Littoral Sedimentary Processes on Kauai,: A Subtropical High Island"

Beach and shallow water sand samples from the island of Kauai, Hawaiian Islands, were studied to ascertain the effects of climate 'on the suppl y 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 comp onents: 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 playa 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 varia tion in rainfall results in a climatic range from trop ic-humid to semi-arid. Rivers draining the wet, windward port ion 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 detail ed 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 separate 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 voIeanic material in the beach sands decreases logarithmically w'irh distance along the coast from the rivers. . K AVAI is the next to westernmost of the win d-ward islands of the Hawaiian Arch ipelago ( Fig . 1) . It is roughly circular in shape, with a diameter of about 25 nautical miles ( Fig. 2) . Th e island is volcanic and has a central plateau whose eastern rim exceeds 5,000 'f t in elevation. This mile-high rim intercepts the trade winds, causing moderat e to extreme precipitation on the windward slope and producing a rainshadow desert along portions of the leeward coast. 1 Published as Contribution No. 24 of the H awaii Institute of Geophysics and, with the app roval 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 versit y of Californ ia, La Jolla, Californi a. 3 H awaii In stitute of Geoph ysics, Universit y of Ha waii ; formerly Exper iment Station, H awaiian Sugar Planters' Association. 106 Th e contrast between windward and leeward sides of the isl and in terms of climate and source of sediment suggested that this island was an ideal locality to study the relation between nearshore sedimentation and the source and type of sediment. It was found that the windward streams, although high in runoff, contributed very littl e sand-size material to the nearshore areas. On this side, the coastal environm ent is characterized by fringing coral reefs, with protected beaches composed of calcareous sand der ived from shells, reef detritus, and foraminifera. Kapaa Reef had been investi gated by Kohn and H elfrich (1 957) and, since it seemed typical of the windward reefs, it was selected for more detailed study. Consistently high concentrations of terrigenous materi al were found on only a few of the leeward beaches. These deposits were particularly conspicuous near the mouth of the WaiLittoral Sedimentary Processes-INMAN, GAYMAN and Cox 107