Recommended Hydraulic Modeling Method

[0000jz]

I am looking for recommendations from experienced users. For cross-pipe analysis of pipes which are not culverts that impact FEMA floodplains, and where HY-8 is invalid due to velocity limitations, would you recommend using HEC-RAS or using HDS-5 Nomographs? Why?

 

[cvg]

what's a "cross-pipe"

 

[0000jz]

The correct terminology in PA would be pipe culvert; a pipe conveying a stream or other surface runoff across a highway right-of-way, apart from the pavement drainage system. The analysis is to determine headwater elevation and flow velocities.

 

[Twinkie]

Still not following this question ... start with culvert master, solve for hw

 

[BigInch]

US Federal highways engineering standards for culvert design,

http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=7&id=13

Independent events are seldomly independent.

 

[TerryScan]

What are the velocity limitations that preclude HY-8?

 

[0000jz]

@Twinkie: CulvertMaster is proprietary software, which is the reason I limited the choice of hydraulic calculation methods to HEC-RAS and HDS-5. If you still do not understand the question, please let me know what further information you need.

@BigInch: HDS-5 is the US FHWA methodology. By posting that link, are you sating that you recommend the HDS-5 method over using HEC-RAS for these pipe culverts? If so, why?

@TerryScan: According to one design manuals, HEC-RAS should be used instead of HY-8 when the stream velocity is greater than 5 ft/s. According to another design manual, HY-8 is invalid when entrance velocity is greater than 5 ft/s. Neither manual gives the reasoning behind this limitation.

 

[LincolnPE]

The technical methods implemented in HY-8 are from HDS-5.

 

[0000jz]

@LincolnPE: Right, and knowing that relationship, would you assume that it is OK to utilize HDS-5 when HY-8 is not allowed?

 

[cvg]

you can use any method you want that is approved by the reviewing agency. you would need to ask the reviewer which methods are approved.

 

[LincolnPE]

No, I wouldn't.

 

[TerryScan]

Ok, upon further reading, HDS-5 does discuss approach velocity. (It may not be sufficient for the conditions being discussed).

"HWo+Vu^2/2g=TW+Vd^2/2g+HL

HWo is the headwater depth above the outlet invert, m (ft)
Vu is the approach velocity, m/s (ft/s)
TW is the tailwater depth above the outlet invert, m (ft)
Vd is the downstream velocity, m/s (ft/s)
HL is the sum of all losses including entrance (He), friction (Hf), exit (Ho) and
other losses, (Hb), (Hj), etc., m (ft)
Note: the total available upstream energy (HW) includes the depth of the upstream water
surface above the outlet invert and the approach velocity head. In most instances, the approach
velocity is low, and the approach velocity head is neglected. However, it can be considered to
be a part of the available headwater and used to convey the flow through the culvert

Likewise, the velocity downstream of the culvert (Vd) is usually neglected."

HY-8 indeed does not account for approach velocity "Entrance limitations -Since HY-8 is not primarily a water surface profile computation program but is a culvert analysis tool, it assumes a pooled condition at the entrance to the culvert."

My experience has been entirely in a coastal plain where I have never encountered significant approach velocity. I alway appreciate getting an education.

 

[beej67]

JZ -

If your culvert is in outlet control, or is in inlet control with high head, then the approach velocity is going to be effectively zero anyway due to ponding at the headwall. The only time you'd have a significant approach velocity is in cases where the inlet headwall isn't flooded, and those honestly aren't very common with event storms.

I would at a minimum try HY-8 and see what the results are, and if you have your inlet flooded I'd use that as a case with the reviewer that the assumptions within HY-8 (which are the same as HDS-5) are valid.

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

http://www.campbellcivil.com