Culverts in Zone AE

[kechha2060]

I have a existing roadway culvert in zone AE. We are proposing improvement on the roadway (widening) but decided (higher officials) to leave in AE zone. Existing culvert is 2-60" pipes for 2.2k cfs flow at culvert. The 100yr flood elev (HEC-RAS) at the culvert determined is 4' above the pavement (different study). Do one has to design the culvert for the 2.2k cfs or can allow overtopping and hence fewer and smaller culvert sizes.

 

[froude]

The fact that the road overtops (or doesn't overtop) doesn't impact the FEMA AE zoning. Your proposed improvement should simply not make the condition worse.

It seems like the community public works official needs to determine whether roadway inundation in post-project conditions is acceptable, or not. Whether it is or not may depend on how heavily the road is used, adjacent properties (and old or potentially new flooding claims), planned development in the area, etc. etc.

 

[cvg]

most standards try and limit the depth of flow over the road to 12 inches or less in order to make the road "all weather". Some also require handling the 50-year storm without overtopping. Suggest this is a better approach and cheaper than trying to handle the entire 100-year storm.

 

[kechha2060]

froude - community PW decide that the roadway can be inundated. If my project is determined to be 4' under water for 100 yr flood, doesn't that mean my culvert is under water as well and hence overtopping?

 

[beej67]

Just because your culvert is overtopping doesn't mean it doesn't carry flow. It still does, just not as much as the road(weir) does.

Where you will run into trouble if you start undersizing the culvert, is that the road will overtop more frequently. If you're trying to VE the size of the culvert, it would be in your best interests to run an analysis of the 1 year, 2 year, 5 year, etc storms to get an idea how often it's going to overtop.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[froude]

Sorry, I must be misunderstanding your question.

If you are asking whether future improvements are allowed to overtop, then I think the answer is yes and your PW official needs to tell you (or perhaps you need to convince them) of what that frequency should be. It seems like CVG and beej67 have provided good guidelines to that end, above.

If you are asking how to size culverts that are going to be overtopped, then it's a matter of calculating your total flow and then determining your culvert capacity and weir flow capacity.

Sorry if I'm confused.

 

[cvg]

download a free copy of HY8 and use it size the culvert

 

[gbam]

Better Yet code the culvert in the HECRAS FEMA model to verify it does not impact the 100-Year Flood Elevation.

 

[Rye1]

If the culvert is in a FEMA adopted "floodway" with the AE zone, additional permitting requirements must be met. If is very unlikely that you will be able to impact the 100 year WSEs by replacing the culverts, you are probably in a backwater area anyway if you have 4' of water over the road. If the existing culverts are not falling apart, the best option may be to leave them in place.

Robert Billings

http://www.newrivereng.com