This blog will outline how to generate a FEMA regulatory floodway, as well as provide tips for manipulating one if you inherit one with an effective model.
First and foremost, realize the floodway is one of the most crucial things out there. Throughout my career, I have changed positions on my opinion of the usefulness of the floodway. On one hand, it's a great tool that allows development in flood fringe areas without a massive burden. On the other hand, it's a made-up concept, is finicky to set up, and can be manipulated in ways that are not useful to floodplain management. In addition, the NFIP is several billion dollars in debt, we are experiencing more intense and severe rainfall events, and the regulations need to change to accommodate. And all that is ignoring the fact that the more and more prominent 2D modeling is a nightmare to try and generate a floodway. Enough with the background, let's get into it.
FEMA allows for two methods, but you should really only use one for a quick and dirty method. HEC-RAS has 5x methods.
Method 4-Target WS Change
This is where I always start if working from scratch, but have never submitted a model with this method. This is the ultimate blackbox, but can also save you time from the jump. This method tries to move the encroachment station to acheive your target water-surface-elevation increase, i.e. 1.0 feet. I will always start by setting this to 1, and seeing what surcharges I come up with. From there, sometimes I will change sections of the stream to try and get close to 1 without going over. Then I quickly switch over to the method 1 using the "import to Method 1..." button shown in the image above.
quick note, for the most streamlined viewing of output tables, I will set up my 100yr as profile 1 in the steady state flow editor, and the floodway as the 2nd profile. This allows the output detailed RAS tables to quickly compare the correct profiles for you. You can always just export to excel, but this saves time.
Method 1-Manually Assign left and right encroachment stations
This is the most common method, and the one I recommend. This method manually sets your floodway where you want it. Here is my setup when running through a floodway, and always remember two major items:
Ensure your model has been QAQC'd at this point. You want your model DONE. Check your n, IFA, flow change locations, drawn flow paths and updated reach lengths, set bridge modeling methodology, really looked hard at structure IFAs (the single biggest error I see in modeling), expansion/contraction coefficients, XS spacing/layout, etc. You do not want to go back and change those inputs after dialing in a floodway.
Start downstream and work your way up....this is a standard step backwater program (think back to hydraulics 101 where you solved the standard and direct step backwater calcs)
Set your FW & 100 profiles on only, select your preferred encoarchmant/floodway table (I use a user defined one) allow a XS view, and work upstream. See below.
On another screen, I will have RAS mapper open, and update my floodway inundation new results map layer on my results (yes you have to add this). This is very rough, and I don't use for final floodway delineation, but it quickly shows you the floodway location plus or minus. I recommend you set your XS stationing tick marks, bank stations, IFA, depth, and floodway on and symbolized as shown below.
Once you approve of your encroachments and surcharges, you can right-click on the XS layer (in results), click "Generate Layers," then choose encroachment points on XS to generate a shapefile of the points along the XS. Always check these, but they are typically pretty dialed in. I will then pull that (and my XSs) into GIS, and hand draw the floodway. There are tools to automate, but none that are that great. The floodway width in between XSs is often ballooned out or too narrow, especially around sinuous streams. Keep an eye out for this.
And that's it, you now have a detailed floodway...as long as you were able to work through all the surcharge issues. Don't forget, you get an extra 4 hundredths, the acceptable range is -0.04 through 1.04.
