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Tidal dynamics of the Zanzibar channel in comparison with a regional modelConnor Robinson Walsh1 , Javier Zavala-garay1, Daudi Mukaka3, Jurgen Theiss2, Katherine Zaba4
IOC / UNESCO
1. Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, NJ, United States, [email protected]; [email protected]
2. Theiss Research, San Diego, CA, United States, [email protected] 3. Institute of Marine Sciences, University of Dar es Salaam, Zanzibar, Zanzibar, Tanzania, United Republic of. , [email protected]
4. University of California, Los Angeles, CA, United States , [email protected]
Introduction
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Tidal propagation
Discussion & future studyShallow water tides
Observations
Tidal forcings
The development of a robust regional ocean model for the Zanzibar channel has far reaching po-tential for solutions to environmental and socioeconomic problems in the region. In particular, cre-ating a basis for local prediction capabilities will allow for more educated marine management schemes and ensure environmental health in a highly tourist-based economy. The Zanzibar channel is located between Unguja Island (Zanzibar) and mainland Tanzania, approxi-mately 6 ° latitude below the equator. The channel has a maximum depth of about 40 meters and is approximately 40 km wide and 100 km long with Northern and Southern entrances opening to the Western Indian ocean. The relatively shallow depths make processes such as tidal motion important for small time scaled dynamics within the channel.Using the Regional Ocean Modeling System (ROMS), a model was developed in 2009 to describethe seasonal cycle of the channel. This was the first installment of a three year project to develop arobust model of the Zanzibar channel. Building on the research of previous years, the current studyfocuses on the predictive capabilities of the tidal model and is one of the final installments for theZanzibar Project. The general dynamics of the tidal propagation are described in terms of flow andchanges in sea surface elevation. Additionally, non-linear interactions such as the presence of shal-low water tides are investigated.In collaboration with researchers from the Institute of Marine Sciences (IMS) in Zanzibar, measure-ments were taken in three locations within the Zanzibar channel. The results were analyzed andused to evaluate the accuracey of the ROMS model predictions of tidal elevation and flow.
Acknowledgements
CitationsShaghude, Y., Wannas, K., Mahongo, S. (2002). Biogenic assemblage and hydrodynamic settlings of the tidally dominated reef platform sediments of the Zanzibar channel. Western Indian Ocean Journal of Marine Science, 1, 107-116.
Le Provost, C. (1991). Generation of overtides and compound tides (Review). In B. B. Parker (Ed.), Tidal Hydrodynamics (pp. 269-295). New York, New York: John Wiley & Sons.
Pugh, D. T. 1987. Tides, Surges, and Mean Sea-Level. Chichester, UK: John Wiley & Sons.
1The Oregon State University TOPEX/Poseidon, Global Inverse Solution, TPXO7, http://www.coas.oregonstate.edu/research/po/research/tide/global.html
2 The University of Hawaii Sea Level Center, UHSLC, http://uhslc.soest.hawaii.edu.
3 Theiss Research: Zanzibar Project, http://www.theissresearch.org/scientists/theiss/zanzibar/
Shallow-water tides are a phenomenon caused by the effects of friction and changes in depth on the propagation of ordinary tidal waves in coastal regions (Le Provost,1991). The resulting tidal constituents are higher harmonic frequencies dependent on the square or higher power of an original tidal constituent (Pugh,1996).A time series of current velocity at Chumbe island from September 2006 was collected by researchers at IMS and shows high energy in the quarter-diurnal tidal constituents of the M2 and S2 when veiwed in the frequency domain (Figure C.1). To further investigate the occurence of these non-linear tidal interactions, an algorithm was developed to create a spatial map from the spectrum of tidal constituents in each model grid-point. Figure C.2 depicts the relative amplitude of the principle semi-diurnal lunar constituent, M2, and the quarter-diurnal lunar constituent, M4. This map essentially shows the loca-tions in the model domain where the M4 quarter-diurnal constituent is expected to occur. In comparison to the model’s bathymetry (Figure C.3), the M4 is predicted to occur in regions corresponding to significant bathymetric gradients. This agrees with the theo-retical explainations concerning the genesis of non-linear tidal distortions.
