How long after a rain event Should my French Drain take to dry up the yard?

Offering reliable landscaping services to homeowners in British Columbia since 2013

Natasha Maerz

Natasha Maerz

Owner/Office Manager

 

One of our most frequently asked questions about yard drainage is how long will a French Drain system take to work?

A very general timeframe is 12-36 hours after a heavy rain event you can expect your yard to be free from pooling water. The factors that effect this will be explained, and by the end of the article you will understand that every French Drainage system is different, and why some areas dry out faster than others.

 

Table of contents:

  • The 3 Factors that impact the speed of a French Drain System
    • How much yard water needs to be drained?
    • How does the water get from the surface of the yard into the drainage pipe?
    • Where does my French Drain water go?
  • Maintenance of a French Drain System
  • Longevity of a French Drain System

 

 

The 3 Factors that impact the speed of a French Drain System

 

A French Drains performance can be broken down into 3 sections and as we are about to discover they all have to function to get results. Often times a system that is not working has a component that has failed, or was not designed correctly.

 

How much yard water needs to be drained?

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.

 

How does the water get from the surface of the yard into the drainage pipe?

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.

 

Clay soil effects a French drain in two main ways.

1: The first is it creates standing water on the surface of a lawn. If you have ever made a clay bowl as a child you will remember that water will stay in this bowl almost indefinitely. Light clay soils will eventually pass water, but heavy clays will almost never allow water through it on its own.

Lawns with healthy grass roots will help to penetrate this clay soil, and provide microscopic paths for the water to get through. It generally takes a year for the grass to establish healthy roots, however if this is in a shady area, or an unmaintained area the grass may never provide adequate roots to penetrate the clay soil.

2: The second reason clay soil is a problem for French Drains is once the water does eventually make it into the ground, it moves laterally very slowly. This slow process causes water to back up on the surface and if not addressed will prevent the system from ever draining the water.

Best practises to overcome clay soil:

  • Install a French drain system with void spaces underground (large round rock). The void spaces will dry out the clay between rain events, and instead of starting a rain event with saturated soil, you will go into a rain event with dry soil that is capable of absorbing much more water.
  • Replace the top 2-4” of soil in the yard with free draining sandy soil. This will allow the water to get below the grass and move laterally to the nearest French Drain trench.
  • Increase the size of a French Drain system. The closer together the Drain trenches are excavated the shorter the distance the water has to travel to be removed. This will increase the speed in which your lawn dries up.
  • Grade the area to slope to a low spot and instal the French drain system there. This way the water will run on top of the grass and move quickly to a centralized location to be collected from.

 

Another component of infiltrating a French Drain system is the fabric layer the water must travel through.

Most landscape filter fabrics do not pass water fast enough to use. This means the fabric will plug up, and greatly reduce the speed in which the area dries up. There are specific fabrics to use that are tested to use in clay soils, and that pass water even under the most extreme cases.

 

In some cases, the highest rated filter fabric may still not be fast enough and you may want to Install a grate over top of the fabric instead of grass. In extreme cases you could have an open French Drain system where there is no fabric or sod on top, just a layer of round river rock. It would be important not to do this in a heavily treed or trafficked area where the system would get plugged up from debris. Installing an open French drain around a patio however would not only look great, it would drain the water faster than it can accumulate.

 

Where does my French Drain water go?

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.

 

Maintenance of a French Drain System

French drain systems that use filter fabric see very little debris from the surrounding soil. The benefit to using a corrugated pipe is that as the water passes through it, it creates ripples. These ripples actually clean the pipe and pull any sediment with it.

 

Smooth walled PVC pipe actually allows debris to build up inside. The gentle smooth flowing water does not have the power to disrupt the debris and it can settle in the bottom or along the inside of corners.

 

PVC also has another drawl back. Because it only has holes on the bottom it needs to be installed on top of 2-4” of gravel. This means that you would always have standing water in the gravel just below the pipe. This water will attract tree roots that will find their way into the pipe. As the roots begin to clog up the pipe, they will need to be cleaned out by a jetting machine. This jetting machine can and will remove the roots from inside the pipe, but the roots will still be present just cut flush with the pipe’s intake holes. Days/months later the roots will have regrown inside of the pipe again. This cleaning and regrowing will continue indefinitely.

 

 

Longevity of a French Drain System

 

The lifespan of a French drain is determined by the materials and design of a system. Most materials will not biodegrade in the ground on their own, but rather will fail from poor installation or design. Pvc for instance is very strong, but because of this will not flex under changing ground pressures from frost or ground settling. A cracked pvc pipe at the bottom of the trench will need to be replaced much sooner than a flexible corrugated pipe that is rated and designed for a French drain system.

 

A properly designed and installed system will outlast and outperform in every category. It will have little to no maintenance, and will last well past a lifetime.

 

If your facing yard drainage issues, and have more questions please read through our drainage category for more information.

 

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