Peak Flow SCS vs. USGS 1

[Roadbuilder]

When comparing the peak flow calculations on a 1 to 2 square mile drainage basin in south georgia I am getting a peak flow based on the SCS method that is 2 to 3 times what I calculate using the local USGS regression equations. The drainage area is type C soils with a mix of cultivated fields, forests, and swamps. Should there be this much of a difference between these calculations? Could someone advise as to which method would most accurately predict the peak flows in my situation?

 

[DamEng1]

In reality you are dealing with two different monsters:
the SCS method was developed based on only a few watersheds and the results of the method are based on the predefined rainfall distribution (i.e. Type II, III, etc.), the standard or modified unit hydrograph you are using, CN, and the time of concentration. Also, you must understand that if you input a 100-yr rainfall amount it will be based on a statistical anlyses of precipitation data.

The USGS method is based on a regression of years of record of streamflow data which are not tied to a particular rainfall event. The reccurence interval of certain flow events allows the determination of the peak flow for a 100-yr event. Meaning that the actual rainfall hydrographs and distributions are completely different than SCS.

Thus, it is highly unlikely that the flows for a SCS analyses and USGS equations will ever match. The solution to your problem though is to look at both and the data behind it to determine which best fits your situation. A lot of times it is accepted in practice to adjust the time of concentration in the SCS method (since it is the value with the greatest uncertainity in the SCS method) such that the peak flow approximately matches that from the USGS method. However, this is based on your engineering judgement.

 

[RWF7437]

When comparing hydrology methods large differences are to be expected. This is at least partly because these are apples to oranges comparisons and because the basic data used in each method are subject to large errors.

The SCS ( now NRCS ) method is a unit hydrograph procedure based on RAINFALL statistics and physical characteristics of the watershed. It is usually also limited to analysis of storms of assumed durations, up to 24 hours. The soil, slope, ground cover, and antecedent moisture conditions must all be guessed at. The total rainfall ( a statistic) is also a guess often plus or minus 30% or more as is the temporal pattern of that rainfall.

Regional regression equations are based on STREAMFLOW data, not rainfall. That data is derived from many storms over a wide range of storm durations with a similarly wide range temporal and spacial rainfall patterns ( ignoring snowmelt in GA ). Unless your drainage basin is identical to a nearby gaged basin you should expect large differences...just a guess but these differences can easily be as large as plus or minus 50%, based on my experience .

Hydrology is a very inexact mixture of art and science and pure guesswork.

As to which one to use perhaps the best answer is to use any and all methods available to you assuming all of them are "wrong" and then ask yourself what are the prctical implications of these errors to the success or failure of your project over its useful life.

good luck

 

[Roadbuilder]

Thanks for the valuable information guys. Your comments were very helpful.