Drainage area for a French drain

Question

I have a muddy backyard that has minimal slope toward the back property line, and I'm considering installing a French drain to improve the situation. Is there a general rule as to how much area a French drain can drain, or is that type of thing totally dependent on soil quality or other factors? The property is between 100 and 200 feet wide, and I'm wondering if a single drain running down the middle of the property would be sufficient to improve the drainage in the whole yard (50-100 feet on either side), or if I'd have to install multiple parallel French drains to serve an area that large.

Is there any rule of thumb, or simple test I can perform to roughly estimate how much area might be drained on either side of a backyard French drain?

Answer 1

This would depend greatly on the slope of the yard on either side of the drain. A (properly sloped) french drain should divert water about 5 times the pipe diameter on either side(assuming slotted corrugated black pipe in sandy soil) which doesn't come anywhere close to the 50-100 feet you need, so you would need a way to feed the drain area. The easiest way would be to slope the surface toward the drain. Plastic sheet buried under the surface, gravel and sand can also help direct water.

But, as already pointed out - french drains need somewhere to let the water out, and also have to be sloped (at least 1/4" per foot) to be at all effective; there are places they don't work and another solution is required.

This is probably a situation where we can provide better suggestions with pictures or diagrams, and a stormwater expert in your area may have even better advice since they're familiar with the soil and conditions in your area.