6.2. Prepare Data

6.2.1. Coefficient of friction

  • The plugin will export the roughness according to these priorities:

    • If reach.coefficient_of_friction is set then it is converted (1/coefficient_of_friction) and exported to SWMM

    • Else if reach.wall_roughness and reach.clear_height are set, it is converted ((1/(4 * SQRT(9.81)*POWER((32/re.clear_height/1000),(1/6))*LOG(((3.71*re.clear_height/1000)/(re.wall_roughness/1000)))))) and exported.

    • Else if reach.swmm_default_coefficient_of_friction is set then it is used

    • Else a default value of 0.01 is set

    • See vw_swmm_conduites.sql for details

Note

The formula to convert from Strickler to Coolebrook is documented in *ATV-DVWK STANDARD ATV-DVWK-A 110E, Hydraulic Dimensioning and Performance Verification of Sewers and Drains September 2001*

6.2.1.1. Populate default coefficient of friction

The table tww_swmm.reach_coefficient_of_friction contains default coefficient of Strickler for the reach materials. The file ../project/qml/reach_coefficient_of_friction.qml is the layer definition, that you can import in your QGIS project. If the default values don’t match your needs you can edit them. The function SWMM Set default coefficient of friction will put in tww_od.reach.swmm_default_coefficient_of_friction these default values. You can choose if you want to overwrite or not the values existing in this attribute.

Launch SWMM Set default coefficient of friction:

  • Overwrite existing default values: If checked the values stored in swmm_default_coefficient_of_friction will be overwrited with new values.