*2.1.4 Lagoon stratification*

the hydrological behaviour of the lagoon depending on the dimension of the chan-

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

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

Early research surrounding coastal lagoons focused on understanding processes of coastal lagoon formation, identification of defining characteristics and the development of classification schemes within which to group water bodies that are similar in geomorphology. Coastal lagoon was described by Kjerfve [32] as: "an inland body of water, usually oriented parallel to the coast, separated from the ocean by a barrier, always connected to the ocean by one or more restricted inlets, and having depths which seldom exceed a couple of metres", although some recent definitions [33, 34] have considered deposition of sediment as well as littoral drift in an attempt to define coastal lagoon. In addition, much of the sediment present in lagoons can be cohesive in composition and will therefore flocculate (e.g., [35]) when resuspended and subsequently produce a range of floc settling velocities (e.g., [36, 37]) that will affect depositional fluxes [38, 39] throughout a lagoon and, similarly, will have an effect on both bed erodibility (e.g., [40]) and subaqueous

Geological evolution of coastal lagoons is typically expressed in terms of the rate of basin fill through sedimentation, and this is thus helpful to consider lagoon fill in terms of maturity [43]. The geological evolution of coastal lagoons from unfilled to deltaic stage is described as a seamless progression [43] that progresses correspondingly to the rate of sediment supply. In addition, Adlam [44] used a model of geologic evolution to explain the formation of the coastal lagoons in geological scale and found that the threshold between the two phases relates to depth and is defined as the depth at which wind waves are able to suspend sediments within the system

nel's cross-sectional area [24, 25].

rise is low [27].

**2.1 Origin and size of coastal lagoons**

*Lagoon Environments Around the World - A Scientific Perspective*

aquatic vegetation slows current movement.

*2.1.1 Definition of coastal lagoon*

bed form sizes (e.g., [41, 42]).

**82**

*2.1.2 Geological origin and formation of coastal lagoons*

However, being a shallow coastal feature, lagoons tend to be well-mixed (mainly by winds rather than by currents), and they vary from brackish to hyper-saline, depending on the geographic location which dictates the level of balance between evaporation, precipitation and river flow. In equatorial regions, lagoons can be hyper-saline during dry seasons as a result of low influx of fresh water and high intrusion of saline water. But the same lagoon may become entirely fresh during rainy seasons [49]. Even though lagoons are shallow water bodies, the Lagos Lagoon that is the lagoon for consideration in this chapter (our research) has some parts around the inlets that are deep (12–17 m) as a result of continuous dredging either for the purpose of sand mining and reclamation or for channel navigation. Likewise, it is considered too brackish during the dry season and a fresh water lagoon during the raining season [50, 51].
