In hydraulics area there is an open space for apply of numerical methods, instead of frequently used empirical formulas. At first place numerical methods may be used in river flow hydraulics and ground water flow. Application of numerical methods is given enough correct results and prediction for all parameters in river flow, that is presented in this paper. Natural phenomenon of flood wave is researched and original numerical method is shown, having calculation of water depth as primary goal. There are different amount of flooding in river in correspondence with event frequency which are monitored through numerical calculation and measuring. Numerical calculation of water depth in huge amount of flooding is important for defined defense areas. In this paper is researched flood wave in river Velika Morava, propagated from city Naissus to measuring station Bagrdan, continuing to city Smederevo where is merged with Danube. Flooding is unsteady flow with dominant part of kinetic energy. Propagation of flood wave is analyzed as plane problem and through numerical procedure are calculated values of water depth. Calculation results of propagated flood wave is treated with measured data water depth H and discharge Q. Presented calculated and measured data are compared and they have small differences, therefor results are acceptable as correct. Whole volume of flooded water in river is analyzed for calculation results correction and average cross section of flow is got.