Reef breakwater

   General introduction

The calculation rule computes the wave transmission of the significant wave over/through a "reef" breakwater, a conventional breakwater, a highly permeable breakwater, a vertical breakwater, an impermeable dam, vegetation or a row of piles. A "reef"-breakwater consists of a homogeneous pile of stones and has a low crest. A conventional breakwater is a layered structure (fine grading in the core overlain by several layers of coarse stone). In practice, a highly permeable breakwater will permit water only to pass though the structure and not over the top (no overtopping). A vertical breakwater consists of a caisson on a permeable foundation of stones. Transmission occurs as a result of overtopping, wave penetration under the caisson and movements of the caisson. Wave transmission through vegetation can be determined for rushes (scirpus sp./ scirpus lacustris) and for reeds, if the stalks extend above the surface of the water. With impermeable dams wave transmission occurs only via overtopping. Finally transmission through a row of piles depends on the distance between the piles, the size of the splits between the piles and the shape of the piles (round or square).

In general:

     (equation 1)

   Reef breakwater

if 1 < Hs/Dn50 < 6 and 0.01 < Hs/L0p < 0.05 then:

     (equation 2)

With 0.15 < KT < 0.6

Where:

• Hs = significant wave height incoming wave [m]
• Dn50 = nominal size of stones (edge of a cube equal in volume to the stone with a mass M50 [m]
• hc = distance between the upper side of the breakwater and the surface of the water [m]
• HsT = wave height of the transmitted wave [m]
• KT = transmission coefficient [-]
• L0p = wave length in deep water = (gTp2)/(2ð) [m]