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Simple French Drain Design
French drains (or filter drains) consist of narrow trenches filled with gravel. The gravel fill includes a large percentage of voids which fill with rainwater. This rainwater is able to percolate through the gravel layers to a perforated drain pipe installed at the bottom of the trench. The runoff water enters this pipe through the perforations and is then conveyed through the pipe to an outfall.
The design and construction of French drains needs to be undertaken with care to ensure that the drain will perform as required throughout its life. The design needs to ensure that the French drain trench and pipe are adequately sized and that the gravel used is suitable. The construction of the French drain needs to be undertaken correctly in order to prevent the French drain from becoming blocked over time.
Common Uses for French Drains
In many cases a French drain presents an effective and cost efficient method of removing surface water runoff. This is particularly the case where a linear drainage feature is required such as along the perimeter of a garden, patio or driveway. They can also be used to intercept groundwater and as curtain drains down slopes.
Design of French Drains
In order for the french drain to perform adequately it must pass three main design criteria;
- The trench must be wide and long enough for the full volume of rainwater to enter the trench and percolate through to the pipe.
- The pipe must include enough percolations large enough to allow the volume of water to enter the pipe.
- Then the pipe must be sufficiently large enough to convey the full volume of runoff water to the outfall.
If any one of these design criteria is not met this will create a bottleneck which will cause the whole French drain to flood during heavy rainfall. Where the French drain is installed to prevent runoff from reaching a house or other building this is particularly critical.
In order to design the french drain first we must decide exactly how much water you are going to design the french drain to handle. Determining the runoff volume from large catchments can be very complex but for small catchments typical for domestic uses, the following rules of thumb will be adequate.
In the UK a typical value of 50mm/m2/hr is often used to determine a rough runoff volume from a design storm which will occur on average once per year. This can vary depending on the location, altitude and design storm return period. Where the French drain is being used to protect a dwelling or other building, it may be prudent to undertake a slightly more rigorous analysis. This can be done yourself using a simple rainfall and runoff spreadsheet.
Trench Size and Gravel Type
The first critical design criteria refers to the size of the trench and the permeability of the gravel. In many cases the length of the French drain is set as the length of the driveway, garden or patio it is serving. Therefore the critical design parameter is the width of the trench which is dependent on the permeability of the gravel fill used.
The trench and gravel fill must be able to percolate the full volume of the runoff entering the French drain. The rate of flow through the gravel can be estimated simply using Darcy’s Law which states that the total rate of flow is equal to the length of the French drain multiplied by the width multiplied by the permeability of the gravel. Typical single size gravels will have a permeability of around 2.5m/s. If the gravel is of a lower quality with a higher percentage of sands and fine particles, this permeability will reduce. For this reason it is important to use a high quality single sized gravel fill.
The minimum width of the French drain trench is often set by the type of excavation materials used. Typically a minimum width of around 500mm is suitable for hand excavation or mechanical excavators with special smaller buckets. This minimum width should be checked against the above calculation and if this is not sufficient then the width should be increased. For general domestic use this minimum width is usually sufficient so long as good quality gravel is used and the catchment area is not particularly large compared to the length of the French drain.
Inflow into the Perforated Drain
The second potential bottleneck is the inflow into the perforated drain. If there are not enough perforations in the pipe the runoff water cannot enter the pipe quickly enough to prevent this becoming a bottleneck with the runoff water backing up through the gravel and flooding the French drain.
Most perforated pipes will include a maximum inflow capacity value with their literature which can be used to check whether the pipe is suitable. If this information is not available, for example when an existing pipe is being used, the maximum inflow into the pipe can be determined using the orifice flow equation. This can be quite tricky to complete as it requires a coefficient of discharge and hydraulic head inputs as well as the precise size and number of perforations. Simple software such as this french drain design spreadsheet can assist you in completing this calculation.
In most cases the manufacturers of perforated pipes align the size and number of perforations to suit typical uses such as for French drains. In general the inflow into the pipe should be okay so long as the French drain is not especially short and handling a large catchment area.
Flow Through the Perforated Pipe
The final potential bottleneck is flow of water through the perforated pipe to the outfall. Experimentation has confirmed that flow through a perforated pipe running full or close to full is very similar to the flow of water through non-perforated circular pipes. The Colebrook-White equation is the most commonly used method in the UK for determining the capacity of perforated and non-perforated pipes.
The Colebrook-White equation is complex however for typical drainage design applications it can be condensed into a chart showing the flow capacity related to the pipe diameter and the gradient. An example of a Colebrook-White chart is shown below which is suitable for the design of French drain perforated pipes.
Again in general the manufacturers of the pipe will design the pipe size to suit the inflow capacity through the perforations. So if the inflow into the pipe is acceptable then it is likely that the pipe size will also be okay, unless the pipe is to be laid at a very flat gradient.
The careful design of french drains is required to ensure that the drain works as it should. The design rests on three main components, the trench width, the pipe inflow and the pipe through flow. Completing these design checks can generally be done with information provided by the manufacturer and some simple calculations. Where required some a commercial design spreadsheet can be used to complete all the calculations.
If any one component is insufficient this will cause a bottleneck leading to flooding of the French drain. For typical domestic French drains serving smaller catchments each component should be okay. However for larger French drain systems, non-typical situations or for critical French drains protecting buildings, completing these three design checks will provide peace of mind and confirmation that the drain will perform as required.