Question on sizing Culvert using FHWA HY 8 program

[cweezorak]

I am currently trying to size a culvert in a FEMA Floodplain. My question is on Allowable Headwater depth and tailwater depth. I am using The FEMA floodplain Elevation as my allowable head water depth. I am interpolating the FEMA floodplain elevation around the Culvert entrance. When looking at the the output data from HY 8 I am sizing the culvert on headwater depth for that particular pipe size/shape and my design Q. I am trying to meet FEMA requirements of 0' rise. Does this seem correct?

My second question is on tailwater. I took a section 50' downstream of culvert end. This part of the stream is more typical. Should I be taking a section closer to end of Culvert?

Also, in HY 8 they ask for road data. I do not have a road just fill. Can I leave this part out?

Thanks,

Chris

 

[saruman26]

Sounds like a reasonable approach to me. Keep performing culvert sizing iterations until you get 0 change in flood elevation, or very nearly 0. I would think it would be harde for somebody to argue against a rise of less than 0.1 feet.

The 50 foot section downstream sounds reasonable, too, though you could take the section closer if you wanted to.

As for the roadway data, the software programmers seem to have assumed that all culverts are under roadways. Just define your fill data (embankment height, top widths, etc.) to ensure correct calculations.

 

[ychenut]

Never used HY 8. My comment on road info.

I guess it need that for check storm, say 100 year storm, when the road will be flooded but act as a weir for water to pass overtop. the overtop capacity is required by authority? Not sure.

 

[civilperson]

The road/embankment needs a design storm,(i.e. 100 year or 50 year return), to use for maximum headwater elevation with the specified freeboard, (or in your case the FEMA elevation). This will size the culvert for inlet controlled case.