Computing HGL utilizing Hydraflow Storm Sewers

[cch02]

I have utilized Hydraflow Storm Sewers 2005 to compute the HGL for a storm sewer system. I set the codes to check for inlet control; however, I have been questioned by the review engineer as to why the HGL does not correspond to that computed using the headwater depth charts from the Hydraulic Design of Highway Culverts as published by the USDOT. The HGLs computed by Hydraflow are lower that the headwater determined by the charts.

In analyzing the results from the 2005 program, it appears that the HGL for the upstream end is computed utilizing the downstream invert. Is this correct?

One final item - I ran the same program in the 2009 Hydraflow Storm Sewers (the AutoCAD extension), and appear to get more accurate results for the HGL. If anyone has any input on this issue, it would be much appreciated.

 

[bpattengale]

HGL is computed starting at the downstream invert for Hydraflow. Hydraflow utilizes a direct step calculation method to determine the HGL starting at the outlet of the system. Most software programs calculate the HGL in the same way.

Second an isolated culvert, which is what the charts from the USDOT are for, and a storm sewer network are two different things.

Most likely it has to do with the entrance loss coefficients that the software uses. The entrance coefficient that was used to create the charts is likely larger than the one that hydraflow uses to check for inlet control. A larger coefficient will increase the HW elevation.

Software is great for doing tedious calculations, but you need to understand what it is doing. I suggest you pick up a good book on open channels (Storm Sewers generally operate as open channels) such as Open Channel Hydraulics by Sturm to get a better understanding of what the software is actually doing.

 

[cvg]

calculation only proceeds in an upstream direction if the flow in the channel / culvert is subcritical. For supercritical slopes it proceeds in a downstream direction. Since you have not given any information necessary to determine if your channel is sub vs super critical flow - it is unclear if it is indeed calculating the hgl based on a downstream section. Either way, the entrance loss to the culvert as mentioned previously is probably the culprit.

 

[tobeace]

where I work we use a simple program to make sure there is pipe capacity for pipe design, then whenever we need an HWE we just use civiltools program culvert design tab.

Our program does not calculate the HGL inside the manhole like Hydraflow does, just inside the pipes, however the upside is our program prints to autocad in a graphically superior look than hydraflow.



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http://www.cemp3.info

 

[beej67]

Second the "Open Channel Hydraulics" (Sturm) recommendation. He was my adviser in grad school. That guy REALLY knows his stuff. I use that very book more than any other reference book.

All hydraflow is doing is starting with a downstream HGL, and figuring a pipe friction loss and a junction loss at each pipe/structure in your network, all the way up.

Also, something else important to remember, is the design storm. I haven't used hydraflow storm sewers much, but I do know that with StormCAD (similar software) it uses the modified rational method, which means it's re-figuring a Tc of the network at each link in the system, based on downstream flow velocity, and it's re-figuring it's Q based on the new I based on the new Tc. It's a good approach at sizing pipes, but in reality it means that each pipe is sized for a slightly different storm event. It also means that the flows don't add up, which can really throw some engineers and regulators for a loop if they don't understand what the software is doing.

 

[bank]

Hydraflow gives you three default options for the starting HGL: crown, normal, or critical. You can also specify a starting HGL when running the program. Design requirements differ in different locations. In Houston the City criteria states that if the actual HGL at the discharge location is not known, one has to use the crown of the pipe on the downstream end for starting HGL. This obviously would give a higher HGL on the upstream end than if the invert was used. That could be a reason for the difference between your results and those in the headwater depth charts.