High-tide flooding in French Guiana due to sea-level rise

Many flat coastal plains are already subject to recurrent high-tide flooding events under calm meteorological conditions. As a consequence of sea level rise (SLR), the frequency of such harmful flooding events is expected to increase. Although not catastrophic, the coastal flooding may become a major adaptation challenge as they could cause substantial transportation and economic disruption. In many low-lying coastal zones in the world, an increasing occurrence of high-tide flooding has indeed been recorded. This is often accompanied by land subsidence. The French Guiana coast, located in northern South America, is a coastal plain where nuisance flooding has been recently observed (Cayenne city). In this study, we first performed a hindcast analysis of this recently observed chronic flooding to better understand the associated drivers. Then, we developed projections over the 21^st century for three cities of this French territory: Cayenne, Kourou and Mana.

Hindcasts and projections of relative SLR (i.e., accounting for vertical land motion) relied on CMIP5 climate model results and IPCC/SROCC estimates for the region, together with records from permanent GNSS stations in French Guiana. The daily maximum tide level was determined from hourly tide gauge observations and tide predictions. SLR and tide water levels combined with digital terrain models have allowed mapping of low-lying areas exposed to coastal flooding, and the associated flooding frequencies.

The hindcast analysis revealed the absence of significant land subsidence/uplift and suggested that the recently observed high-tide flooding in Cayenne have been fostered by climate change-induced SLR. Over the 21^st century, our results showed that urban zones exposed to high-tide flooding will progressively expand in the three cities, although at different rates. The sectors most exposed to flooding are located along urban channels and within swamps. Conversely, sandy coastlines are less exposed to chronic flooding due to their fast and dynamic evolution in response to changing hydrodynamic forcing factors. The results of this study provide the first scientific basis in French Guiana available for decision-makers and stakeholders in order to develop coastal management and adaptation strategies.