INTRODUCTION 



The most important water movements off exposed 

 coasts are due to waves, and long shore and rip currents. 

 The purpose of this study was to determine the movements 

 due to wave motion acting along the bottom in a specific 

 area off an exposed coast; namely. South Beach, Martha's 

 Vineyard, Since waves cause oscillatory motion along the 

 bottom, this report is concerned with calculations of the 

 lengths of these oscillations and the maximxom speeds which 

 particles attain in such an oscillation. This information 

 is important, not only in its effects on objects resting 

 on the bottom, but also in its effects on erosion, trans- 

 portation and deposition of sediments. 



DISCUSSION 



Wave height and period observations from July 19l|.6 

 to May 19k-l (Seiwell I9I4.8) and from January to April 19^4 

 (Bigelow 19i^-7) were used as representing average monthly 

 conditions of waves in the vicinity of South Beach, Martha's 

 Vineyard, Seiwell' s observations were made with an under- 

 v;ater pressure recorder located about one and one-quarter 

 "miles south of Cuttyhiink Island in 75 feet of water; Bigelow' s 

 were surf observations at South Beach. 



Deep water is defined as water of depth greater 

 than 1/2 of the wave length, shallow water as water of depth 

 less than l/ZO of the deep water wave length and intermediate 

 water as water between l/20 and l/2 of the deep water wave 

 length. 



Both sets of observations were first reduced to 

 their equivalent values in deep water using the plates in H.O. 

 No, 23k- and the Supplement to H.O. 23i|, The areas are in 

 such close proximity that the same deep water waves approach 

 both areas even though they have different values at the two 

 observing places due to differences in depth and topography. 

 Refraction was not considered, due to lack of sufficient ob- 

 servations and reliability (within the accuracy of the work) 

 for calculations in such a complex topography as that exist- 

 ing from No Mans Land out to deep water. 



The deep water heights and periods for March and 

 April from the two sets were then averaged. 



Using the plates from H.O. 23i|. (and Supplement), the 

 heights and lengths for each month (except June) were deter- 



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