HGL calculations

[Gumhead]

I'm doing HGL calcs for a series of storm sewer pipes. It is my understanding that you need to check for both inlet control and outlet control of each storm sewer segment (from one junction to the next).

For outlet control I am adding friction losses and junction losses to my starting HGL from the previous segment. For inlet control I am using HDS-5 nomographs to get an HW/D. Whichever gives me the higher elevation I choose as the HGL for that particular junction. I have found that my inlet control check is regularly the higher (controlling) elevation.

I am concerned that I may be oversizing my pipes. When I check Manning's Eqn for pipe capacity I get relatively large Q's as compared to the Q's I'm getting into my pipes through inlet control. It seems to me that HDS-5 inlet control nomographs are extremely conservative for storm sewer design since the velocity and 'channelization' of the incoming flow through a junction are not taken into account. Is there a more reasonable check for inlet control? Thanks for any suggestions.

 

[LHA]

It sounds like you are counting entrance losses twice. The losses through a structure should be applied as a coefficient to the exit velocity head (the v1 term in Bernoulli's) from that structure. The previous pipe is controlled by its own entrance conditions, minus its friction losses. The HDS tables (which I too have always found to be ultra-conservative, but alas seem to be universally accepted) are meant to be applied to a quantity of water waiting to get into an entrance...like a headwall in the terminal end of a channel. I don't think they are even applicable to water already moving through a pipe network.

 

[gbam]

A good source for storm drain analysis is HEC-22 From the Federal Highways Admin. FHWA. You can download it in pdf format from:

http://www.fhwa.dot.gov/bridge/hydpub.htm

I would review the proceedures outlined in HEC-22 IT should help you alot.

 

[SiPaul]

I agree with lha it sounds like you are counting the loss twice. I believe that you want to use the friction velocity for the loss calculations. This would be closer to the actual flow in the pipe once friction is accounted for.

I generally do an initial sizing with mannings and then plot start at the outlet crown (assuming no TW) for a conservative estimate. Then I plot the friction slope to the next upstream structure and compute the loss at the structure, plot the loss and then continue upstream along the friction slope.

It is a quick and dirty approach. I find that many engineers do not show the velocity once friction is accounted for. If you don't do this you may get a higher velocity when in reality the velocity is below minimum (typically you want 3-4 fps as a minimum).

 

[Gumhead]

Thanks for the advice so far. I'm going to go through the procedures in HEC-22 this weekend. gbam thanks for the reference.

SiPaul, I did what you suggested for HGL up through the pipes and junctions as you suggested. And from what I understand so far, this works fine until you get to a point where the pipe isn't surcharged. Once you get to a point where there is a free water surface, a different approach is required to determine HGL. I suppose I could use the HDS-5 nomograph to conservatively estimate HGL at this point. However, I think it's too conservative. Hopefully HEC-22 will clear this up...

 

[Maury]

HDS-5 is for culverts, a single pipe through an embankment, not for storm sewers. Use the procedures described in DEC-22 for storm sewer systems.

 

[SiPaul]

I agree that HEC-22 will give the most complete procedure for HGL computations. If the pipe is not surcharged the HGL won't fall below the normal depth. Usually, you take the HGL along the friction slope until it intersects the normal depth and then continue along the depth until the next structure.

I am curious if anyone has an easy way to deal with the friction velocity issue? If you compute the velocity using mannings you get a higher velocity. In other words, the velocity upstream of a 1000' pipe will be far less then a 20' pipe for the same conditions (slope, Q etc).