NormanDGutter
Civil/Environmental
- Apr 22, 2008
- 4
Greetings, hydrology and hydraulics engineers!
I am modelling a 6x6 reinforced concrete box culvert under a two lane highway in a rapidly urbanizing area. The original FEMA model had some errors in the location of cross-sections 1, 2, 3, and 4 of the culvert model. I have inserted new cross-sections 2 and 3 in the appropriate places, and re-termed others.
The problem I have is with the elevation of the ineffective flow areas for cross-section 2. They are supposed to be set at the top of the roadway. In my case, this is elevation 77.5; culvert invert is 69.0 and inside top 75.0.
With the elevations this high the program routes only 0.5 cfs (out of a total flow of 972 cfs) through the culvert, and puts the rest over the highway, greating an unrealistic BFE. When I drop the ineffective flow area elevations to the top of the culvert, 75.0, about 40-45 percent of the flow goes through the culvert, as you would expect. My downstream channel sections are such I have no channel for over 30 feet, as side ditches with 4:1 sideslopes enter from both directions.
Has anyone encountered this before, where HEC-RAS refuses to route flow through the culvert when using the suggested ineffective flow elevations with the downstream culvert section?
Has anyone modelled the culvert with lower ineffective flow elevations than recommended and had FEMA accept it?
Thanks in advance,
NDG
P.S. Might come back and try to add a figure later today.
I am modelling a 6x6 reinforced concrete box culvert under a two lane highway in a rapidly urbanizing area. The original FEMA model had some errors in the location of cross-sections 1, 2, 3, and 4 of the culvert model. I have inserted new cross-sections 2 and 3 in the appropriate places, and re-termed others.
The problem I have is with the elevation of the ineffective flow areas for cross-section 2. They are supposed to be set at the top of the roadway. In my case, this is elevation 77.5; culvert invert is 69.0 and inside top 75.0.
With the elevations this high the program routes only 0.5 cfs (out of a total flow of 972 cfs) through the culvert, and puts the rest over the highway, greating an unrealistic BFE. When I drop the ineffective flow area elevations to the top of the culvert, 75.0, about 40-45 percent of the flow goes through the culvert, as you would expect. My downstream channel sections are such I have no channel for over 30 feet, as side ditches with 4:1 sideslopes enter from both directions.
Has anyone encountered this before, where HEC-RAS refuses to route flow through the culvert when using the suggested ineffective flow elevations with the downstream culvert section?
Has anyone modelled the culvert with lower ineffective flow elevations than recommended and had FEMA accept it?
Thanks in advance,
NDG
P.S. Might come back and try to add a figure later today.