What is a French Drain? How to Install a French Drain

What is a French Drain

French Drain

What is a French Drain? French drains are linear drainage features which take rainfall runoff away from buildings, gardens and other sensitive features and alleviates flooding from any low areas where water tends to pond.

The design of simple french drains in standard conditions is very simple, a procedure is outlined below. Only when the french drain protects your home should a more detailed analysis be required and this can also be undertaken yourself with the assistance of some simple commercial software such as this french drain design spreadsheet.

What is a French Drain?

French drains consist of a linear trench which is excavated into the soil. The trench is then backfilled with gravel which contains a significant quantity of voids between the large gravel particles.

These voids allow water to enter the trench and to percolate down to the bottom. At the bottom of the french drain there is a perforated pipe. The perforations in the pipe allow the water to enter the pipe and then travel down the pipe to an outfall.

In some cases the french drain will allow water to collect at the bottom of the trench which then infiltrates into the surrounding soils rather than being taken away by the pipe. This reduces the amount of water which the pipe and outfall need to handle.

The surrounding soils must however be suitable for infiltration of rainwater. Some soils such as clays and silts are made of very fine particles which form an almost impenetrable layer to water.

It should also be noted that the infiltration performance of the french drain will slowly decrease over the life of the drain as the voids in the surrounding soils are slowly blocked by sediments washed into the french drain with the runoff water.

This process can be slowed by including a suitable geotextile around the perimeter of the drain or some form of sediment interception such as a grass verge before the runoff enters the drain.

No matter what precautions are taken the infiltration performance of the surrounding soils will deteriorate and the design of the perforated pipe should make allowance for this.

This prevents the french drain from failing when the surrounding soils can no longer take a large percentage of the rainfall runoff water.

Where are French Drains Used?

French drains are often used alongside roads, driveways, patios and around the perimeter of gardens. French drains are a linear drainage feature which makes them very suitable for areas where rainfall runoff does not run to a concentrated low spot.

Where a concentrated low spot can be formed to collect all the required runoff a traditional drainage gully and pipe or a soakaway might be more appropriate. However where the runoff simply runs down a slope or needs to be collected along the length of a driveway, french drains are often used.

Design of French Drains

The design of french drains is relatively straightforward. Generally the length and location of the french drain is fixed as it will need to extend along the length of ground where the runoff needs to be intercepted. This is often the full length of a driveway, patio or one edge of a garden.

There are two main factors which are usually critical when designing a french drain. These are the width of the trench and the capacity of the pipe.

The trench needs to be wide enough to accept the full volume of runoff required. The volume of runoff is generally determined by multiplying the total catchment area by a rainfall intensity of 50mm/hour/m2. This will give the volume of water in l/s which equates to an average yearly design storm.

This does mean that the french drain is likely to flood for short periods on average once per year. For critical drain designs where the drain provides protection for a home, building or other critical feature a more detailed runoff assessment should be undertaken. This can be undertaken by a suitable qualified engineer or can be done yourself with the help of a simple runoff analysis spreadsheet.

Once the maximum volume of runoff has been determined, the required width of the french drain can be calculated. The total flow of water through the trench can be determined by multiplying the width of the drain by the length to get the plan area.

The plan area is then multiplied by the permeability of the gravel fill to calculate the maximum flow through the french drain. Good quality gravel fills have a permeability of around 2.5m/s. If the french drain is backfilled with rubble, soil or other poor quality fill this will decrease the permeability and therefore the flow capacity of the drain.

The second critical component to design is the perforated pipe. Generally perforated pipes are designed for uses such as typical french drains so they are sized accordingly.

The manufacturer of the pipe will be able to advise the maximum inflow rate into the pipe through the perforations and the flow capacity of the pipe at different gradients. These will not be critical for most small french drains. The only cases where this can become a problem is when large catchments are focussed on short french drains or where the pipe is laid to a very flat gradient.

In these cases or where the design is critical a more detailed analysis can be undertaken by a qualified engineer. Alternatively a more detailed analysis can be undertaken yourself with the assistance of a simple spreadsheet.

Conclusion

The design of simple french drains for non-critical uses is a simple process but is also an important one to get correct. If any component part of the french drain is undersized this will lead to a bottleneck effect which will cause failure of the french drain and flooding of the surface.

Using the simple procedure outlined above this risk can be mitigated with some very simple design checks. Only when the french drain is protecting your home or another critical feature should more detailed analysis be required.