The ROMS tidal model is configured with boundary forcings obtained from the Oregon State Univer-sity TOPEX/Poseidon Global Inverse Solution, TPXO71. The TPXO7 global tidal model uses a least-squares fitting scheme to best fit the Laplace Tidal Equations and averaged altimetry data from tracks of Topex/Poseidon satellites. The figures below show the TPXO7 (A.1-A.3) and the ROMS (A.4-A.6) model grids for the Zanzibar channel region along with the harmonic amplitudes of the three domi-nant tidal constituents, M2, S2, and K1. For reference, the coastline has been plotted on the TPXO7 grids to show the coarse resolution in comparison with the ROMS model.
A.
B.
C.
D.
38.8 38.9 39 39.1 39.2 39.3 39.4 39.5 39.6
−6.8
−6.6
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−6.2
−6
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B
C
A
Sampling Locations
Longitude
Latit
ude
Tidal measurements were taken at three primary locations during the 2011 observational campaign. Mea-surements included sea surface eleva-tion for all three sites and bottom cur-rents for sites B and C. The measure-ments obtained were analyzed and compared with model measurements for the corresponding grid-points in the ROMS model.
Site A: Stown Town
Site B: Chumbe Island
Site C: Dar es Salaam
Measurements of sea surface elevation from a tidal gauge at the Stown Town shipyard were obtained from the Univer-sity of Hawaii Sea Level Center OPeNDAP server2. Figure D.1 shows tidal elevations for July- August 2011 plotted with pre-dicted measurements for the correspond-ing grid-points in the ROMS model. The residuals are plotted below in green.
With the collaborative help of researchers from IMS, an Acoustic Doppler Velocime-ter (ADV) was deployed off the coast of the small island of Chumbe for 13.708 days. The sea surface elevation, obtained from the pressure sensor on the instru-ment, is shown in Figure D.2 along with the ROMS prediction for the correspond-ing grid-point. The residuals are plotted below in green. Figure D.3, shows the tidal spectra of the observed and model currents for Chumbe. Note: The S2 constituent was not resolved because the record length required to dif-ferentiate between the M2 and S2 is ap-proximately 14.79 days.
A second ADV was deployed off the coast of Dar es Salaam for almost 18 days during August 2011. Figure D.4 shows the tidal el-evation along with the ROMS predictions and residuals. Additionally, the spectra of observed and model tidal currents is plot-ted in Figure D.5.
The empirical evidence suggests that the prediction capa-bilities of the ROMS tidal model are sufficiently accurate. The coefficient of determination, R2, was calculated for site B and C tidal elevations in comparison with ROMS mea-surements. The results imply that the ROMS model is 99% effective in predicting tidal elevations in these two loca-tions. Larger disparities, however, exist between the tidal current spectra of the model and observations. These differences may be accounted for by the lengths of the time series obtained and also the resolution of the model bathym-etry. With longer tidal records, more frequencies can be resolved and thus improve the distribution of energy in the spectrum. Problems associated with model bathym-etry result from limited resolution. For example, the spec-trum of tidal currents observed at Chumbe island does not match the model, however Chumbe is not included as a land mass in the model due to resolution limitations. In the future, a more detailed investigation of high resolu-tion tidal interactions may be pursued by developing a nested grid model within the current ROMS model.
Since 2009, the Institute of Marine Sciences (IMS) in Zanzibar has been accommodating and vital in the success of the Zanzibar Project. We would like to extend our gratitude to Margaret Kyewalyanga, Narriman Jiddawi, and Ntahondi Nyandwi for their support and helpful administrative efforts. The entire staff of IMS has been very helpful in establishing a fruitful col-laboration. A special thanks to Tuju Sharali for his enthusiasm and dedica-tion.A gesture of thanks and appreciation would like to be conveyed to the ad-ministration of Chumbe on behalf of the Zanzibar Project. We graciously thank Khamis Khamis for supplying his boats for research purposes and his ongoing dedication to the project. Without the generosity of Clint Winant and Ralf Goericke, who lent an ADCP and a CTD respectively at no charge, most of the measurements would not have been possible.Additionally, we would like to thank the Arresty Reaserch Center at Rutgers University for providing funding towards the presentation of this poster.Finally, thanks to all who have contributed in the past to the Zanzibar Proj-ect.Funding for this project has been provided by the National ScienceFoundation (OISE-0827059, OCE-0927472).
