This International Standard specifies methods for the determination of discharge in open channels in steady flow conditions using vertical underflow gates on a flat horizontal floor between vertical side walls under modular or non-modular conditions. NOTE Generally, gate controls are not designed with discharge determination as a primary function and this International Standard is intended to help the development of stage-discharge relations at existing gate structures, and to incorporate into new structures features which will provide good facilities for the establishment of stage-discharge relations. When a better accuracy of measurement (i.e. better than specified in 9.4) is required, the structure is to be calibrated with actual measurement of discharge using an appropriate method.