There are five encroachment methods within HEC-RAS, but using Method 4 initially and then switching to Method 1 can be the most efficient in our experience.
#Hec ras what file are boundary conditions in plus
For the boundary conditions, all floodway profiles should start with a downstream boundary condition equivalent to the starting water surface elevation from the unencroached profile plus the target surcharge (up to 1.0 ft.). More than two profiles may be used depending on if the modeler chooses to run several floodway runs at one time, but all profiles should have the same flows. The flow data file should include at least two profiles – the base flood (100-year) and the floodway. Starting this process includes creating a floodway flow data file and setting boundary conditions. After the HEC-RAS geometry has been finalized and the 100-year storm event has been executed, the modeler can then start the process of creating a floodway. The first step to develop a floodway is to establish the 100-year (or base flood) using USACE’s HEC-RAS software. If a floodway exists for an area, it is typically shown on a FIRM (Flood Insurance Rate Map) panel as a hatched area, as shown in the figure below. If there is an updated model for the floodway, encroachment stations may not need to be changed however, if the project involves drastic changes to the existing model or it is a completely new model for the area, developing a floodway may be necessary. Keep in mind that there may already be a regulatory floodway for the study area. FEMA defines the regulatory floodway as “the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a desired height.” Local agencies and municipalities have the option to specify a smaller surcharge, but they can go no larger than FEMA’s limit of 1.0 ft. FEMA also does not allow development to occur within the floodway, which has a maximum allowable surcharge of 1.0 ft. Communities that participate in the NFIP must prevent excessive development within the floodplain that could cause increases in potential flood damage. The Federal Emergency Management Agency (FEMA) regulates development within the 100-year floodplain by administrating the National Flood Insurance Program (NFIP). I authored this article on the Freese and Nichols' stormwater blog Ebb and Flow.