A 2D streamwise velocity model based on the Reynolds Averaged Navier–Stokes (RANS) is a useful approach to predict the boundary shear stress and the streamwise velocity in a free surface stream where secondary flows are not relevant. Boundary conditions treatment is a key aspect implementing these models. A low computational cost and fully predictive numerical model with a novel treatment of boundary conditions is presented. The main features of the modified model are the employment of a modified law of the wall valid for any roughness condition, the estimation of the boundary shear stress is done only focusing on the near-contour region, the use of a full-predictive physical based model for the eddy viscosity distribution and the incorporation of the free surface shear stress due to water–air interface. The validation of the proposed changes was performed with a substantial number of experimental cases available in the literature using different cross-section shapes (circular, rectangular, trapezoidal and compound section) and roughness condition with quite good agreement. Preliminary results suggest that the influence of the free surface boundary layer has a significant impact on the results for both the streamwise velocity and boundary shear stress in windy conditions. The proposed approach allows its considerations in practical applications.

Boat-mounted Acoustic Doppler Current Profilers (ADCP) are commonly used to measure the streamwise velocity distribution and discharge in rivers and open channels. Generally, the method used to integrate the measurements is the velocity-area method, which consists of a discrete integration of flow velocity over the whole cross-section. The discrete integration is accomplished independently in the vertical and transversal direction without assessing the hydraulic coherence between both dimensions. To address these limitations, a new alternative method for estimating the discharge and its associated uncertainty is here proposed. The new approach uses a validated 2D RANS hydraulic model to numerically compute the streamwise velocity distribution. The hydraulic model is fitted using state estimation (SE) techniques to accurately reproduce the measurement field and hydraulic behaviour of the free-surface stream. The performance of the hydraulic model has been validated with measurements on two different trapezoidal cross-sections in a real channel, even with asymmetric velocity distribution. The proposed method allows extrapolation of measurement information to other points where there are no measurements with a solid and consistent hydraulic basis. The 2D-hydraulic velocity model (2D-HVM) approach discharge values have been proven more accurate than the ones obtained using velocity-area method, thank to the enhanced use of the measurements in addition to the hydraulic behaviour represented by the 2D RANS model.

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