Sorry to take a while to get back to you….had a friend in the hospital, a son graduate from NCSU, loads of work piling up….
So, I’ll give you the bottom line numbers first, then explain the details for those of you eager to be bored. There are approximately 191 acres flowing to your west fork driveway crossing yielding a 2 year storm flow of 265 cfs (cubic feet per second) and 70 acres flowing to the east fork driveway crossing yielding a flow of 65 cfs. Assuming you’re using your 36” plastic pipes and assuming they are corrugated, the west side would need 2 36” pipes at 5% slope. You could use 1 36” pipe at 15% slope but that increases the outlet velocity to an unacceptable range which can scour out the area just downstream of the pipe. The east side needs the 36” pipe to be at a 3% slope. Let me know if they are smooth wall pipes which would reduce size/slope need. I can give that to you.
Another thing that would help to keep the pipes from washing out would be to add a flared end section to each end. This would help to channel the water into the pipe. Kind of difficult for water that is in a channel to be funneled into that circular opening without disturbing the bank slope at your driveway. May also want to add some riprap around the opening to "armor" the bank....anything to reduce the impact and velocity of that water flowing down the channel.
Now, some further explanation. The Alamance County GIS was very helpful but didn’t have existing storm drainage system info available for the streets so I did my best to determine where the water flows based on the topography. I do see the detention pond for the hospital and took that into account for the flow on the west side……which will reduce the actual storm water runoff volume getting to your driveway.
2 year storm vs. 10 year storm vs. 100 year storm….the “blank” year storm does not mean the storm that comes every “blank” years. It really translates to a probability of occurrence or percent chance. In other words, the 100 year storm has a 1% chance of occurring every year, the 50 year storm has 2% chance of occurring every year, 10 year – 10% chance, 2 year – 50% chance. So you could get a 100 year storm occurring consecutive years, as some of you guys have seen out east. The usual accepted “design” storm for driveway pipes is the 2 year storm, for subdivision street systems is the 10 year storm, and 25, 50, and 100 year for roadways depending on how minor or major they are and the governing agency. I did the 2 year storm for your driveway so for the big storm that has a 50% chance of occurring every year, you should be good.
There are many variables in storm drainage design and we try to take all of them into account. One that is very recently noticeable is the amount of rain we’ve had and the overly saturated ground. Obviously, the ground loses ability to absorb any more rain which produces more runoff. Can’t really account for that in calculations. Another variable is the type of terrain in the large areas running toward your property. Of course, the developed areas (buildings, pavement) produce more runoff, grassy areas while absorbing much more than developed areas do not absorb as much as a wooded area. I estimated as best as I could these areas based on the aerial photos.
Lesson finished. Please let me know if you have questions or of something here doesn’t make sense or if I did something wrong.
Mark
