Recent coastal sea-level variations and flooding events in the Nigerian Transgressive Mud coast of Gulf of Guinea

Significant warming and sea-level rise that will greatly impact the coastal zone is expected to intensify the coastal flooding occurrence on low-lying coasts in the next few decades. This challenge necessitates improving understanding of the possible trends and contributions of different oceanic forcing to the local coastal sea-level change at different time-scales at different locations. The present study seeks to understand different processes responsible for sea-level variations and coastal flooding events along the low-lying Nigerian Transgressive Mahin Mud coast in the Gulf of Guinea. The contributions of astronomical tides, altimetry-derived sea level, meteorologically forced non-tide residuals and wave run-up to coastal sea-level variations were assessed over the period between 1993 and 2016. The results show a distinct seasonal cycle, with more occurrences during summer/wet season and fewer events in winter/dry season, typically connected to the annual cycles of swell-waves. The leading contributor to total sea-level variations along this mud coast, at daily, seasonal and inter-annual time-scales was wave run-up. However, the leading contributor to total sea-level variations at the monthly (subannual) time scale was astronomical tides. Results further show that while the altimetry-derived sea level was responsible for the trend in total sea-level variability, the wave run-up was the major forcing responsible for the extreme coastal flooding events. However, the interaction between wave run-up and astronomical tides further contributed to the extreme/coastal flooding events along the mud coast. It is concluded that the increase in the coastal flooding events in the last 12 years of the study may have rendered the study area vulnerable to chronic erosion and flood hazards, which may have triggered the accelerated rate of coastal degradation in the study area.