Abstract
A pocket beach is known to be a relatively stable system in terms of longshore sediment transport and shows only a seasonal variation (swing) of a shoreline configuration depending on the dominant wave. Kagoshima Prefecture Government Office has conducted studies on an artificial (submerged) reef project in a pocket beach of Nagasakibana Coast since 1996. Coastal structures that affect a longshore sediment transport system are supposed to be constructed from the down-drift side. However, a field study and a theoretical analysis on optimal set-up of artificial reefs in the pocket beach revealed that artificial reef and also detached breakwater should be installed in the middle of the pocket beach first to minimize the maximum shoreline recession at a terminal beach. INTRODUCTION The study area, Nagasakibana Coast, opens to the Pacific Ocean and has been influenced by a number of intense typhoons recently. As a result, significant damage to coastal structure and bluff had occurred during the 1990s. Therefore, a local government office decided to install a number of artificial reefs to dissipate the high wave energy as shown in Photo. 1. As a rule of thumb to coastal engineer, coastal structures that affect the longshore sediment transport are supposed to be constructed from the down-drift side (see, Uda 1997). However annual dominant wave angle could not be defined at the site from engineering point of view, since the dominant wave directions change in each season. Therefore, an alternative guideline for the optimal design of coastal structures in the pocket beach system becomes necessary. 1 Chief of River Section, Civil Engineering Dept., Kagoshhna Prefecture Government Office 2 Assoc. Prof. Dept. of Ocean Civil Engineering, Kagoshuna University, 1-21-40 Kotimoto, Kagoshhna-shi,JAPAN,Fax.+(81)-99-285-8484, Email: scdiment~oce.kago~hima-u.ac.jp 3 Director, River Department, Public Works Research Institute, Asahi-l,Tsukuba, lbaragi, Japan, Email:uda~wri.go.jp
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