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Modeling Junction Losses in HydroCAD

Modeling Junction Losses in HydroCAD

Modeling Junction Losses in HydroCAD

Hello All,

I'm looking for advice in modeling minor losses through closed conduit junctions (manholes) in HydroCAD. I have ponds with storage draining to a storm sewer system. Each junction in the storm sewer system is modeled as a catch basin zero-storage pond. My understanding is the peak elevation HydroCAD generates at each junction will account for only the inlet loss at the catch basin culvert outlet, and will ignore other minor losses due to turbulence or velocity/momentum changes within the junction. Since some of the junctions I am modeling have multiple inlets with complicated geometry, I would like to account for these additional minor losses to verify flooding is not occurring, and to ensure I am getting an appropriate tailwater for nodes upstream of each junction.

My questions:

Is there a method to model these minor losses in HydroCAD?

Can I simply add the "rule-of-thumb" head loss coefficients published for varying manhole geometries to the inlet loss coefficient I've chosen for each junction's culvert outlet to account for the additional minor losses?

Finally, some of my peers have approached this problem by modeling a storm sewer system in HydroCAD to obtain the peak flow from each node's hydrograph. The peak flow is then used in a hydraulic grade line analysis of the system where minor losses at junctions are determined by the HEC-22 method (or similar) to produce a "true" peak elevation at nodes of interest. Can anyone comment on the validity of this approach? It seems to make sense, however, if the minor losses were ignored when obtaining the peak flow from HydroCAD, it seems to me you would have to perform this exercise a couple of times to find an iterative solution to peak flow (from HydroCAD) and tailwater (from the HGL analysis).

Any input you have is appreciated.

RE: Modeling Junction Losses in HydroCAD

You can adjust the culvert entrance loss coefficient to allow for losses in the CB as well as the actual geometry of the inlet.

However, keep in mind that the normal outlet calculations are based on zero approach velocity at the outlet device. This means that no momentum or kinetic energy is carried over from the inlet to the outlet. So if you have a very efficient CB with aligned inlet/outlet, you could probably reduce the Ke to represent a more efficient transition. But increasing the standard values should not be necessary, since they are already assume a zero approach velocity.

From the help system:

The entrance to a pipe or culvert is frequently characterized by an energy loss coefficient, Ke. A Ke value of zero indicates ideal flow with no energy loss at the inlet. Values typically range from 0.2 to 0.9, as listed below. Higher values are possible for riser structures or drop inlets with more turbulent flow.

Ke Description

0.9 CMP, projecting, no headwall
0.5 CMP, square edge headwall
0.7 CMP, mitered to conform to fill
0.5 CMP, end-section conforming to fill

0.2 RCP, groove end projecting
0.5 RCP, sq.cut end projecting
0.2 RCP, groove end w/headwall
0.5 RCP, square edge headwall
0.1 RCP, rounded edge headwall
0.7 RCP, mitered to conform to fill
0.5 RCP, end-section conforming to fill

0.5 Box, headwall w/3 square edges
0.2 Box, headwall w/3 rounded edges
0.4 Box, 30-75° wingwalls, square crown
0.2 Box, 30-75° wingwalls, rounded crown
0.5 Box, 10-30° wingwalls, square crown
0.7 Box, 0° wingwalls, square crown edge

CMP, RCP, and Box culvert data was compiled from the Concrete Pipe Handbook, 1981, American Concrete Pipe Association.

Peter Smart
HydroCAD Software

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