Effects of Flood Plain Encroachments on Peak Flow,

When land development encroachments into river flood plains are permitted, the magnitude of the flood peak discharge will be increased due to removal of flood plain storage. When the flood plain encroachment is limited (for example, when no more than one foot of water surface rise is permitted), the study results indicate that the increase in the flood peak is usually relatively small; it is generally less than 10 percent. The arrival time of the flood peak can be affected to a much greater extent, and in stream systems with significant tributary flows, consideration must be given to the effects of encroachments on the coinciding of the flood peaks. Various hydrograph shapes and mean river slopes were used in the investigation of encroachment effects. Both unsteady flow simulations and steady flow profile calculations with hydrologic routing were used in the analyses. (Author)