Time of Concentration - The amount of time that it will take for water landing on the roof area to enter the system. The inflows from the area will then be evenly spread over this time when the area is analyzed. This amount of time can be calculated with the Time Of Concentration Calculator.
Percent Impervious - The impervious portion of the Inflows area. When the Use Land Uses/Soil Types layers box is checked, the Percentage Impervious is calculated from weighted averages of Land Uses.
Urban Creep - This scales the impermeable area by the amount specified. This can be used to take the increase of urban areas, or other factors into account. This value is only enabled (and used) when the analysis creep of the Analysis Criteria is set to "Use Catchment Areas ".
Analysis
The analysis is the same as the Time Area Diagram method with the time area diagram being built with an even spread of the area over the time of concentration.
Eg. Time of Concentration = 3 mins, Area = 0.75ha
If the timestep is 1 minute, A1 = 0.25, A2 = 0.25, A3 = 0.25.
The rainfall profile and the time area diagram are combined with the volumetric runoff coefficient in the following way:
Q1 = (I1 x A1) x 2.78 x Cv
Q2 = (I1 x A2 + I2 x A1) x 2.78 x Cv
Q3 = (I1 x A3 + I2 x A2 + I3 x A1) x 2.78 x Cv
and so on.