HEC-RAS Floodway Encroachment 2

[joshlorge]

I have a study for a stream. The water surface (WS)elevations were given with the data I received. Trying to utilize the Methods 4 and 5 to determine my encroachments. I set the most downstream WS elevation to one foot above the given to determine the encroachments. I then try to manipulate the next upstream profile by using methods 4 and 5 by increasing the encroachment values. No matter what number I enter, the WS does not get to 1. Am I doing this correctly? What am I doing wrong?

 

[Crystalct1]

I know how frustrating this can be - and the manual makes it seem so simple and intuitive. But it actually does NOT do what they lead you to believe it will do.

Your approach is correct. But remember that the limit for the WSE is 1. That does not mean it must be 1, it must be at the most 1.

Squeeze the encroachment at the next-to-last d/s section, as you are doing, trying moving floodway to the right or left, keeping the distance between encroachments the same. Play around with it until you get say to 0.8 or 0.9. Then move on. Do the next cros-section the same way. Run it. If this section will not cooperate, go back to the next-to-last section and open or squeeze the encroachments again. You may get the number a next section little higher.

Remember that this is a step backwater calculation. Whatever is d/s affects what is u/s. It is tedious as all he11. You'll spend two or three days on it and the best you'll get is 0.8 and 0.9 all the way through.

I'm from NJ, where we only want a 0.2 rise with encroachments. It is still just as hard.

 

[NinetyWt]

Are you doing a brand new study from scratch? What I mean is, often the floodway must stay the same as the previous study. If this is a FEMA mapped stream, then you probably can't increase the floodway elevations without submitting a request for Letter of Map Revision. In that case use Method 1 and set the encroachment stations to match the previous limits. (You may or may not get the 1.0-ft rise with that).

If you *are* doing a new study, start with Method 4. Set all of the encroachments with a target of 1.0 foot initially. Run to see what surcharges you get. Then begin moving upstream, tweaking the model to see if you can squeeze the floodway in further. You will probably see a few upstream cross-sections get to 1.0 before the others. You will reach a limit where you can't squeeze any more without sending some surcharges over 1.0 foot.