Hydraulic behavior and unsteady sediment transport within dam break outburst floods are inherently inter-dependent, yet poorly constrained. This study experimentally determined how the mobility and clast size of channel bed sediments affected the peak flow depth and flow velocities, wave propagation and attenuation, sediment entrainment and duration of transport, flood power, energy transfer and geomorphological impact of outburst floods. Due to our choice of clast size and of a unimodal size distribution the predominant sediment transport mechanism was bedload. We found that frontal wave propagation was slower and attenuated faster for flows over a mobile bed versus those over a fixed bed. Peak flow depth was greater and was attained earlier for flows over a mobile bed and with increased clast size. Geomorphological power declined logarithmically with time after an initial period where gravitational forces exceeded frictional forces. Both peak and time-to-peak geomorphological power varied inversely with clast size. Peak deposition rates were greatest with coarser bed sediment but time to peak deposition rate was inversely proportional to clast size. Volume changes down channel were described by an exponential curve, which we suggest indicates that sediment transport proceeded in waves via both translation and dispersion mechanisms. Peak suspended load occurred coincident with peak flow velocity, peak bedload occurred coincident with peak flow depth, and peak sediment discharge lagged behind peak water discharge. These descriptions of kinematic transformation, multi-phase flow and of geomorphological work due to outburst floods help to provide data for numerical modelling, for mitigating outburst flood hazards and for informing on the immediate and longer-term geological legacy of outburst floods.
# We quantify frontal wave propagation over a mobile bed versus those over a fixed bed.
# We quantify spatial and temporal variations in hydraulics over a mobile bed versus those over a fixed bed.
# We quantify erosion and deposition as controlled by channel substrate character and hydraulics.
# We calculate energy and power expenditure and hence geomorphic work achieved.
# We describe the controls on sediment volume changes down channel and note that it is described by an exponential curve.