Yard/storm drain pitch question

[rfb321]

I have a backyard storm drain system that runs from a catch basin at the back of the yard to a storm drain manhole in the street with the following:

Total length - 188 feet

Pipe and size - 8" PVC SDR 26

Pitch or slope - 2.5% (about 19.5") for the first 65 feet starting at the back yard catch basin, and the dropping off to .5% (about 7.4") for the next or last 123 feet to the street.

So, my question is, what real slope does this represent. I'm thinking that the first 1/3 of the run at 2.5% will provide a push to the remaining 2/3 of the run (at .5%) that will add to the volume of water able to be carried in that .5% run. But, will the total flow capacity be greater than if it were just a .5% slope for the entire 188 feet?

 

[frodo]

8'' storm drain minimum slope is 1/16 per foot
which equals 3/4'' per 10'
the fall needs to be uniform threw out the run

 

[rfb321]

8'' storm drain minimum slope is 1/16 per foot
which equals 3/4'' per 10'
the fall needs to be uniform threw out the run

Thanks, the system is already in place and cannot be changed. So I guess my question is, does the last 123' pitched at .5% benefit from the first 65' pouring into it at 2.5% pitch rate?

 

[frodo]

if my math is correct than .5% is a little less than 1/8'' per foot and should be fine
i suggest you check my math to be sure

 

[rfb321]

The system works in fairly heavy spring rains, so I know it is running at least at a .5% slope rate. I've never seen it in one of those 50 year rains we seem to get every five years now. I'm just curious as to what advantage that first 65' of pipe running downhill into the .5% section offers. Does it provide extra push that would increase the overall flow rate to something similar to a .75% slope?
Probably a question for a sewer and water engineer

 

[plumberscott]

I would run 1/8 per foot

 

[frodo]

I would run 1/8 per foot

I agree, even though code says a 1/16 is ok,