Assessing coastal vulnerability in Ghana

The more immediate problem with sea level rise for low-lying coasts is not permanent submersion, but the extra height it is already adding to extreme events. Assessing the risks is thus of foremost importance for all threatened areas.

The Ghanaian coast is a low-lying sandy coast. In recent years, it experienced more and more coastal flooding, with, as in many countries, repercutions on populations and vital infrastructures. The eastern coast of Ghana, and in particular the area of the Keta City, close to the Volta estuary, is the most vulnerable coast in the country, with coastal flooding experienced no less than twice a year. This frequency, and the intensity of coastal flooding and erosion, have increased in recent years. 

To assess coastal flooding heights using altimetry, some of the usually applied corrections must be "re-added" to sea level anomalies (which includes sea level rise). Tides, Dynamic Atmospheric Corrections (DAC), including both the atmospheric pressure (Inverse Barometer correction) on the sea surface and high-frequency effects, but also wave heights. Moreover, other inputs can play a role, such as river discharge and rainfall. 

Used with a Pleiades imagery-derived DEM, taking into account the tide level at the moment of the measurements, and model wave run-up (ERA Interim), the data enabled to compute Extreme Coastal Water Levels and flood extent maps. Results of the study shows that only one of the several factors which can cause a extreme coastal water level in needed to have a severe episode on the Keta area. If several occur (e.g., high tide and very low atmospheric pressure at the same time), the episode can become catastrophic. Both tides and atmospheric pressure can be predicted, though. 

Altimetry can help in assessing vulnerabilities, even if the coastal data are not as precise as the open ocean ones. Swot will help in a better assessment, even if several years of its measurements will be needed to get the full range of possibilities, and add river discharge to such site study.