On momentum and energy fluxes in dam-break waves

Dam-break waves are widely used to study unsteady flows like (flash) floods, tsunamis and storm surges. This technical note advances the classical theory of dry-bed dam-break waves, providing new analytical insights into the momentum and energy fluxes: key quantities governing impact loads and debris motion during extreme events. Building on previous seminal work, explicit expressions are derived for the maxima of momentum and energy fluxes and their occurrence is linked to the propagation characteristics of dam-break waves. Analysis is further extended by incorporating existing models that consider frictional effects in the wave-tip region. This allows the derivation of a transition criterion between the ideal-fluid region and the friction-dominated tip, identifying when friction alters peak fluxes. Comparison with new laboratory data confirms that simplified models capture the dam-break waves’ essential dynamics and results show that friction particularly reduces flux maxima for smaller impoundment depths, with important implications for structural loading, hazard assessment and engineering design.