How to make a garden drainage channel

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Drainage channels are a narrow trench, gulley or depression which channels water to a specific location. 

Typically drainage channels allow unwanted water to flow to drainage networks or the landscapes lowest point. This is to prevent flash flooding, damp and standing water from affecting a given area.

​If you want to know how to make a drainage channel you must identify what kind of channel you need. 
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This will depend upon the potential volume of water you wish to channel and the type of catchment area.

For example if you are draining a patio you will more than likely need a grated drainage channel.

In this article we will discuss the drainage considerations and implications surrounding drainage channel installation. We will also describe the four main types of drainage channel and how to install them. 

Considerations for building drainage channels

Catchment area

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The type of drainage channel you install will really depend upon your catchment area. This is the garden area or zone which needs to be drained. This will not only dictate the volume of water runoff but velocity of that runoff. 

Catchment area surface

​An impervious surface such as a patio will accelerate storm water, surface, runoff. In contrast, if you are draining a lawn, hydraulic discharge will be slower and longer lasting.

Therefore each catchment scenario is different. If you are dealing with impervious, hard surfacing drainage channels such as grated gulleys are best.

These provide deep channels which do not create hazardous level changes for pedestrian traffic.

They are also designed not to decrease the aesthetic appeal of paving and other surfaces. For lawns and flowerbeds French drains or perforated pipe drainage channels are best.
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Water volume 

​When considering what garden drainage channel to install, predicted runoff volumes are a noble consideration.

You certainly do not want your drainage channel to be overwhelmed in times of heavy rainfall. Therefore it is a good idea to get an idea of your potential run off volume during times of heavy rain.

In order for you to do this you will need to work out your catchments surface area. This can be done by multiplying its length by width.

The catchment area must then be multiplied by the depth of rainfall during a period of extreme rainfall. 
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The UK record rainfall total in 24 hours was Storm Desmond at 341.4mm in 2015. However I would say that 100mm is more of a typical, extreme rainfall event.
 
Therefore if you have a catchment area of 10 metres by 10 metres the calculation would be:

10m x 10m = 100 square metres   100sqm x 0.100mm = 10 cubic metres of water.

In such a scenario a drainage channel would have to accommodate 10 cubic metres of water within 24hours.  Your proposed drainage channel should be sufficient to accept such volumes in times of heavy rainfall.

​It is also vital that your destination drain also has the capacity to handle such storm events. This is more relevant if you are relying on soakaways as a destination drain for a channel. 

Destination drain

All drainage channels have a primary function of channelling water from the catchment to a destination drain. Destination drains could be the local drainage network, a soakaway or even a retention pond.

All drainage channels should be planned and installed with the destination drain in mind. If you expect to collect large volumes of water your drain must be able to handle such run off.

Grated drainage chambers typically plumb straight into the guttering water network. However sustainable water policies now require you reabsorb storm water back into the landscape.

For specific drainage policies in your area contact your local authority. 
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Types of drainage channel

Drainage gulley

A drainage gulley is traditionally a shallow, semi-circular, depression which collects and directs surface water.

These are generally formed into areas of hard standing such as paving and concrete. In their simplest form they are formed with a fillet of concrete formed into a bowl. This can be executed with a pointing trowel or by pressing a wide pipe onto a mortar surface.

For added decoration concrete gulley’s can have flat pebbles pushed into the surface. To make a drainage gulley you will need to install a rigid base similar to that of a patio.

This is so the concrete does not crack due to ground movement. 
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How to install a drainage gully

Make sure your gulley route is excavated to a depth of 200mm with no loose soil present.

Peg out your levels so you can engineer a fall so water flows downhill to is destination.

Supply and spread a 100mm layer of compactable hardcore such as mot type 1.

Supply and lay a 100mm layer of wet concrete mix with impervious, sealant, admixtures.
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Work the concrete to a consistent depression or semi circular shape to accommodate the flow of water. 

Drainage grate channel

​Drainage grate channels typically come under many different brands and names. One of the most common of these is the Acco drainage chamber.

These drainage channels are designed to receive surface run off from paving and other surfaces. Grated channels generally consist of a narrow gully with a removable grill for cleaning.

There are now so many on the market with materials ranging from plastic to stainless steel. 
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​Nonetheless they are all very similar with a grate covering a depth and width of around 100mm.

Drainage channels with grates also require some excavation to be installed. To be as rigid as possible these require at least 100mm of base underneath them. This means a complete excavation of around 200mm.

