Characteristics and hydrodynamics of boulders transported by storm waves at Kudaka Island, Japan

Abstract

This study investigates the size, position and the long axis orientation of 210 boulders at Kudaka Island, Japan. These boulders were deposited from the reef crest to the slope of the back reef moat, distributed within 275 m from the reef edge. Most boulders were rectangular to ellipsoidal, without sharp broken edges. They are reef rock fragments estimated as < 63 m 3 (< 127 t). The second largest boulder (54 t) was not observed in aerial photographs taken in 1977 and 1993, although it appears in photographs taken in 2005 and 2007. Considering that no large tsunami event occurred during 1993–2005, the second largest boulder is expected to have been emplaced by typhoon-generated storm waves. Moreover, the positions of many boulders were found to have shifted after 1977. These boulders were highly likely to have been repositioned by the storm waves. Results showed that boulders' motion follows an exponential fining trend shoreward. This trend fits well with the distribution of the height of the storm wave after breaking on the reef flat. The largest storm waves after 1977 (typhoon 0704 in 2007) were probably responsible for the current boulder distribution. Using the relation between the distributions of boulders and the significant wave height of typhoon 0704, the approximate transport distance of boulders by an arbitrary storm wave at the island can be estimated. The storm wave boulders' distribution is also useful to estimate the storm wave properties: we estimated the maximum current velocity distribution of waves generated by typhoon 0704 on the reef flat as up to 6.5 m/s using the boulder distribution.