All of these beach profile types may be found in any season of the 

 year. The maturity of a beach profile is a direct consequence of the 

 frequency and strength of northeasterly storms. 



VIII. STORM PROCESS MEASUREMENTS AND PROFILE CHANGES, 

 19 TO 26 FEBRUARY 1972 



A large northeaster passed over Plum Island on 19 February 1972 and 

 caused considerable erosion of the backshore and foredune ridges in the 

 study area. Although continuous bihourly measurements were not taken, 

 closely spaced beach processes were measured by a small field crew and 

 these reflect the major changes in beach process variables. 



The beach process variables measured during and after the storm 

 (Fig. 72) included barometric pressure, windspeed and direction, wave 

 period, breaker height, longshore current velocity, and breaker angle. 

 At the beginning of process measurements at 11:00 p.m. on 18 February, 

 the low was centered off Cape Hatteras, North Carolina. As the low 

 moved northward, barometric pressure steadily decreased over the study 

 area. With the decrease in pressure, wind velocity increased from the 

 northeast, wave period decreased, and breaker height increased (Figs. 72 

 and 73) . The wind shift to the northeast resulted in breaker angles 

 shifting from negative to positive readings and longshore current veloc- 

 ity increasing in a southerly (positive) direction. As the pressure con- 

 tinued to drop, windspeed increased to a peak of 39.4 miles per hour at 

 noon on 19 February. This strong onshore wind moved a large amount of 

 sediment by eolian transport and deposited this material in the back 

 dune area. The maximum wave height and lowest wave period occurred 

 about 2 hours after the highest wind measurement; however, this could 

 be a function of the time between readings. The short-period waves 

 generated by the strong onshore winds increased in height to an average 

 of 330 centimeters. After a change in wind direction from near 90° to 

 near 45°, the breaker angle increased from +2° to a maximum of +6°. 

 Although the breaker angle did not greatly increase, longshore current 

 velocities increased considerably. Strong longshore currents moved 

 large dock sections, picnic tables, and telephone poles down the beach. 

 High tide occurred at 3:00 p.m. on 19 February near the peak in energy 

 conditions and resulted in abnormally high tides. Logs were carried by 

 the swash and deposited near the PL-0 backup stake, 9.1 meters above MLW. 



Late in the day on 19 February, as the low-pressure system began to 

 move farther offshore, the barometric pressure began to rise. With the 

 rise in barometric pressure, a decrease in the onshore winds led to a 

 lower breaker height and an increase in the wave period. Longshore 

 current velocity and the breaker angle also decreased as the pressure 

 rose. As the low moved farther north (Fig. 74) the winds changed to the 

 northwest and began blowing offshore. For several hours, wave height 

 increased due to the strong offshore winds; however, with time the tops 

 of the waves were blown off and the breaker height decreased. Wave 

 period increased as high pressure moved into the area. 



87 



