Because of the lack of an accepted standard settling tube, rapidly 

 changing technology, possible changes in tube calibration, and the uncertainty 

 about fluid mechanics in settling tubes, it is recommended that all settling 

 tubes be carefully calibrated by running a range of samples through both the 

 settling tube and ASTM standard sieves. After thorough initial calibration, 

 the calibration should be spot-checked periodically by running replicate sand 

 samples of known size distribution through the tube. 



III. LITTORAL WAVE CONDITIONS 



1. Effect of Wave Conditions on Sediment Transport . 



Waves arriving at the shore are the primary cause of sediment transport in 

 the littoral zone. Higher waves break farther offshore, widening the surf 

 zone and setting more sand in motion. Changes in wave period or height cause 

 sand to move onshore or offshore. The angle between the crest of the breaking 

 wave and the shoreline determines the direction of the longshore component of 

 water motion in the surf zone and, usually, the longshore transport 

 direction. For these reasons, knowledge about the wave climate — the combined 

 distribution of height, period, and direction through the seasons — is required 

 for an adequate understanding of the littoral processes of any specific area. 



2. Factors Determining Littoral Wave Climate . 



The wave climate at a shoreline depends on the offshore wave climate, 

 caused by prevailing winds and storms and on the bottom topography that 

 modifies the waves as they travel shoreward. 



a. Offshore Wave Climate . Wave climate is the temporal distribution of 

 wave conditions averaged over the years. A wave condition is the particular 

 combination of wave heights, wave periods, and wave directions at a given 

 time. A specific wave condition offshore is the result of local winds blowing 

 at the time of the observation and the recent history of winds in the more 

 distant parts of the same waterbody. For local winds, wave conditions off- 

 shore depend on the wind velocity, duration, and fetch. For waves reaching an 

 observation point from distant parts of the sea, wave height is reduced and 

 wave period is increased with increasing travel distance. Waves generated by 

 local winds have short crest lengths in a direction perpendicular to the 

 forward wave velocity and a wide directional spread of energy. Waves arriving 

 from distant parts of the sea are characterized by long crests and a narrow 

 directional spread of energy. (Wave generation and decay are discussed in 

 Chapter 3.) Offshore vave climate varies among different coastal areas 

 because of differences in exposure to waves generated in distant parts of the 

 sea and because of systematic differences in wind patterns around the Earth. 

 The variations in offshore wave climate affect the amount of littoral wave 

 energy availably and the directions from which it comes. 



b. Effect of Bottom Topography . As storm waves travel from deep water 

 into shallow water, they generally lose energy even before breaking (Vincent, 

 1981). They also change height and direction in most cases. The changes may 

 be attributed to refraction, shoaling, bottom friction, percolation, and 

 nonlinear deformation of the wave profile. 



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