To prevent the secondary scour even when earth and sand in the upper part is carried in a flood without interrupting a river-bed water by molding a substantially square concrete block, when seen from above, of porous water-permeable concrete so as to have mutually opposed edges consisting of a convex surface and a concave surface, and laying this concrete block in the earth of a multiple natural type river terrace.

This concrete block 1 has a square form, when seen form above, and it is molded of porous water permeable concrete so as to have a convex surface 2 and a concave surface 3 consisting of circular arcs having the thickness as the diameter. When a plurality of flat concrete blocks 1 are laid over a flat river terrace 8, a water-permeable connecting sheet 6 is laid on the laying surface of the river terrace 8, and each concrete block 1 arranged adjacently thereon through an adhesive material so that the opposed convex surface 2 and the concave surface 3 are fitted and connected. Thus, since the connecting surfaces can be moved in connection so to adapt a free angle, the concrete blocks 1 can be freely and easily connected.