NRCS TR-60 Rainfall Curve 4

[RWF7437]

NRCS TR-60 (2005) Earth Dams and Reservoirs

I am attempting to design an earth dam near Salem, Oregon, USA using the National Resource Conservation Service’s TR-60.

TR-60 includes a dimensionless rainfall curve ( Fig. 2-4, page 2-12) which is prescribed to be used in the design of the dam spillway and to calculate required freeboard. TR-60 does NOT, however, provide any information on the origin of this curve or the range of its applicability.

Any experienced dam designers:

1. Where does Fig. 2-4 come from?
2. Is it applicable to all 50 United States ?
3. Is it the most conservative method available; assuming a 50 year design life and a desired confidence interval of 95% ?
4. Is it based on the NRCS “standard” 24 hour storms such as Types I, II, III and IA described in TR-55 ?
5. Is it necessary, or desirable, to route a series of storms through the dam / reservoir in order to determine the critical storm duration for design ?

 

[RWF7437]

"...the first thing you should do is to check with the state dam safety office and determine what criteria they require for the design of dams."

Oregon has no "presciptive" design standards for dams; or any other clear standards that I've been able to find. This is why I've sought out other standards, and the advice of experienced dam designers.

" Also, the temporal distribution of the rainfall can have a large effect on the resulting hydrograph. The emergency spillway distribution with a 40+" rainfall is essentially like designing for a Type II rainfall of around 30".

It's pretty rainy in the Willamette Valley of Oregon. Annual rainfall is about 42 inches. The 24 hour Probable Maximum Precipitation is about 13 inches. Also, in Oregon,
the Type IA rainfall is most commonly used but orographic effects often render such "standard" storms highly suspect.

Where in the world would 40" and 30" rainfall occur? Are these PMP amounts ?
For what duration ?
What temporal pattern are you comparing to the Type II storm ?

Thanks for your interest.

 

[RWF7437]

http://www.weather.gov/oh/hdsc/PMP_documents/HMR39.pdf

 

[NinetyWt]

I'm a little late to the party RWF, but where I am, the PMP is a 24-hour storm. We are using a different distribution than the 'standard' SCS 24-hour storms.

What we generally do is set the E/S and top of dam using criteria found in the TR-60 documentation. For a high hazard dam here, that is the 100-year 24-hour storm for the principal spillway (sets the E/S crest elevation) and the full PMP for the top-of-dam.

13 inches for a 24-hour PMP seems low, but then I haven't been a student of hydrology out there in Oregon. Our PMPs here are all over 40 inches. For years, the standard was 30.5 inches but the regulations were changed a few years ago. I do think that it is different across the country; it actually varies by county here.

Interestingly, for small watersheds the new PMP & distribution predict a smaller peak.

I'm not sure if you ever got answers to your questions, but I do some dam design, and we have a fellow on staff of my partner company who retired from NRCS and has designed probably hundreds. I can pass along any further questions you may have to him, if you'd like.

 

[RWF7437]

Thank you NinetyWt,

"... but where I am, the PMP is a 24-hour storm."

1. Where is that, please ?
2. Do you ever investigate storms of other durations ?

" We are using a different distribution than the 'standard' SCS 24-hour storms."

3. Are you using the distribution shown in TR-60, Fig. 2-4 ?

"What we generally do is set the E/S and top of dam using criteria found in the TR-60 documentation. For a high hazard dam here, that is the 100-year 24-hour storm for the principal spillway (sets the E/S crest elevation) and the full PMP for the top-of-dam."

4. Much confusion here. TR-60 uses the term " principle spillway " to describe the outlet(s) at or near the bottom of the dam. What you and I call the "emergency spillway" TR-60 calls the "auxiliary spillway". To confuse things even more, NRCS talks about freeboard above the aux. spillway to ensure no overtopping.

"13 inches for a 24-hour PMP seems low, but then I haven't been a student of hydrology out there in Oregon."

5. My PMP numbers come from HMR 57, Map 4 for a 24 hour general storm over a drainage area of at least 10 mi^2. Where do you get the PMP numbers you use, please?

http://www.nws.noaa.gov/oh/hdsc/studies/pmp.html

"Interestingly, for small watersheds the new PMP & distribution predict a smaller peak."

