© 2020 The Royal Society of New Zealand. Existing hydrodynamic flow transport equations for coastal boulder transport are useful for estimating post-event the characteristics of extreme storm waves and tsunamis. However, the effect of suspended sediment concentration (SSC) on seawater density is normally ignored. This is unrealistic given that turbulent runup flows easily entrain available fine sediment. Proper consideration of SSC can be encouraged by including a mixed-fluid density coefficient (Cρ) as a multiplier for clear-seawater density, where elevated sediment content can be assumed and estimated. Minimum flow velocities required for boulder transport are shown to reduce as sediment concentrations increase.