The purpose of the invention is to eliminate water-permeable sheets and thus to eliminate various bothersome works related to the water-permeable sheets.To achieve the purpose, the surfaces of the chutes 21, 22 are roughened by sandblasting. The chutes 21, 22 are made of metal such as stainless steel, for example. Processed metal surfaces are generally smooth with almost no friction. Moreover, metal has its inherent water repellency. If water is flowed on a bare metal surface, some parts do not get wet while some parts have concentrated surge water flow thereon. However, by sandblasting the surfaces of the chutes 21, 22, adequate friction resistance is generated and inherent water repellency of metal is eliminated.