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

Abstract

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Terrestrial or oceanic forcing? Water level variations in coastal lagoons constrained by river inflow and ocean tides

“…Although the net river flow considerably varies from the dry to the wet season, an exact quantification remains uncertain as rivers are not regularly gauged. Previous studies report river flow variations from a few tens of m 3 /s in the dry season to 1000 m 3 /s in the wet season (Chaigneau et al, 2022;Le Barbé et al, 1993;Djihouessi and Aina, 2018;Lawin et al, 2019). This strong seasonal variation of freshwater inflow results in a complete seasonal reversal of the salinity and ecosystem of the lagoon (Lalèyè et al, 2003;Okpeitcha et al, 2022) and can lead to major floodings.…”

“…This strong seasonal variation of freshwater inflow results in a complete seasonal reversal of the salinity and ecosystem of the lagoon (Lalèyè et al, 2003;Okpeitcha et al, 2022) and can lead to major floodings. In the wet season, the water level (WL) in the rivers increases by several meters (Le Barbé et al, 1993) and in the Nokoué Lagoon, the average depth increases from 1.3 m in low-water period to 2.1 m in flood period (Chaigneau et al, 2022). Interannual variations in rainfall can lead to exceptional flood with disastrous impacts for local populations.…”

“…Similarly, in 2019, a major flood was less intense but extended over several weeks (Chaigneau et al, 2022), causing significant damages and affecting tens of thousands of households (Chaigneau et al, 2022). Thus, in order to protect the population and avoid these numerous human and material damages, it is necessary to set up early warning systems for floods, based on both continuous observations and numerical models.…”

“…Recently, Chaigneau et al (2022) described the main patterns of the WL variations observed in Nokoué Lagoon, based on 2 years of high-frequency WL measurements. They showed that the lagoon, constrained by tide and river flow, is tidally choked.…”

Coastal Zone Changes in West Africa: Challenges and Opportunities for Satellite Earth Observations

“…The assumption of a 1 m rise in sea level would lead to a considerable increase in coastal hazards, mainly affecting low-lying sandy and muddy coasts and mangroves. Major lagoon systems will also be affected (Chaigneau et al 2022).…”