In my blog of 11 June, I mentioned the erosion that had occurred on Bird Island’s north-west beach. Bird Island is a sand cay, a low-lying island formed by the deposition of sand of animal and plant debris derived from a coral reef. The calcareous deposits vary considerably in coarseness, ranging from shell and coral chunks and fragments down to sand so fine it can almost feel like talcum powder. The finer sand particles are susceptible to movement by wind but all of the sand moves under the influence of water, sometimes affected by heavy rain but perpetually moved by waves and marine currents. Vegetation cover confers stability inland but beaches are in perpetual motion, so that a sand cay changes shape over time and, in Bird Island’s case, these changes can occur over years, but also more regularly under the influence of seasonal changes in ocean currents and wind direction.
Bird Island is subjected to two seasons. The south-east trade winds blow from roughly May to October, reaching peak strength in July and August. This south-east is a steady wind, normally fairly constant in direction, and it can lead to very rough seas at its peak. The predominant current direction during the south-east is westerly, under the influence of the South Equatorial Current. From roughly November to April Seychelles experiences the north-west monsoon, normally characterised by periods of calm interrupted by storms and heavy rainfall, but generally with light winds. At this time of year the ocean current is dominated by the Equatorial Counter Current that flows eastwards.
These seasonal changes have a huge impact on the distribution of sand around the island with tonnes of it on the move, sometimes very quickly. As the south-east season progresses a sand spit often forms at the northern end of the island, sometimes extending close to the drop-off into the deep ocean off the north of the Seychelles Bank. The north-west monsoon winds, currents and waves redistribute sand further south, shortening the sand spit and broadening the beaches on the east and west coasts.
Periodically there are more significant changes. For example, in the early 1990s erosion of the western beach was so severe that the sea began to wash away the hotel’s first chalets. Eventually, all of them had to be removed and the present hotel was built on firmer ground, further from the sea. On the other side of the island, the beach along the northern edge of the airstrip eroded in 2012-2013, taking away much of the vegetation bordering the airstrip; this has now stabilised again. Currently, however, erosion of the north-west beach is encroaching into the Sooty Tern breeding area. This has always been a tendency but has been particularly noticeable since 2015. We cannot predict how far this will go or how long it will last!