Inlet Control Nomographs

[Brown1975]

I have been looking at the "Headwater Depth For Concrete Pipe Culverts With Inlet Control" Nomograph. Is the headwater distance from the invert of the pipe or the top of the pipe? It appears to me that as long as the headwater elevation is lower than the top of pipe, it would simply be free flowing and there would be no pressure head to factor in. However, if the headwater distance is measured to the invert of the pipe, my thinking is incorrect. Can someone help me to understand this? Thanks.

 

[bltseattle]

I've only ever seen HW measured from the pipe invert when using nomographs. For inlet control, there is head needed to get the water into the mouth of the culvert, even if unsubmerged; ie it will be free flowing but still have head losses at the inlet. You need the nomograph check to determine IF the head is beneath the culvert crown, and if not how deep is the ponding at the culvert inlet.

 

[Brown1975]

Thanks for the response bltseattle. I have an Engineer who is trying to use this nomograph in sizing pipes within a storm drainage system (approximately 20 connected pipes). It is my understanding that this nomograph is for culverts and not really for pipe systems. I have told him that he could not just take each pipe individually to evaluate it using the nomograph but would need to do a hydraulic grade line analysis for the entire system. What are your thoughts on this?

 

[LCruiser]

It depends on how long and steep the pipes are, as well as the efficiency of the connections. Inlet control is only one factor. Flow and outlet control need to be considered as well.

 

[bltseattle]

The issue with using culvert nomographs for pipe systems is that you may not fully account for bend losses and any benching in the structures (which would tend to give an HGL lower than expected), and the inlet conditions (eg headwall with wingwalls) won't match in name to what you might find in piped SD systems. (However if you use the nomograph with the Ke equivalent to the condition in the CB/manhole you are okay for that on). Using culvert nomographs you also don't get any "credit" for approach velocities that keeps water moving through the system, which could yield a higher HGL from using culvert nomographs. So , In a straight run of pipe your culvert nomographs would overestimate the HGL elevation. In a system with bends, who knows?


The FHWA manual HEC-22 is the "bible" for analyzing and designing piped storm drain systems. All the charts and nomographs you would need are there, enabling you to account for bend losses, inlet losses, benching/sumps at junctions, plunging flow, etc.

That said, if you are designing a new SD system, often normal flow can be used to size pipes without considering inlet/junction losses. This is valid for systems of moderate or steep slope and no funky junctions. The way to see if this approach is valid should be to size pipes based on normal flow (Manning's eqn). If the flow in a pipe is subcritical or close to pipe full be careful and analyze more closely; supercritical and less than pipe full should be okay. If all your pipes are 2% or greater you will likely be in the second scenario. Flatter systems, systems with manhole channeling to improve hydraulics, and systems with a downstream tailwater require more scrutiny.

 

[cvg]

be careful with steep, supercritical flow - if you have a flat section, or large inflow from a junction or some other type of head loss, you could have a hydraulic jump. If so, the pipe will seal and cause backwater.

Most every system I design has tailwater and subcritical flow. Our clients demand that we design for full flow and limit the velocities. The ground is flat and difficult (impossible) to use 2% slope.

Using mannings equation with normal depth in a system of pipes can be an tedious, iterative process. While it can be done using a spreadsheet or even by hand, it is probably much easier and quicker to do with the proper software. Even at our lowest EIT billing rate, it is more economical to buy commercial software to do the analysis.