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Natural Drainage
Drainage trenches are a useful means of draining excess water from land. Traditionally, French drains are used and require a trench to be dug with a perforated pipe being laid at the bottom and a drainage aggregate placed on top with a few inches of top soil replaced on the surface. A brash drain is a more natural and equally if not more long lived means of draining land. The brash material can be sourced on site or locally.
One of the main benefits of using a brash drain is that it is made from completely natural materials, costs next to nothing and can be forgotten, altered or removed with great ease.
Brash
The most suitable brash species have thorny stems and are widely found in hedgerows across Great Britain.
They are:
- Blackthorn/Sloe (Prunus spinosa)
- Hawthorn (Crataegus monogyna)
Blackthorn is the better of the two as it suckers readily and repeatedly from the main plant, these suckers making ideal length material after 3 years of growth.
The thorns help knit the brash together and ensure there is space in between the brash to allow water to drain through. Thorns also take a very long time to decay. The brash can be any length although for ease of use 3 to 4 ft (90-120cm) long material is easiest to handle and is mature enough to last beneath ground.
Cutting time
Brash material should be cut during late winter and early spring (January-March). This way you are cutting the trees/shrubs when dormant and can avoid leaves and fresh growth. One area of interest is that trees from the Cherry family should not normally be cut during late autumn, winter and early spring. The reason being that at this time of year these trees are most likely to be affected by a disease known as Silver Leaf. However, Blackthorn is a durable hardy hedging species that is traditionally laid during the winter. Therefore winter is the only practical time of year to prune.
Longevity
Drainage using brash material is an ancient technique. Blackthorn and Hawthorn are both hardwoods and in the case of Blackthorn is surprisingly slow growing for a native shrub. Coming from the Cherry family Blackthorn has a dense and durable wood. Despite the durability of the material, it is the lack of oxygen beneath ground and the damp, sometimes waterlogged environment that prevents aerobic decomposition and so ensuring the long term efficient use of the drain.
Brash drains could last decades.
How to construct a brash drain
Key points
- First of all, make sure you have access to plenty of material (preferably of regular size and tough and woody. Huge quantities of material are required even for short lengths of drainage, so try and judge what you have.
- Decide what your soak away will be and where it is/or will be located. A soak away could take the form of a ditch or a pit filled with drainage aggregate/brash, topped with soil.
- Mark out your trench with string lines.
- For every 100metres of drainage trench the trench needs an incline of 1 ft (30cm). A ratio of 1 : 100. If the ground is level, then at one end, the end where the ditch or soak away is located must be 1 ft deeper than the other.
- Locate the soak away in the lowest point of the land.
- Dig the trench at least 8” (20cm) wide and 12” (30cm) deep.
- Make sure the trench leads into a soak away
- Keep the trench far away from building foundations, especially historical buildings that have shallow foundations. Otherwise subsidence can occur from the soil drying more than normal.
The Brash material should be laid in a brickwork pattern, along the length of the trench. After lying two to three layers of material, tread down the brash to flatten and consolidate as best you can. If the material is compacted there will be minimum soil subsidence on the trench later on. Repeat this process until you are 3” (7.5cm) from ground level.
The remaining 3” can be back filled with topsoil (not clay) and can be consolidated by treading. Leave the surface of the soil slightly proud to allow for natural consolidation from rain etc.