Venturi flumes are measurement structures commonly used in water systems to measure the flow discharge. Some of them are not well installed or present some geometrical defects. The objective of this study is to investigate the hydraulic influence of a number of typical wrong installations and geometrical defects of long-throated Venturi flumes: significant positive or adverse slopes, humps and hollows on the walls of the throat, hump or hollow on the bed of the throat. The geometric tolerances corresponding to an acceptable tolerance on the discharge of 2% and 5% are calculated for each defect. A number of corrections of the head –discharge relationship are proposed to avoid the destruction of a flume if the geometric tolerance is not respected.