzed effects on crculaton and flushng, and thereby water qualty n the Mbta locale lkely resulted from ts constructon. Longer term hydrodynamc modelng and ncorporaton of bogeochemcal models wll provde addtonal nsght nto the effects of the Causeway on the local water qualty. References Acknowledgements Ths work was completed under the consultancy Plot Study of the Hydraulc Condtons over Rusnga Channel and Wnam Gulf of Lake Vctora by the Centre for Water Research for the Kenya Agrcultural Research Insttute and the Lake Vctora Envronmental Management Project. The authors would lke to thank the numerous people nvolved n the successful completon of the study ncludng Peter Njuru-Gkuma, Davd Njoroge, Felx Sangele, and Rueben Ngessa of the Lake Vctora Envronmental Management Program (LVEMP); the study s communty partcpaton offcers Jean Boroto, Carol Adhsa, John Onyango, and Mary Kura; the Kenya Marne and Fsheres Research Insttute (KEMFRI) for the use of the Utaft Research Vessel; and the crew of the Utaft RV. Antenucc J. P., Romero J. R., Imberger J., Ewng T., Khsa P. & Njuguna H., The Physcal Lmnology of Wnam Gulf and Rusnga Channel of Lake Vctora durng Apr.-May and Aug. of Proceedngs on the 11 th World Lakes Conference, 31 Oct.-4 Nov. 2005, Narob, Kenya. Hodges B. R., Imberger J., Saggo A. & Wnters K.B., Modelng basn-scale nternal waves n a stratfed lake. Lmnol. Oceanogr. 45, Laval B.E., Imberger J. & Fndkaks A.N., Mass transport between a sem-enclosed basn and the ocean: Maracabo system. J. Geophys. Res. (Oceans) 108, Romero J. R., Imberger J., Antenucc J. P., Ewng T., Khsa P. & Njuguna H., 2005a. Management Implcatons of the Physcal Lmnologcal Studes of Rusnga Channel and Wnam Gulf n Lake Vctora, Proceedngs on the 11 th World Lakes Conference, 31 Oct.-4 Nov. 2005, Narob, Kenya. Romero J.R., Feaver S.F., Boroto J., Antenucc J.P. & Imberger J., 2005b. Plot study of hydraulc condtons over Rusnga Channel and Wnam Gulf of Lake Vctora: Report of Frst Feld Study. Prepared for Kenya Agrcultural Research Insttute, Lake Vctora Envronmental Management Program. CWR, UWA Reference WP1623JR. 78