A contribution to the Africa-America GLOSS News - 2002
The case of the Maputo harbour
By Jafar Ruby and Cândida Sete (*)
(*) National Institute of Hydrography and Navigation - INAHINA
Figure 1- Tidal station network in the coast of Mozambique.
Four sea level observation stations are presently operating in permanent basis in the Mozambique coast, and have provided an important contribution supplying data for operational and research applications. Apart from the actual observation network, tidal data have been obtained for different locations from the past and have been used to generate different products and inputs to different studies. Two of the tidal stations (Inhambane and Pemba) are also part of the GLOSS network.
Tides in the coast of Mozambique, are of semi-diurnal type (two low waters and two high waters a day) with a daily inequality of about 10-20 centimeters in Maputo, Inhambane, Chinde, Quelimane and Angoche, to about 30-40 centimetres in Beira, Pebane, Ilha de Moçambique, Nacala, Pemba and Mocímboa da Praia.
For the purpose of this study, the main tidal characteristics of the Maputo harbour will be summarised and the seasonal variation of sea levels will be presented in this report.
The tidal range in the Mozambique coast varies quite considerably between extreme neap tides and extreme spring tides. Tidal amplitudes vary over the year and along the coast.
Figure 2- The variation of tides at Maputo harbour.
The tidal range at Maputo harbour varies from less than 30 centimetres to about 380 centimetres (reference to chart datum) during extreme spring tides. Tidal amplitudes vary over the year between 80 centimetres during neap tides and about 300 centimetres during spring tides (see figure below).
Time (day x 2): Because of the semidiurnal type of the tides, two to obtain the exact should divide each scale number of days.
Mean Sea levels
The determination of the mean sea level is of high interest, particularly for geodesy.
Mean sea level is obtained by means of filtering the oscillations due to astronomical tide. It means that the mean sea level is also influenced by meteorological (or oceanographic) conditions.
For the estuaries in particular, the mean sea level varies with the river flows. For example, during floods the mean sea level at Chinde in the Zambezi River mouth increases by some 50 centimetres.
Long period mean sea-level variation can be caused by isostatic movements of the land, ice melting, sea water density variation, pressure and wind regime variation, etc.
In order to obtain a mean sea-level value that can be used with satisfactory severity as geodetic vertical reference, long time series of mean sea-level values are required to resolve (by averaging) the oscillations observed during a large number of years.
Fixed benchmarks were implemented in the main port locations along the coast of Mozambique. The objective was to help monitoring the relative changes of the mean sea level as function of balance between the sea level rise due to the global warming of the planet, and the vertical displacements of the coast due to geological movements.
Daily mean sea-level values are filtered from the hourly-observed data. Monthly and yearly mean values are calculated and used to monitor the mean sea level along the coast.
Consistent results as to the investigation of sea level trends in the coast of Mozambique do not show up with clear evidence as yet, relying in the need for longer and uninterrupted time series, and subject to the application of improved quality control techniques.
Pre-processing techniques were developed and have been used at the Institute of Hydrography and Navigation of Mozambique, envisaging exceeding observed diversions in heights (use of cubic and lagragian polynomials), and diversions in phase (use of the Spline function). The effectiveness of these techniques can be read from the inherent residuals.
Processed mean sea level values were used to plot and analyse its variation against different factors as follows:
Maputo Tidal Station (Lat. = 25º 58.5' S, Lon. = 32º 34.2'E)
Figure 3- mean sea level at Maputo all over the year obtained from about
21 years of data.Characteristic
Figure 4- Annual mean sea level plots for Maputo harbour observed data from 1958 to 1999.
Adopted mean sea level at Maputo area is 200 cm above Chart Datum.
Mean sea level in Maputo may be influenced by the total rainfall/runoff pattern of the catchments areas of the Incomati, Maputo and Matola river basins, as the tidal station lies in the delta of these rivers. It is otherwise, quite clear from the Figure 3 the influence of the rain regime of these rivers. The monthly mean sea-level variation curve indicates a minimum during May to October and a maximum during November to April, which corresponds exactly to the dry and wet seasons of the above-mentioned rivers, respectively.
Manual de Hidrografia - J. A. Barahona Fernandes - Instituto Hidrográfico, Lisboa 1971.
Marés. Fundamentos, Análise e Previsão - A. S. Franco - Directoria de Hidrografia e Navegação, Brazil 1ª edição 1997.
The alternating open and closed inlet of Bilene lagoon, Mozambique; influence of waves and tides and of human intervention - M.Achimo, J. Mugabe, F. Cuamba, D. Chemane, J. Ruby, E. T. James, P. L. de Boer.
Physical Oceanography of coastal waters - K. F. Bowden, 1983.