Littoral Sedimentary Processes on Kauai, 
A Subtropical High Island 1 
D. L. Inman , 2 W. R. Gayman , 2 and D. C. Cox 3 
ABSTRACT: Beach and shallow water sand samples from the island of Kauai, 
Hawaiian Islands, were studied to ascertain the effects of climate on the supply 
of sediment, and of wave action on the dispersal and transport of sand along the 
shores of this circular island. The littoral sediments of the island are made up of 
two components: biogenous material, such as shell, coral, and foraminiferal sands, 
formed near the shoreline; and terrigenous material, consisting mostly of volcanic 
mineral and rock fragments brought to the beach by rivers. 
The northeast trade winds play a dominant part in the climatic and oceanographic 
processes affecting the island. On the windward side of Kauai, the annual rainfall 
ranges from 30 inches near the coast to over 460 inches at higher elevations; on 
the leeward side, annual rainfall is less than 20 inches. This large variation in 
rainfall results in a climatic range from tropic-humid to semi-arid. Rivers draining 
the wet, windward portion of the island carry relatively little sand, and the shoreline 
on this side is characterized by intermittent fringing reefs and beaches of calcareous 
sand. A detailed study of several reefs on the windward side of the island shows 
that each pair of fringing reefs, divided by a relatively deep inlet, constitute sep- 
arate cells for the circulation of water and distribution of sediments. 
The highest concentrations of volcanic sand occur on the lee side of the island, 
where streams draining the semi-arid region enter the coast. The concentration of 
volcanic material in the beach sands decreases logarithmically with distance along 
the coast from the rivers. 
KAUAI is the next to westernmost of the wind- 
ward islands of the Hawaiian Archipelago ( Fig. 
1). It is roughly circular in shape, with a di- 
ameter of about 25 nautical miles (Fig. 2). 
The island is volcanic and has a central plateau 
whose eastern rim exceeds 5,000 ft in elevation. 
This mile-high rim intercepts the trade winds, 
causing moderate to extreme precipitation on 
the windward slope and producing a rain- 
shadow desert along portions of the leeward 
coast. 
1 Published as Contribution No. 24 of the Hawaii 
Institute of Geophysics and, with the approval of the 
Director, as Paper Number 88 in the Journal Series 
of the Experiment Station, Hawaiian Sugar Planters’ 
Association. Manuscript received November 22, 1961. 
2 Scripps Institution of Oceanography of the Uni- 
versity of California, La Jolla, California. 
3 Hawaii Institute of Geophysics, University of Ha- 
waii; formerly Experiment Station, Hawaiian Sugar 
Planters’ Association. 
The contrast between windward and leeward 
sides of the island in terms of climate and 
source of sediment suggested that this island 
was an ideal locality to study the relation be- 
tween nearshore sedimentation and the source 
and type of sediment. It was found that the 
windward streams, although high in runoff, con- 
tributed very little sand-size material to the 
nearshore areas. On this side, the coastal en- 
vironment is characterized by fringing coral 
reefs, with protected beaches composed of cal- 
careous sand derived from shells, reef detritus, 
and foraminifera. Kapaa Reef had been investi- 
gated by Kohn and Helfrich (1957) and, since 
it seemed typical of the windward reefs, it was 
selected for more detailed study. 
Consistently high concentrations of terrige- 
nous material were found on only a few of the 
leeward beaches. These deposits were particu- 
larly conspicuous near the mouth of the Wai- 
106 
