HydroCAD Stormwater Modeling - Since 1986

Sample Culvert #1: Inlet control in action

The following photographs illustrate simultaneous conditions at the inlet and outlet of a typical roadway culvert.  While the outlet photo indicates that the barrel of the culvert is operating at only a fraction of capacity, the inlet is almost fully submerged and is very close to capacity.

Roadway culvert under inlet control Roadway culvert operating with free discharge

The photographs illustrate that the limiting condition for this crossing is the inlet, rather than the barrel flow or outlet conditions.  The use of a square-edged flush inlet (with a typical energy loss coefficient of .5) will often result in such inlet control.  If a rounded inlet had been used (with a lower coefficient) the inlet capacity could have been increased without the need for a larger culvert barrel.

To model this situation in HydroCAD requires the use of a pond with a culvert outlet.  The pond is required because the stage-storage characteristics upstream of the culvert must be evaluated in order to determine the head at the culvert inlet.   A culvert outlet also has the ability to consider entrance loss conditions (which are closely related to inlet control) as well as certain tailwater situations.

By comparison, a pipe reach considers only normal Manning's flow within the barrel, and cannot be used to evaluate inlet or outlet conditions.  A pipe reach is designed for situations where 1) open channel flow is assured, and 2) the inlet and outlet of the pipe are not controlling factors.  (To avoid misuse of a pipe reach, this information is presented by HydroCAD each time a new reach is created.)

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