Hydrographs and Time to Peak
Hydrographs and Time to Peak
(OP)
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?
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?





RE: Hydrographs and Time to Peak
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.
RE: Hydrographs and Time to Peak
RE: Hydrographs and Time to Peak
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.
RE: Hydrographs and Time to Peak
RE: Hydrographs and Time to Peak
RE: Hydrographs and Time to Peak
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!
RE: Hydrographs and Time to Peak
RE: Hydrographs and Time to Peak
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.
RE: Hydrographs and Time to Peak
Herb
RE: Hydrographs and Time to Peak
Read "Voodoo Hydrology"
http://ww
RE: Hydrographs and Time to Peak
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.
RE: Hydrographs and Time to Peak
RE: Hydrographs and Time to Peak
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.
RE: Hydrographs and Time to Peak
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:
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Hydrographs and Time to Peak
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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RE: Hydrographs and Time to Peak