HEC-RAS Model - Bridge (Culvert) Replacement 1

[mvill]

I have a new prjoect where I am replacing a bridge and have to show that the Water Surface Elevation will not increase by replacing the existing bridge. (It is more of a culvert than a bridge). I have a HEC-RAS model set up with cross sections for the existing and proposed conditions. These models need to be used to do a floodway determination and create a Flood Hazard Area Plan showing the floodplain. This will be done using encroachments in the HEC-RAS model.

For this project, the bridge runs over a “stream” that flows into “Main Channel”. I have not included this "Main Channel” in my HEC-RAS model because its flow will not have an effect on the bridge however, based on Flood Insurance Rate Maps, the bridge is within the “Main Channel” floodplain (The bridge is about 80 feet from the "Main Channel".

In my HEC-RAS model, my cross sections extend across the “Main Channel”. Therefore, on some cross sections it looks like there are two channels (which there are). When I run my HEC-RAS model, the "stream" does not overbank and the "Main Channel" is filling up simultaneously with the stream. When I do not extend the cross sections out far enought to include this "Main Channel", the water surface elevations are higher than the highest ground surface elevation.

Do you think I need to have two reaches and actually model the "Main Channel” even though it will not have an effect on the bridge? And more importantly, for my Flood Hazard Area Plan, how do I show the area within the “Main Channel” flood zone since the flood plain is already existing or should I ignore this Flood Insurance Rate Map and create my own flood limits for the “stream” using my existing and proposed condition models?

Thank you for any help, advice, or suggestions you can provide.

 

[mvill]

Would adding levees at every station help my analysis so that the flow is contained within my "stream" until it overbanks and goes into the "Main Channel"?

 

[gbam]

Your main channel will impact your tributary stream that you are studying with the backwater. You do not need to model the two reaches but you must account for the tailwater condition. FHWA's HEC-22 has a table (Frequency of Coincidental Occurances) that address what frequency should be used for your tailwater base on watershed areas. Also, is there currently a FEMA model for your stream. If there is you should start your modeling using that.