First, unless the pipe always flows full, it IS an open channel.

Second the relationship between depth and flow is well known and may be calculated or found in any good hydraulic reference such as Kings Handbook of Hydraulics.  You may also derive it by calibrating the gauge with actual known flows.

Third, if the pipe is "overloaded", i.e. if it sometimes flows under pressure you may still use Manning's equation to calculate the flow provided you use the slope of the hydraulic grade line, rather than the slope of the pipe.

good luck

RWF7437,

Perhaps I did not make my questions clear.  I am aware of (and frequently use) the myriad of modeling tools available(Rationale, SCS-TR-20, TR-55, mannings,etc).  What I am after is information correlating the model Hydrology with observed flows in pipe.  Particularly, observed flow with depth gauges.  USGS and others have stream gauges at several areas and there are studies of stream response.  My interest is in the response of the closed conduit (although it is functioning as an open channel - as you point out).

I am responsible for an urban area with many large diameter pipes which drain over 1 sq. mi.  Many models indicate undersized pipes but we never observe any flooding conditions in the area (even under the heaviest of storms).
In some cases the model indicates that the pipe has a capacity of less than a two year storm.  While some may use this as a basis for a Capital improvement to a system it is much more common to base these decisions on observations and complaint tracking.  Gauging may be another useful tool.

I believe storm models are meant to be conservative so when you design you CYA.  Just like safety factors in structural or geotechnical.  I am currently experiencing the same type of problem trying to model existing conditions at a DOE facility.  Normal design modeling practices would predict big time flooding but this has never actually been observed.  So we are in the process of placing some depth guages in a few locations to adjust our curve numbers to make the model more realistic.

Sam 74 -
Sounds interesting the work at DOE!  Keep us posted (so to speak).  I agree that there is some serious "uncertainty factor" in the models and it is frustrating.  
We currently have a gauge in a 10' diameter storm drain and are waiting for some big storms.  We had one storm (.2 in/hr) and the depth hardly moved.  After we get some data I will look at some curve formulas.  This is the part I am not sure about and was wondering if anyone had experience with analyzing gauge inforation for storm drains.  I would imagine that there has been research to analyze the models with field run data.  As you state, you can always attribute the descrepancy to curve numbers or times of concentration and make the model work.

We use a curve number of .85 for much of the urban residential area.  It may be useful to adjust this per the gauge readings.

I have seen at least one good published example of calibration. If I remember correctly it was done by the Corps of Engineers, Sacramento District.  Published in the HEC-HMS User Manual if memory serves.  You may also find SMADA to be a very useful program. It will allow you to input actual rainfall data, for any duration storm, and produce several different kinds of hydrographs.  Also, it doesn't cost thousands of dollars.

good luck

The SMADA programs were written to accompany the textbook
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