HGL calculations
HGL calculations
(OP)
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.
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.





RE: HGL calculations
RE: HGL calculations
http://www.fhwa.dot.gov/bridge/hydpub.htm
I would review the proceedures outlined in HEC-22 IT should help you alot.
RE: HGL calculations
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).
RE: HGL calculations
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...
RE: HGL calculations
RE: HGL calculations
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).