Top tips for Mole DrainingCotterill Civils
After an exceptionally hot summer, this winter is set to be the coldest and harshest in a decade (according to long-range forecasters). But how will your land drainage system fare if we get a lot of rain?
If you’re looking to prevent water-logged fields, now’s the ideal time to ensure your field-drainage system is in tip top condition. A very effective way to ensure this is by undertaking mole draining.
What is mole draining?
Mole draining is an operation to optimise the performance of your field-drainage system. Mole draining is carried out across the direction of an installed land drainage scheme and joins up the land between the lateral drains.
As land drains are typically spaced at 20m intervals across a field, rainwater that falls between them can often struggle to reach the drains. As a result, this rainwater can end up sitting on the surface of the soil with nowhere to go.
Mole drains help counter this by guiding the excess rainwater into the drains. Typically, they are opened up at centres of around 2.75m across the field. Used in the right soil type and installed correctly, mole ploughing will help reduce water logging.
When should it be carried out?
For mole draining to work, it is essential that it is carried out under the right soil conditions – these being dry enough in the top 12 – 18” to allow good traction and soil fissuring whilst moist or plastic enough at depth to form and maintain the channel. The best time to do this is in the two month window after harvest and before drilling, when the land is bare. Another good time to mole plough is in spring on heavy clay soils although it’s important to bear in mind that the top soil needs to be dry enough to travel.
How often should mole draining be carried out?
Ideally, mole draining should be carried out at least every 5 years. During the first few years of its installation, we recommend undertaking an annual programme. This is because the cumulative effect of doing this in its initial few years will bring the greatest long-term benefit. Carrying out this operation will increase the effectiveness of your land drainage system. A good drainage system should last at least 40 years.
How to identify if your drains are in good working order?
This is not always as straightforward as you may think but the best way to identify whether a drainage system is working effectively is after a heavy downpour. If the system is working correctly, the drains should be running freely and the drains should dry up within 72 hours after the rain. If the outlets are still trickling water after this period of time, it could suggest that the rainwater is struggling to make its way to the drains. Also, if the drains do not release any water, this could be as a result of a blockage.
How does mole draining work?
The aim of mole channels is to run them into either an open ditch or above a pipe drain back-filled with gravel. Before you even attempt to undertake mole ploughing though you need to gather as much information about the field as possible. Local knowledge should never be underestimated and anyone new to an area should find out as much as possible. The chances are that if you have a problem on the site, then at some time in the past, your predecessors will have tried to make improvements. Here’s some examples of ways in which to gather information:
- Looking for old plans hidden away in a cupboard
- Contact your predecessors to see if they have any plans
- Investigate local authority records
- Contact ADAS – they sometimes keep records
If you don’t have maps then it’s probably not a good idea to mole drain at all. Land drainage systems are carefully worked out when they’re installed and take into account all the field undulations.
If you do have a drainage map, there will be a gravel depth measurement and this indicates your mole depth. If the system is more than 20 years old, you should use a spade to check this is still accurate – it may have sunk and if so, you may need to mole a bit deeper. The maps may also include the direction of the moling on them.
View the video below to see mole draining in action – it shows a farmer installing mole drains at Overbury Enterprises in the Cotswolds.
Length, gradient and spacing of mole drains
The length of mole drains can vary and depends on a number of factors, including soil texture, surface gradient and soil suitability. The table below provides you with an indication of mole length based upon the soil profile and surface gradient. If an outfall requires a long mole drain, then a collector pipe is recommended.
Recommended lengths & gradients for mole drains in pasture soils
|Soil profile||Surface gradient (%)||Mole length (m)|
|Clean||0 – 1||40 – 50|
|Clean||1 – 2||50 – 60|
|Clean||3 – 5||80 – 100|
|Stony||0 – 1||30 – 40|
|Stony||1 – 2||40 – 50|
Table source: Agriculture Victoria
In the case of steeper gradients (greater than 3 %), mole draining should be relatively trouble free. This is because minor surface undulations won’t cause blockages with negative gradients, and the risk from erosion is reduced. On steeper gradients moles should cross the direction of the main slope to help intercept surface runoff and to avoid the possibility of channel scour and erosion. Generally speaking though, surface slopes are normally below 3 % and drains should run parallel to the slope fall. When the gradient is much flatter, the more even the soil surface has to be and collector pipe drains may be needed.
In terms of spacing between mole drains, this is normally around 2 m in dairy pastures. In less intensely grazed areas the spacing can be up to 5 m apart but it is important to note that the wider the spacing, the less effective the drains will perform.
The effectiveness of the system is underpinned by the outfall. The drain can be designed to discharge into open drains, collector pipe systems or interceptor drains filled with gravel.
If you use an open drain outlet, this should be fenced off from stock and kept clean to ensure the outfall is above the drain water level. This is required to avoid water backing up into the mole outlets, which will soften the clay channel and eventually cause their collapse. You can, however, insert short lengths of plastic pipe in the ends to protect them.