Generally, sediment grain size of shoreface deposits decreases in an 

 offshore direction because of wave energy distribution along the profile. 

 Exceptions occur where coarse materials that occur in deeper shoreface areas 

 are left as finer sediments and moved onshore, or where outcrops of relict 

 substrate material are exposed on the sea floor. 



Inlets 



An inlet is a small, narrow opening, recess, or indentation into a coastline 

 or a lake through which water penetrates into the land (Bates and Jackson 

 1980). Although inlets range in size from the narrow short breaches in sandy 

 barrier islands to the wide entrances of major estuaries, most geologic studies 

 concern tidal inlets, which are interruptions in barrier beaches maintained by 

 tidal flow (Fisher 1982). Tidal inlets are most characteristic of sand-domi- 

 nated barriers, and may originate as natural interruptions in a developing 

 barrier beach or baymouth spit, or as breakthroughs caused by storm waves 

 (Fisher 1982). Gravel -dominated barriers tend to lack inlets and tidal passes 

 because they have greater structural stability and more seepage than do sandy 

 barriers (Carter and Orford 1984). 



Tidal inlets usually consist of a gorge or throat, and several shoals includ- 

 ing a shallow one flanking the gorge, a landward flood-tidal delta shaped 

 mainly by flood-tidal currents, and a seaward ebb-tidal delta shaped mostly by 

 ebb-tidal currents and waves. Sand in the alongshore drift system, which is 

 intercepted by the inlet tidal currents, supplies the inlet shoals (Figure 21). 

 Nevertheless, some material may bypass the inlet. Morphological features 

 associated with tidal deltas include small tidal spits and topographically high 

 semicircular ridges, which extend across the breadth of the sandbank as flood 

 or ebb shields (Hayes 1980). 



Inlets exchange water between the ocean and back barrier during each tidal 

 cycle. Therefore, several aspects of inlet morphology are related to tidal 

 processes. Inlet spacing, for example, decreases with increasing tidal range, 

 and inlet migration is influenced by tides and the quantity of littoral drift. 



Microtidal barrier inlets tend to be widely spaced and ephemeral, migrating 

 rapidly with the longshore drift direction. The primary process is by erosion 

 of the updrift margin of the inlet and deposition on the downdrift margin 

 (Galloway and Hobday 1983). Deeper mesotidal inlets are less subject to 

 longshore migration, especially if they are incised into harder underlying 

 strata. At some sites, updrift inlet migration has been noted (Aubrey and 

 Spear 1984; Carter 1988). 



Variations in tidal delta morphology can also be related to differences in 

 tidal range, and thus the tidal prism, and to differences in wave-energy flux 

 impinging upon an inlet (Boothroyd 1985). Microtidal areas are thought to 



Chapter 3 Variable Coastal Features 



55 



