This article provides the fundamentals required for designing the stormwater management facility, which includes the calculation of **Time of Concentration, t _{c, }**

**Time of Overland Flow Time,**

**t**and

_{o}**Time of Drain, t**.

_{d}

Project file example

Project file example

# Time of Concentration, t_{c}

According to MSMA, t_{c} is the travel time of runoff flows from the most hydraulically remote point upstream in the contributing catchment area to the point under consideration downstream. There are** two methods** of calculating the t_{c} which are

**First Method (total of t**_{o} and t_{d})

_{o}and t

_{d})

### Time of Overland Flow Time, t_{o}

T_{o} is estimated based on the land surface condition which determines the catchment roughness and also is affected by the distance and slope of the water flow in the catchment. The Horton’s Roughness for various land surfaces are given in Table 2.2.

Formula to calculate the t_{o}:

**t_{o}=\frac{(107 . n . L^{1/3})}{S^{^{1/5}}}**

t_{o} = Overland sheet flow travel time (minutes)

L_{o} = Distance of flow path travel from the catchment area towards the drain (m)

n = Horton’s roughness value (Table 2.2)

S = Slope of overland surface (%)

**Example of calculation of t _{o} based on Catchment 1:**

**How to calculate the L _{o}:**

Based on the project file attached, see Catchment 1. The distance (L) is calculated based on the estimation of the flow path in the catchment towards the drain. The unit used is in meters.

The L is also auto calculated in the MiTS. You can refer the L in the catchment spread input.

**How to calculate the S:**

The S is the slope of the overland flow in the catchment and it is calculated in percentage (%). It is calculated from a point from the upstream to another point of the downstream which is at the drain. The S is calculated by using the same line used to calculate the L_{o}. For example,

Upstream elevation : 31.921 m

Downstream elevation : 30.946 m

Basic equation of gradient is applied to calculate the slope.

Slope, S_{o} = \frac{Vertical Distance}{Horizontal Distance} x 100

= \frac{31.921 - 30.946}{133.692} x 100

= 0.729 %

The slope is also auto calculated in MiTS. See catchment spread input.

How to determine the Horton’s Roughness, n:

Typical values of Horton’s Roughness are given in Table 2.2 in MSMA 2.

Select appropriate n for the surface condition. In this example, we will be choosing n for Average Grassed 0.045. **Therefore, t _{o} of Catchment 1:**t_{o}=\frac{(107 . n . L^{1/3})}{S^{^{1/5}}}

t_{o}=\frac{(107 . 0.045 . 133.692^{1/3})}{0.729^{^{1/5}}}

= 26.225 min

T_{o} is auto calculated in MiTS.

Time of Drain, t_{d}

The t_{d} can be calculated by using Constant Average Velocity with the assumption of Velocity, V 1 m/s, or by using Manning Equation. You can change the settings in the “Options > Project Parameters > Drainage > Design > Drain Flow Time”.

For this example, we will be using Constant Average Velocity for the t_{d} calculation. The equation used is:

**T**_{d} **=** **L/V**

_{d}

L_{d} = Length of Drain (m)

V = Velocity (m/s)

**How to calculate the L _{d}:**

The L is the length of the drain. For example, the length of Drain 1 is the L_{d} for that drain.

It is auto measured in MiTS. See Drain Spread Input.

**Therefore, t _{d} for Drain 1:**

T_{d = L/V}

T_{d} = 100/60

= 1.667 min

T_{d} is auto calculated in MiTS.

**Therefore, t _{c} of Drain 1 is the total sum of t_{o} and t_{d}. Thus,**

T_{c} = t_{o} + t_{d}

= 26.225 + 1.667

= **27.892 min**

T_{c} is auto calculated in MiTS.

How about the consecutive drains? How to determine the t_{c}?

The next drain t_{c} calculation will consider the t_{c} from the previous drain as well. See Drain 2 as an example.

- Calculate the t
_{o}for Drain 2 which is from Catchment 2 and results in 23.537 min.

2. Calculate Drain 2 t_{d}. The t_{d} is 1.667 min

3. Take the longer t_{c} between these two equations:

**Equation a** **:** **t _{cD1} + t_{dD2}**

= 27.891 + 1.667

= 29.558 min

**Equation b** **=** **t _{oD2} + t_{dD2}**

= 23.537 + 1.667

= 25.204 min

** Equation a results to higher value, therefore 29.558 min is adopted for t_{c} of Drain 2. **MiTS will detect whichever higher to be the t

_{c}.

4. Repeat the steps towards the end of the network.

### How to adapt the first method to t_{c} calculation in Pond Design?

If you have multiple branches of networks, there would be multiple values of t_{c} which carries the t_{c} of each of the networks.

Based on MSMA 2 Chapter 7 Detention Pond, the t_{c} is chosen based on the longest t_{c} among the three different networks. Since drainage module auto calculates the t_{c}, you can simply refer to the table and find the longest _{tc} among those three networks.

Based on the output, the longest t_{c} lies at the network 2 with the longest t_{c} 53.44 min. Therefore, override the t_{c} 53.44 min in the pond module.

If you still need to plug in the values of t_{o} and t_{d} in the pond design, therefore you only need to fill the input based on the network 2.

**Second Method (Bransby Williams)**

Another method of calculating t_{c} for Pre Development is by using Bransby Williams Equation. This method is better suited for catchments which consist of natural streams or catchments without well-defined drainage channels.

You can change the method of Pre Development t_{c} in Pond Module by changing the settings in “Options > Project Parameters > Det. Facility > Pond > Design > Pre Development > T_{c} Calculation Method”

The equation of Bransby Williams:

**t_{c} = \frac{F_{c}.L}{A^{1/10}.S^{1/5}}**

T_{c} = time of concentration (min)

Fc = conversion factor, 92.5 when area is in ha (MiTS is using ha)

L = Length of flow path from the summit of catchment to outlet (km)**input the L in MiTS in meter, it will be converted automatically to km*A = Catchment area (ha)

S = Slope of stream flow path from the summit of catchment to outlet(m/km)

**How to calculate L:**

The L is calculated from the peak of catchment towards the downstream. The unit used in the equation is in km. However, you only need to key in the value in MiTS in meters, which software will automatically change the unit to km to suit the equation.

The L is measured as example below:

How to determine the A:

Measure the catchment area in ha. Then insert the value in A box. For this example, the catchment area is 18.755 ha.

How to calculate the S:

S is the slope of the flow path which is calculated in m/km. It is calculated in m/km from the upstream point towards the downstream point. The S is calculated by using the same equation of slope.

Upstream elevation: 34.729 m

Downstream elevation: 23.389 m

S = \frac{Vertical Distance in m}{Horizontal Distance in km}

= \frac{34.729-23.389}{530.82/1000}

= 21.36 m/km

**Therefore, the t**_{c} by using Bransby Williams is:

_{c}by using Bransby Williams is:

t_{c} = \frac{F_{c}.L}{A^{1/10}.S^{1/5}}

tc= \frac{92.5 x 0.53082}{18.755^{1/10} x S^{1/5}}

= 19.85 min

MiTS result:

You may also be interested in :**How to determine the ‘S (%)’ and ‘L (m)’ value for catchmentModelling Detention Drain in MiTS 2**