How does SWMM5 assign curb lengths to subcatchments?

[MaitreyaYadav]

For example, I have a subcatchment with with 3 land use categories: Developed(20%), Semi-developed (30%) and Undeveloped (50%). I set the normalizer as curb length for developed and semi-developed; and as area for Undeveloped. Since I have to specify the curb length for the entire subcatchment, lets say the totla curb length is 100. Will SWMM allocate a curb length of 20 to developed and 30 to semi-developed (this will leave a curb length of 50 not covered) or will it allocate 40 to developed and 60 to semi-developed?
Also while calculating curb lengths in a subcatchment, should I also include the length of curb that is inside undeveloped (even though the normalizer for undeveloped is area and not curb length)?
Any help in this regard will be appreciated.
Thanks,

 

[MaitreyaYadav]

Got this reply from Mr. Lewis Rossman of EPA:

The curb length for a specific land use within a subcatchment is the subcatchment's total curb length multiplied by the fraction of total area devoted to the land use. In your example, the curb lengths would be 20 and 30 for the developed and semi-developed areas, respectively. The curb length assigned to the subcatchment should equal the sum of the curb lengths in each land use so that the proportions work out correctly. Curb length is only used along with land uses to compute pollutant build-up -- it's not used for runoff or any other calculations.

The allocation of curb length to land uses is based on the fraction of total area devoted to each land use. In your example, if you want 40 units of curb length devoted to the developed land use, and it makes up 20% of total area, then the total curb length you need to assign to the subcatchment is 40/0.2 = 200. The curb length for the semi-developed area, which comprises 30% of the total area, will then be 200 * 0.3 = 60.

Whether one or both sides of the street gets counted as curb length depends on how you want to define your buildup function. I believe that the curb length metric was first used when researchers actually vacuumed the curb areas along a street to determine the amount of pollutants on the surface. In this case the buildup function would be based on both sides of the street, since pollutants will accumulate wherever there is a curb.

 

[tsgrue]

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SWMM5 methodologies are generally implemented in the same conceptual manner as in SWMM4.

I suggest you download and review the EPA SWMM4 Manual.

This can be downloaded from "

http://eng.odu.edu/cee/resources/model/swmm.shtml".

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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

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