of the low resulted in southerly and southwesterly winds. Winds blowing 

 from the south or southwest (negative alongshore components) resulted in 

 negative longshore currents flowing toward the north. As the low-pressure 

 system moved offshore, the wind shifted to the north, and positive along- 

 shore wind components and positive longshore current velocities were 

 produced. 



Sudden changes in wind direction will sometimes generate short-period 

 waves. These short-period waves break on the beach at greater angles 

 than longer period waves and cause strong longshore currents. Waves 

 measured on 8, 15, and 21 January show an inverse relationship between 

 breaker angle and wave period (Fig. 51). 



Breaker type and breaker depth are shown in Figure 52. Breaker types 

 between 4 and 6 are either surging, surging-plunging, or surging-spilling. 

 Surging waves and breaker depths less than 50 centimeters characterize 

 wave conditions on the high tide beach face for 1 to 2 hours on either 

 side of the high tide. The semidiurnal tides at Plum Island are evident 

 in Figure 52, especially between 20 and 24 January. On these days, two 

 daily periods of surging breakers along with breaker depths less than 50 

 centimeters occurred at high tide conditions. Under higher energy condi- 

 tions, plunging waves will break on the beach face and this relationship 

 is invalid. 



Several process variable relationships discussed under summer beach 

 process variables were similar for both summer and winter study periods. 



VI. WINTER BEACH MORPHOLOGY 



During the winter study period, strong offshore winds and subfreezing 

 temperatures resulted in changes in beach morphology that were more rapid 

 and pronounced than during the summer period. 



A small northeaster on 5 January resulted in a poststorm beach pro- 

 file at all profile locations. The poststorm profile is characterized 

 by a flat, concave, upward profile caused by wave energy acting upon a 

 wider beach zone than during nonstorm periods. Heavy minerals are con- 

 centrated at the upper swash limit of the waves, usually at the base of 

 the dune scarp. The gradient of the high tide beach face after a storm 

 has passed is between 5° and 6° (Fig. 53). The high tide beach-face 

 gradient increases after a storm until a stable condition is reached or 

 until another storm occurs. The reason for the pronounced change in 

 gradient was because a neap berm formed 3 days after the storm. As the 

 neap berm grew, the gradient of the beach face steepened until 15 January, 

 after which time the gradient changes were due more to changes in wave 

 energy than to the effect of an enlarging neap berm. Changes in profile 

 PL-0 between 8 and 18 January are shown in Figure 54. The upper profile 

 shows the beach morphology 4 and 5 days after a storm. A neap berm 

 began to form on 7 January and by 10 January was large enough to con- 

 siderably affect the beach face gradient (Figs. 55 and 56). Sediment 



68 