OS43B-1545
Tidal flow in the Zanzibar channel can be described as convergent/divergent in the center of the channel. The series of figures to the right depicts the flood and ebb cycle in three hour increments showing the vector fields for the depth averaged currents and sea surface heights from the ROMS model. The Figure B.1 begins with ‘hour 0’ at the onset of the tidal flood. As the series progresses, a tidally induced bulge in sea surface height develops in the center of the channel. It is important to note that the scal-ing of the colorbars are not uniform in order to capture the incremental changes in sea surface height. Past observations of current velocity on both sides of the convergence/divergence zone agree with these modeled dynamics (Shaghude et al., 2002).
MSF Q1 O1 NO1 K1 OO1 M2 S2 MO3 MK3 MN4 M4 MS4 2MK5 2MS60
0.02
0.04
0.06
0.08
0.1
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0.14
0.16
Am
plitu
de
(m
/s)
Constituents
Power Spectra of tidal constituents at Chumbe Island September−2006 (95% Confidence)C.1)
C.2
14.5
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Elev
ation
(m)
Tidal elevations: predicted and observed near Dar es Salaam (July− August 2011)
R2= 0.99
ROMS predictedObserved
07/20 07/21 07/22 07/23 07/24 07/25 07/26 07/27 07/28 07/29 07/30 07/31 08/01 08/02 08/03 08/04 08/05 08/06 08/07−0.3
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0
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time (days)
residuals
07/03 07/10 07/17 07/24 07/31 08/07
0
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5
time (days)
Elev
ation
(m)
ROMS predicted and observed tidal elevation (July−August 2011)
ROMS predictedObservedResiduals
5
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7
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8
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9.5
Elev
ation
(m)
Tidal elevations: predicted and observed at Chumbe Island (July 2011)
R2= 0.99
ROMS predictedObserved
07/01 07/02 07/03 07/04 07/05 07/06 07/07 07/08 07/09 07/10 07/11 07/12 07/13 07/14−0.2
0
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time (days)
Residuals
O10
0.02
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0.06
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Veloc
ity (m
/s)
K1 M2 M3 M4 2MK5 2SK5 M6 3MK7 M8
power spectra of tidal constituents at Chumbe (95% confidence)
Constituents
ROMSObserved
O1 K1 M2 S2 M4 MS4 S40
0.02
0.04
0.06
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0.1
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power spectra of tidal constituents near Dar es Salaam (95% confidence)
Constituents
Veloc
ity (m
/s)
ROMSObserved
D.1
D.2 D.3
D.4 D.5
B.1
38.8 39 39.2 39.4 39.6 39.8 40 40.2 40.4 40.6
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Longitude
Latit
ude
S2 Amplitude: TOPEX/Posiedon Global Inverse Solution
0.2
0.4
0.6
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1
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1.4
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2
38.8 39 39.2 39.4 39.6 39.8 40 40.2 40.4 40.6
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−5.6
Longitude
Latit
ude
K1 Amplitude: TOPEX/Posiedon Global Inverse Solution
0.15
0.155
0.16
0.165
0.17
0.175
0.18
0.185
0.19
0.195
0.2
38.8 39 39.2 39.4 39.6 39.8 40 40.2 40.4 40.6
−6.8
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−6.4
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−5.6
Longitude
Latit
ude
M2 Amplitude: TOPEX/Posiedon Global Inverse Solution
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
A.1
A.2
A.3
A.4
A.5
A.6
Hour 0 Hour 3 Hour 6 Hour 9 Hour 12
C.3
