An Improved Algorithmic Approach to Iterative Floodway Modeling using HECRAS and GIS

Abstract

Hydrologic Engineering Center's River Analysis System (HEC-RAS) software is commonly used to perform hydraulic analysis for floodplain delineation studies. In addition to floodplains, the hydraulic analysis also includes modeling a floodway in detailed floodplain study areas. Floodway modeling is an iterative process in which the 1% annual chance flood discharge is restricted within a floodway without exceeding a designated increase, called the surcharge (usually 1 foot), in water surface elevation. An engineer models flows along a reach to meet Federal Emergency Management Agency's (FEMA) surcharge requirements.

We present a tightly coupled system comprising of a commercial GIS (ArcGIS) and HECRAS that automates HECRAS's floodway encroachments modeling. The coupled system takes an automated approach, in which an initial floodway is developed by running HEC-RAS in an iterative fashion with minimal user intervention. A customized ArcGIS visual environment has been developed to edit, remodel, spatially analyze and map floodway boundaries. Four different encroachments fine-tuning options are provided which eliminates the need for a modeler to switch between HECRAS and GIS in the floodway modeling process. Thus, the tool increases the productivity of a modeler by cutting down on manual modeling time during floodway iterations and transition between HECRAS and ArcGIS. The transfer of HECRAS model output into the ArcGIS environment facilitates quick and efficient spatial analysis.

The final step in the floodway modeling process is to develop a smooth floodway boundary that can be mapped on a DFIRM. We have developed automated mapping algorithms that accomplish this task. Some manual fine-tuning is required to finalize the floodway to be printed on FEMA's Flood Insurance Rate Maps (FIRMs).