Sand Beaches 



191 



Figure 164. Relationship of grain size of beach sand to 

 offshore topography, wave height, and reworking of 

 beach. Note that the finest sand occurs near the pier 

 buih at the head of Newport Canyon where waves were 

 smallest. Great increase of grain size is at left where 

 beach was being cut by spring tides, leaving a concentrate 

 of coarse grains at the surface. Samples were collected 

 on July 22, 1957, about 30 cm above midtide level. 



grain, steepness of beach, and amount of 

 wave energy. Some of the spread in results 

 is attributable to the difference in slope of 

 the beach when it is being eroded and when 

 it is being built up during the biweekly tidal 

 cycle, although the sand is nearly identical in 

 both stages. Differences in slope at the two 

 times can be detected from repeated field 

 measurements of profiles and from the man- 

 ner in which the laminae which compose 

 the beach are truncated and overlain by 

 later laminae. 



One of the distinguishing characteristics 

 of beach sands is their high degree of sort- 

 ing. Rarely do appreciable quantities of 

 sand fall into more than three Wentworth 

 grade sizes. In the suite of samples collected 

 along the mainland coast in November 1956 

 the sorting coefficient ranged between 1.11 

 and 1.40 and averaged 1.22. As pointed out 

 by Inman (1949a), the median diameter likely 

 to be most highly sorted in water is 180 



microns, owing to the unique interrelation- 

 ship of settling velocity, roughness velocity, 

 and threshold velocity for grains of that size. 

 Silt (finer than 62 microns) is so fine that it 

 tends to remain in suspension and be carried 

 beyond the beach. Pebbles are so much 

 coarser than most beach sands that they tend 

 to be bypassed by moving sand, and when 

 included in grain-size analyses they form a 

 double maximum in the size frequency curve. 



In addition to their geographical varia- 

 tions in grain size, beach sands undergo sea- 

 sonal variations produced by winnowing 

 away of finer grains by storm waves of winter 

 and replacement of these grains by the 

 smaller waves of summer (Kerr, 1938; Shep- 

 ard and Inman, 1953). A side effect is the 

 greater concentration of heavy minerals on 

 some winter beaches by the "panning" 

 action of wave erosion. A less extreme 

 biweekly cycle probably also occurs as a 

 result of cutting and filling of beaches dur- 

 ing the lunar cycle of spring and neap tides 

 (Fig. 30). Still another cycle results from 

 differences in transporting ability of individ- 

 ual waves. The effects of this cycle are most 

 easily seen in cross sections cut through the 

 beach, where they show up as the thin lam- 

 inae of alternating coarser and finer sands 

 noted by Thompson (1937) and others (Figs. 

 165, 166). Because most of the dark and 

 heavy minerals are concentrated in finer grain 

 sizes, 50 to 200 microns, the fine layers are 

 darker than the coarse ones. As pointed 

 out by Emery and Stevenson (1950) the 

 presence of the layers complicates precise 

 sampling of beaches because a grab sample 

 must have a grain size intermediate between 

 the grain sizes of the many pairs of coarse 

 and fine laminae that were penetrated and 

 yet have a poorer sorting coefficient than 

 either kind of lamina. 



Preferred orientation of beach sands dif- 

 fers from that of beach gravels. As shown by 

 Nanz (1955) and Curray (1956) elongate and 

 particularly wedge-shaped grains tend to act 

 as weather vanes, aligning their long axes 

 with the water movement. Because the 

 waves wash more or less directly across the 

 beach, the grains tend to align themselves at 

 right angles to the beach trend; the mean 



