Manning N for overland flow in urban settings

[BBened]

Hi all,

I'm searching for documentation regarding Manning N values for overland flow in urban settings.

This is to use with PC-SWMM while modeling storm drainage in a suburb.

Thanks.

 

[Lih]

What sort of resolution/detail are you modelling the overland flows in? Will you be modelling at a sufficiently fine scale where you can differentiate between flowpaths in roads vs through yards (I'm not really familiar with SWMM)

Generally when modelling at a broader scale (lumping roads/yards together) I would adopt a relatively high manning's n 0.1 - 0.15 to account for fences and other obstructions.

 

[BrunoB]

As you guessed, I would like to model at a relatively broad scale, fixing one Manning coefficient for every 3-4 lots. This would include the sidewalk, roofs, fences, yards, etc.

Runoff coefficients for surburban, urban, commercial or residential areas are easy to come by, but Manning's n are more rare.

 

[psmart]

What type of flow are you trying to model? The effective Manning's value can vary considerably depending on the depth of flow.

For example, channel flow with short grass cover might have a value of 0.03, while sheet flow across the same surface is typically 0.15.

 

[BrunoB]

Definitely sheet flow.

Do tables displaying the various types of surfaces and their corresponding coefficients exist?

 

[Lih]

Google "mannings n overland flow". A number of items came up.

 

[psmart]

See TR-55 table 3-1, sorted by Manning's value as follows:

0.011 Smooth surfaces
0.05 Fallow
0.06 Cultivated, Residue<20%
0.13 Range
0.15 Grass, short
0.17 Cultivated, Residue>20%
0.24 Grass, Dense
0.40 Woods, light underbrush
0.41 Grass, Bermuda
0.80 Woods, dense underbrush

 

[BrunoB]

Great. Thanks for the help!

 

[dcnnng]

Dear ALL,
If we are referring to overland flow, i.e. sheet flow for depth less than 0.1 ft. The Manning's "n" is for the previous surface and its value depends on the type of surface. I note that there are large discrepancies especailly for the grassed surface, e.g for dense grassed surface n=0.06 based on the AR&R 1977, whereas n=0.35 for dense turf based on Crowford and Linsley 1966(obtained by calibration of Stanford Wwatershed Model)and n= 0.15 for short grass prairie and n= 0.41 for Bermuda grass based on Engman (1986). In other words, the former will give us a much higher runoff duscharge than the other two which have very similar n value. Can anyone comment and advise which set of n values be used in order to obatin more accurate runoff.I am using runoff block of EPA SWMM to calculate the runoff. Thank you!

 

[psmart]

I think the TR-55 table (posted above) covers the full range of conditions for shallow flow.

dcnnng: Your Manning's value of 0.06 for "dense grass" sounds a bit low for sheet or shallow flow. It's closer to the channel-flow value.

 

[dcnnng]

Hi pssmart,
Thank you for your reponse.
That is exactly what puzzle me. In fact I was told that we should use 0.06 for desnse grass as per Australian rainfall and runoff (AR & R): a guide to flood estimation of Institution of Engineers, Australia, c1987.
As I said there are 2 sets of values, which one would give use more accurate runoff?