Drainage

Drainage can be tested easily by observing the way in which water moves through soil. Simply observing soil conditions where you wish to develop a garden, particularly after several days of wet weather, will give you a good idea of any potential drainage problems, such as boggy areas or high levels of surface runoff.

Another simple method is to simply place a sample of the soil in a pot and water. Keep in mind that the results of the top soil will be easily observed, but when soil is disturbed by digging, its characteristics may change. Another more reliable way to test the soil drainage is to use an empty tin can. With both the top and bottom removed it forms a parallel sided tube which can be pushed into the soil to remove a relatively undisturbed sample. Leave a little room at the top to hold water. Add some water to see how it drains and then saturate the soil and add some more water to the top. You will often note slower drainage on saturated soil. If the water passes through quickly, it is a fast draining soil. If it moves slowly, the soil may have a tendency to bog and become too wet. Either way, this test will help to indicate if the soil needs to be improved for water retention or if additional drainage is needed.

 

If mending the soil or drainage is too costly, then be very careful in plant selection and limit your choices to those plants that thrive in the conditions that naturally occur. This sounds like an easy alternative, but can prove to be very limiting in what you can do with your landscape.

 

SOLVING DRAINAGE PROBLEMS

There are several ways to solve drainage problems, including:

 

1. Reshape the surface of the land so water flows somewhere else where it won't be a problem, such as a creek or stormwater drain. It is very important that you don't divert water onto your neighbour's property, unless they are happy for you to do so. You are legally responsible for problems you cause on your neighbours' property.

 

2. Improve the soil structure so it will drain more freely.

This can done by adding organic matter to most soils, and by cultivation to break up crusted surfaces and hardpan layers.

 

3. Add Soil Ameliorants (eg. Gypsum and Lime).

Soil ameliorants are chemicals which cause clay particles to aggregate together into small crumbs, thus opening up the soil and letting water in. They normally take months to show any effect, and they only work if the soil is kept moist.  Lime or gypsum are other alternatives, but these can have side effects (both increase calcium levels and lime changes soil pH). For water repelling sands, where the water just sits on the surface, a wetting agent can be added to the soil.

 

4. Install drainage pits in low areas

A drainage pit is a large hole filled with sand or rubble. Water collects in the pit and gradually seeps away into the lower layers of the soil. The hole is best to be long and deep, not square or circular if possible, and at least 1‑2 cu. metres in volume.

 

5. Install surface drains

These can be spoon drains, which are normally half pipes (or concreted depressions set along at the bottom of a slope, or at the edge of a paved area), or square or rectangular plastic channel with a surface grate . Water runs into the drain and is carried to a disposal point, such as an underground pipe in the stormwater system or a drainage pit.

 

6. Install sub-surface drains

A sub-surface drain is a pipe buried below the ground surface which collects water and  moves it away from where it is not wanted. Sub-surface drains are rarely needed in  sandy soils, but may be very important in heavy clay soils.

 

Layout Of Drains

a/ Herringbone ‑ lateral drains feed from 2 sides (generally at an angle of around 45 degrees) into a central main drain. This system has the advantage of minimising the depth that some of the drains need to be placed. If there is likely to be any subsidence after construction, laterals should be kept at less than 30 m in length....on undisturbed ground, up to 90 m lengths can be acceptable.

b/ Grid ‑ has laterals all running into one main drain from one side. This is less complicated, has less junctions and the main drain can run along the boundary (minimising interference with the main parts of the landscape).

 

Outlet

Any drain must have a low point which the water can be drained to....from which it can be disposed of. If it is impossible to find such a low point (ie. outfall), there are only two possibilities...

a/ Pump the water from the outlet to a suitable disposal point.

b/ Create a soakaway pit ‑ (see item 4 above).

 

Gradients

A drain should never have a slope less than 1:200....it is advisable it should never be less than 1:100 if possible. The slope of a drain should be uniform. Sharp bends should be avoided.


Distance Between Drainage Pipes

In heavy clay, 3 to 5 metres might be necessary.

In average soils, 6‑9 metre spacings are adequate.

In wet spots, it may be necessary to add an additional line and decrease the spacing.

 

Depth Of Drains

There is no firm rule, though 600 mm is usually considered reasonable. If sub-soil cultivation or sand slitting might be carried out at a future date, it is advisable to place drains deeper (ie. 750 mm).

 


Types Of Drains

a/ Clay Pipes ‑ Rigid; should be covered with stones; are generally long term; water soaks through the porous pipe and between joins; joins can sometimes allow soil in, causing blockages.

b/ Corrugated Plastic ‑ Flexible; should be covered with stones; water soaks through holes in the pipe; the plastic can break down after a number of years.

c/ Fibreglass Strip Drain ‑ Flexible; stones are optional; water permeates through outer case; relatively new so long term effects are unknown.

d/ PVC Pipe ‑ Rigid; cover with stones; water enters through slits; slippery surface tends to reduce blocking; generally long term.

