Sand-Barrier Islands
8 THE NATURAL SAND-BARRIER ISLANDS OF THE NIGER DELTA
- The Formation of the Sand Barrier Islands
- The Balance Between Deposition and Erosion
- The Natural Ecosystems
- The Beach Strand Sub-Ecozones
- Animal Communities of the Sand Barrier Islands
8.1 THE FORMATION OF THE SAND BARRIER ISLANDS
The Sand-barrier Islands form a 200km. chain along the Niger Delta coast, protecting the mangrove forests from the battering of the Atlantic Ocean. Without the barrier islands there would be merely a bay of low, crumbling cliffs along the boundary of the old coastal plain; they are both a guardian and a product of the Niger Delta itself.
Eroded sand, borne by the Niger/Benue river systems eventually reaches the sea. However it may then be driven back towards the shore by wind and waves, and may collect in deposits along the shallow bed of the continental shelf.
Long-shore Drift: the 'backwash' of a wave is always at right angles to the waterline, straight back out to sea. However wave direction is almost always at an angle to the shore, so that the 'swash' runs diagonally up the beach. This means that sediment tends to be carried along the shoreline. Longshore drift is responsible for the erosion of some shorelines, and the deposition of features such as spits and barrier beaches.
Sand Bars: ridges of submerged sand crossing the mouths of estuaries as a result of long-shore drift. They restrict the entry of ships, and one of the most well known was the bar that restricted entry to the Lagos lagoon until a channel was first dredged in 1908.In this case, sand brought down to the ocean by the Niger/Benue is carried back to the 'nose' of the Delta by the Guinea current (see Maps 1 and 3B). Longshore drift spreads it to the East and West, where it joins sediment contributed by lesser river systems such as the Imo and the Osse (Ovia).
The sand spreads as a band of islands along the coast. They are mostly 'true islands', completely surrounded by water, be it the Atlantic itself or the water of lagoons and of the creeks that twine their way around islands formed further inshore. Where they form in the mouth of an estuary, they may be called sand bars.
Sand thus thrown up by the Atlantic often forms a high beach, behind which streams running from inland may form a lagoon. Eventually the beach is higher than the level of the highest tide, and is colonised by vegetation that further stabilises the new island.
This continuing process forms a low corrugated plain, no more than a few metres above sea level. Sandy ridges running parallel to the coast line alternate with troughs
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