Urban Flood Risk Mitigation model-CN Method

Science, Research & Data
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Hi Everyone,

I am addressing some challenging comments from some tough reviewers and I need your help to clarify something. The Urban Flood Risk Mitigation model is based on the CN method, which according to the reviewer assumes 24h rainfall event. For the study I used a value of 24mm of rainfall depth that triggered a flood in my study area. It was a rain event that occurred in 1 hour. Does the CN need to be adjusted since it is a less than 24 h rainfall event? Thanks for any feedback!!
Laura

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Thanks for the question @laura78!

Maybe @chris or @b.janke would be able to speak on this?

Cheers,

Doug

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Hi Laura,

Sorry for the slow reply here. My understanding of the Curve Number method is that, while it is often applied using 24-hour storms (because that is a common design standard for stormwater infrastructure), it is intended to be applied at the time scale of a single storm, whatever the duration might be. According to Chapter 10 of the US Department of Agriculture’s National Engineering Handbook (the document that outlines the Curve Number method – CN tables are in Chapter 9 of this report):
“The NRCS runoff equation was developed to estimate total storm runoff from total storm rainfall. That is, the relationship excludes time as a variable. Rainfall intensity is ignored.”

This states pretty strongly that there is not an implied duration. I did a quick lit search and found one paper that states that duration and intensity do have some effect on the CN method but that these effects are not consistent (further, they used 60-minute and 90-minute storms, similar to your storm). You might poke around the literature and see what else there is, though, this method has been used and adapted for a long time.

Incidentally, your storm size seems really small to me, so you must be in an arid watershed? I think there are some considerations to make when applying curve numbers in more arid regions or in cases of drier climate (i.e., Antecedent Runoff Conditions, and I think you would use ARC Type I for dry conditions – see page 10-5 in the handbook).

I hope this is helpful.
Ben

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Thank you Ben. I just need to address some comments from a very hard reviewer. Hopefully, there will be a very relevant paper published using this modeling tool!