Stability of Armor Units for a Leeward Mound of Composite Breakwaters

Abstract

: The armor units on the rubble mound foundation of a composite breakwater should be of sufficient mass to ensure stability against storm waves. Toward this, the stability mass of seaward units has been calculated by an equation developed from hydraulic model tests and the mass of leeward units has been designed empirically as half mass of seaward units. This study uses numerical simulation to show that large eddies generated by wave overtopping results in a rapid caissonward flow above the leeward mound crown and that the velocity of this flow is a very important factor in the stability of armor units. Hydraulic model tests were used to formulate a function of wave conditions and structural geometry. From the balance of forces acting on an armor unit, the stability number NS in the Hudson formula was formulated to allow design of armor units on the leeward mound. INTRODUCTION The rubble mound foundation of a composite breakwater is covered with armor units of concrete or stone whose mass is sufficient to prevent wave action from displacing rubble stones. Tanimoto et al. (1982) and Takahashi et al. (1990) developed a calculation method for stable mass of armor unit using hydraulic model tests. The mass for leeward armor units has been set empirically at half that for seaward units. However, as there are few examples of damage to leeward armor units, it might 1 Port Planning Section, Hokkaido Regional Development Bureau, Ministry of Land Infrastructure and Transport, Kita 8 Nishi 2, Kita-ku, 060-8511 JAPAN 2 Civil Engineering Research Institute, Independent Administrative Institution, 1-3 Hiragishi Sapporo, 062-0931 JAPAN 3 Division of Environmental Engineering, Muroran Institute of Technology,27-1,Mizumoto-cho, Muroran, 050-8585 JAPAN 4 Division of Environmental Resources Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, 0608628 JAPAN