French Drain Clogged

There are many potential causes of french drains becoming clogged. This includes clogging of the filter material, the perforations in the pipe or within the pipe itself. In some cases a combination of all three specific causes will result in french drain becoming clogged. A critical part of french drain design is minimizing the fine sediments able to enter the french drain system. Further information on sediment control methods are included in our French Drain Water Quality post.

Do French Drains Clog?

Yes! The most common reason for french drains not draining is because the french drain is clogged with mud or fine sediments or leaf/vegetation mulch.

A french drain is clogged when the sediments are able to enter the filter material or the perforated pipe. This is one of the most common causes of french drains not draining and clogging is one of the main reasons that suitable french drain maintenance needs to be undertaken on a regular basis.

If a french drain becomes clogged the performance of the drain will rapidly deteriorate until ponding on the surface after rain becomes common due to the french drain not draining. Ponding occurs because the french drain is unable to handle the volume of water due to the reduced permeability of the filter material, reduced inflow into the perforated pipe due to clogging of the perforations, or due to the reduced water conveyance capacity of the pipe due to sediments building up within the pipe.

French Drain Repair

If a french drain becomes clogged the required repair will depend on which part of the french drain is blocked. If the filter material has become clogged, this can be repaired by removing the filter material and either replacing with new or better the filter material can be washed and relaid.

Special machines are available that can remove, wash and relay filter material from french drains which reduces costs and removes the need for imported material to be used and old filter material being disposed of. French drain cleaning services are available in some areas though a large french drain run may be required in order to make the french drain cleanout service cost effective.

If the perforations in the pipe are clogged it may be possible to temporarily improve performance by jetting the pipe, though it is unlikely that this will remove all the material clogging the pipe. If the pipe is full or partially full with sediments this can be cleared by jetting the pipe. In some cases the sediments clogging the pipe cannot be removed by jetting. In these cases it is usually required for the clogged section of pipe to be removed and relaid.

Fin drains are subjected to similar issues with clogging and repairing clogged fin drains will usually require the fin drains to be removed.

A french drain maintenance schedule should be devised which can significantly extend the life of the french drain even in conditions where sediments are able to enter the drain. Timely maintenance is also preferable to the situation deteriorating and the french drain not draining, requiring a full repair of the drain.

How to Prevent French Drain Clogging

As well as the permeability, the filter material must be chosen to minimize the risk of the filter material clogging with fine materials from the runoff or from the surrounding soils. Typically this is done with the use of a french drain filter fabric but the filter material can also be designed to avoid clogging where a geotextile filter fabric would not be suitable.

In fine grained soils such as silts filter materials cannot be designed to prevent clogging as any materials which would prevent migration of these fine grains would also prevent sufficient infiltration of water for the french drain to function. This is a particular concern for french drains in clay soils. In these cases the performance of the filter material should be monitored closely and the filter material replaced when it has become clogged with fine particles from the surrounding soils. This may lead to the filter material being regularly replaced or washed as part of a french drain maintenance schedule.

How to Prevent Perforated Pipe Clogging

Similar precautions can be taken to minimize the risk of fine sediments from clogging the perforated pipe, which is another common cause of french drain clogging. Obviously if the fine sediments are prevented from passing through the filter fabric or the filter material, they cannot enter the perforated pipe.

However in some cases it is the filter material itself which is able to enter the perforations in the pipe and lead to a blocked french drain. The filter material must be sufficiently sized to avoid entering the perforations in the pipe. Coarse graded material such as Type B does not contain significant quantities of material which could enter the perforations in the pipe. Test have also indicated that filter materials graded to a Type A standard also do not generally enter the perforations of french drain pipes. However where finer filter materials have been specified for water quality reasons for example, a check must be done to ensure that the filter material is not washed into the pipe.

Poor quality filter material must also be avoided as any unwashed materials containing significant quantities of fine particles can lead to blocked french drains.

The french drain grade can also be adjusted in some cases to prevent the perforated pipe from becoming clogged. Circular pipes installed at a steep enough grade will clean themselves of fine sediments during storm events. These self-cleansing conditions can be determined in the same manner as for normal non-perforated pipes. A spreadsheet such as the CivilWeb Pipe Flow Calculator can be used to determine the required french drain grade for self-cleansing conditions. Further information can be found in our minimum velocity post.

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