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# Same culvert, different results???

## Same culvert, different results???

(OP)
I have modeled a culvert as a 'Reach' and as a 'Pond' outlet device.  All variable are the same (i.e. culvert dimensions, Manning's #, inverts, slopes, lengths) for both models, with the exception of the Entrance Loss Coefficient and Contraction Coefficient, which are only specified for the pond outlet model.

The inlet flow from the upland 'Subcatch' is approx. 131 c.f.s for both models, and both outlets have a free discharge of tail water

The culvert in the 'Reach' model had a maximum flow capacity of 255 c.f.s, and easily passes the flow from the upland watershed.

The culvert in the 'Pond' Outlet model has a maximum capacity of 94 c.f.s., and causes ponding behind the outlet device.

Why is there such a large difference when I model the same culvert two different ways?  Am I missing something?

Thanks,
Nick

Replies continue below

### RE: Same culvert, different results???

The "Reach" in HydroCAD is used to simulate open channel flow and essentially all it does is delay the peak flow from the hydrograph.

The reach model does not take into account the type of control that the culvert is operating under.  The culvert can operate under three conditons

1 Orifice Control
2 Barrel Control
3 Outlet Control

Modeling the culvert as "catch basin", a pond with little or no storage volume, then using the outlet to define the culvert will allow HydroCAD to calculate which control the culvert is operating under the given conditons.

Personally I would use a program like CulvertMaster or do the Calcs by hand to determine what type of control the culvert is operating under.

### RE: Same culvert, different results???

When you use a pipe reach, HydroCAD always generates a "Hint" to remind you that reach calcuations are based soley on open-channel Manning's flow.  Please click the message for full details.

For many pipes, the inlet conditions will restrict the flow to less than the Manning's capacity.  For details see www.hydrocad.net/pipes.htm

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