The evaluation of the impact of floodplain encroachments on water surface profiles can be of substantial interest to planners, land developers, and engineers. It is also a significant aspect of flood insurance studies. HECRAS contains five optional methods for specifying floodplain encroachments within a steady flow analysis. This chapter describes the computational details of each of the five encroachment methods, as well as special considerations for encroachments at bridges, culverts, and multiple openings. Discussions are also provided on a general modeling approach for performing an encroachment analysis.
For information on how to enter encroachment data, how to perform the encroachment calculations, and viewing encroachment results, see "Modeling Gated Spillways, Weirs and Drop Structures" of the HEC-RAS user's manual.
The HEC-RAS floodway procedure for steady flow analyses is based on calculating a natural profile (existing conditions geometry) as the first profile in a multiple profile run. Other profiles in a run are calculated using various encroachment options, as desired. Before performing an encroachment analysis, the user should have developed a model of the existing river system. This model should be calibrated to the fullest extent that is possible. Verification that the model is adequately modeling the river system is an extremely important step before attempting to perform an encroachment analysis.