Calculating runoff for stormwater control

Step-by-step:

The goal in the catchment design process is to determine how much surface area of a catchment is impermeable, and how much runoff is caused by those impermeable areas. To do so, you must calculate the percentage impervious value, or “PIMP value,” which is the percentage of the catchment that is covered with tarmac, concrete, or other materials that rain cannot penetrate. InfoDrainage only accounts for runoff from those impermeable areas.

1. On the ribbon, Build tab, Data panel, click Tables.
2. In the Tables dialog box, Inflows tab, expand the Inflows node.
3. Expand the Catchment Area node.
4. Expand the Time of Concentration node.
   
   
5. Click the checkboxes to enable these options:
• Preliminary Percentage Impervious
• Summer Volumetric Runoff Coefficient
• Winter Volumetric Runoff Coefficient
• Percentage Impervious
  

6. Review the values in the columns of the table for accuracy.
   
7. The two Percentage Impervious columns should both be set to 60 percent, but if they are not, change them.

InfoDrainage assumes:

• 60% of each catchment is hard standing impermeable surface, such as roads or roofed areas.
• A fixed amount of runoff is caused by impermeable surfaces for the Summer and Winter Volumetric Runoff Coefficients.
• The ground is drier during the summer; therefore, during a rainfall event, 75% of the runoff will make it into the manhole in that subcatchment.
• The ground is wetter during the winter; therefore, during a rainfall event, 84% of the runoff will make it into the manhole in that subcatchment.

Keep in mind that these values work for this exercise, but they may need to be increased or decreased for other situations. These values are critical because they influence how much rainfall enters a drainage system in a model, and therefore dictate design elements like pipe and storage sizes.

8. When finished, Close the Tables dialog box.