HEC-RAS Floodway Encroachment for LOMR

[Rcox220]

Good afternoon,

I have been working on a LOMR for a small tributary due to (2) new culverts and the physical change in the basin. I managed to produce the duplicate effective, corrected effective, and new existing conditions plans just fine. At my first attempt at a Floodway Analysis using the encroachment tools on the new existing conditions information, I have so far been unsuccessful.

My approach is as follows: Using the 100-year storm flows in the steady flow data as the "base profile" I duplicated these flows and labeled it Floodway (so that I could use the encroachment tool). When I begin the encroachment analysis, I choose Floodway as my profile and method 4 with a W.S. increase of 1.0 feet.

This is where the problems arise. Do I then apply this to just the most downstream x/section as a start or every section on the tributary all at once? It seems when I try the approach I get values both well above 1.0 feet and others less than 0, not to mention the downstream x/section W.S. not increasing at all. I have the intention of getting this information close and then fine tuning with Method 1 and actual x/section station adjustments.

The HEC manual tends to lead me this far and then suggests "Good Luck." This could certainly be a beginners issue and an easy overlook but any advice and help would be greatly appreciated, I have some HEC experience but by no means would be an expert.

Thank you,

RC

 

[DavisMurphy]

Hello RC,

For your floodway woes I fully suggest you pick up a copy of "Floodplain Modeling Using HEC-RAS" published by Bentley (used to be Haestad). This is an invaluable tool for any hydraulic modeling project related to FEMA or otherwise.

For now though, I would suggest using method 5 (read about this in the HEC-RAS Hydraulic Reference) instead of method 4. Method 5 will allow you to set a maximum change in energy in addition to a surcharge target. This can help in many cases, especially if you have critical depth water surface elevations in your model (which you should try to remove from the model anyway). Use several different surcharge targets for the entire profile. Start with 1 and then 0.9, etc. working your way down until you reach 0. Throw the surcharges into a spreadsheet for comparison. You will see that for some cross-sections the surcharge will not be exactly 1.0 but will still be within your surcharge limits depending on the target surcharge used at a particular cross-section and at cross-sections downstream.

Last item: for a subcritical profile, you will see that reducing the widths between encroachments (reducing conveyance area) will increase the surcharge for the next upstream cross-section. The opposite is also true: if you increase the width between encroachments (increase conveyance), the surcharge will decrease for the next upstream cross-section. There are exceptions to this rule for more complex situations, but in general it holds for a subcritical profile. You can use this to tweak your floodway as needed.

Hope that helps

Davis Murphy, EIT, CFM
Water Resources Engineer

 

[Rcox220]

Thank you for the input Davis, I will be sure to pick up a copy of the book you mentioned and also try Method 5.

Have a great day,

RC