Impact of Mangrove on Tidal Propagation in a Tropical Coastal Lagoon

Abstract

Very complex aquatic ecosystem, the mangrove forest colonizes coastal lagoons, estuaries and deltas in tropical and subtropical zones. This study investigates the impact of mangrove vegetation on tidal propagation in the Somone Coastal Lagoon, located on the Senegalese small coast. The analysis of new field data is complemented with the application of a 3D circulation model accounting for vegetation. The analysis of well-reproduced water levels and currents over the whole lagoon make it possible to spatially study the amplitudes of the main tidal harmonics, semi-diurnal (M2), diurnal (K1) and the major overtide (M4). The comparison between our baseline model accounting for vegetation and a configuration without vegetation reveals firstly that the mangrove strongly attenuates tidal propagation in the lagoon, whit a stronger impact on semi-diurnal waves than diurnal waves. Tidal currents are also strongly reduced when the mangrove is accounted for, even close to the inlet mouth, far away from vegetation areas. The mangrove also enhances tidal asymmetry and promotes ebb dominance, both in terms of duration (ebbs last from 0.5 up to 3 h more than floods) and current magnitude (ebb currents are about 50% stronger). Spring tides accentuate ebb-dominance compared to neap tides, where tidal asymmetry is weaker.

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

“…The accumulation of water upstream of the estuary could accelerate its salinisation. It has been verified in a tropical lagoon with a reverse estuary (Samou al., 2024). Thior et al (2019) demonstrated the reverse salinity character of the Casamance estuary, with salinity increasing from downstream to upstream is due to the absence of freshwater inflow during the dry period and also high evaporation of seawater flowing up the estuary.…”

“…Recently, estuarine environments in Senegal have gained increasing scientific interest. The Somone lagoon is a notable example where the impact of mangroves on tidal wave propagation and hydro-sedimentary processes has been studied (Samou Seujip et al 2024;Sakho et al 2011Sakho et al , 2015. The study conducted by Brunet-Moret in 1970 examined the propagation of the tidal wave in the Casamance River up to 200 km from its mouth over two consecutive years.…”