# Can someone help me confirm that a CERM erratum is wrong?



## K19 (May 31, 2012)

Hello Friends,

I'm using the CERM 12th Edition (1st printing), and believe that Equation 20-11 (Hydrology - Time of Concentration from Watershed Lag Method) is correct in it's original form and that the following erratum posted on PPI's website is wrong.



> Change "100" to "(60 min/hr)", since the NRCS Lag Equation determines the time of concentration in hours.


The equation is for tc (Time of Concentration) in minutes, which is empirically correlated to lag time by tc = 1.67 tlag.

The expression for tlag in hours presented later in the chapter (Equation 20-23) is correct - I consulted the NRCS National Engineering Handbook to confirm this.

Hence the 100 coefficient (1.67 x 60) in the numerator of Equation 20-11 converts tlag in hours to tc in minutes.

Is this (and the equation as written) correct, or am I missing something?

Thanks,

K


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## ptatohed (May 31, 2012)

K, I don't have a CERM in front of me, can you please post Eqn 20-11 as it is printed in the CERM12? Thanks.


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## civilized_naah (May 31, 2012)

K19 said:


> The expression for tlag in hours presented later in the chapter (Equation 20-23) is correct - I consulted the NRCS National Engineering Handbook to confirm this.
> 
> Hence the 100 coefficient (1.67 x 60) in the numerator of Equation 20-11 converts tlag in hours to tc in minutes.
> 
> ...


At the risk of answering without having the documents in front of me, I believe you are correct. The version which gives answer in hours has the coeff 1.67 and the version which gives answer in minutes has coeff 100


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## K19 (Jun 1, 2012)

Thanks guys. Here's Eqn 20-11 as published:

tc (minutes)* =* 1.67 tlag* =* [ 100 L0.8 (Sin + 1)0.7 ] */* [ 1900 √(S%) ]

Later in the chapter tlag is given as:

tlag(hours) *=* [ L0.8 (Sin + 1)0.7 ] */* [ 1900 √(S%) ]

After conferring with the NRCS National Engineering Handbook Part 630 - Hydrology ("straight from the horses mouth"), I've sufficiently convinced myself that 100 is the correct coefficient, and that the erratum is wrong.

I'll probably submit this to PPI as an erratum since there doesn't appear to be any way to report an incorrect erratum / false positive. I thought it was standard due diligence for authors to verify errata before publishing them...


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## ptatohed (Jun 12, 2012)

I am confused what Sin is. Is this related to the NRCS curve number (CN)?


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## K19 (Jun 13, 2012)

ptatohed said:


> I am confused what Sin is. Is this related to the NRCS curve number (CN)?


Correct. S = (1000 / CN) - 10, where S (inches) is defined as "potential maximum retention after runoff begins" in TR-55. I've yet to see this term properly explained in plain English (engineers, amirite?) but I understand it to be representative of the soil's capacity to uptake rainfall via infiltration (higher CN -&gt; lower S -&gt; more runoff).

Eqn 20-11 is also written in terms of CN instead of S (also with the 100 coefficient).


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