Time of Concentration Calculator

  • Tony Ingold

Time of Concentration is the time it takes for water to flow to its outlet from the farthest point on a catchment.

12d Model will calculate the Time of Concentration for a catchment based on the ‘Finished Surface’, and the flow lines that you draft.

There are three steps:

  1. Draft the flow line that shows the route that a drop of water would take from the farthest point on the catchment to the node.
    These flow lines are referred to as ‘Tc Strings’, and are all drafted into a model
  2. Tell 12d Model that strings in the model are Tc Strings, and are to be used to calculate Time of Concentration
  3. Tell 12d Model which method is to be used to calculate the Time of Concentration (Kinematic Wave, Friend Equation, etc)

Draft Flow Lines.

This requires a little background.
12d Model allows you to specify five different ‘Catchment Sets’ that deliver water to a node.  This allows you to identify different land uses with different runoff characteristics, and to model each land use separately.
For Example, in an area of residential land development different land uses might be roads, residential properties and parkland.

Catchment boundaries (polygons) would be created for catchments in each of the land uses and each of the catchments would have a proportion that was impervious, and a portion that was pervious.

So we have different three different catchment sets, each with a % impervious and a % pervious – conceivably six different Tc strings.

The reality is usually a little more simple.

  • A single Tc string can be drafted for a catchment, and used for both the pervious and impervious part of the catchment.
  • Tc may not need to be calculated for each catchment.
    Often road catchments have a minimum Tc of five minutes, (sometimes six minutes) and this can be made a ‘Direct’ setting. If Tc is not being calculated then a Tc String is not required.

Workflow for Creating Tc Strings

  • In the Cad Control Bar, type the model name for the Tc string that you plan to draft.
    Set a linestyle- ‘flowline’ is very popular.
    Set a colour – whatever colour pleases you.
  • Use the ‘CAD Line String’ tool to draft a flow line from the high point on the catchment, to the node

In the example the model is ‘SW Tc2 Lot Pervious’.  This model will contain Tc Strings for the Pervious part of Catchment Set 2 (which is the residential lots).

Model of Flow Lines to be Tc Strings

This is done in the Catchment File, which is accessed from the Global/Utility Model tab on the Water Network Editor

In the screenshot below, 12d Model is being told that the model ‘SW Tc2 Lot Pervious’ contains Tc strings relevant to the Pervious part of Catchment Set 2

[Screenshot of the Catchment File]

Choose the Tc Calculation Method

This is done in the Water Network Editor, and can be done for an entire catchment Set (using the Default/Catchment tab) or for an individual catchment (using the Catchment tab)

In the screenshot below, the Default/Catchments/Set 2 tab is shown, and 12d Model is being told to use the Friend Equation to calculate the Tc for the Pervious part of all catchments in Catchment Set 2.