Shoaling of Kaneohe Bay, Oahu, Hawaii, in the period 1927 to 1976, based on bathymetric, sedimentological and geophysical studies

Abstract

A comparison of a 1976 bathymetric survey of Kaneohe Bay, Oahu, Hawaii, with that of a 1927 survey indicates that there has been an average shoaling of the lagoonal area by 3.3 feet. Shoaling for the north and middle bay at 1.4 ft/49 years is at a considerably lower rate than for the south bay at 5.2 ft/49 years. Sedimentological studies, based on dredging history and detailed chemical, mineralogical and grain-size analyses of 251 bottom samples, suggest that the total volume of fill into the bay during the 49 year period is approximately 25,500,000 cubic yards. Of this amount, approximately 63% is carbonate detritus from the barrier, fringing, and patch reefs as well as by the living corals, 11% is dredging spoils, and the remaining 27% is attributed to terrigenous detritus. The terrigenous fraction, contributed primarily by stream loading and sheet-wash, brings approximately 69,300 tons of sediment to the lagoon per year. Seismic reflection profiles, which span the length of the bay and include additional lines in the south bay, show that most of the infilling material is unconsolidated sediment trapped between and burying coralline structures built during ancient, lower stands of the sea. All information suggests that shoaling rates within the bay have increased since 1927 due to higher stream loads from increased urbanization as well as to extensive dredging and disposal operations. This is particularly evident in the area of the south bay.