Sand Beaches 



193 



little preferred orientation. A related phe- 

 nomenon was studied by Bryson (1956), 

 who measured the permeability of beach 

 sands vertically, horizontally parallel to the 

 beach, and horizontally at right angles to the 

 beach. He used a constant-head permea- 

 meter and measured permeability according 

 to the equation P = QLii/TAH, where Q 

 is the quantity of water of viscosity ju, that 

 passes through a column of sand of length L 

 and cross section A in the time T. The aver- 

 age of 24 sets of measurements at 9 stations 

 along the beaches of Santa Monica Bay was 

 36 darcys vertically, 61 darcys parallel to the 

 beach, and 75 darcys transverse to it; all 

 except two sets of measurements corres- 

 ponded to this relative order. The lowest 

 permeability, in the vertical direction, is 

 doubtlessly due to the presence of horizontal 

 laminae, some of which are fine-grained and 

 thus of low permeability. However, a priori 

 we might have expected that the orientation 

 of long axes of grains at nearly right angles 

 to the beach would have produced the great- 

 est permeability in that direction. The fact 

 that the greatest permeability is horizontal 

 and parallel to the beach may be due to 

 blocking of the seaward flow of interstitial 

 water by the grain imbrication as suggested 

 by a comparison of measurements of perme- 

 ability and grain orientation and imbrication 

 by Rabinovitz(1958). 



The horizontal direction of great permea- 

 bility produces several unique minor features 

 on sand beaches. When waves on an in- 

 coming tide wash across a largely dry beach, 

 a high percentage of the water seeps down- 

 ward into the sand. Where the sand is 

 coarse, the air readily escapes through small 

 holes that sometimes have been mistakenly 

 identified as animal burrows. Beaches com- 

 posed of fine-to-medium sand having few 

 coarse layers and characterized by small 

 waves, such as in largely enclosed bays, trap 

 most of the air as subspherical pockets many 

 times larger in diameter than the sand grains 

 (Fig. 167). A porosity of 73 per cent was 

 measured in such sand, half of which was in 

 the form of large air pockets (Emery, 1945fl). 

 Compaction of the sand underfoot leaves 

 footprints as deep as 20 cm and disturbance 



Figure 1 67. Beach of fine sand near former entrance to 

 Mission Bay, San Diego, iiaving a thickness of about 15 

 cm of highly cavernous sand with subspherical pockets 

 filled with air. From Emery (1945fl. PL 3). 



under water releases clouds of air bubbles. 

 On beaches having well-developed laminae 

 of alternating coarse and fine sand the water 

 drives the air ahead of it in the coarser and 

 more permeable laminae. Frequently, the 

 air is so compressed by the water movement, 

 probably aided by capillarity, that it is able 

 to lift up several centimeters of overlying 

 sand, forming a sand dome, a sort of air lac- 

 colith which usually has a diameter between 

 5 and 15 cm (Fig. 168). Later waves crossing 



Figure 168. Double sand dome in sand of Scripps Beach 

 formed when air was driven along coarse laminae by 

 water seeping into the surface of beach. 



