The Influence of tidal distortion on extreme water levels in Casamance estuary, Senegal.

Abstract

The Casamance estuary in Senegal is experiencing hypersalination and silting, which are negatively impacting its mangrove ecosystem and maritime navigation. This is a critical issue for both biodiversity preservation and economic development. However, research and documentation on this estuary, particularly with regard to tides, have been relatively neglected. The focus of this study is on the impact of tidal asymmetry on extreme water levels in the region. Tide data was collected at the river entrance and at Ziguinchor, and analysed using digital tools such as Tide Peaks Toolbox and UTide. The results indicate that the average tidal range at Ziguinchor varies from 33 cm to 65 cm, with peaks in February and March and troughs in January and March. The duration of the ebb tide varies, with a duration of over 4 hours during spring tides and 2 hours during neap tides. During the dry season, the duration is extended to 3 hours and 30 minutes, leading to a greater accumulation of water upstream of the estuary, particularly in July. Maximum velocities of up to 30 cm/s are observed at the start of high tide, mainly in February. Harmonic analysis conducted over a six-month period indicates a reduction in amplitude of over 45% for semi-diurnal and diurnal waves. Additionally, there is a significant amplification of the upper diurnal waves, with a more asymmetry upstream of the estuary. Additionally, the analysis of residual water levels and currents emphasizes the complexity of tidal dynamics, particularly due to non-tidal influences. This study constitutes a good contribution to the understanding of the hydrodynamic functioning of estuarine systems in West Africa marked by a strong climatic contrast.