It started road constructions around river in 1990s. These maintenances concentrate on cityriver. Because river lives no living things and men don’t come near there. But in spite of theseriver environment go to rack, river maintenances still keep on using preexistence method since1990s. Only a part of city river environment maintenances consider environmental ability ofpassive river, river maintenance of a purpose of flood control still don’t consider in theconcrete. Because propulsion device that consider environment ability of passive river andpossible application techniques don ’t complete.In accordance, A natural river maintenance needs absolutly a series of river projects. Becausea natural river maintenance prevents a damage of environment ability.This study is to assume the flood really happened and to carry out the flood damagesimulation needed in overflow simulation about the inundated zone. Also, This study examineunstable part about the hydraulic characteristic as velocities, stream power, shear, hydraulicdepth, flow area in basin. And this study applied the HEC-RAS(river analysis system) model topredict flood overflow in youngsan river basin. Project flood is used the return period 100 yearand inputed data that was calculated in intensity figures of illumination.