​This allows for 50mm of compactable sub-base and 50mm of bedding mortar. Alternatively these drainage channels can be bedded on a thick layer of wet concrete. 

How to install a drainage grate channel

​Make sure your gulley route is excavated to a depth of 200mm with no loose soil present.

Peg out your levels so you can engineer a fall so water flows downhill to is destination.

Supply and spread a 50mm layer of compactable hardcore such as mot type 1 and compact.

Supply and lay a 50mm layer of wet concrete mix to the sub-base.

Bed down the drainage channels onto the mortar mix. Make sure they are connected well and laid to a desired and consistent level.

Haunch up around the bottom half of the channel to secure them into position. 

French drain 

​A French drain is a drainage channel which allows both surface run off and ground water to drain away.

French drains consist of a linear trench which hosts a perforated pipe encased by free draining gravel. The benefit of this drainage channel is it can relieve hydraulic pressure within the ground and drain waterlogged soil.

As water seeps into the channel and through the aggregate it flows into the perforated pipe. This allows the water to be channelled away quickly to its destination drain.

French drains do work and are very effective drainage channels for both ground and surface water. However their effectiveness rests upon the correct installation and specification of the channel.

​Furthermore the level of ground saturation could require multiple French drain channels to be effective. This is particularly so on clay soil where soil particles soak up water like a sponge. 
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French drains should ideally reach a minimum of 600mm deep and a width of 300mm. They should encase a 100mm diameter perforated pipe encased in a free draining 10mm, non calcareous, aggregate.

In order to prevent silting the channel should then be capped with filtration fabric.
The beauty of French drains is they have the ability to hold large quantities of water as they drain.

This means they can act as linear soakaways which allow water to reabsorb into the landscape as they drain. This is particularly the case over long distances as their volume becomes increased due to extrusion.

​These are extremely effective combined with soakaway drainage systems.
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How to install a French drain

​Make sure your gulley route is excavated to a depth of 600mm x 300mm with no loose soil present.

Peg out your levels to engineer a fall so water flows downhill to is destination drain.

Supply and spread a 200mm layer of free draining 10mm pea shingle gravel to the excavation.

Lay a 100mm diameter perforated pipe on top of the gravel and plumb in where appropriate.
Supply and spread another 200mm of gravel on top of the perforated pipe completely encasing it.

Cap the top of the gravel with a geo-textile filtration membrane.
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Fill the remaining 200mm of trench with topsoil and consolidate to prevent settlement. 

Swales 

A swale is a landscape depression which is designed to collect, channel and drain away storm water.

Thought more of as landform than a drainage channel they allow water to pool and be reabsorbed slowly. These can be engineered with overflow outlets so they do not flood in times of excessive rainfall.

​The real benefit of swales is they can become landscape features and valuable wildlife habitat. Swales generally become seasonal flooding wetlands which provide valuable habitats for wetland species. 
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Bio swales take this concept a little further and are designed with ecology in mind. Urban and polluted runoff is channelled through wetland plants which cleans the water naturally.

Swales can be a creative and ecological solution to conventional drainage channels. On a more domestic scale these can be landscaped into attractive rain gardens.

For more information on rain gardens read our article on the subject here. 
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How to install a swale drainage channel

Analyse your sites existing topography to see where surface water collects and runs to.

Plan to enhance this natural course way or re-engineer levels to create a new water collection route.

When you have formed your landform, excavate out a smoothly graded swale to the lowest contour.

Grade the topsoil to an evenly graded surface for planting, turfing or seeding.

Monitor your new swale in times of heavy rain, if it gets too full install and overflow.
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At this point you can plant wetland plants or execute further aesthetic landscaping to the swale. 

Thank you for reading our article on how to install a drainage channel. If it helped in your search why not share it via the tabs below.

If you require more specific information on diagnosing garden drainageread our article here. Planning on building your drainage channel yourself?

Why not read our article on the best tools and equipment for garden drainage projects. 

Garden drainage Contractors & Services in Buckinghamshire

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​Buckinghamshire landscape gardeners are experts in garden drainage and provide a number of garden drainage solutions. Our garden drainage services include;
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• Garden drainage systems
• French drains
• Garden soakaways
• Lawn drainage
• Patio drainage
• Garden drainage gulley installation
• Sustainable & eco drainage

Our garden drainage services cover most of Buckinghamshire including:
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• ​Amersham
• Aylesbury
• Beaconsfield
• Berkhamsted
• Chalfont
• Chesham
• Gerard's Cross
• Great Missenden
• High Wycombe
• Princes Risborough
• Wendover

Further reading

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