Bottom step enlarging horizontal momentum flux of dam break flow

Dam break flow over the horizontal bed with a bottom step is solved using both computational fluid dynamics and shallow water equation models. The bottom step is a scenario related to greenwater overtopping of offshore structures, in which a sharp variation of water depth exists from the location in front of the ship to the position on deck. The effect of bottom step level on the horizontal momentum flux of dam break flow, representative of its impact loading on plate-type structures, is investigated. Both these models predict a trend that the increase of bottom step level leads to an increase of the maximum horizontal momentum flux, being larger than that of the classical dam break solution. The increasing rate is large for a small bottom step level but becomes less sensitive for a large bottom step level. This trend can be attributed to the fact that the rarefaction wave associated with draw down of water upstream of the dam travels faster with a bottom step.