Rapid Floodplain Delineation

A command-line program was created that enables a backwater model to be calculated directly from a set of GIS cross-sections. The program furthermore generates a GIS representation of the floodplain, a simple set of elevation plots of the cross-sections and water surface in PDF format, and a set of GIS cross-sections with the calculated hydraulic parameters attached as attributes. The program does not fully provide the functionality of a hydraulic model such as HEC-RAS. However, it can generate a reasonably accurate floodplain very quickly, typically in about ten seconds once the input files (centerline and topography) have been created. Cross-sections can be added, moved or realigned in GIS and the script will ascertain the sequence of the cross-sections and the reach lengths. The program is also robust and will almost always generate a floodplain. An optional feature also generates a shapefile of evenly spaced cross-sections based on the stream centerline using either of two algorithms, although some cross-sections may be later adjusted manually. The rapid delineation of the floodplain using this technique is useful for the approximate delineation of floodplains and fulfills all requirements for FEMA submittal as an approximate study. Although many simplifying assumptions are made, the script computes a backwater analysis (equivalent to the standard-step approach), and therefore is more accurate than any delineation based on normal depth approximations. Furthermore, the program is useful in rapidly obtaining reasonable alignments of the stream centerline and cross-sections and an accurate estimate of the floodplain limits even when a detailed hydraulic model will be created later.