I am working on a CLOMR for the first time and I need some technical guidance. The project proposes to change a trapezoidal concrete channel into a rectangular channel with fish passage. I have prepared the natural run of existing condition and the proposed condition in HEC-RAS.  When I compare the natural run of existing against proposed, I see variations both up and down.  Now, how do I show that the BFE will not increase by more than 0.0'? How do I prepare the Floodway run? I also need to show that the BFE will not increase by more than 0.0'. I think I need to use encroachment for these two purposes. But, what should be the target increase? If I input target WSE increase of 0.0 foot and use encroachment, I see no changes to the WSE or any encroachment station. If i use 1.0', I see up and down variations. What is tie-in within 0.5' at the transitions and how to show that?
So, my questions are as follows:
How to prepare the Floodway run?
How to show that the BFE will not increase by 0.0' or more than 1.0'?
What is tie-in within 0.5' at the transitions and how to show that?
Can you give me some ideas and/or send me any written procedure? I very much appreciate your help.
 

1.  To generate a floodway model you will need to do part 2 below first.  After you have all the models built you can begin with using method 4 using 1' as a target increase.  Then you can use method 1 to refine the floodway delineation.  

2.  The first thing you need to do is obtain H&H data from FEMA, if available.  There is a standard form that you fill out to request information from the FEMA Library.  

After you obtain the H&H data from FEMA you will need to generate a duplicate flood model.  After you build the duplicate model compare the BFE's to the H&H data that you obtained from FEMA (usually a PDF output from HEC-2).  Your duplicate model BFE's will need to be within 0.5' of the effective BFE's.  

If you encounter any errors in the duplicate model you will need to build a corrective effective model that address these errors.  After that you will need to build an existing conditions model which will reflect any changes that has occured since the effective model was generated.  If no modifications have been made then the existing conditions model will match the corrected effective model.  

After these models have been built you will then need to produce a Proposed Conditions model that incorporates any proposed changes to the floodplain (Cut or Fill, etc).

After you have created these models you will compare the existing model to the proposed model to verify there are no increases in the BFE.  

3.  The 0.5' tie-in is refering to your begining and ending cross sections of you study area (tie-in points).  The BFE's in the effective model compared to the existing/proposed should be wihtin 0.5'.

Hope this helps,

Matt





 

Thanks a lot for your input. I will try those. May I have your email address to ask you a question or two later on regarding this.

Thanks again.