of headlands^ the latter of which acts as a littoral barrier, whereas the 

 former permits passage of drift even though no beach exists on the headland 

 itself. Features to look for are wave cut cliffs and on which side of 

 the headland they are located, usually marking the downdrift shore, and the 

 existence of relatively wide stable beaches commonly found on the updrift 

 shore, 



7^0 Evidence At Tidal Inlets . - The location and formation of tidal 

 inlets may also be indicative of the direction of littoral drift. In 

 general, over a long period, such inlets tend to migrate in the direction 

 in which littoral drift is moving. Brief reversals, associated with the 

 shifting of the bar channel, are often observed. Natural closure and 

 break-through at an updrift location may confuse the evidence. Figures 

 ii8 and k9 show typical inlet formations and the manner in which they in- 

 dicate generally the direction of littoral drift. 



760 Wave Analysis. - A complete knowledge of the directional com- 

 ponents of wave energy acting upon the littoral zone will permit deduction 

 of the direction and energy value of the longshore component of the primary 

 force responsible for littoral drift, i^'o satisfactory instrument for 

 measuring wave direction has yet been devised, thus there are no continuing 

 instrumental records which can be used for this purpose. Two methods 

 have been employed for developing statistical wave data, the first being 

 reports over a long period by ships at sea, compiled by the U.S, Navy 

 Hydrographic Office and published as "Sea and Swell Charts." The second 

 method, applied first by Scripps Institution of Oceanography on the 

 California coast and later by the Beach Erosion Board for the Great Lakes 

 and a portion of the north Atlantic coast, involves applying wave fore- 

 cast techniques to produce wave statistics from historical synoptic weather 

 maps, (See paragraph 13 to 21 on Wave Action). Both methods provide 

 statistical data on wave heights and directions in deep water. The latter 

 method also provides wave periods associated with height and direction, 

 enabling energy evaluation, 



77, When other reliable evidence as to predominant littoral drift 

 direction is lacking, the longshore wave energy component has been 

 employed for that purpose. There are two general methods for making 

 this computation, the first by simple vector force diagrams to determine 

 the resultant force in deep water parallel to the general shore alinement. 

 The second method is a refinement of the first, involving projecting the 

 deep water wave energy to a position near the shore by refraction analysis., 

 and computing the longshore component at that point. For comparatively 

 regular shore alinement and bottom topography, see Figure $0, there will 

 be little difference in results from the two methods. For more complex 

 topography, local inconsistencies are usually found. There is at 

 present no proven technique for employing refraction analysis to determine 

 drift direction, and as results of this method are sometimes in conflict 

 with other more reliable evidence, its use for general practice does not 

 appear warranted. 



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