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Time of concentration and Curve Number through Railroad Ballast

Time of concentration and Curve Number through Railroad Ballast

(OP)
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?

RE: Time of concentration and Curve Number through Railroad Ballast

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 . . .

RE: Time of concentration and Curve Number through Railroad Ballast

(OP)
I will look into that.  Thank you.

RE: Time of concentration and Curve Number through Railroad Ballast

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.

RE: Time of concentration and Curve Number through Railroad Ballast

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.    

RE: Time of concentration and Curve Number through Railroad Ballast

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!

RE: Time of concentration and Curve Number through Railroad Ballast

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)

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

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