Hydrographs and Time to Peak 2

[zebco]

I guess I need some reassurance or correction.
I'm developing hydrographs.
I've got a project under review, the Tp or times to peak are being questioned, I kinda question them too.
Scenario, 0.22 acre site, pre-development is all woodland (0.4C), post-development is a roadway with C&G (0.96C).
Time to peak flow calculates as 4 minutes pre and 26 minutes post development. 300 foot reach 5% slope. Does this sound reasonable?

 

[Trackfiend]

Intuitively, this doesn't sound right. Without having any other information (routing or redirecting of post development runoff), it would seem that going from a wooded area (pervious) to a C&G which I assume is concrete and gravel (impervious), the times to peak would increase. However, you haven't given enough information to make that deduction.

First, what method and formula(s) are you using to determing tp? If you are using the rational method (which it looks like you are), then the time to peak should match the time of concentration. What changed from pre to post development to cause the time of concentration to increase? Are there obstructions, a new meandering ditch, detention area, etc.? Secondly, are you using software or calculating by hand. I know sometimes when using hydraulic software (hydraflow, SWMM, etc.), I always backcheck to ensure the proper IDF curve, precipitation data, or even the return period are correct for each iteration I use.

 

[zebco]

The Tc does decrease from pre to post. I'll have to post the formulas later tonight, I'm at work without the info.

 

[cvg]

0.22 acres is much to small to analyze with a hydrograph. If you are using rational method, there is no hydrograph or time to peak, instead there is a time of concentration.

Never the less, all other things being equal, increase in impervious area generally results in a reduction in time for the runoff to occur and a corresponding increase in volume and peak flow.

 

[Trackfiend]

I agree with cvg. The rational method was not intended for use with forming hydrographs, only with giving a peak discharge (Q). However, a lot of software programs will generate a hydrograph for whatever method is used. This can turn into the whole "garbage in, garbage out" conflict and another reason why I asked about the method being used and if software was involved.

 

[zebco]

Actually, I'm using the curve numbers (CN). I'll supply more info when I get back to my machine at home. Its all inside excel.

 

[zebco]

here is my spreadsheet and graph.
The volume under the Hydrograph is Vol = 1.39*Qp*Tp

Qp = peak dischrge
Tp = Time to peak flow

This is derived from an SCS dimenionless unit hydrograph.
(look up McCuen 1982)

The spreadsheet solve for Tp. aka 4 min. and 26 min.

The Q* you see on the sheet is the runoff depth from the site, not the total discharge. Akin to acre-feet, different units of course.

All the math seems to work out fine.

I am dealing with a very anal city government.
Hoops from HELL!

 

[zebco]

Well the spreadsheet did not post , I'll try it here

 

[Trackfiend]

I haven't had a chance to really "dig" into your spreadsheet but it seems that you are mixing the SCS method and the rational method in order to generate a hydrograph. Basically the SCS method (which is now called the NRCS) is used in order to determine a runoff volume. Again, the rational method gives a peak flow, not a volume. I would take another look at how your generating your hydrograph (including the formula for tp as there are several out there).

What are you designing for? If you are trying to determine if the ditches or culverts can handle the flow, then just use the peak flow (rational method). If you have to detain (or retain) any runoff, then stick with the SCS (NRCS) method so that you can route that volume.

 

[zebco]

It's all about retaining the flow and the volume to predevelopment conditions. The rational method is only used to determine peak discharge. From that Tp is derived through the posted formula. I'm thinking about just "forcing" the time to peak back to the Tc, or 5 minutes, which seems intuitive to me, I mean it just "feels" better. The spreadsheet is the result of classes and seminars taken from the Godfather of urban stormwater design in NC. Now deceased, Dr. H. Rooney Malcolm. Some of you may have heard of him. Thanks kindly for your responsews and wisdom. I'll take more at any time.
Herb

 

[cvg]

as I said before, trying to develop a hydrograph for such a small area is not recommended. Also, in my world, Q's get rounded up to the nearest 1 cfs. Calculating runoff to the nearest 0.01 cfs is virtually impossible. Therefore, your pre and post runoff are both rounded to 1 cfs. There, you are done, no detention necessary to reduce the peak runoff to pre-existing conditions.

