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# Modified French Drain with perforated pipe.

## Modified French Drain with perforated pipe.

(OP)

Hi,

I have a 5 ft deep and 7 ft wide drain with two perforated pipes placed on 1 ft over cobble base. Basically entire drain in filled with cobble. Head loss on one side is taken into count and the flow will be above the perforated section of the pipe the entire time. The pipe is schedule 40- 16 inch (dia).

the manufacture says " To calculate transmitting capacity in gpm at 0.1 ft/sec flow velocity, multiply the open area in sq in by 0.31. Slotted screen opening in sq in per foot of pipe is 76.9 for selected pipe which is about 0.052 cfs per ft.

Pipe is placed flat to allow flow from both sides. One side is river and other side is pond. Pipe will be capped at the ends and the flow is only through the slotted area.

What is the max flow that the pipe can allow? n value is (0.26).

I am confused with if adding extra slotted length to the pipe will add to flow through the pipe.
*Pipe will flow from river to pond. Starting at head difference of 5 ft till equalization occurs.
* The middle section of the pipe is solid and has compacted fill over it.
* flow will occur only through the pipe.

### RE: Modified French Drain with perforated pipe.

Max velocity is (2 g h)^0.5
Where h is the difference in head of each end, at the interior end of the pooferations.
There may be significant head loss across the entrance and exit.

Added pooferated length can increase flow if additional length reduces the losses of water entering and exiting the pipe through the rock and pooferations.

Maximum flow will fall to zero with decrease of the square of the head difference.

### RE: Modified French Drain with perforated pipe.

the unperforated solid pipes are way oversized for the small amount of flow that enters through the screen. n value of pvc pipe is not 0.26. it should be closer to 0.013. I assume you are not using corrugated pipe. however, regardless of your n value, your flow is controlled by the size and number of perforations in the slotted screen and by the head differential at the inlet and outlet. making the slotted screen pipe longer should increase the flow

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