Drifting chunks of ice can get caught near them, clogging up the flow of water forming a dam.
漂流的大块冰可以在它们附近阻塞水流形成一个大坝。
英语百科
Shear forming
(重定向自Flow Forming)
Shear forming, also referred as shear spinning, is similar to metal spinning. In shear spinning the area of the final piece is approximately equal to that of the flat sheet metal blank. The wall thickness is maintained by controlling the gap between the roller and the mandrel. In shear forming a reduction of the wall thickness occurs.