Computing HGL utilizing Hydraflow Storm Sewers
Computing HGL utilizing Hydraflow Storm Sewers
(OP)
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.
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.





RE: Computing HGL utilizing Hydraflow Storm Sewers
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.
RE: Computing HGL utilizing Hydraflow Storm Sewers
RE: Computing HGL utilizing Hydraflow Storm Sewers
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.
Want to Learn More About Civil Engineering? Visit: www.cemp3.info
RE: Computing HGL utilizing Hydraflow Storm Sewers
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.
RE: Computing HGL utilizing Hydraflow Storm Sewers