SECT. 3] BEACH AND NEARSHOKE PROCESSES 539 



The approximate quantitative agreement in this single instance is obviously 

 fortuitous, but the result suggests that a consistent understanding of what 

 happens on the sea bed might best be got by combining further quantitative 

 field measurement with laboratory experiments of extended scope designed to 

 sort out the sej)arate effects of wave, wind and boundary conditions. For 

 instance, the field experiment of Inman and Chamberlain gave the seemingly 

 anomalous result that the marked grains migrated, not only shorewards, as 

 might be expected, but in all directions. In part this dispersion is associated 

 with the onshore and offshore movement of sand, both in suspension and as bed 

 load, by the orbital velocity of the waves. Also, a hint as to the reason for this 

 may be given by the incipient horizontal circulation of bed water noticed by 

 Russell and Osorio (1958) in their wave experiments. If such a circulation is 

 real, the scatter due to wave drift alone would be expected after an elapsed 

 time, t, to show the pattern sketched in Fig. 10b. Assuming no appreciable mass 

 drift on the part of the sea-water as a whole, an onshore wind, as discussed 

 previously, would be expected to suj)erimpose an offshore diift along the bed, 

 thus producing a modified pattern such as is sketched in Fig. 10c. However, no 

 appreciable wind was observed during the field experiment. 



It is possible that each of these effects can be studied separately in the 

 laboratory, j^rovided the equipment is designed with the specific objects in 

 view, rather than in isolation. 



Again, in the irradiated sand experiment the marked grains were detected by 

 direct emission photography of samples of the bed surface taken on adhesive 

 jDaper. This disclosed the location of the active grains on the paper, and their 

 sizes should be measureable by microscopic observation; and since, by the 

 reasoning leading to relation (19), bed surface samples should be representative 

 of the sub-surface distribution also, it may be possible by this means to throw 

 valuable light on the differential migration of sediment grains of different sizes 

 — a subject on which no certain information is now available. 



Following the experiment described above involving the dispersion of 1 kg 

 of irradiated sand outside of the surf zone, a further series of experiments i 

 using dyed sand was conducted on the swash zone of the beach face. In the 

 first swash-zone experiment, 3 kg of dyed sand were placed in the wave uprush 

 where the maximum depth of water in the uprush was about 20 cm. The 

 second experiment involved 4 kg of sand and was conducted where the 

 maximum uprush depth was about 15 cm. 



The results of the two experiments showed that: (a) dispersion of sand 

 grains in the uprush zone w^as extremely rapid ; 4 kg of sand dispersed over 

 30 m2 of beach in one minute ; (b) individual sand particles had about 10 times 

 the onshore-offshore trajectory as longshore trajectory; and (c) very small 

 beach features, such as small cusps, drastically affected the dispersion of 

 quantities of sand of the order of 4 kg. 



In contrast to the area just outside the surf zone, where 1 kg of irradiated 

 sand was sufficient to observe successfully sand movement for about 24 h, in 



1 As yet unpublished. 



