HEC-RAS Starting elevation question

[Anonymous]

I have a unique situation. I am analyzing a bridge on a stream which is on Zone A. The stream merges in to a river 2000 feet downstream of the crossing. Cross-section I have only goes 1000′ downstream of the crossing. The river at downstream is a Zone AE with Special Flood Hazard area. The regulatory high water elevation near the confluence location is approximately 6 feet higher than the normal high water elevation. FIS profile only shows regulatory 100-year high water profile.

So what do I use as my starting elevation which is 1000′ upstream of confluence point which is a special hazard area with regulatory HW Elevation which is 6′ higher than regular 100-year HW elevation. Also I only have information for 100-year elevation from FIS.

Any suggestion?

[Lonnie A]

Are the two stream watersheds similar in size? Meaning do you expect to have the peak flows occurring near the same time at the confluence? If so then you might consider using the BFE in the receiving stream as the starting WSE on your stream. If they aren’t going to likely be coincidental then normal depth should work. This is how FEMA typically maps things. You run the trib as normal depth and then project the backwater elevation of the receiving stream back up the trib until it intersects with the profile of the normal depth SWSE run.

With the distance away from the receiving stream does either choice impact the elevation at the crossing? In coastal areas it probably would but in areas with steeper channel gradients your backwater assumption typically only impacts WSE over a short distance.

[PT]

The study area has drainage area of 128 sq. mi. compare to receiving stream has much larger drainage area of 2804 sq. miles

[PT]

In this particular case, channel slope is very flat, 0.00125 ft/ft so backwater effect would carry up to and beyond the project location.

[Author]

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