Overland Mannings n for Farm Fields

I am modeling runoff from some tilled farm fields and am looking for a typical value (or range) for the overland flow Mannings n. I reviewed Table 5-13 in the CHI handbooks for Hydrology that list some values from Engman (1986), but none of them say tilled farm fields. I was wondering if anyone has any experience or other references that may be helpful or some updated values.

Thanks,

Roger Leventhal, P.E.
FarWestEng@AOL.COM

Try HDS-2, Highway Hydrology, 1996.

Barry Palynchuk
Barry_Palynchuk@CPR.CA

Roger

You may not have overland flow pre-se, since the runoff would concentrate in the furrows. In the prairies, runoff from tilled fields is often nil for rainstorms less than the 5 year extreme even. Even in areas with relief, contour plowed furrows typically run perpendicular to the natural ground slope (to prevent erosion) and the furrows may intercept all of the flow if the furrow depth exceeds the depth of rainfall. I have often seen furrows overtopped and a easily-defined erosion channel following extreme rainfalls, but this would not be applicable for general modelling.

Manning 'n' values for tilled farm fields in the flood plain are available, but would not be applicable since these would be based on deeper flood plain flow and not runoff.

You may wish to cook something up using the SCS/NRCS Curve Numbers from TR 55 (i.e. ratio the flow from a known area CN by the estimated CN for your tilled field). See table 2.2b for Curve Numbers for Fallow ground, row crops, grain crops, etc. Also note that sheet flow would turn into shallow concentrated flow (after 300 feet of sheet flow, using TR 55 rules of thumb).

The TR 55 manual is available on-line . . . http://www.il.nrcs.usda.gov/engineer/enggsoft.htm <http://www.il.nrcs.usda.gov/engineer/enggsoft.htm>

Regards,

Kirby McRae, P.Eng.
kmcrae@UMAGROUP.COM

Just a small caution with respect to one of Mr. McRae's comments, namely "...sheet flow would turn into shallow concentrated flow (after 300 feet of sheet flow, using TR 55 rules of thumb)." 300 feet is rather long for sheet flow except under the most favorable conditions (e.g., VERY smooth surface).

NRCS recognizes this, despite what is written in TR55. For example, Hydrology Technical Note No. N4 says: "The maximum flow length of 300' with a most likely length of 100' should be used in overland flow computations for unpaved areas." Further, USDA/SCS in Pennsylvania (Bulletin PA210-0-5, 1990) indicates that the maximum for unpaved areas should be 150', with most likely length being 50 to 100 feet.

Jeffrey M. Rice, P.Eng.
jrice@URBAN-SYSTEMS.COM

As an aside, this "detail" is extremely critical in modeling.

In my experience there is typically an order of magnitude difference between velocities in the sheet flow regime as compared to shallow flow, therefore what you pick for a length is the prime determinant in what time of concentration you end up with. The shortened overland path is one of the key impacts of development. Simplifying an entire watershed into a single "representative" path, and treating the transition from overland flow to shallow as a hard divide rather than a continuim are both significant simplifications, which seems to raise this one variable to the level of a "black knob" that should be considered carefully.

Bill Heatherman
bheather@OPKANSAS.ORG

With regard to sheet flow legnth, McCuen recommends that the sheet flow length be given by L n/sqrt(S) = 100 ft. This is for use in the iterative equation for time of travel that contains L n/sqrt(S) in the equation. This approach has some appeal because intuitively, it seems that the length of sheet flow should increase with slope and decrease with surface roughness.

The effect of using this is that the time of travel for sheet flow ends up being a function only of the IDF curve. This means that the sheet flow time of travel does not depend on surface roughness or slope. McCuen's paper "Policy Considerations in Time of Concentration Estimation," (J. of Floodplain Management, Vol.2, No. 1, July 2000) makes for interesting reading.

Warren Campbell
Warren.Campbell@HSVCITY.COM

>>>> Warren Campbell wrote:"With regard to sheet flow length, McCuen recommends that the sheet flow length be given by L n/sqrt(S) = 100 ft. "

Interesting.

Actually, I have a handwritten note in my June 1986 copy of TR55 concerning the the computation of Tt (Eq. 3-3 on Page 3-3). The text says "For sheet flow of less than 300 feet, use Manning's kinematic solution..." The note, to be inserted between "300 feet" and "use," reads "or, more precisely, Tt less than or equal to 0.4 hours,"; as I recall this information was provided by the NRCS' former (?) state hydraulic engineer in Minnesota, Sonia Johnson.

Jeff Rice
jrice@urban-systems.com