how to interpolate an IDF curve? - Water Resources & Environmental - Engineer Boards
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# how to interpolate an IDF curve?

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I'm trying to obtain the intensity value from the IDF curve for a 25 year storm. The time of concentration is 17 min. Since 25 year storm is not there, how would I interpolate? Do I just take the average of the 20 year and 50 year value? Please let me know what approximate value you get =] Thank you.

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4 hours ago, saraxo said:

I'm trying to obtain the intensity value from the IDF curve for a 25 year storm. The time of concentration is 17 min. Since 25 year storm is not there, how would I interpolate? Do I just take the average of the 20 year and 50 year value? Please let me know what approximate value you get =] Thank you.

You will have to carry out three interpolations.

1. at 10 min interpolate between 20 yr and 50 yr storm to get intensity at 10 min for 25 yr frequency

2. at 20 min interpolate between 20 yr and 50 yr storm to get intensity at 20 min for 25 yr frequency

3. Interpolate between above calculated values to get intensity at 17 min for 25 yr frequency.

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@NikRCouldn't I just take the values for the 20 year and 50 year storm at 17 min?

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11 hours ago, saraxo said:

@NikRCouldn't I just take the values for the 20 year and 50 year storm at 17 min?

You definitely can. If a plot is provided, just read those values and interpolate. If a table is provided, you will have to do as my mentioned previously. For some reason I thought you were reading off a table.

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11 hours ago, NikR said:

You definitely can. If a plot is provided, just read those values and interpolate. If a table is provided, you will have to do as my mentioned previously. For some reason I thought you were reading off a table.

@NikRYeah I read the values from the curves but my textbook doesn't have the same answer. Here is how I did my interpolation:

At 17 min:

20 year    5.7 in/hr

25 year       X

50 year    6.8 in/hr

Interpolation equation:

[20 - 25] / [20 - 50 ] = [5.7 - x] / [5.7- 5.8]

Solving for x gives me roughly 5.9 in/hr. But my textbook says interpolation should arrive at 6.4 in/hr. I can't figure out what I'm doing wrong :/

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1 hour ago, saraxo said:

@NikRYeah I read the values from the curves but my textbook doesn't have the same answer. Here is how I did my interpolation:

At 17 min:

20 year    5.7 in/hr

25 year       X

50 year    6.8 in/hr

Interpolation equation:

[20 - 25] / [20 - 50 ] = [5.7 - x] / [5.7- 5.8]

Solving for x gives me roughly 5.9 in/hr. But my textbook says interpolation should arrive at 6.4 in/hr. I can't figure out what I'm doing wrong 😕

I'm not sure how to derive to the according intensity value but the plot is based on a curve. 5.9 in/hr was derived through linear interpolation so I think another method would need to be used to derive to the needed intensity value. I'm just not sure which method to use..

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8 hours ago, jgiang said:

I'm not sure how to derive to the according intensity value but the plot is based on a curve. 5.9 in/hr was derived through linear interpolation so I think another method would need to be used to derive to the needed intensity value. I'm just not sure which method to use..

not sure how thats the answer. unless they are using a different figure.

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@NikRIm only using the figure provided in my original post. Can you try interpolating it based on this curve and see if you get the same answer as mine?

@jgianghmm I never thought of "curve" interpolatiom but i have no clue how to do that either lol o.O

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On 3/25/2019 at 7:48 AM, saraxo said:

@NikRYeah I read the values from the curves but my textbook doesn't have the same answer. Here is how I did my interpolation:

At 17 min:

20 year    5.7 in/hr

25 year       X

50 year    6.8 in/hr

Interpolation equation:

[20 - 25] / [20 - 50 ] = [5.7 - x] / [5.7- 5.8]

Solving for x gives me roughly 5.9 in/hr. But my textbook says interpolation should arrive at 6.4 in/hr. I can't figure out what I'm doing wrong 😕

Using above methodology the 25 year storms curve would be just slightly above the 20 year storm which is incorrect. In reality it would be roughly half way between the 20 and 50 year events.

From the chart I get 20 = 5.8,  50 = 6.9

(6.9-5.8)/2=.55 (average)

5.8+.55=6.35 (20 yr event + avg.)

Reason being that the curve is based on the probability of that storms recurrence and not just the difference in years. A 25 year storms chance of recurrence is somewhere in the vicinity of halfway between a 20 and 50.

Hope this helps.

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@Jbone27 PEthanks! I think i'll go with your explanation as it makes more sense than linear interpolation haha

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@saraxoYeah long way for me to just say take the avg. lol

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