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I was wondering if there are any studies/research out there where a runoff curve number (CN) has been developed for Railroad Ballast. The only thing that is published by the SCS is a number for gravel roads which seems extremely high. I would appreciate any information that you may have come across.
You may want to consult the ASCE Publication Curve Number Hydrology, State of Practice Edited by Richard H. Hawkins, Timothy J. Ward, Donald E. Woodward, and Joseph A. Van Mullem. http://www.asce.org/templates/publications-book-detail.aspx?id=7996
First, about gravel roads. Gravel roads generally are paved with a broad mix of gravel sizes to encourage the aggregate to lock together and spread load. Most bases use a small amount of dust or choke to assist this process. In Ohio we generally use a material from the Ohio Department of Transportation specified as 304 Aggregate Base. It is meant to exclude water from penetrating the aggregate because a high moisture content causes the subbase to emulsify when loaded or when frozen.
The curve number for railroads is unrelated to gravel roads. Railroad base consists of large, broken or angular aggregate that is designed to lock together and spread the load of a train on the rails. It is highly pervious. However, the subbase for a railroad has been compacted and usually graded to prevent penetration of water and to drain the water from under the railroad. A reasonable estimate for a curve number under the base would be a compacted (or remolded) soil with no vegetation.
Use the in situ soil (on either side of the railroad) with bare earth and bump the hydrologic soil on level to compensate for remolding the soil. One of the appendices in TR55 explains the reasoning behind this for construction sites.
I think you will find that in most cases, you can use the curve number for a gravel road as a substitute. If you are in the western states, you might have Hydrologic Group A or B soils which might give you some benefit in reducing the runoff load, but generally not in Ohio.
https://www.codot.gov/programs/research/pdfs/2012/runoff.pdf is a paper that studied the rational runoff coefficient "C" for railroad ballast. One conclusion was that railroad ballast is designed to remove runoff water quickly and the observed runoff coefficient was higher than generally assumed.
I suspect that the railroad ballast question is not too far removed from a composting pad consisting of 6” of crush-run aggregate over a compacted clay soil. Infiltration occurs until the pore space in the aggregate is filled, then runoff occurs at a rate of (rainfall – drainage) from the aggregate. The question of drainage from the aggregate depends on the slope of the pad (in the case of a windrow compost pad). Because the infiltrated water volume maintains such a profound influence on subsequent behavior, I believe we have to declare the curve number method not applicable to these situations, unless we can discover a some meaningful subcategories based on length of run, slope, etc. How this may extend to a railroad ballast situation would likely depend on similar subcategory attributes.
I had a student play around with a Hydrus-2d model to get some perspective on the compost pad problem.
I found the question to be good grist for the mill,