There are numerous types of settling tubes; the most common is the 

 visual accumulation tube (Colby and Christensen, 1956), of which there 

 are also several types. The type now used at CERC (the rapid sediment 

 analyzer or RSA) works in the following way: 



A 3- to 6-gram sample of sand is dropped through a tube filled with 

 distilled water at constant temperature. A pressure sensor near the 

 bottom of the tube senses the added weight of the sediment supported by 

 the column of water above the sensor. As the sediment falls past the 

 sensor, the pressure decreases. The record of pressure versus time is 

 empirically calibrated to give size distribution based on fall velocity. 

 (Zeigler and Gill, 1959.) 



The advantage of settling tube analysis is its speed. With modem 

 settling tubes, average time for size analyses of bulk lots can be about 

 one-fifth the time required by sieves. 



It is often claimed that a settling tube also provides a physically 

 more realistic size analysis than a sieve, since the fall velocity takes 

 into account the hydrodynamic effects of shape and density. However, 

 this claim has not been documented, and may be questioned in view of the 

 limited knowledge concerning the fluid mechanics of a sand sample falling 

 in a settling tube - the lead particles encounter effectively laminar flow, 

 the trailing particles encounter turbulent flow, and all particles inter- 

 act with each other. 



Because of lack of an accepted standard settling tube, rapidly chang- 

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

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

 tling 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. 



4.3 LITTORAL WAVE CONDITIONS 



4.31 EFFECT OF WAVE CONDITIONS ON SEDIMENT TRANSPORT 



Waves arriving at the shore are the primary cause of sediment trans- 

 port in the littoral zone. Higher waves break further offshore, widening 

 the surf zone and setting more sand in motion. Changes in wave period or 

 height result in moving sand 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 

 lon^chore 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. 



4-27 



