Manning's Roughness for Lawn Area

[TheEnginerd]

Hi all,
Got into a debate with a municipal review engineer about roughness coefficients for Lawn area.
I do most of my stormwater calculations using the HYDROCAD program. The typical list of values for grass manning's coefficients is:

Short grass prairie - 0.15
Dense grasses = 0.24
Bermudagrass = 0.41

I always go with 0.24 for our residential jobs which will usually have sodded lawns or very well established grasses by the time construction is completed (ie. very dense). Others seem to like to use 0.15 for "Short Grass". The sheet flow section of the TC can have a huge effect on the total flow. It's always a little bit "smoke and mirrors" if you ask me in that 2 people doing it will get totally different answers, but 0.15 seems a little too conservative in my opinion. What do you all think and is there a document that would justify the use of any of these coefficients for lawn area?
Thanks!

Andy

 

[cvg]

unless the lawn is planted with prairie grass or is not fully established or is dead, than I think your assumption is valid.

http://water.me.vccs.edu/courses/civ246/table3.htm

 

[TerryScan]

HydroCad is adopting these values straight from SCS documents. The SCS/NRCS documents cite long established studies by USDA and others. The footnote in TR-55 documentation for dense grass states: "Includes species such as weeping lovegrass, bluegrass, buffalo grass, blue grama grass, and native grass mixtures." Various SCS documentation refers to sheet flow being 0.1 ft. or less.

If the reviewer needs construction stage calculations - before lawns are established, that is another matter. However, for typical established lawns, it would be difficult to argue dense grass is NOT the appropriate choice based on NRCS documentation.