French Drain Grade

The french drain grade is the total fall from the top of the french drain run to the bottom, divided by the length of the run. So for example a french drain which drops a total of 1.0m over a length of 100m has a grade of 1% or 1 in 100.

The french drain grade is used in french drain system design during the perforated pipe design process. The perforated pipe is assumed to behave in a similar manner to non-perforated drainage pipes when running full. As such the hydraulic capacity of the perforated pipe can be found using the Colebrook-White Equation or the Manning Equation which includes the pipe grade as an important input. A steeply laid perforated pipe will allow the water to run much more quickly than a flat graded pipe, greatly increasing the total volume of water which can run through the pipe at any one time. The design process is outlined in our Perforated Pipe Design post.

The CivilWeb French Drain Design spreadsheet includes an embedded perforated pipe design element using the complex Colebrook-White Equation. This is sufficient for the vast majority of french drain design conditions. However in some cases a more detailed analysis may be required. This can be undertaken using a traditional drainage pipe design and analysis tool such as the CivilWeb Pipe Flow Calculator spreadsheet.

Typical French Drain Grades

Typically the french drain grade is determined by the levels of the ground above the french drain. In most cases the french drain is laid at the same grade as the overlying ground, then the perforated pipe is designed to suit this grade. The alternative is that the french drain grade is increased relative to the overlying ground but this leads to a french drain which is shallower at the top of the run and deeper at the bottom. This is quite possible to achieve in practice but in most cases the costs of laying a deeper trench than is necessary more than out ways the costs of simply installing a slightly larger perforated pipe. Only in cases where a very long french drain would be laid very flat or occasionally where the french drain needs to run in the wrong direction relative to the slope of the existing ground is the french drain grade artificially altered. This might be in some cases where the only available outfall is at the wrong end of the french drain.

Typically french drains are laid quite flat, as explained above this usually reflects the overlying ground contours rather than aiming for a particular design grade. French drain grades of 1%-0.1% are common, though steeper grades are unlikely to present a problem. Where the grade becomes very steep the possible risk of a hydraulic jump occurring within the pipe or erosion issues can a problem. Further information on maximum velocities for water in drainage pipes is included in our Maximum Velocity in Pipe post.

Usually the most important aspect of french drain grade design is ensuring that the french drain is laid at a consistent grade across the whole length of the run. This is important because the perforated pipe will only be able to handle flows at the rate of the shallowest gradient along its length, not the overall total grade. So for example if a french drains overall grade is around 1% but there is a small length of pipe laid at a grade of 0.1%, the perforated pipe will have a capacity consistent with the 0.1% grade, not the overall grade of 1%. This is because the shallow section of pipe will act as a bottleneck, reducing the capacity of the whole pipe.

For this reason it is important that the french drain installation process is closely controlled to ensure that the french drain is laid to a consistent grade. In most cases this constant grade will lead to small variations in the total depth of the perforated pipe relative to the overlying ground. Where this would lead to significant variations in depth, particularly where the french drain depth would be left shallow in places, the structural design of the french drain and the minimum depths should be checked. This is explained in further detail in our French Drain Depth post.

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