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I'm trying to get the Time concentration for the ModClark method in Hec-HMS, and one of the variables is Slope in feet per mile.

I have a DEM for the sub basin that I've download, the elevation was in metres but I've used ArcGIS Spatial Analyst - Math to convert to feet.

How could I get the slope value (preferably an average for the basin) that is in feet per mile?

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According to your question,I guess that the unit of slope should be "feet/mile",so you should set the output measurement as "PERCENT_RISE "when you use the ArcTool of "Slope".Next,you need to set the Z factor.For the fact that 1 meter is equal to 0.0006214 mile and is also equal to 3.2808399 feet,the Z factor should be 5279.76(3.2808399/0.0006214≈5279.76)。

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    Right so by adapting the Z-factor as you've detailed above I should then get a slope dataset with the units of feet/mile. Then by doing Zonal statistics I can calculate the average of the basin which would be in feet/mile? Thanks. – Iain Varney Apr 17 '17 at 17:10
  • Sorry, I just couldn't understand the last words, "which would be in feet/mile?"Could you give me a clear explanation? if you want to calculate the average of the basin,I think the "in_value_raster" of Zonal statistics should be the slope layer which you calculated above. – Scheng L Apr 18 '17 at 0:51

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