5 minutes is commonly used as the minimum value for the Time of Concentration. This is due to the storm duration used in the IDF curve equation which is valid between 0.0833 hours (5 minutes) to 72 hours and is referred in Storm Management Manual for Malaysia (MSMA).
By reason of that, minimum time of concentration setting is implemented in the MiTS which can be adjusted in the following directory “Options > Project Parameters > Drainage > Design > Min. Time of Concentration”. This setting is applied to ensure that the tc is always more than the minimum value setting.
However, if your value of duration is less or higher than the range, do exercise your judgement on the validity of the result and confirmation regarding this should be referred to the authorities.
Other supportive reference regarding minimum time of concentration:
- MSMA 1 Chapter 14 Flows Estimation and Routing provides a table of recommended minimum tc for the different drainage elements.
- MSMA 2 Chapter 2 Quantity Design Fundamentals states that the recommended minimum tc is 5 minutes for roof drainage.
2. MSMA 2 Chapter 4 Roof and Property Drainage states that a storm duration of 5 minutes should be adopted for all roofs. (based on Point 1)
Roof Drainage: Drainage systems which are located at the top of property drainage systems.
3. MSMA 2 Chapter 1 Design Acceptance Criteria. Based on the Point 1, it does not state for the property drainage, but refers to Chapter 1, which mentions that usually 5 minute duration design rainfall of 20, 50, and 100 year ARIs shall be used for the sizing of property drainage components.
Property Drainage: System which transfers runoff from roofs, paved areas and other surfaces of a premise to a suitable outlet or disposal facility.