A STUDY ON THE EFFECTS OF TIDE ON SEDIMENTATION IN ESTUARIES OF THE NIGER DELTA, NIGERIA

Abstract

Estuaries in the Niger Delta-Nigeria are influenced by tidal currents and wave actions due to their proximity to the Atlantic Ocean. Tidal current provides the energy required to move sediments in and out of the estuaries from the seashores, while wave actions curtails the deposition of sediments at the bottom of the estuaries as the bed get shallower, resulting in the modification and long-term regulation of siltation through erosion or accretion. The interaction between estuaries bathymetric shapes, tidal currents and wave actions can only be fully understood through analytical or mathematical relationships. Analytical methods are less accurate in predicting future tides, while none of the existing mathematical relationships can accurately predict tidal behavior in the Niger Delta region due to the fact that parameters governing tidal actions vary from region to region, hence the need for this study. This study shows the behavior of the estuaries in response to variation in tidal heights, currents and wave actions through mathematical modeling, a knowledge which is useful in planning and timing of marine activities that requires pre-knowledge of tidal levels, direction and current velocities. The models were formulated and calibrated using parameters generated from the hydrographic, hydraulics and geotechnical investigation, including local field observations and measurements conducted within the study area. Soil samples taken from the area are composed of peat, organic clays, silty clays and sand. Peat constitutes the dominant soil which is locally known as 'chikoko' with high compressibility and color ranging from dark brown to dark gray and texture from soft to firm. Typical bed material size (D50) is approximately 0.2mm. Tides in the region are mostly semidiurnal with tidal prism ranging from 0.4 to 1.5m. The tidal strength is strongest at the inlets and decreases with distance inland. The depth of the estuaries is controlled by the strength of the tidal currents. Areas very close to the ocean with stronger tidal effect are very deep; while shallow canals and creeks predominates the hub of the estuaries. The results obtained using the present models compared favorably with the field measurements. Average correlation coefficients of 0.9 were obtained in some cases. The results showed that the estuaries erodes, accretes, or remain stable, depending on the rate at which sediment is supplied or removed from them.