We present and apply a new discharge monitoring system that uses H-ADCP measurements and numerical simulations to flood-discharge monitoring in a compound channel. In the numerical simulation, we introduce a numerical procedure for data assimilation using an additional term Fa, called the Dynamic Interpolation and EXtrapolation (DIEX) method. This method was applied to flood-discharge monitoring in the middle reach of the Edogawa River. Furthermore an index-velocity method, one of conventional methods for river-discharge monitoring, was also used to evaluate river discharge under flood conditions. The results indicate that DIEX method perform good for discharge monitoring under large flood conditions, showing the applicability of the present method into flood-discharge monitoring in a compound channel.