Abstract: There are many cascade hydropower dams on the main rivers in China. If cascade dam break happens, the peak flood discharge will be greatly enhanced as compared to the single dam break flood. Therefore, it is important to study the mechanism of peak discharge enhancement of cascade dam break floods. In this study, we developed a well-balanced numerical solver for the one-dimensional shallow water flow model with shock and wetting-drying capturing ability, to study the transformation mechanism of mass and energy in cascade dam break flood. A series of simulations with different combinations of upstream and downstream reservoir capacities imply that a jet-water tower mechanism greatly enhances the peak discharge of cascade dam break flood. The upstream dam break flood adds much mass and momentum into the downstream reservoir, and a tailing jet with large momentum behind replenishes additional mass and momentum continuously into the downstream reservoir during the downstream dam breaks. Based on the jet-water tower mechanism, we propose a novel simplified single dam break flood model with jet and water column for cascade dam break flood. Several simulations show that the new model is able to reproduce almost equivalent peak depth and discharge for small-scale cascade dam break floods. Applications for real-scale cascade dam break flow with dam distance of hundreds of kilometers reveal that the equivalent single dam break model could predict the peak water depth and discharge in cascade dam break flood within acceptable errors, which may provide scientific foundations for flood control and design of cascade dams.