WS or EG

[SMIAH]

Designing a culvert : most software (e.g. HydroCulv, HY-8) gives the Upstream Head Elevation as final result.

Do you calculate the Upstream Water Surface Elevation (susbtracting v^2/2g) and use this water level to see if the conditions are ok (e.g. under the maximum allowable elevation) ?

Or rather use the Upstream Head Elevation ?

 

[cvg]

HY8 reports the HGL at the inlet. It does not know what the upstream hydraulic conditions are, so velocity / flow depth or width of the channel or reservoir are unknown. there will be losses in friction and momentum at the transition into the culvert that must be added to get the true HGL at the transition. That should be compared with any open channel hydraulic calculations you have done to see how it compares. It may result in tailwater that controls the upstream HGL in a channel. Losses through a grate or trash rack should also be considered to make sure flow is within the allowable freeboard. If your channel is supercritical, than you may have a hydraulic jump at the transition. HY8 cannot handle that either.

 

[SMIAH]

This almost means that you cannot evaluate a culvert without knowing the open channel (or else upstream) conditions.

Now, let's assume that the open channel HGL is lower than the culvert upstream HGL : do you use the WSE or the HGL to check if it's above or under the maximum allowable elevation?

 

[cvg]

Now, let's assume that the open channel HGL is lower than the culvert upstream HGL...

well, we both know that is impossible, so that would require a new analysis of the upstream channel HGL to estimate the true HGL in the channel.

remember, unless you have supercritical flow, water surface calculations must proceed upstream. that's why tailwater is so important and HY8 requires an estimate of your culverts tailwater conditions but not for headwater...

 

[SMIAH]

I'm not sure to follow you on that one:

The culvert might "choke" the flow to the point where the upstream water level is higher than the open-channel water surface level (calculted without a culvert there). Isn't?

Anyway, this wasn't my real question : In a culvert analysis report, do you calculate the WSE from the HGL (substracting the v^2/2g) or do you only use the HGL to compare with the maximum allowable water level?

 

[Drew08]

I always thought the HGL is the WSE, where the EGL (Energy Grade Line) is HGL + velocity head. Also, I say use the EGL at the culvert opening because there is momentum associated with the approach velocity that is "pushing" the water into the culvert as much as the static pressure is. The catch is to concider the EGL at the culvert opening, which is not neccesarily the same as the EGL in the approach channel. If the culvert is scewed relative to the channel, or the channel widens suddenly at the culvert, or there is a hydraulic jump approaching the culvert inlet, or any number of other reasons, the velocity in the channel could be killed just before it gets to the culvert opening. Point: use EGL, but EGL might be very close or equal to HGL if there is no vector velocity inline with the culvert.

 

[SMIAH]

Yes sorry meant : EGL (Energy Grade Line) = HGL + velocity head.

Ok thanks!