How to Customize the IDF Coefficient for Detention Pond

Climate change is having an impact on Malaysia’s rainfall patterns, with the country now experiencing higher levels of rainfall year-round due to the rising global temperature. The intense rainfall will definitely exacerbate the risk of flooding and other water-related disasters, especially in areas that are vulnerable to these events.

To mitigate these risks or disasters, it is essential for engineers to carefully study and consider the severe rainfall, especially when designing detention ponds that are intended to regulate the flow of runoff into the nearby water bodies. Hence, this explains why there is a need by the authorities to have the engineers take the Climate Change Factor (CCF) into account when making rainfall intensity calculations.

So, how do we incorporate these values in MiTS 2 Detention Pond design? #

There are two ways of integrating CCF values in our software for detention pond design (using Rational Method and Time-Area Method) which are (i) modifying the IDF coefficients for the location of your project, or (ii) selecting the option “include CCF” under the Project Parameters, with the latter only available starting with MiTS 2.9.60.0.

Both methods involve the multiplication of the CCF value from the guidelines to the rainfall intensity, with the main difference being that the first method requires manual computation by the user, while the second method involves automatic computation by the software.

Modifying IDF Coefficients #

Example project file here

Using this method, users will need to multiply the CCF value to the lambda, λ, one of the coefficients for rainfall intensity computation, which can be edited in our MSMA Detention Pond project.

In MSMA module, for Pre-development > scroll to the ‘Stormwater Quantity Design Criteria’ > Click on the Edit button (highlighted in yellow) > edit the λ value > Click OK

Example of the computation:

Location: Kuala Lumpur – Puchong Drop, K Lumpur

t_c = 24.33min

λ = 69.6500

K = 0.1510

θ = 0.2230

ɳ = 0.8800

CCF (ARI 50 years) = 1.21

CFF (ARI 100 years) = 1.22

ARI 50 years:

λ = 69.6500 x 1.21 = 84.2765

K = 0.1510

θ = 0.2230

ɳ = 0.8800

Rainfall Intensity:

$$\begin{aligned} i\; &= \frac{\;(\lambda T^K)}{{(d+\theta)}^ƞ}\ \\ i\; &= \frac{\;(84.2765\times50^{0.1510})}{{({\displaystyle\frac{24.33}{60}}+0.2230)}^{0.8800}}\ \\ i\; &= 228.951\;mm/hr \end{aligned} $$

ARI 100 years:

λ = 69.6500 x 1.22 = 84.973

K = 0.1510

θ = 0.2230

ɳ = 0.8800

Rainfall intensity:

$$\begin{aligned} i\; &= \frac{\;(\lambda T^K)}{{(d+\theta)}^ƞ}\ \\ i\; &= \frac{\;(84.973\times100^{0.1510})}{{({\displaystyle\frac{24.33}{60}}+0.2230)}^{0.8800}}\ \\ i\; &= 256.314\;mm/hr \end{aligned} $$

Include CCF Option #

Example project file here

In MiTS 2 later versions, the option to include the CCF in the calculation of the rainfall intensity for detention pond design is available for your project. To enable this option, users can follow these steps: