From seasonal flood pulse to seiche: Multi-frequency water-level fluctuations in a large shallow tropical lagoon (Nokoué Lagoon, Benin). (5th April 2022)

• Record Type: Journal Article
• Authors: Chaigneau, Alexis; Okpeitcha, Olaègbè Victor; Morel, Yves; Stieglitz, Thomas; Assogba, Arnaud; Benoist, Morgane; Allamel, Pierre; Honfo, Jules; Awoulmbang Sakpak, Thierry Derol; Rétif, Fabien; Duhaut, Thomas; Peugeot, Christophe; Sohou, Zacharie
• Is Part Of: Estuarine, coastal and shelf science. Volume 267(2022)
• Publication Date: 2022-04-05
• Subjects: Water-levels -- Lagoons -- Tides -- Seiches -- Nokoué -- Wavelet analysis; Estuarine oceanography -- Periodicals; Coasts -- Periodicals; Estuarine biology -- Periodicals; Seashore biology -- Periodicals; Coasts; Estuarine biology; Estuarine oceanography; Seashore biology; Periodicals; 551.461805
• DOI: 10.1016/j.ecss.2022.107767
• Languages: English
• ISSNs: 0272-7714

Abstract

Abstract: This study investigated the main water-level (WL) variability modes of Nokoué Lagoon in Benin (West-Africa). The average WL ranges between 1.3 and 2.3 m between the low- and high-water seasons. Seasonal as well as weak interannual variations between 2018 and 2019 are driven by rainfall regime over the catchment and associated river inflow. At sub-monthly scales, the lagoon is tidally choked: ocean tides can reach 90 cm, whereas in the lagoon semi-diurnal and diurnal tides hardly reach few centimeters. Choking conditions vary with river inflow and ocean tide amplitude, correctly represented by a simple tidal choking model. Diurnal modulation and asymmetry of the tide are stronger (weaker) during high (low) water period. We also observed WL variations of ±5–10 cm at a fortnightly frequency, stronger during wet (high-water) season. Superimposed on the seasonal, fortnightly and tidal WL variations, we further observed short-term high-frequency seiche events. Mostly observed during dry (low-water) conditions, they are characterized by typical standing-wave oscillations of 5–10 cm amplitudes and 3 h periods. They are forced by the passage of fast-moving squall-lines that induce strong wind variations, heavy rainfalls and rapid drop-off of the air temperature. Results obtained in this study provide useful metrics for the validation of flood forecasting models to be implemented in Benin, and elsewhere on the West African coastline. Highlights: Nokoué Lagoon is strongly tidally choked due to bottom friction in the Cotonou channel. WL and tidal choking conditions depend on extreme seasonal hydrological cycle. Fortnightly WL variations (±5–15 cm) are observed and are stronger during wet season. High-frequency seiche events have 5–10 cm amplitudes and 3 h periods. Seiche events are forced by the passage of fast-moving squall-lines.