Too much water lying on the garden or on the lawn can create all sorts of problems. Aside from making it unpleasant to walk on the lawn, weeds and diseases can flourish and plants will find it hard to get the oxygen they need.
If you have a waterlogging problem in your garden during winter, then you will need to take steps to take the water away from the site.
WHAT TO DO?
The first thing is to observe how and where the water collects in your garden.
- Does the water flow from higher parts of the garden and collect in low spots?
- Does rainwater percolate into the soil or does it just lay on top?
- Are some parts of the garden more prone to waterlogging than others?
Once you have done this, it is a good idea to dig a hole in the problem areas to find out about the properties of the soil. In some cases the reason the water can not drain away is because the topsoil is a thick clay that can only absorb small amounts of water at a time. In other places the subsoil might not be able to absorb water as quickly as the topsoil.
When you have identified where and why the problems are occurring, you can start remedying the situation.
There are number of ways to improve soil drainage:
- By cultivating the soil to break up clay pans. This method has some short term benefits, but it can sometimes make matters worse and will not solve the problem of water ponding in low lying spots.
- By adding sand, organic matter or gypsum to the soil. These solutions are really only practical when you are starting a new garden.
- Making grooves or holes in the soil and filling the gaps with sand. This can improve garden soil over the long term, but it is time consuming and may damage plant roots.
- The best solution to most garden drainage problems is to install a drainage system.
FIRST DECISION: SUB SURFACE OR SURFACE DRAINAGE
There are two basic types of garden drains – those that move water along the soil surface and those that move it underground. Deciding which one to use will depend upon the waterlogging problem in your garden.
If you have a problem with water moving from higher parts of the garden into lower areas, then the answer could be a spoon drain that redirects the water away from the lower part of the garden. If you have a lawn that is slippery or soggy during winter because of a heavy subsoil, then the answer might be a sub-surface agricultural drain.
WHAT IS A SPOON DRAIN?
A spoon drain is a channel cut into the soil surface. Excess water runs along the soil surface to an outlet point. One advantage of a spoon drain is that it can be made to look like a natural part of the landscape. This can be done by growing a lawn or by filling the drain with a bed of pebbles or stones.
WHAT IS AN ‘AGI’ DRAIN?
Sub-surface drains can be made from clay pipes or even fibreglass. However, for most domestic situations, the best method is to install either flexible corrugated plastic or rigid PVC agricultural (agi) pipes. Set in a bed of gravel, the water enters these pipes through slits and then flows down the slope to an outlet point.
TYPES OF SUBSURFACE DRAINS
- Clay Pipes rigid, covered with stones, long lasting, water soaks through porous pipe and between joins, joins can sometimes become blocked with soil
- Fibreglass Strip Drain flexible, stones are optional, water permeates through outer case, relatively new product, so durability is unknown
- Corrugated Plastic flexible, covered with stones, water comes through slits in the pipe, slits can become blocked with soil, plastic can break down after a number of years
- PVC – rigid, covered with stones, water enters through holes in the plastic, long lasting
THE WATER OUTLET
Any drain must go to a dispersal point. To be effective, this has to be at the lowest part of the drainage system. Before entering a stormwater system, drainage water should pass through a holding pit. In some cases, you will need a plumber to connect your drain to the stormwater system.
If it is impossible to find an outlet for the drainage water, there are still things you can do:
Turn the problem to your advantage by installing a pond or other water feature to collect the water.
Pump the water from the outlet to a suitable disposal point.
Create a soakaway pit a large deep hole filled with porous material such as sand or stones that collects the water and then slowly disperses it into the surrounding soil.
Whether above or below the ground, a drain should never have a slope of less than 1:200. For example, if the drain is twenty metres long, the oulet point should be at least 10cm lower than the top of the drain.
The slope of a drain should be uniform. Sharp bends should be avoided. On relatively level sites it will be difficult to create a suitable slope with a spoon drain and a sub-surface drain will have to be installed.
LAYOUT OF DRAINS
- Herringbone lateral drains feed from 2 sides into a central main drain. This system has the advantage of minimizing the depth of some of the drains. If there is likely to be any subsidence after construction, laterals should be kept at less than 30m in length. On undisturbed ground, lengths up to 90m are acceptable.
- Grid has laterals all running into one main drain from one side. This is less complicated, has less joins, and the main drain can run along the boundary (minimizing interference with the main parts of the garden).
LAYING AN AGI DRAIN
The drain is laid as follows....
- Dig the trench (with an even slope on the base)
- Lay a base of gravel or crushed rock or scoria (particle size should be between 10 and 25mm)
- Lay drainage pipes in the trench
- Cover pipes with the crushed rock
- Place a layer of sand over the coarse material to fill within a few centimetres of the top of the trench
- Cover the sand with a well draining topsoil
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 additional pipes
DEPTH OF DRAINS
There is no firm rule, although 600mm is usually considered reasonable. If cultivation or other types of digging might be carried out in the future, it is advisable to place drains deeper (ie: 750mm)