Hi,

A good starting point to get values of Manning's n for overland flow, depending on land cover, is USDA's TR-55 report, Table 3-1. Get it here: ftp://ftp.wcc.nrcs.usda.gov/wntsc/H&H/other/TR55documentation.pdf (or Google "USDA TR-55" to get other links to the manual).

Slope length can be calculated from flow accumulation maps created inside a GIS. If this is an option for you, I can point you to some bibliography.

Regards,

Joao

Thanks a lot.. I have created the catchment that way some months earlier... And i had the flow accumulation maps what should i do with those ?i know how to fill sinks on a dem find flow accumulation flow direction ects.. with archydro .. is the slope length so important?,, maybe i ll read the gis help.any help is good thanks a lot

Table 3.1 of TR-55 are of general (common) use and will give you a Manning's N based on surface cover. It is a widely use reference. There are many other similar and traditional references available with more detailed information about land cover.

In the other hand, the N values in vegetated areas is very sensitive to type and height of vegetation, flow depth and water velocity. There are many other references that relates Manning's N to Flow intensity (Product of Water Velocity x Hydraulic Radius-VR) for different vegetation covers. This is especially important if the vegetation is Grass-type. As an example, the following paper have a nice and simple explanation:

http://www.waternz.org.nz/documents/sigs/stormwater/stormwater%20conf%202011/Peter%20McCarten.pdf

Back in the 90's I compiled a significant amount of data for many previous studies (lab, field, Natural grass (many types), synthetic grass, etc.) and found that for very low water velocities manning's N increase with increases in water velocity until certain point where the N value suddenly drops. The reason is that, for very low translation velocity, the flow behaves in two dimensional (really three dimensional)fashion while flowing through the vegetation stems. Water depth increases with increments in flow rate and translation velocity also increases. This relationship continues until reaching a vegetation submergence ratio that is high enough to put down the vegetation. When that happens, water velocity increases suddenly because resistance to flow rate (or Manning's) is much lower. So, at some point, for almost the same water flow rate, the Manning's N have significantly different values. In summary, my recommendation to estimate Manning's N for vegetated waterways, regardless of the reference you use, is to monitor at least the results obtained for water velocities and its relationship with vegetation type and height and adjust Manning's N accordingly.

I am sorry I could not provide you with an advice because I am working in arid environment were the land cover is sand and silt with sparsy vegetation.

I Would like to know how to calculate slope length from Flow accumulation ! thanks

And something else.. is mannings number the same with 1,49/n? cause other values i looked in the software where in m^1/3/s same as the conversion factor . so i should divide 1,49 with n to put it on my software .

I can give you a good source for mannings number from southern germany. In the federal country Baden-Württemberg you cand find mountains and some kind of lowlands too, so perhaps you might find it usefull. The only problem is that it is in German, but the values you need are listet in tables on pages 12-14:

http://bit.ly/Op9JNB (8.5 MB)

The publication is a series of four books, all published as pdf:

http://bit.ly/U78RtS

hth, regards, Andre

Thank you Very much Andre !Thank you all !

in the general situation Manning number changes with the kind of land ,and whats cover in this land,i's just appoximation number,but,in the precise values, you have to take "echantillon" in the labory and do the measures

You may want to have a look at the WetSpa hydrologic model documentation, for the range of Manning's coefficients for sheet flow:

http://www.vub.ac.be/WetSpa/downloads/WetSpa_manual.pdf

Table 3.2, page 51

The thesis is finished now i uploaded on my page but it is in Greek .. the abstract is in English . and some diagrams are shown only if you download the document , i do not know why this is happening .. but if someone whats to take a look its better if he or she downloads it all. Thanks again.