Sheet Flow in Undeveloped Areas and small developed Area in 230 acre subbasin with Tr-55

[Andy J]

I'm fairly new to TR-55 and I have a couple of questions as to what general practice is:

1) Do you usually use a short distance of sheet flow for an undeveloped, undisturbed area. The area has some short grass prairie, cactus, prairie, juniper and would call it arid rangeland essentially. I was thinking the max of 100-feet, but doubt that it really sheet flows much at all. So then that brings up the question if I should even put anything in for the sheet flow length for undeveloped arid lands. Slopes range from 1.5 to 5.6 percent.


2) The site I have has 7 subbasins, but I have 1 sub-basin of 230 acres where we are putting a 2.3 acre gravel pad with some small storage buildings on it with roofs covering about 4,000 sf. The county insisted that I use different manning values for the pre and post development conditions even though the water flows over 3500 feet before the property line. Specific manning values for different surface types as far as I can tell can only be entered for sheet flow conditions and if I say I have 100-feet of sheet flow for the sub-basin with a manning value for a smooth surface (gravel parking lot essentially) it seems like a gross misrepresentation of reality when I only have a small 2.3 acre pad vs. 230 acre subbasin. Sooo....

I then was thinking I would make the building pad area a seperate subbasin of the 230 acre subbasin with a reach extending through the original subbasin to the outlet. This would get me calculated runoff values for the building pad area. Does this seem like a reasonable approach?​

 

[gbam]

I recommend that you consult with the reviewing agency to determine their minimum criteria for the hydrology. Base your analysis on acceptable practice for your specific area.