71. Determination of Direction of Littoral Drift . - It is not only 

 necesvsary to know the direction of littoral drift at any one time -- 

 which can generally be determined by observation of existing structures — 

 but the predominant direction of littoral drift over a normal climatic 

 cycle must be established. This may also involve locating the position 



of natural and unnatural littoral barriers and those areas called nodal 

 zones in which the net littoral transport changes direction. In these 

 zones the net littoral drift is zero or, in other words, the downdrift 

 components of littoral drift are equal to the updrift components. Although 

 the methods used in determining the direction of littoral drift may vary 

 in each section of the country, deterraination of the instantaneous and 

 predominant directions of littoral drift and the location of littoral 

 barriers and nodal zones may ordinarily be accomplished by analysis of 

 such of the following factors as may be required to reach conclusions: 



Accretion or erosion effects of existing structures; 



Shore patterns in the vicinity of headlands; 



The configuration of the banks and beds of inlets; 



Statistical analysis of wave energy; 



Characteristics of beach and bed materials; 



Ciirrent measui-ements, (in the vicinity of inlets). 



72. Shore Effects of Existing Structures . - This provides the most 

 reliable means of determining littoral characteristics, and will ordinarily 

 outweigh all other evidence. The use of existing structures to determine 

 the direction of littoral drift is illustrated in the series of sketches. 

 Figures i|0 to ii5 inclusive. Considering the evidence presented by groins. 

 Figures kO ^nd ij.1, the condition, of the beach at the time of inspection 

 probably indicates the direction of drift during the immediately preceding 

 period. To determine the predominant direction of littoral drift requires 

 observations at regular intervals over a period of at least one year to 

 avoid misinterpretation due to seasonal effect. 



73. Considering the evidence presented by breakwaters and entrance 

 jetties. Figures lj.2, l43j and UU, the quantities involved are generally 

 large enough so that the condition observed at any one particular time 

 probably is indicative of the predominant direction of littoral drift, 

 with incremental changes showing the short term direction variability. 



7ij.o Evidence of Headlands o - The evidence presented by headlands 

 as to the direction of littoral drift is not ordinarily as clear as that 

 presented by structures because of the frequency of rocky shores on both 

 sides of headlands. In vSome instances the headland is so oriented as to 

 cause a reversal of direction of littoral drift under all wave conditions, 

 thus compartmenting the coast line. Figures I46 and ii7 show two types 



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