Applying Roughness Values to River Sections
    • 16 Aug 2022
    • 3 Minutes to read
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    Applying Roughness Values to River Sections

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    Article Summary

    Roughness factors may be applied as multipliers of the Manning’s n coefficient for any number of river cross-section nodes in an Flood Modeller model. This is achieved by defining the roughness factor information in a separate text file. This is the roughness factor file (.rff). This is then cross-referenced in the Flood Modeller data (.dat) file.

    Defining a Flood Modeller Roughness Factor File

    The individual components that are used to construct a new roughness factor file are defined as follows:

    1. The same factors can be applied to a GROUP of any number of nodes:
      • Different factors may be applied to different GROUPs.
      • Within each GROUP, one has to specify the factors and corresponding depth; the node labels to which the factors apply and the definition/measurement of 'depth'.
      • The GROUP is delimited in the rff file by GROUP and ENDGROUP markers.
    2. 'Depth' may be specified as a measurement of RELATIVE depth, ABSOLUTE depth or STAGE:
      • This keyword is specified between the DEPTH and ENDDEPTH markers, which are in turn inside the GROUP ... ENDGROUP markers.
      • RELATIVE depth is defined as the ratio of water depth to bankful depth; zero being at bed elevation; bankful being at the higher of the left/right bank markers (if either exists) or maximum section elevation [excluding dflood ] if neither exists. Thus a relative depth of one applies to water level being at bankful.
      • ABSOLUTE depth is defined as the water depth (in m) above minimum bed level.
      • STAGE is the water level (in m) above datum.
    3. The 'depth' and factor are specified in pairs, between FACTORS and ENDFACTORS markers, which are in turn inside the GROUP ... ENDGROUP markers:
      • There is no limit to the number of such pairs, although the must be specified in ascending depth order.
    4. The nodes to which the factors apply are to be specified in between NODES and ENDNODES markers, which are in turn inside the GROUP ... ENDGROUP markers:
      • Any nodes not contained in the roughness factor file will be assigned a roughness factor of 1
    5. The rff file can be punctuated with comment lines beginning with the hash character (#), although this must not be present within a node list or factor list.
    6. Each item (or item pair in the case of the factor table) in the rff file must be on its own line, and left-aligned.

    Roughness Factor File Format

    The format of the .rff file is

    Line 1 - Title

    Line 2 - #REVISION#1

    and then the following block:



    <depth type>



    <factor list>



    <node list>




    1. the GROUP ... ENDGROUP block can be repeated any number of times.
    2. <depth type> can be RELATIVE, ABSOLUTE or STAGE.
    3. the format of <factor list> is :
      rel. depth, factor(format 2f10.0),
      repeated m times, where m is the number of factors per group.
    4. the format of <node list> is :
      node label (format a12 [or a8]),
      repeated n times, where n is the number of nodes per group.

    An example of a roughness factor file is shown below:


    Application of Roughness Factors

    Roughness will be applied as a [piecewise] linear function of the 'depth', linearly interpolating between specified (roughness factor, 'depth') pairs. 'Depths' outside those specified will take the roughness factor of the highest or lowest 'depth' specified, as appropriate.

    Flood Modeller Data File Format

    To facilitate compatibility with the current version of the interface, the roughness factor file can be specified on the line beneath the RAD FILE header, with a mandatory file extension of .rff.

    The format of the first seven lines of the dat file will be

    Line 1 - Title - Up to 40 characters

    Line 2 - #REVISION#1

    Line 3 - Nnodes Frmin Frmax Dmin Stol

    Line 4 - T Qtol Htol ? P ? ?

    Line 5 - RAD FILE

    Line 6 - rff or rad file name

    Line 7 - END GENERAL

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