I sometimes get questions about how I calculated the Catchment Tc's for examples. Generally these are examples to represent a process of using DRAINS, not necessarily the absolute answer of what the time of concentration should be.

Typically in practise, modellers might assume 5 minutes minimum for impervious surfaces. I could specify 1-2 minutes for RIA areas if they were related to small garden sheds or small patios that run off onto grass (PA).

If I look at possible Kinematic Wave inputs to estimate the Tc for CatE1 from this example, I estimated the inputs in the attached image. Sometimes rear yards are landscaped / retained so slopes might be flatter than the contours. You may also add a couple minutes to PA for slowing down the time due to sealed gaps under fences or other items that may impede flow.

After analysis, I get the following reported timings in the 2nd figure (Edit Menu > Copy results to spreadsheet). Yes they are a little different to what has been manually specified into CatE1, but it is generally a reasonable approximation.

If you want to learn more about specifying catchments, I discussed Kinematic Equation briefly in the ARR Seminar, Part 3 (@ approx. 15 minutes), when to apply Kinematic VS Manning's n Equations. When the flow is sheet flow = Kinematic, otherwise it then becomes speed related (kerb & gutter, channels, swills, etc, this is Manning's. This is also discussed in QUDM 2013 Section 4.6 - Page 4-12.