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Drainage Systems
Drainage systems are vital in the garden for healthy plants, to stop waterlogging, and protect the integrity of your home’s foundations. Learn more about drainage and the types of drainage systems that are available.
Why Drainage?
Drainage is vital within the garden, to stop “boggy” areas, to protect the integrity of the foundations of the home, and to ensure that plants are growing to their optimal capacity. Poor drainage can occur for a number of reasons – compacted soil as a result of building or renovation, heavy clay soils, or a very shallow water table, to name a couple of reasons. To check your drainage, dig a hole 600mm deep and 300mm square. Fill it half full with water. Leave it for 24 hours, during which time it should fully empty if your soil is well-drained. If you have poor drainage, water will still be in the hole, and in some cases, there may actually be more water.
Types of Drainage
There are two forms of drainage – surface or subsurface. Surface drainage collects and redirects the excess water that is on the surface of the ground. It is used to catch rainfall before it causes damage and direct it elsewhere. Spoon drains are shallow channels that are generally made from concrete and can be installed around the garden to redirect this excess water to other parts of the garden or to inlets for subsoil drainage systems. Large paved or concreted entertainment areas use spoon drains covered by a small grille which then directs the water to a stormwater system. However, another option is to direct the water to a specially-built rain garden, preventing the water from entering the stormwater system and potentially polluting waterways.
Subsurface drainage systems are buried beneath the surface of the soil. They can be installed in new or existing gardens. Subsurface drainage is commonly used in grassed areas and it must be used behind all retaining walls. Specially designed pipes are used to collect and move the excess water away. Trenches need to have a slight angle in order for the water to flow in one direction towards an outlet system. Subsurface drainage systems commonly use geotextile fabrics around the pipes in order to stop soil and other particles from blocking the drains.
Land drainage works by providing an open conduit for groundwater to follow to a point where it can be disposed of. Dispersal drains lead to a dispersal area or leach field. The disposal point – a soakaway, leach field or manhole, or connection to the stormwater system is always at the lowest point of the system. This is because water will always flow downhill.
Drainage Solutions
The type of drainage that you choose will depend on how severe your drainage issues are. If you have excess surface water, this may be able to be controlled by shaping the garden surfaces so that the water flows into ditches. Ditches should be 0.9-1.2m deep, with sloping sides. A soakaway is another solution. The soakaway is simply a large hole at the lowest point of the garden filled with rubble or broken bricks and then covered with 300mm of soil. To have the intended effect, the soakaway will have to be quite large- up to 1.8m deep and the same across. You will not always have to run underground pipes to a soakaway but if the land does not have enough slope to drain into the soakaway, piping will be required.
Modern plastic drainage pipes are made from crush-resistant moulded plastic, usually covered in a geotextile fabric that stops the system from becoming clogged with sediment. The pipes come in lengths up to 15m long, which can be laid into a trench and surrounded by gravel. If a large area needs draining, it is most efficient to lay the pipes in a herringbone pattern. Underground pipes should always be laid above the water table otherwise the pipes will be moving groundwater instead of draining excess surface water. If you have level ground, ensure that you lay the pipes with some slope so that water drains away quickly.
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