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PACIFIC SCIENCE, VoL XVII, January 1963 
Fig. 10. Generalized sketch of surge channel on Kapaa Reef. Note sediment fill and ripples in bottom of 
channel. Sand in channels is moved shoreward by the small waves, which reform from the mass of water 
.surging over the reef as larger waves break on outer reef. Prevailing currents, which flow from north to south 
(to the right of the viewer), are relatively ineffective in transporting sand on the reef flat. 
Investigations indicate that the transportation 
■of sand and the circulation of water over Kapaa 
Reef do not follow the same paths or cycles. 
While the water circulation is principally from 
north to south over the north reef, the east-west 
trending channels, which are common on the 
reef flat, act as traps for the sand, and prevent 
any appreciable southerly transport across the 
reef flat by water currents. The floors of the 
channels are covered with sand and reef rubble, 
in some places to a thickness of several feet. The 
sand is rippled by small waves moving across 
the reef flat from east to west, resulting in a net 
transport of sand toward the beach (Fig. 10). 
These low waves, which move sand across the 
reef flat, are formed from the mass of water 
which surges over the reef. The characteristic 
pattern is for waves of several feet or more in 
height to break on the outer reef, their momen- 
tum carrying a considerable amount of water 
over the reef in the form of a surge of white 
water. Over the reef flat, the surge first forms 
into a single wave crest, which may, in turn, 
disperse into a train of several smaller waves as 
the wave front moves toward the beach. The 
reef waves thus formed have heights in the 
neighborhood of Vl to 1 ft, and wavelengths In 
the order of 20 ft. Water level and shoals on the 
reef control the height of the waves, for waves 
higher than the approximate depth of water tend 
to break and re-form as lower waves. Observa- 
tions made while swimming over the reef indi- 
cate that the small waves are solitary in nature 
and that their particle or orbital velocity is 
greater in the onshore than in the offshore direc- 
tion. This differential between onshore and off- 
shore particle velocity, that accompanies the 
small waves, results in a step by step movement 
of sand toward the beach. 
The east-west trending channels of the reef 
