A very general timeframe is 12-36 hours after a heavy rain event you can expect your yard to be free from pooling water.
How surface water gets into your French Drain drainage pipe is called infiltration. This is by far the most important and complex part of the system.
Almost every yard that has a drainage issue is caused because of clay soil. Clay soil does not percolate well, and is the reason you have standing water. Yards with sandy soils drain into the earth as fast as it can rain.
The first question we must ask when designing a system is how much water will the French drain be responsible to drain? French drains are installed in the yard or lawn, and may also be responsible to drain rain water that has run onto the lawn from a patio, sidewalk, and sometimes even a driveway.
In most cases a French Drain is not responsible for collecting rain water that falls onto the roof, and is collected in the gutters and downspouts.
That drainage system is called a downspout drain. However, it is not uncommon for downspouts to be directed onto the surface of the lawn.
While a French drain can eventually drain this, it will create more standing water that will take longer to dry after a rain event.
When setting your expectations for how fast your French drain dries up an area, consider how much water is involved. Limiting the amount of water directed onto your lawn will decrease the waiting time after a rain event.
Once the water is inside the fabric of a French drain system the water will flow virtually unrestricted to the lowest point in the system.
The best French drain systems use a pipe at the very bottom of the trench and surround it with large 1.5” round rock. There is so much void space underground that between the pipe and the round rock the system can move 400-800 gallons per minute.
The French drain system itself is almost never what effects the speed in which your yard dries out, as it’s so much faster than the intake portion of the system.
Most French drain systems discharge to a concrete sump where any sediment or debris can settle to the bottom of the basin.
From here its drained to a city storm water connection, ditch, lower area that can be flooded, etc. As long as this capacity to intake the water isn’t exceeded, then the water will not back up the French drain trench.
What Can I Do Make My Lawn Dry Out Quicker?
While French Drains dry out within 12-36 hours after a rainfall, if you want to dry out your yard even quicker, you may want to upgrade your system.
A drain grate can be added to your French Drain system to allow water to flow directly into the trench, and let the water be discharged faster than having to wait for it to go through the clay or sandy soil, and then into the system.
Another option is to actually leave your trench open. Leaving your trench open (not backfilling with soil) means that the water will be able to find your trench right away, and the exposed rock adds a nice décor element to your space.
The option of leaving your trench open is a good one for homeowners who have an open space in their yard, with little vegetation.
If your yard has a lot of trees around the area, an open trench may cause you problems, due to leaves and other dirt and debris getting into the system. This debris can clog the rock, and not allow the rock to drain the water going into your trench.