May 5, 2006

Time Area Method-losses

Hi Dr Quek,

Q 1:
I want to ask you about Time Area Method calculation for pre development as in your spreadsheet, for losses, why did you minus by 0.4 x rainfall not 0.2 as in Table 14.4 (MSMAM) ?

I want to confirm with you about the temporal pattern, as MSMAM said we need to use the nearest temporal pattern, for example if our tc = 25 minutes so we need to use temporal pattern for 30 minutes (MSMAM, 13.3.5)


Filed under Design Rainfall, Detention Basin, Time-Area Mthd by Rahiman

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Comments on Time Area Method-losses »

May 5, 2006

Dr. Quek @ 11:17 am

Hi Rahiman,

Thanks for your question.

Reply to Q1:

The Loss Model I used in my worked example is based on "Impervious Areas" which according to Table 14.4 involves an initial loss of 1.5 mm and a loss rate of 0 mm/hr.

The Loss Model of 20% rainfall you referred to is for "Pervious Areas".

Kindly refer the above table for verification.

Reply to Q2:

You brought up a good point!

Cl 13.3.5 states that:

For other durations, the temporal pattern for the nearest standard duration should be adopted. It is NOT correct to average the temporal patterns for different durations.

So to answer you question: if tc=25 min, we need to use the temporal pattern for 30 min.

So in hindsight, what I told you all to interpolate in the workshop may not be necessary at all!

My suggestion to interpolate is based on my rational thinking- if you have if the standard durations are 60 and 120 min. And you have a storm of 90 min. The “rational” way would be to interpolate it between 60 and 120 min. This was the way we did it when using the temporaral patterns in HP1. But apparently this is not what MSMAM recommends us to do.

If you any more question, please feel free to add your comments below.

Dr. Quek

March 19, 2007

Rahiman @ 11:51 am

Dr. Quek,
I had met JPS Johor to submit the detention pond design based on your spreadsheet. They comment about the outflow, (in my case) why did the after 10min the pond start to discharge and why the Q stay the same for several minutes?

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