have poorly developed ebb-tidal deltas and relatively larger flood-tidal deltas 

 because of the dominance of wave energy over the small tidal prism. Along 

 mesotidal areas, ebb-tidal deltas are more prominent, but are more elongated 

 and extend farther seaward in areas of lower wave energy. 



On a smaller scale, variations in bedforms, or deviations from a flat bed, 

 and accompanying sedimentary structures are associated with inlet hydrody- 

 namics including the effects and interaction among tidal currents, wave char- 

 acteristics, and longshore drift. Bedforms show varying wavelengths and 

 amplitudes, and have been divided into categories largely on the basis of 

 wavelength. Ripples are small bedforms with spacings to 60 cm, which can 

 be generated by both waves and currents. 



In the past, terminology for large-scale bedforms has been confusing and it 

 has recently been suggested that large bedforms be called subaqueous dunes 

 (Ashley 1990). First-order descriptors of such subaqueous dunes include size 

 and shape. Descriptors based on spacing include small (0.6-5 m), medium 

 (5-10 m), large (10-100 m), and very large (> 100 m) dunes with cor- 

 responding heights. Shape is distinguished by two-dimensional and three- 

 dimensional descriptors. Second-order descriptors should be used where 

 feasible to characterize superposition (simple or compound bedforms) and 

 sediment type (size and sorting). Third-order descriptors characterize bedform 

 morphology, bedform behavior, and flow. 



Bedforms are sensitive to flow velocity and somewhat independent of 

 depth, which allows them to serve as a powerful tool in estimating flow 

 velocities in estuaries when field current measurements are not possible 

 (Boothroyd 1985). The shape of bedforms can also vary in response to 

 increasing flow strength (Hayes and Kana 1976). The orientation of these 

 shapes and associated slipfaces also provides clues to flow direction. 



Inlet stability plays an important role in coastal geomorphic variability. 

 The effects of inlets on coastal hydraulic and sedimentation patterns may 

 extend to areas lying some distance from the inlet itself. Historical studies 

 have shown that barrier inlets are ephemeral features, which may be closed or 

 created at susceptible places by storm overwash during the course of a single 

 storm. Newly breached inlets may be temporary or may be maintained by 

 tidal currents and may persist for many years. Inlet shoals also are not fixed, 

 and changes in form and dimensions occur as a result of varying currents, 

 waves and sediment supply factors. 



Barrier inlets and associated shoals are of great importance in barrier 

 sedimentation because they act as sinks for sediment in the longshore transport 

 system. The opening of an inlet can thus greatly reduce downdrift sediment 

 supply by trapping large amounts of sediment moving in the longshore drift 

 pattern and storing it in inlet shoal complexes. This loss of nourishment to 

 downdrift beaches can result in serious erosion and shoreline regression as the 

 sediment supply decreases. Inlet closure can restore downdrift sediment 

 supply and ebb-tidal delta materials may gradually return to the alongshore 



Chapter 3 Variable Coastal Features 



57 



