Table 1. Daces of overwash events at Nauset Spit-Eastham 

 between January 1976 and January 1980. 



9 Feb. 1976 1 10 May 1977 1 25 Jan. 1979 



10 Mar. 1976 1 10 June 1977 28 to 29 Jan. 1979 



16 Mar. 1976 2 18 Nov. 1977 11 Aug . 1979 



2 Sept. 1976 9 Jan. 1978 



12 to 13 Nov. 1976 6 to 7 Feb. 1978 2 



1 Event observed (monitored), 

 2 Event recorded (monitored) 



c. Analysis of Data . 



(1) Small-Scale E vents . The first observed overwash occurred as dis- 

 crete pulse6 of water during the 5:56 a.m. high tide on 9 February 1976. Com- 

 mencing at 5:00 a.m. and continuing for 2 hours, flow depths were generally 

 less than 15 centimeters. The average wind velocity was 30 knots from the 

 northeast, which resulted in blowing snow and large drifts. Measurements were 

 not attempted during this threshold event. 



The storm center of the 10 March 1976 northeaster passed approximately 370 

 kilometers offshore of Cape Cod. The central pressure was 992 millibars, 

 and 40-knot winds were recorded at the offshore weather station "Hotel." 

 Approximately 25 overwash surges were observed during the 2-hour period 

 bracketing the 5:50 a.m. high tide. Surge depths were only 5 to 10 centi- 

 meters so current meter readings were not possible. This northeaster also 

 represented the threshold conditions necessary for an overwash at preexisting 

 dune breaches. 



The third overwash on 16 March 1976 was monitored at the first breach in 

 the dunes south of Orleans parking lot on North Beach (Fig. 3). Nearshore, 

 2-meter waves approached from the east, and northeast winds averaged 30 knots. 

 The stonn, which had a central pressure of 968 millibars, tracked less than 

 37 kilometers eastward of Cape Cod, producing a high stoma surge. During a 

 2-hour interval (10:55 to 12:54 p.m.) bracketing the 11:30 p.m. high tide, 

 more than 200 surges were recorded for an average of 1.6 surges per minute. 

 Average flow depth was 15 centimeters with a maximum of 25 centimeters. 



The velocity probe of the electromagnetic current meter was located 5 

 centimeters above the bed surface and was periodically adjusted during over- 

 wash to maintain a constant height as the sand surface eroded and accreted. 

 The initial response of the dry probe to surging water resulted in a velocity 

 spike on the recorder. Since this velocity maximum wae only apparent and not 

 real ( D. Aubrey, personal communication, 1980), the curve was smoothed by 

 filtering out this instantaneous response. 



Table 2 lists the surges, time of occurrence, and maximum velocity for 

 the 16 March 1976 overwash. The highest velocity surges occurred within 30 

 minutes of the predicted high tide (11:30 p.m.), but overwash continued for 

 another 1.5 hours. Although the highest instantaneous velocity recorded waa 



30 



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