Littoral Sedimentary Processes— Inman, Gayman and Cox 
125 
flat appear to play a unique role in the cycle of 
sand transportation over the reef. They function 
effectively as traps for sand that might other- 
wise be transported laterally along the reef by 
water currents and, at the same time, act as open 
conduits for the onshore transportation of sand 
by wave action. 
Once on the beach, the sand probably under- 
goes a slow net southerly drift under the com- 
bined action of currents and waves impinging 
on the beach face. After migrating to the inlet, 
the sand is carried out to deeper water by the 
relatively strong currents that flow seaward 
through the inlet. 
Thus, the transportation cycle of sand on the 
reef appears to- include: (a) migration of sand 
along the channels in the reef flat, which is in- 
duced by wave action; (b) southerly transport 
of sand along the beach face and at the toe of the 
beach by waves and currents; and (c) seaward 
transport of sand through the inlet and into 
deeper water by the currents which converge 
from the north and south reef and flow seaward. 
The sand on the beaches apparently is derived 
from organisms living on the reef, and from ma- 
terial transported in suspension by waves break- 
ing over the reef. Diving observations indicated 
that the numerous solution cavities and the gen- 
eral rough outer edges of the reef cause con- 
siderable turbulence in the breaking and near- 
breaking waves. This turbulence increases their 
capacity for carrying material in suspension. Un- 
doubtedly some reef material also undergoes a 
net offshore migration by processes of turbulent 
diffusion. Whether this loss is greater or less 
than the supply of material to the reef from off- 
shore is not known. 
The position and growth of the reef, in recent 
geologic time, appears to be governed by the 
combined effects of land runoff and the water 
circulation and sand transportation over the reef 
system. Inlets tend to occur in the fringing reefs 
where streams enter and bring fresh water and 
mud, which are detrimental to the reef building 
corals and algae. The reef builders flourish where 
wave action is most vigorous. Therefore, the reef 
platform becomes more extensive in the areas 
not influenced by land runoff. The formation of 
a wider reef platform causes more and more 
water and sand to cross the reef and flow into 
the inlets. The scouring action on the inlets by 
water and sand derived from the reef maintains 
the inlets even when rivers are not flowing. Thus 
it would appear that once the position of a po- 
tential inlet is determined by land runoff and 
coastal configuration, the pattern of water cir- 
culation and sand transportation on the growing 
reef will tend to perpetuate the location of the 
inlet. 
Kapaa Sediments 
Samples collected from the beach, reef flat, 
and sea floor outside the reef all consisted of very 
coarse, coarse, and medium-grain sands, contain- 
ing about 9'0% calcium carbonate of biogenous 
origin. The following trends in the sediment 
were observed from north to south along the 
north Kapaa reef flat: (1) the sand becomes 
finer, (2) the sands become better sorted, (3) 
the magnesium content of the cal cite decreases, 
and (4) the foraminifera content in the sand 
decreases. In general, the reef flat and offshore 
samples were coarser than the beach samples. 
These trends, although not necessarily definitive 
in themselves, are in agreement with the modes 
and cycles of sediment transport as discussed pre- 
viously. The samples from the Kapaa area con- 
tained a higher percentage of magnesium-poor 
cal cite than any of the other unconsolidated sed- 
iment samples collected from Kauai. In general, 
the content of magnesium-poor calcite increases 
southward on Kapaa beach and seaward across 
the reef. The distribution of magnesium-poor 
calcite is consistent with the supposition that the 
supply of beach sand is predominantly from 
transportation over the reef opposite the beach, 
rather than from transportation along the shore. 
The only completely identifiable biogenous 
components present in significant quantities in 
sands were the foraminifera Heterostegina sub- 
orbicularis and Amphistegina madagascariensis, 
A rough estimate indicated that these species 
made up 3% to 20% of the grains in the sam- 
ples. All of the tests were considerably worn, 
making it quite difficult to separate the two 
species under a microscope. A more accurate de- 
termination of their relative abundance, based 
on their skeletal composition, was obtained by 
X-ray diffraction. In sample 59, about 55% of 
the foraminiferal carbonate consisted of magne- 
sium-rich calcite, indicating deposition by Het- 
