A garden should have good drainage as an overly wet landscape can create problems as its unhealthy for plant life and for humans as well as creating potential safety hazards.
Excess water can be prevented by ensuring that rainwater moves through the topsoil quickly. Once through, it will need to be removed from the subsoil either by passing to even deeper layers or by collecting it in drains that flow away from the site.
There are several ways to test a soils drainage abilities; the most affective way is to dig a hole about 1 foot deep in the desired test location. Then fill with water and allow it to drain completely. Immediately after, refill the pit and measure the depth of the water with a ruler. 15 minutes later, measure the drop in water in inches, and multiply by 4 to calculate how much water drains in an hour.
Less than 1 inch per hour is poor drainage, indicating the site may stay wet for periods during the year. Plants that don’t tolerate poor drainage will suffer. 1 to 6 inches of drainage per hour is desirable. Soils that drain faster than 6 inches per hour have excessive drainage, and you should consider choosing plants that tolerate dry conditions and “droughty” soils.
Drainage can be improved in a number of ways. These include:
• Increasing the infiltration rate of water into the soil by improving the permeability of the soil. There are several ways in which this can be achieved; cultivation, mixing in soil additives (e.g. sand) and chemical treatments (e.g. lime)
• The provision of sub-surface drainage to facilitate removal of water from the soil, e.g. drainage pipes. There are many types of pipe to consider including those constructed from materials such as clay, corrugated plastic, fibreglass and PVC. Pipes are usually placed at a depth of around 600mm and the distance between them can be adjusted according to the type of soil.
• Managing surface water runoff effectively and efficiently. This can be simply achieved by sloping surfaces within the landscape so that the surface water is directed to an area where it can be efficiently disposed of (e.g. a surface drain)
When creating a drainage system, its important to check any council regulations that may be in place as well as considering how it will affect other drainage patterns already in place. The drainage system should also be designed to withstand the amount of water that may result from heavy rainfall in the local area.
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