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c-factor vs roughness height extremes

In the bookAdvanced Water Distribution Modeling and Management” I found the following:

If the differences in pressures and flows between actual conditions and predicted conditions are so great that unrealistic and unexplainable pipe roughness values (less than 30 or more than 150) or major adjustments in demands must be used to achieve calibration, then chances are good that the discrepancy is the result of a closed or partially closed valve or errors in system mapping.

The numbers less than 30 or more than 150 are Hazen-Williams c-factor. But when talking in terms of Darcy-Weisbach roughness height (e) what would be the extremes that one could draw the same conclusions from?

  • The key here is that you should not focus your calibration solely on pipe roughness but determine WHY the model and the field data differ and correct the parameter that is different.  Here is a list of a few of the things that can make the model and field data differ. You need to be a bit of a detective to determine which applies in your situation.

    Physical

    Pipe size/location

    Pipe connectivity

    Pipe roughness

    Pressure zone boundary

    Pump curves

    Pipe material/age in GIS

    System changes since model built

    Elevation data

     

    Operational

    Valve open/closed/throttled status

    Control valve operation/settings

    Transient events

    Actual operations not matching control rules

    Unusual operations when data were collected

    Tank water levels

    Pump status/speed

    Lack of sufficient sensors/gages

    Water quality reaction rates

     

    Demands

    Spatial allocation

    Model does not reflect conditions when data collected

    Large customers with atypical demand patterns

    Not accounting for seasonal changes in demand

     

    Data

    Inaccurate/uncalibrated gages/meters

    “Latched” data from SCADA

    Understanding SCADA data – average vs. instantaneous

     

     

  • Note that I have incorporated Dr. Walski's advice into the following related article: Water Model Calibration Tips


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.