from 1 deg to 30 deg, depending upon the sediment character of the foreshore 

 and the processes acting upon it. Steeper beach faces generally occur with 

 coarse materials and in higher energy environments. A small and sometimes 

 subtle step, typically marked by a concentration of shell debris and/or coarse 

 sediments, may be present at the line where the waves plunge before surging 

 up the beach face. A sandbar and landward trough, known as a ridge-and- 

 runnel system, may also be present at times (Figure 2). The ridge-and-runnel 

 system is most common on beaches with an abundance of sand. Seaward of 

 the ridge near the low-water line, there may be a nearly horizontal morpho- 

 logic zone called a low tide terrace. 



Rhythmic topography changes also occur in varying scales on the fore- 

 shore. Beach cusps (spacing of 10-30 m) and giant cusps (spacing of 

 100-200 m) are examples. Both cusp features have a similar morphology and 

 a non-tidal genesis, with beach cusps being related to accretionary processes 

 during swell conditions, and giant cusps being formed during storms under 

 erosional wave conditions. Spacing of beach cusps may be related to rip 

 current spacing, the wavelength of edge waves, and the spacing between 

 waves arriving at the beach from different directions. Larger rhythmic fea- 

 tures called beach protuberances (100 m-100 km) are characterized by subtidal 

 components and sandbar movements. Non-rhythmic morphologic features, 

 forming aperiodic protuberances, may also occur at the shoreline, often in 

 response to the longshore drift patterns associated with cells. Other non- 

 rhythmic features, such as salients, may form in sheltered areas behind rock 

 outcrops or offshore breakwaters. 



Morphologic features of the nearshore zone include a number and variety 

 of stable and ephemeral subtidal bar forms. These often show relationships to 

 beach topography. The formation of submarine bars is favored by, but not 

 entirely dependent on, a gentle shoreface slope, low tidal range, ample sedi- 

 ment supply, and a low incidence of long swell waves. Bars are absent only 

 where these factors are lacking, and are less stable in shallow water than in 

 deeper water. 



Several types of submarine bars have been recognized. They are differen- 

 tiated mainly on the basis of plan view shape, alignment, and continuity. 

 Longshore bars are the most common, consisting of a 1- to 4-meter relief 

 linear ridges aligned parallel to shore. Their continuity is broken only by 

 narrow rip channels which may migrate alongshore. The origin of longshore 

 bars is believed to be related to breaking waves, and most bars seem to be 

 located at the breaker line (Miller 1976). Where multiple longshore bars 

 exist, their positions are often associated with the breaker lines at high- and 

 low-tide stages, multiple breaker lines, or breaker positions for various wave 

 characteristics. 



In the nearshore zone, crescentic bars are the next most common form. 

 They are shaped convex-seaward, and may be in-phase or out-of-phase with 

 shoreline protuberances. Lunate bars form a half crescent, with the bar 

 initially extending from the shore protuberance, then bending to a more or 



Chapter 3 Variable Coastal Features 



39 



