Methods

Subbottom profiles:   High-resolution acoustic-reflection profiles were collected along tracklines that generally are perpendicular to shore, with some tie lines parallel to shore. We used a tunable, swept-frequency (0.6 to 3 kHz) acoustic profiler specifically designed for Hawaii's sandy carbonate deposits, which have proven difficult to resolve clearly with other acoustic systems (Ericksen et al. 1997).

Vibracore samples:   Sediment samples were collected in Kailua Bay in 1997 with a 2-m maximum length vibracorer belonging to the Marine Minerals Technology Center at the University of Hawaii. The cores were collected in 8.25-cm- inside diameter (3.25-in) aluminum barrels. In 2000, vibracores of 6-m maximum length were collected in Kailua Bay and off the south, leeward, and north coasts with a device owned by TEG Oceanographic Services of Santa Cruz, CA. The cores were collected in 9.52 cm inside-diameter (3.75 in) aluminum barrels.

Grain size analysis:   Grain size was determined for a composite sample from each vibracore, obtained by mixing equal-volume samples from 20-cm intervals, as well as for samples from the top and bottom of each core plus a few samples at intermediate depths. For comparison, grain size was determined for 60 beach samples from around Oahu. For all samples, the gravel-size grains were separated as a unit with a -1.0 phi (2 mm) sieve and weighed, then a laser-diffraction instrument was used to determine the size distribution of the sand-size and finer grains, in 1/2-phi intervals (Agrawal et al., 1991).

Composition analysis:   Organic carbon was removed from sediment sample, which was dried then passed through a sample splitter until a few hundred representative grains were separated for identification under a binocular microscope.

Insoluble residue:   Organic carbon was removed from a weighed sediment sample, which then was repeatedly treated with hydrochloric acid of increasing concentration (10%, 50%, and 100%) at 60°C until digestion ceased. After three additions of distilled water, centrifuging, and decanting, the sample was dried and weighed.

Abrasion tests:   Laboratory tests using a bench-top wrist shaker were run to assess the relative abrasion resistance of samples, particularly to compare the offshore sediment with the beach sediment. The tests consisted of placing 75 ml of distilled water and 5 gm of sediment in a plastic centrifuge bottle and agitating the mixture at 700 cpm with the wrist shaker for a period of 24 hours. Agitation during a test was relatively violent, perhaps simulating grain collisions in the surf zone. Moberly (1968) used a jar mill for abrasion tests, which he stated might represent sliding grain interactions in the surf zone. Comparison of the two methods shows similarity of results (Hampton et al., 2002).

Note:   The subbottom profiles and the vibracore samples are archived at the U.S. Geological Survey, Western Coastal and Marine Geology Team in Menlo Park, CA. (To learn more about the Western Coastal and Marine Geology Team, visit the web site at http://walrus.wr.usgs.gov/.)