New Fish Reef Block Ensuring Accurate Placement by Free-Fall Method

Abstract

A new fish reef block ensuring accurate placement by a free fall was developed to enable a more economical construction of nursery reefs for snow crabs. The block is based on a cube-shaped frame structure with extra shelf areas attached to the upper and lower frames to act as stabilizers. The falling behavior of the block in water was investigated by hydraulic model experiments and three-dimensional (3D) numerical computations. A stochastic model based on the experimental data was used to confirm a narrow distribution of the placement position as a result of the swing motion of the block during its fall. The numerical computation results indicate that the upper stabilizer produces a restoring moment for a tilted block and the lower stabilizer reduces instability in the wake of the falling block. In addition, a quick estimation method for the placement position was developed to determine the release point of the block during actual construction in the presence of ambient currents.