**2.1 Origin and size of coastal lagoons**

The genesis of coastal lagoons and the barrier island enclosing them depends primarily on the sea-level history of a region [26]. In terms of climatic setting, there is no restriction to the formation of coastal lagoons. Coastal lagoons exist where coastal embayment are separated from the adjacent sea by a barrier [27]. The barriers that separate the lagoons from the sea could at times be sand or gravel deposited by erosion and flood or are created by vegetation, coral growth or tectonics [28]. Lagoons are best formed on transgress coasts going towards the landward area, especially where the continental margin has a low gradient and sea-level rise is low [27].

In terms of spatial distribution, they occur in tropical, temperate and cold coasts extending along 13% of the world's coastline [29]. Even though coastal lagoons are found everywhere all over the world, however, they are more common in low-lying coastal parts of the world where sea level, shore-face dynamics and tidal range are common parameters that influence their formation [30]. Also, coastal lagoons can be recognised either in coasts where sea level has been rising (transgressive) or dropping (regressive). Formation of coastal lagoons was discussed by Anthony et al. [30] as a system formed and nourished through sediment transport. The transported sediment is carried by rivers, waves, currents, winds and tides [31] and gathers either in tidal deltas and rivers or on marshes and flats where immersed aquatic vegetation slows current movement.
