Time of concentration and Curve Number through Railroad Ballast

[lpvaa]

I am trying to compute the time of concentration (Tc) through railroad ballast. The ballast section is only 6" thick and includes a gradation of 2.5" minus. Currently we are thinking that we need to compute the Tc verticaly and then horizontaly. The railyard varies from 100' - 120' wide.

Does anyone have any experience determining time of concentrations and CN values for railroad ballast?

 

[BigH]

I may be wrong, but I would think that you would need to determine the permeability of the railroad ballast - and see how much water it passes and compare to the volume of water being produced on the upstream face . . . then determine if there is a "barrier" to the flow. Flow nets . . .

 

[lpvaa]

I will look into that. Thank you.

 

[gbam]

lpvaa - are you looking to compute the discharge assoiciated with runoff from the railroad or are you including this area in a larger area calculation? If I were computing discharge from a raised rail section, I would probably ignore the slightly increased Tc and compute runoff (Tc & etc..) from the underlain soils. This would provide a conservative discharge for design; but the I would try to determine if the conservative approach significantly increases costs. If the ballast area is a subarea of a larger watershed, I would probably ignore it.

 

[oldestguy]

Is this ballast on a geotech filter fabric? If not, I'd give it a permeability about the same as the soil below because, with time due to the traffic, these usually get filled with soil from below.

 

[fattdad]

I'd say there's only an issue if there is more than a 2 in rain event. For a porosity of 1/3, there'd be enough headroom for 2 inches of rainfall before the ballast has a potential to yield runoff at which point, you'd have 100 percent runoff (assuming that there's no infiltration from the subgrade).

Just some morning thoughts.

f-d


¡papá gordo ain’t no madre flaca!

 

[beej67]

If you'd like to use fatdad's approach, and there is no positive drainage to the field (meaning water is trapped in the ballast and doesn't run off) consider choosing a CN based on gravel and adjusting your initial abstraction factor in the model. (initial abstraction = the birdbath effect)

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