Sub-mesh is a critical component of the Flow-3D sediment scour model for the estimation of bed-load transport. The model uses a sub-mesh technique to calculate the bed shear stress and estimate the amount of sediment that is transported as bed load. In this technique, a finer mesh is superimposed on the original mesh to capture the small-scale turbulence and flow structures near the bed surface. The sub-mesh technique works by dividing each cell in the original mesh into a number of smaller cells. The number of sub-cells can vary depending on the resolution required for the simulation. Typically, a sub-mesh with 4-8 times more cells than the original mesh is used for accurate estimation of bed-load transport. The sub-mesh cells are placed near the bed surface where the velocity gradients are high, and turbulence is dominant. These cells are also used to calculate the bed shear stress, which is a critical parameter for estimating sediment transport. The bed shear stress is calculated by solving the Reynolds-averaged Navier-Stokes equations using a turbulence closure model such as k-epsilon or k-omega. Once the bed shear stress is calculated, it is compared with the critical shear stress required to initiate sediment transport. If the bed shear stress exceeds the critical shear stress, then sediment particles are entrained and transported as bed load. The amount of sediment transported as bed load is estimated using empirical relations such as Shields' curve or Meyer-Peter and Muller's formula. The sub-mesh technique has been extensively used in Flow-3D sediment scour models to accurately estimate bed-load transport in various hydraulic structures such as bridge piers, culverts, and spillways. The accuracy of the model depends on several factors such as mesh resolution, turbulence closure model, and empirical relations used for estimating sediment transport. References: 1. Abreu, T., Ferreira, R., and Cardoso, A. (2017). "Numerical modeling of sediment transport in hydraulic structures using Flow-3D." Journal of Hydraulic Engineering, 143(7), 04017023. 2. Zhang, Y., and Cheng, L. (2014). "Numerical simulation of sediment scour around bridge piers using Flow-3D." Journal of Bridge Engineering, 19(6), 04014005. 3. Wu, W., and Liu, H. (2015). "Numerical simulation of sediment transport in a culvert using Flow-3D." Journal of Hydrodynamics, 27(2), 305-312.