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X Axis = 80 mm/s 



Figure 9. Scrip-chart record of overwash surge velocities, 6 February 1978. 



The horizontal axis of the chart represents the time scale, and the 

 recorder speed was set at 80 millimeters per second during the overwash event. 

 Figure 9 shows a series of high velocity s-.-rges (69 to 71) in rapid succes- 

 sion, followed by a short pause. There appears to be some periodicity to thj 

 record, which may be interpreted as a surf be^t. 



Flow depth measurements were not possible due to technical problems with 

 the capacitance wire wave gage. From visual observations, it was evident that 

 all surges were less than 0.5 meter deep; the average is generally less than 

 0.3 meter. Based on the relationship of low flow depths to high surge veloc- 

 ities, the frontal bore of the flow was supercritical as it passed through the 

 washover throat ( Leatherman, 1977, 1979d). These surges were eroslonal and 

 removed most of the vegetation. As surges proceeded toward the fan, the 

 velocity dropped due to the lack of horizontal constraints and percolation 

 losses; the overwash surges were principally depositional at this point. This 

 information helps to explain the pattern of vegetative changes during the 

 event. 



Because of its exposure to the Atlantic Ocean, Nauset Spit received the 

 full force of the 2-day storm on 6 and 7 February 1978 (Fig. 10). Fortu- 

 nately, the beach was surveyed just before the onset of the storm (5 February 

 1978) and soon after (9 February 1978). Figure 11 illustrates the magnitude 

 of beach erosion resulting from this large-scale northeaster. The beach was 

 in a winter (high energy) beach profile configuration before the February 

 storm. As a result of the storm, the benn crest was pushed approximately 20 

 meters landward, with a loss of 30 cubic meters of sand per meter of beach. 

 The dunes flanking site 1 washover on Nauset Spit-Eastham were eroded by an 

 average of 9 meters. At North Beach, dune erosion was somewhat less with 

 recession distances of 5 meters (Fig. 12). Entire sections of the dune line, 



36 



