The results of running the floodway can be shown in cross section plots, the X-Y-Z perspective plot, or with encroachment tables. Using that surcharge, the modeler updates the “Target WS Change” for the downstream cross sections. After running a few scenarios, the modeler can work from downstream to upstream in selecting the target surcharge that worked the best in resulting in a surcharge of 1.0 ft. It is important to remember that when developing a steady state floodway model, it is generally better to work from downstream to upstream to develop the encroachment stations.Īnother way to increase the efficiency in creating a floodway is to start with multiple profiles in Method 4, where the target surcharge is close to 1.0 ft.
HEC RAS MODEL TRIAL
Method 1 allows the user to adjust the stations of the encroachments, but requires some trial and error iterations. After creating a “rough” floodway with Method 4, the modeler can switch to Method 1, which allows the user to fine-tune encroachment stations. The modeler specifies a target increase in water surface elevation (surcharge), and the encroachments are determined so that an equal loss of conveyance occurs on each bank.
![hec ras model hec ras model](https://www.jbaconsulting.com/wp-content/uploads/2018/06/HEC-RAS-particle-tracker-function-Mekong.jpg)
With Method 4, HEC-RAS computes the encroachment stations by keeping the conveyance of the encroached cross section equal to the conveyance of the original cross section. 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 MODEL 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.).
![hec ras model hec ras model](https://www.hec.usace.army.mil/confluence/rasdocs/rasum/files/latest/39298843/39298845/1/1605806433277/worddav610201a76fc925ae35a8a5a2e3e0cdf4.png)
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.
![hec ras model hec ras model](https://html.scirp.org/file/7-9403674x12.png)
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).
HEC RAS MODEL SERIES
Be sure to check back for subsequent series articles. This article is the first in our blog series on HEC-RAS modeling.