Modeling flows over gravel beds by a drag force method and a modified S-A turbulence closure

► We developed a numerical model for depth-limited open channel flows over gravels. ► The wall function approach (WFA) is not appropriate for flows over large gravels. ► The drag force method (DFM) is inaccurate for flows over sparsely distributed gravels. ► The DFM incorporating a modified Spalart-Allmaras turbulence closure is proposed. ► The then computed velocity profiles agree well with the corresponding measurements.