III. SUMMER BEACH PROCESS VARIABLES 



1. Introduction . 



Throughout the summer study, few periods of high -wave energy condi- 

 tions were encountered with the result that changes in beach morphology 

 during this time were related mostly to uninterrupted accretion. Changes 

 in beach process variables were caused primarily by the passage of high- 

 and low-pressure systems through the study area. 



2. Meteorological Variables . 



Continuous measurement of barometric pressure showed the passage of 

 five high-pressure systems and three low-pressure systems over Plum Island 

 during the study period. The effects of these pressure systems on beach 

 process variables were multifold. As a result of the cyclonic flow asso- 

 ciated with low-pressure systems and the anticyclonic flow common to high- 

 pressure systems, the dominant wind direction varied in relation to these 

 two factors. Also important was the path of the air mass in the study 

 area. Low-pressure systems such as the one on 14 July resulted in wind 

 directions between 240° and 270°. As the low-pressure system moved past 

 Plum Island, the winds shifted from dominantly southwest to between 270° 

 and 340° or dominantly northwest. Minor fluctuations in the relationship 

 between wind direction and barometric pressure were due to the path of 

 the pressure system over Plum Island. 



Barometric pressure measurements and alongshore wind components are 

 shown in Figure 3. Alongshore winds with a negative component commonly 

 preceded low-pressure systems as they moved into the area. As a high- 

 pressure system moved in, the winds shifted to a westerly or north- 

 westerly direction and changed from a negative to a positive alongshore 

 wind component. 



Windspeed and onshore-offshore wind components are compared in 

 Figure 4. Wind data were recorded in this form by applying techniques 

 discussed in Section 11,4. The results were plotted in a CALCOMP plotter. 

 Qualitative correlations were made by visually analyzing data printouts. 

 The highest wind velocities recorded during the summer period, which 

 exceeded 20 miles per hour on several occasions (e.g., 25 and 31 July), 

 were associated with offshore winds. Onshore winds rarely exceeded 10 

 miles per hour. During the summer period the resultant of the onshore- 

 offshore wind components was -3.52 miles per hour with a maximum onshore 

 reading of 9.8 miles per hour and a maximum offshore reading of -21.03 

 miles per hour. The resultant of the alongshore wind component readings 

 was -1.03 miles per hour with a northerly maximum of 9.03 miles per hour 

 and a southerly maximum of -14.359 miles per hour. 



The relationships between air temperature and surface water tempera- 

 ture are shown in Figure 5. Air temperature readings had a 51° range of 

 48° to 99° Fahrenheit; surface water temperature had a 21° range of 47° 



20 



