material moves from the back to the front of a barrier. However, seaward- 

 flowing currents may cross barriers if landward water levels become elevated, 

 a phenomenon sometimes termed the storm surge ebb-residual flow (Hayes 

 1967). 



Coarse-grained clastic beaches are distinctive in terms of their grain size, 

 wave energetics response, and profile characteristics, but are also distinctive 

 in sediment transport and coastal evolution. Such beaches are usually morph- 

 ologically reflective, dominated by plunging breakers with no surf zone, 

 strong longshore bedload transport in the swash zone, and seepage as a major 

 process (Carter 1988). While sand beaches undergo tidal and seasonal 

 changes, gravel beaches have arrested swell profiles, which are maintained 

 through most storm periods but can show aseasonal responses. Coarse clastic 

 beaches may show alongshore grading. Sediment transport processes are 

 poorly understood in these cases, but are likely related to the wave parameters 

 as on sand beaches. 



Longer term cyclic and noncyclic events affecting beach and nearshore 

 morphology may extend over a period of decades or centuries. Variations in 

 climatic patterns, wave characteristics, sediment supply, and relative sea level, 

 as well as the lasting effects of coastal engineering works, inlets, and extreme 

 events may cause long-term cyclic and noncyclic changes. Studies of histor- 

 ical maps, charts, and aerial photographs are valuable to show relatively long- 

 term trends of such factors as shoreline or shoal migration, and bathymetric 

 changes that should be considered in project design. 



If a beach has been subject to environmental forces for an adequate period, 

 the beach profile will respond to both the long-term and short-term changes in 

 a manner that tends to restore an equilibrium profile. In equilibrium, the 

 amount of sediment deposited by waves and currents will be balanced by the 

 amount removed by them. Some researchers have proposed that the slope of 

 natural profiles fits the following equation: h(x) = Ax"", where A is a scale 

 factor primarily dependent upon sediment characteristics and m is a shape 

 factor, proposed to be 2/3 (Bruun 1954; Dean 1977; Dean and Maurmeyer 

 1983). While the equilibrium profile may be disturbed by unusual and 

 exceptional conditions, such response models can assist in predicting future 

 shoreline positions and in interpreting shoreline history. 



Over historic, and particularly over longer time scales, shoreline response 

 to sea level rise may follow one of three generalized models (Figure 16). The 

 erosional response, or Brunn Rule model, assumes offshore dispersal of 

 eroding shoreline materials such that the rates of sea level rise and sea bed 

 rise are commensurate. In the rollover model, a transgressive barrier moves 

 landward at a rate controlled by the rate of sea level rise (Dillon 1970). The 

 barrier overstepping model suggests that the barrier may be drowned, remain- 

 ing on the shoreface, as sea level rises above it. 



42 



Chapter 3 Variable Coastal Features 



