Figure 20. A three-dimensional view of some features commonly associated with a 

 barrier island system, including the back barrier, overwash fans, and 

 lagoons 



classified according to hydraulic balance, where inflow is equal to, greater 

 than, or less than outflow. The volume of water being exchanged is known as 

 the tidal prism (Carter 1988). The salinities of lagoons are highly variable, 

 depending upon water exchange with sea water and the amount of inflowing 

 fresh water. Sediment brought into lagoons may be washed over from the 

 barrier island, blown into the lagoon from the barrier island, introduced by 

 tidal currents through inlets, or transported from the mainland areas by rivers. 

 Generally, the coarsest sediments are found closest to the barrier. 



Mudflats, Salt Marshes, and Mangrove Coasts 



Mudflats, despite their name, are not entirely flat, nor do they consist 

 entirely of mud. They occur predominantly in areas of medium to large tidal 

 range, areas sheltered from the effects of wind -driven waves, or in areas of 

 abundant suspended sediment supply (Pethick 1984), including in the vicinity 

 of deltas. Mudflats show a marked break in slope about mid-tide level, below 

 which the surface slopes steeply toward the low-water mark. Moving seaward 

 and downslope, sediment size increases, with upper mudflats being replaced 

 by sandy mudflats at slighUy lower levels. These sandy mudflats in turn 

 change into more steeply sloping sand flats below mid-tide level. Fine- 



Chapter 3 Variable Coastal Features 



51 