 

Laying The Drain

A drain can be laid as follows....

1. Dig the trench (you must achieve an appropriate even slope on the base of the trench).

2. Lay a thin layer of screenings or scoria at the bottom of the trench on which to lay your pipes. This help prevent soil from the bottom of the trench blocking up slots and  joins.

3. Lay the drainage pipes in the bottom of the trench.

4. Cover the pipes with a coarse material (e.g. screenings, three quarter inch scoria).

Be sure that there is no fine material as this may pack down greatly reducing drainage of water to the pipe. Particle size should ideally be between 10 and 25 mm). This material should fill 30‑50% of the trench.

5. A layer of newspaper, weed mat, old felt underlay (or similar material) can be laid on top of the coarse material to help prevent finer material working downwards and reducing drainage.

6. Place a layer of sand over the coarse material, to fill within 15-20 centimetres of the top of the trench.

7. Cover the sand with a well draining topsoil.

 

 

IMPROVING SUB-SURFACE DRAINAGE AFTER CONSTRUCTION

 

a/ Sand Slitting ‑ Provision of vertical channels through which surface water can flow quickly. This normally involves cutting 1‑4 cm wide slits (filled with sand) across the slope at an angle to sub‑surface drains, allowing water to move quickly into sub‑surface drains. Special machinery such as a ditch witch is used. Sand is fed into the slits through a hopper.

 

b/ Aerating ‑ A long term process of spiking and sanding (repeated applications lead to an increase in sand % in soil).

 

c/ Sub Soiling ‑ Using single deep tine cultivators to rip the sub soil.

This can cause damage to the surface of the ground...repair work becomes necessary.

 

 


SOIL STRUCTURE

 

Soil structure is the aggregation of individual soil particles into crumbs (or peds), and the arrangement of these crumbs in the soil. A well structured soil will have a good mix of small and large crumbs from about 0.2 to 3mm across, that leaves some spaces (known as pore spaces) between them. In undisturbed soils many of the crumbs fit loosely together to form larger ones that can easily be broken apart using your fingers or when cultivating the soil. A poorly structured soil will be more difficult to cultivate and the soil may be powdery when cultivated, or break up into large solid clods.

 

Soil structure usually changes from the surface of the soil as you move deeper down into the earth. One reason for this is that surface soil usually contains more organic matter than deeper soil. Surface layers frequently drain better....drainage rate decreases as you get deeper. This natural change means that water moves quickly away from the surface of soil but slows down it's rate of flow as you get deeper. Poor cultivation procedures in soil can damage this characteristic of a gradation in soil structure through the soil profile by destroying the structure at the surface.

 

Soil structure is commonly improved in the home garden by adding plenty of well rotted organic matter to the soil, by minimising cultivation, avoiding cultivating when the soil is wet (should be no moister than about the level of a squeezed sponge), and adding soil ameliorants such as gypsum and lime (if the soil is acid).

 

 

The improvement of soil structure may use two approaches. First, where the soil has not been badly leached (still retaining good nutrition), the addition of organic material, use of a crop cover prior to final planting, to add nitrogen and proper (not excessive) cultivation, will normally give the best long term results. However, where soils have been leached and have become very acid, or very alkaline, the use of soil ameliorants such as Lime and Gypsum may be required. These cause a temporary change in soil chemistry to make the structure looser or easier to break up. As such, earthworms can move and mix organic matter easier and deeper, causing a more permanent improvement.

 

SOILS CAN USUALLY BE IMPROVED BY:

* Adding sand to clay soils can improve drainage. This is only practical on a small scale as a lot of sand is generally needed to be effective. The sand must also be thoroughly mixed.

* Adding clay or organic material to sandy soil to improve its ability to hold water. Adding organic matter, while improving water holding capacity, will not adversely affect drainage to the same degree as the addition of clay will.

* Adding sand or organic matter will help break up a clay soil, making cultivation easier. Although the two will act in different ways.

* Adding organic matter will improve the nutritional status of the soil.

 

 

Want to Learn More?

 

Check out horticultural and landscaping publications in the ACS Bookshop: http://www.acsbookshop.com/

 

For more information on Courses on Landscaping, Horticulture and Plants:

 

In Australia: http://www.acs.edu.au/Courses/horti.aspx

 

In the UK: http://www.acsedu.co.uk/Courses/horti.aspx

 

 

 


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