15 Apr 2015

Garden Drainage

Posted by Leo

The weather is starting to cool and with April being a typically wet month, it’s time to sort out your yards drainage.

When correctly designed and implemented, drainage works cannot be seen, but that does not change the fact that garden drainage is one of the most important elements of the garden to get right.


Drainage Problems in the Garden

Poor drainage creates a range of problems for gardeners. These include boggy lawns, the death of plants, and even undermining the foundations of your house. Excessive amounts of water running off garden beds or off overhanging roofing or canopies can bring silt and clay to the surface, causing mud to form and waterlogging of the subsurface.
Sloping blocks can be the cause of unique drainage problems. Water at the base of the block can be drained / soaked away using an agg pit, which is a deep pit filled with aggregate (like cost effective recycled aggregate Fig 1.) and covered with topsoil. If the problem is severe, a subsurface drain may be required. Soil that is naturally water retentive such as clay soil can result in a drainage problem. This may be combated by placing pits at the edges of the garden to drain off accumulated surface water. Human factors also affect the natural drainage of the garden. Driving vehicles over it, the building process, or even heavy foot traffic, can compact the soil and stop it from absorbing water like it should.
Be aware that drainage is essential if you have recently built or renovated your home and the natural contours of the soil have been altered. Legally, you may be responsible for any alteration or change to the natural flow of water that causes damage to your neighbour’s property.
To check how well your soil drains, dig a hole that is 30cm by 30cm by 60cm. Half fill it with water. If you have good drainage, the hole should be empty within a day. If you have a severe drainage problem, the hole will actually contain more water.


Forms of Drainage in the Garden

There are two forms of drainage for the garden – surface or subsurface.
Surface drainage, as the name implies, 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 usually 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.

Subsurface drainage systems are buried beneath the surface of the soil so ideally a landscaper would install this in the early stages of landscaping. However, they can be installed in existing gardens.
Subsurface drainage is commonly used in grassed areas and must used behind all retaining walls. Specially designed pipes, called Ag Flow Pipe, 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.

It is a good idea to have a professional such as a plumber or a landscaper (if you need a referral of a reputable plumber or landscaper, give us a call and we can recommend one to you, to install a drainage system to ensure that it is laid out correctly. If it is badly designed or installed, the system may block up, direct water to the wrong outlet system, or may be insufficient.


Choosing a Drainage System

The type of drainage system that you choose to use in your garden will depend on the extent of the problem. If it is just a mild problem, the problem may be solved by designing the garden so that the water flows into pits up to 1.2m deep.
An agg pit or drainage cell (product name VersiCell, Fig 3.) are other options. An agg pit is a large hole at the lowest part of the yard filled with aggregate, like blue metal or recycled aggregate, and then covered with 30cm of soil. The agg pit drainage may need to be as large as 2m by 2m to have the desired effect.
You could also choose a subsurface system that makes use of Agg pipe Fig 2. Made of crush resistant moulded plastic, they are covered with geo fabric to stop the pipes becoming clogged.
Always lay pipes above the water table otherwise they will just be draining groundwater, instead of water coming from the surface. The pipes should be slightly sloped to encourage drainage. All pipes should terminate in an, agg pit or drainage tank.
For help in regards to your drainage project, feel free to give us a call or drop into our Taren Point or Mascot branches.


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