Low flow rate 2

[arabojan]

I am designing a detention storage pond (pipe storage), the pre-development flow rates are as followa:
years pre-dev flow rate post-dev flow rate
2 0.0000 0.6733
10 0.001 0.9889
25 0.0245 1.2203
100 0.0959 1.4503

the required orifice for adjusting flow rate in pre development for 10 years storm event condition is 3/16 inch that is low below of the requiremnts,i want to put the orifice under the standpipe to start draining my storm runoff at the begining without accumlating any extra water in the pipes, what design measures you guys are suggesting in this case how can we model our stormwater when the peak flow at the predevelopment condition is two low, in addition infilitration is not allowed.

 

[RWF7437]

Just another illustration of the absurdity of many local detention "standards".

For all flows in the minimum size orifice allowed. In my geographic area, that would be 2-inches.

You can size the orifice using:

d inches = 6.166*(SQRT(Qcfs)/H(ft)^0.5)

Assuming a head of 1 foot, and a 2-inch minimum this works out to:

n =2 d < 2.00"
n=10 d < 2.00"
n=25 d < 2.00"
n=100 d < 2.00"

Your flows look VERY low. Are you sure about them ?
This must be a very small drainage area. Are you sure about the area ?
What is the minimum area for which local detention is required in your part of the world ?
By what method did you calculate such small flows ?

good luck

 

[cvg]

and how do you get a runoff of 0.0000?

I would round everything to the nearest cfs and if less than one, round up to 1 cfs. If you do that, you do not need any retention

 

[arabojan]

The predevelopment area that produce such a small flow rate is small and I want to discharge my post development basin flow that is from the new building roof's area to the detention pond and then release to the discharge point of pre-development rate, I modeled the basins by pond pack and base my calculation we need a 3/8" orifice.

 

[cvg]

I would not recommend 3/8 orifice for storm drainage as it will not function well once it gets plugged up. Anything less than about 2" will be problematic unless you can figure out how to screen out all of the debris. Again, I have no confidence that your Q's are accurate, hydrology is not an exact science.