Culvert Computations

[Anonymous]

Hello

I have a question with the discharge through the culvert – partially submerged case. I am trying to model culvert as a lateral outlet pipe and its going out of the system, therefore, there is no question of downstream submergence of this lateral structure.

Now, this culvert / opening is partially submerged and i believe it should behave like an open channel rather than a pipe. I have the culvert US invert, mannings, hydraulic radius, upstream water level, length and elevtation difference of culvert (slope) etc, i use mannings and i get a discharge. This discharge never matches with what hec ras is giving me. Can anyone tell me is my approach incorrect?

Now, if i go to the manual, so hec ras refers some US HIGHWAY NOMOGRAPHS and i have downloaded that actual document. When i use the equation for partially submerged culverts given in the manual for my case, even then the answers never matches, rather the equations (nomographs & mannings) are always over estimating. Can anyone tell me what computational scheme / equations hec ras using for culvert flow?

P.S: i am taking inlet control + please remember culvert is modelled as a lateral structure out of the system.

Hopefully someone can help. Thanks in advance.

Regards
NA

[Anonymous]

By the way, HEC Ras equation for unsubmerged inlet:

HW/D = K(Q/A*D^0.5)^M

where K=0.0098
M=2 (for circular concrete square edge headwall)

HEC RAS MANUAL pg, 6-11

[Anonymous]

Just because the culvert flows out of the system doesn’t mean it is under inlet control. For instance, imagine you have a long, flat culvert. The barrel capacity is going to be the limiting factor.

The nomographs only apply if the inlet is the limiting factor.

So are you sure whether your culvert is being computed as inlet control?

RAS reports whether it is inlet or outlet control. Also, you can force it to use the inlet equation, if you want.

If you have a long enough culvert on a slope, that is not flowing full, it will eventually get to normal depth. But this doesn’t happen instantly. (Assuming it is a shallow/subcritical culvert) it will start with critical depth at the outlet and then go to normal depth as you go upstream.

Even for outlet/barrel control, there is still an entrance loss at the inlet.

[Author]

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