National Institute for Land and Infrastructure Management, Ministry of Land, Infrastructure and Transport Japanese Page
River Department

Research themes of the Coast Division
[5] Technological support

1. Revising the Guideline for Designing Artificial Reefs

(1) Introduction

Consideration of the environment and usage of coasts has become increasingly important when constructing coastal conservation facilities, and the revised Seacoast Law enforced in 2000 stipulated that the purpose of coastal conservation is the harmonious balance among protection, the environment, and usage.

Regarding the environment and usage when constructing coastal conservation facilities, it has been pointed out that detached breakwaters and wave-absorbing dykes degrade the landscape. Artificial reefs have therefore been constructed to absorb waves without affecting the scenic beauty of coasts. Artificial reefs are coastal conservation facilities installed offshore for mitigating the height and the overtopping volume of tidal waves and stabilizing the shoreline for a long time. They were installed for the first time from 1983 to 1984 at Keinomatsubara Beach, Hyogo Prefecture, because they would not affect the landscape as their crests lie underwater. According to the results of a questionnaire survey of coastal managers around the nation in fiscal 2001, a number of artificial reefs have been built to control waves and drifting sand, including 130 artificial reefs either completed or being planned within the past 5 years.

Artificial reefs are designed based on the Artificial Reef Design Manual (issued by the Seacoast Division, River Bureau, Ministry of Construction (at the time) in 1992, hereafter called the "Manual"). NILIM revised the Manual at the end of fiscal 2003 to reflect new knowledge about artificial reefs accumulated during the intervening decade, and because consideration of the environment and usage had become increasingly important, and to meet calls for stipulating performances in the standards.

(2) Addition of new knowledge

When the Manual was first published, model experiments were generally conducted using two-dimensional water channels. Accordingly, when designing artificial reefs, model experiments focused on validating wave height transfer coefficients and wave resistance performances at the onshore-offshore two-dimensional cross section passing through the center of the artificial reef body. Nevertheless, as actual artificial reefs have a finite length in the coastal direction, the results of evaluating the lateral stability against oblique waves and waves passing through the opening section were not necessarily applicable to the end sections of the artificial reefs. Accordingly, the revised Manual explicitly states that the behavior of waves and drift sand due to the installation of artificial reefs should be examined three-dimensionally. In addition, a method for checking the performance of drift sand control is proposed in the revised Manual. Furthermore, in view of the progress of artificial reef installation and the accumulation of data on the effects of artificial reefs obtained by on-site observations, the changes in the ecosystem and the effects of the artificial reefs on coastal usage are described in the revised Manual in addition to the effectiveness of wave and drift sand control, which is the main purpose of artificial reefs.

(3) Consideration of the environment and usage

In line with the revision of the Seacoast Law, the necessity of striking a harmonious balance among protection, the environment, and usage of coasts was added in the revised Manual, which was not stipulated in the first Manual edition. Regarding the properties of artificial reefs, the first Manual stipulated only the advantage that artificial reefs would not adversely affect the landscape, but the revised Manual explicitly states that artificial reefs of large-scale structures will inevitably affect the seabed topography and the ecosystem. Furthermore, the revised Manual clearly stipulates the requirements when designing artificial reefs, i.e., proper understanding of the growth and habitats of fauna and flora in the coastal areas, thorough examination of the impact of artificial reef construction, and consideration of the harmony with the environment, together with the points to be noted to achieve them.

(4) Stipulation of performance

The first Manual edition gave only general specifications such as "natural stones or concrete blocks should be used" or "the structure should be without surface projections." In the revised Manual, however, the performances are given as specifically as possible in the main text with appropriate headings to facilitate searches from the Table of Contents.

The performances were decided with reference to the ICC Performance Code; the contents of "Purposes," "Functions," "Required Performances," and "Checking Items" of artificial reefs were explicitly defined; and "Checking Methods" were provided. Finally, the contents of "Structural Details," were revised under the heading of "Important Reminders regarding Structures" for ease of understanding when designing artificial reefs.