This International Standard specifies a method for the estimation of subcritical flow of clear water in smooth, essentially horizontal, straight open channels with a vertical drop and discharging freely. Gentle positive slopes not greater than 1 in 2 000 are admissible. This International Standard covers channels with the following types of cross-section, the nappe being unconfined: a) trapezoidal; b) triangular; c) parabolic; d) circular. Using the measured depth at the end, the flow can be estimated.