“…Several studies at the NS level of refinement advocate the use of the "partial slip" (Navier, 1827) condition or related formulations in which the near-bed slip velocity is either proportional to the shear stress (Jäger and Mikelic, 2001;Basson and Gerard-Varet, 2008) or depends on it in a non-linear way (Achdou et al, 1998;Jäger and Mikelic, 2003). Other works plead for "no-slip" conditions (Panton, 1984;CasadoDiaz et al, 2003;Myers, 2003;Bucur et al, 2008Bucur et al, , 2010 or suggest the separation of flow domains within or outside bed asperities, with a complete slip condition (non-zero tangential velocity) at the interface (Gerard-Varet and Masmoudi, 2010). A wider consensus exists at the RANS level, calculating bottom friction as the local grain-scale values of the "Reynolds stresses" (Kline et al, 1967;Nezu and Nekagawa, 1993;Keshavarzy and Ball, 1997), which has proven especially relevant for flows in small streams over large asperities (Lawless and Robert, 2001;Nikora et al, 2001;Schmeeckle et al, 2007).…”