Characterizing the interaction of mobile sediments with a turbulent boundary layer driven by waves and currents represents an important scientific and engineering challenge. To approach this, Balachandar's scaling relations for particle Reynolds number and Stokes number (IJMF, vol. 35, pg 801-110; 2009) are recast in terms of Shields parameter and particle Galileo number. This allows for the modified Shields diagram to be partitioned into at least five regimes, where distinct primary mechanisms of sediment turbulence interaction are identified. Practical guidelines are provided for selecting an appropriate direct or large-eddy simulation approach in the Shields-Galileo phase space. (C) 2016 The Authors. Published by Elsevier Ltd.