Manning's n - Urban, Sub-urban, and commercial areas

[redneckengr]

While every textbook and reference in the world has extensive descriptions and methods for determining Manning's n in natural floodplains there seems to be nothing out there for urban, suburban, or commercial areas. I've worked through some different methods making guesses at some parameters but my "gut" is I'm coming in low.

Anyone have suggestions or references? With the ever growing population in floodplains I find it hard to believe no research has been done.

Thanks

 

[DMcGrath]

For open channel flow, I think there are adequate descriptions for n values such as concrete, grass-lined, etc. For sheet flow (different equation, different n), it is kind of limited, but for urban areas you would probably be in the "smooth surface" category with n=0.011. If you are thinking floodplains composed of urban "surfaces" such as roads, cars, billboards and houses, then I agree, it would be very difficult to calculate an n value because it is so highly variable. I doubt we will see any new n values anytime soon, so your best SWAG is probably all you've got.

 

[LHA]

For floodplain channels, paved or otherwise, I use USACOE's numbers. They are on pp 3-13-3-13 of the HECRAS Hydraulic Reference Manual, available free from here:

http://www.hec.usace.army.mil/software/hec-ras/hecras-download.html

For overland flow, paved or otherwise, I use NRCS's TR-55, which are what DMcGrath referred to. They are on page 3-3 of TR-55, available free from here:

http://www.wcc.nrcs.usda.gov/hydro/hydro-tools-models-tr55.html

Remember, amateurs built the ark...professionals built the Titanic. -Steve

 

[francesca]

The Nashville storm water manual has a formula for calculating Manning's n based on five different characteristics of the channel. Their open channel hydraulics chapter can be downloaded in pdf format here.

Actually, their equation is for natural channels, but their Table 3-3 on page 36 has Manning's n values for artificial channels. The values change for different flow depths.

 

[redneckengr]

Thanks to everyone! I see everybodys experience, training, and the occasional Google search turned up the same info I had. When Chow released his Book in '59 he had that very nice methodology using n0,n1,n2,...etc. Apparently FHWA and USGS got together and released WSP 2339 (there is another number for FHWA which is twice as long) which borrowed Chow's method but also created a version for floodplains. The crazy thing is that if you follow this methodolgy in a very developed area (no vegetation, lots of impervious and man-made obstructions) you can easily calculate a mannings between 0.04 and 0.08. Which in my limited experience seems very low (try it sometime when you are bored). It would be interesting to see someone do some model studies with highly developed floodplains.

 

[LHA]

0.016 is generally accepted for compacted bituminous concrete, 0.013 for trowelled concrete. So, no 0.04-0.08 doesn't seem at all low, but probably just about right for highly paved areas, when weighted for some grass, curbs and other obstructions.

Remember, amateurs built the ark...professionals built the Titanic. -Steve