mean velocity. In order to obtain current velocities that are representa- 

 tive of the beach as a vmole, it is necessary to take as many m.easiare- 

 raents and at as many different stations along a beach as possible. 



The momentum approach to the prediction of longshore currents by 

 Putnam, 'lunk and Traylor leads to le ef ul forecasts provided the beach 

 friction coefficient k is permitted to vary with the longshore velocity, , 

 V. The indicated relation is k-^v3/2. As an aid in computing longshore 

 currents, three alignment charts have been prepared incorporating the 

 above relation. Two are for natural beaches, vri_th slopes ranging up to 

 3 per cent, in one case, and up to 10|^ per cent in the other. The third 

 chart is for use on model beaches -with slopes ranging from to lof- 

 degrees and breaker heights from 0-.1 to Oo5 feet. Copies cf thesa charts 

 are available upon request, 



"^.tarine Beaches of the United States 



Further study has been made of the large suite of samples collected 

 in a series of trips along the beaches of the United States. Figures 2 

 and 3 show respectively the relation of the foreshore slope to grain size 

 and the relation of sorting to grain size. These are a compilation of 

 all the samples. Figure 3 differentiates between the samples from the 

 west coast, the Florida and Gulf of Mexico beaches, and New England 

 beaches. Some of the variation from the average curve showing increase 

 of slope with increase in grain size (figure 2) appears to be related to 

 protection of the area from the violence of wave attack. The protected 

 areas show steeper slopes for corresponding grain sizes. The grain size 

 of the sands, on the other hand, shows little relation to the exposure 

 to wave att-^ck, but is decidedly related to the source material. The 

 reason that many pocket and cove beaches have coarse sand is tljaU only 

 coarse neiterial is available to make these sands. The typical fine sand 

 of long beaches is in nsny cases due to the derivation from a larger river 

 which transports predominantly fine sando 



Changes in Submarine Canyon Heads 



Continued soundin:;s along the accurate range lines at the heads of 

 Scripps Canyon during the past three months liave revealed an interesting 

 series of depth changes. The canyon heads had been filling in during 

 the previous three-month period. Observations on 11 December, directly 

 after a series of high waves, showed continued fill, amounting to as much 

 as 5 feet. An earthquake of moderate intensity was felt on 2$ December 

 in the San Diego area. A survey on the follov/ing day shov/ed a deepening 

 of as mnch as 7 feet in one canyon head and of 2 or 3 feet in the adjacent 

 head. The roiled condition of the water over the canyon heads during the 

 survey was in marked contrast to the clear water on either side, indicat- 

 ing that the sediment had not yet settled in the l6-hour period which 

 intervened between the earthquake and the survey. It was estimated that 



33 