Read "Voodoo Hydrology"

http://www.stormh2o.com/july-august-2006/urban-hydrology-methods.aspx

 

[zebco]

See my post above about anal retention from hell. I'm talking Chapel Hill, NC. They actually believe what ever the calculator will produce is the gospel. They want drainage areas in square feet instead of the standard acre.
The entire design approach was never intended for that accuracy. So you see my delimma. I have to account for virtually every drop of water.

 

[coloeng]

Are you sure you don't have a decimal point off on your time to peak. It seems that if you go from a wooded channel or overland flow to a paved section your velocity would increase with the improvements and your time to peak would be sooner, maybe 2.6 minutes instead of 26 minutes.

 

[Drew08]

The theory might be sound but problem lies in the fact that the time to peak and peak flow are based on two different storms events. This is one reason why it is never a good idea to mix methods.

The Qp is based on a 5+ minute duration event of continuous constant intensity rainfall: 0.38" over 5 minutes (4.61 in/hr x 5 min). The Volume is based on a 24-hour event of 2.88" and should result in a 24+hour hydrograph dependent on the rainfall hyetograph.

The point is that a single hydrograph with your peak, and your volume does not exist.

Also, the equation V = QpTp/K [K=0.75] (eq NEH-630 16.4) is the volume of the triangular unit hydrograph, not the actual hydrograph. The unit hydrograph represents 1-inch of runoff. Because the unit hydrograph is for a unit of runoff, both the pre- and post-development Tp’s should be based on the same runoff depth. Making this change, results in a more intuitive answer. But, this is still not the actual time to peak, which would also be based on the assumed rainfall hyetograph.

So really you’re mixing 3 methods and for each method using a different storm event.

If I was reviewing this, I'd throw it back too.

 

[beej67]

Caveat: Haven't looked at your spreadsheets at all.

Don't mix rational and SCS methods. They were developed independently based on different studies and different data and have different basic assumptions that go into them, so they are completely meaningless with regard to each other. The biggest major difference is the storms - one uses a 24 hour statistically generated rainfall hyetograph, the other uses a flat one of duration equal to the Tc. Different storms = different everything else. Hydro starts with the storm.

There's nothing fundamentally wrong with using the SCS method on a small watershed, particularly if your goal is to develop a hydrograph for a detention facility, but the rational method (and rational peak flow) should have nothing to do with your analysis whatsoever.

Your Tc should be developed with the same method for existing and proposed. Could be Kirpich, could be the sheets released in the TR-55 handbook, whatever, but the reviewer is going to call BS on you if your proposed Tc is shorter than your undeveloped Tc.

If you're just looking for a peak, and you're in SCS land, use the method presented in TR-55 to generate a peak, not the rational method.

If you're trying to do this all by hand, download TR-55, read it through front to back, and follow that procedure step by step. Everything you need is in that document, don't let anything else slip in. Doing it by hand can be quite a task your first time, which is why there's software available. Like this:

http://www.wsi.nrcs.usda.gov/products/w2q/h&h/tools_models/wintr55.html

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[fel3]

Zebco…

For a 300' length at 5% slope, the Kirpick Equation gives Tc=2.0 minutes. The FAA Equation gives Tc=12.8 minutes for C=0.40 and 2.6 minutes for C=0.96. Depending on circumstances and the agency, we also add a roof-to-ground time that is anywhere from 2 minutes to 15 minutes. Around here, if Tc<5 minutes, we use 5 minutes.

There are several equations used for estimating Tc, but these are the two I seem to use the most for small areas. "Applied Hydrology" by Chow/Maidment/Mays (1988) presents 7 different equations.

BTW, just in case you don't have the equations handy:
Kirpich: Tc=0.0078*(L^0.77)*(S^-0.385)....S in ft/ft
FAA: Tc=1.8*(1.1-C)*(L^0.50)*(S^-0.333)....S in %

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[zebco]

Thanks for the reponses. My computer got a virus and crashed badly. I think I'll have to start over anyway.