Limiting discharge

Video quiz

When calculating discharge for a drainage system, which of the following controls the receiving watercourse and dictates the amount of water you are allowed to discharge into it?

(Select one)
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Step-by-step:

When you design a drainage system, you must often meet certain requirements that limit the discharge from the new design, into whichever receiving water course it discharges into. This could include rivers, streams, or another pipe network.

A graphic of runoff moving from pipes to storage to a discharge control to a water course.

A common design criteria is the so-called “no detriment” theory, which states that once a drainage system is developed, the discharge from it should not be any greater than what it was before it was built. InfoDrainage can calculate what those pre-development discharge levels are so you can make a post-development comparison.

  1. On the ribbon, Results tab, Reports panel, click Inflow Summary.
    The InfoDrainage ribbon, Results tab, Reports panel, with Inflow Summary selected.
  2. In the Inflow Summary dialog box, note the total area of the catchments; in this case, 4.2 hectares.
    The Inflow Summary dialog box showing the total area of the model catchments in hectares.
  3. Close the dialog box.
  4. In the ribbon, Preliminary Sizing tab, Calculators panel, select UK and Ireland Rural Runoff.
    The Preliminary Sizing tab, Calculators panel with UK and Ireland Runoff called out
  5. In the UK and Ireland Rural Runoff Calculator dialog box, set the Method to ICP SUDS.
  6. Set the Area to 4.2, the total area previously listed in the Inflow Summary dialog box.
  7. Set SAAR to 623.
  8. Set Soil to 0.45.
  9. Expand the Region drop-down and select Region 6.
  10. Set the Return Period (years) field to 30.
  11. Click Calculate.
    The UK and Ireland Rural Runoff Calculator dialog box with settings configured and the Calculate button called out.

It is worth noting that the most common value used in runoff studies is the QBAR Urban value, which represents the discharge rate you are allowed in your drainage system. This provides good design criteria to prevent detrimental effects downstream, so in this example, the discharge from the site would be limited to 16.1 l/s.