6. What is meant by "new PMP" here ?

"We have a fellow on staff of my partner company who retired from NRCS and has designed probably hundreds. I can pass along any further questions you may have to him, if you'd like."

7. Please do, and thanks again.

 

[RWF7437]

Salem Rainfall Data

 

[NinetyWt]

'1. Where is that, please ?'
We are in Central Mississippi.

'2. Do you ever investigate storms of other durations ?'
Not usually - if you mean, to set the E/S or top-of-dam crest for a high hazard dam. Some smaller dams can be designed to some percentage of the PMP.

'3. Are you using the distribution shown in TR-60, Fig. 2-4 ?'
I wasn't sure until I looked it up, but yes, it appears to be the same distribution. If you will check out the regulation at this link:

http://www.deq.state.ms.us/newweb/MDEQRegulations.nsf/RN/LW-4

you will find that distribution on the next-to-last page. It looks to me like the old 6-hour E/S distribution is used as a "dimensionless" distribution.

'4. Much confusion here. TR-60 uses the term " principle spillway " to describe the outlet(s) at or near the bottom of the dam. What you and I call the "emergency spillway" TR-60 calls the "auxiliary spillway". To confuse things even more, NRCS talks about freeboard above the aux. spillway to ensure no overtopping.'
Well, I was trained by old SCS and Corps engineers to I'm used to those terms. We recommend at least 1 foot of freeboard over the E/S storm peak, to allow for uncertainties. We also allow another 6 inches for settlement of the embankment.

'5. My PMP numbers come from HMR 57, Map 4 for a 24 hour general storm over a drainage area of at least 10 mi^2. Where do you get the PMP numbers you use, please?'
Our PMP numbers can be found on the last page of the LW-4 regulation I linked to above. According to that, the numbers were developed from HY-51. Looks like you are using the correct publication for your area(HMR 57).

'6. What is meant by "new PMP" here ?'
Sorry about that, I meant the newer values which are 40+ inches (From 40 inches at the TN line to 48 inches on the Gulf Coast) vs. the "older" value of 31 inches which used to be used state-wide (I mis-spoke in my previous post, it was not 30.5). One of the previous posts above mentions that - the peak of a 40" rain with the new distribution is similar to that of a 30" rain with the SCS Type II distribution.

'7. Please do, and thanks again.'
I will probably see Phil tomorrow.

I should also tell you that we are using the NRCS program SITES to do our dam design. Previous to the development of SITES, we used DAMS-2. I don't know if you are familiar with those programs (please forgive me if you are) but they have "built-in" distributions for the standard SCS storms. In order to model this newer PMP in DAMS-2, what we did was use the rainfall data mentioned by Mr. Smart above (mass-curve values) as a rainfall data table; we identify that it is of 24 hour duration; then we give the total rainfall (for example, 42 inches) as input.

 

[RWF7437]

Thank you NinetyWt,

Having spent two years in Vicksburg, at the Waterways Experiment Station, I'm a little familiar with that Red Carpet City of the South ( AKA The Gibralter of the Confederacy).

The reason for my many questions is that I like to look "behind" the regulations as well as at them. Yes I have used "SITES.

I'll look forward to any insights your friend Phil may choose to offer. Meanwhile, Ill look up the HMR for Ole Miss.

Thanks again.

Russ

 

[NinetyWt]

The reason for my many questions is that I like to look "behind" the regulations as well as at them.

That is an excellent philosopy, and I imagine that it has served you well.

I haven't ever worked at WES in Vicksburg, but I know a lot of folks there. I'm working over in Jackson.

The only things I know about Oregon I learned from a friend who lives in Corvallis. He says that it's a great place to live.

Anyway, Phil and I had an interesting conversation about these rainfall distributions this morning.

If you have access to a copy of TR-55, there is a figure in it which shows the rainfall distributions for the Type I, Type IA, Type II and Type III standard SCS storms. The six-hour ESH distribution which used to be in TR-60 is very similar to the middle six hours of the Type I storm. We think that is its origin.

The Figure 2-4 which is in the 2005 version of TR-60 appears to be a dimensionless representation of that same six-hour distribution.

As to its applicability out west, Phil was of the opinion that it would give a reasonable answer, if a bit conservative over the Type IA storm distribution you would use for the 100-yr 24-hour storm.

Hope that helps.

 

[RWF7437]

Thank you NinetyWt and Phil,

You have at least confirmed some suspicions.