Use Water Runoff to Your Advantage With Slopes and Drainage

No designer wants to see their perfect plot washed out by untamed water flow, and no resident wants their home ruined by an seeping pool. Good drainage is a must for any garden. Here, we introduce you to its fundamentals.

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Predicting how water moves around, and how it can be directed out of harm's way, is the basis of drainage design. As a general rule, all man-made surfaces should be on an incline and water must flow away from buildings. In most cases, the water runs off hard surfaces, such as terraces or steps, into the soil where it is absorbed. However, sites on hills or with heavy, compacted soil can present drainage problems, and you may need to seek specialists to help you avoid waterlogging or flooding your property.

Drainage Issues

All waterproof surfaces (roofs and paved areas) prevent water from draining naturally. They should be designed so that water flows to public drains, rainwater harvesting barrels or, in small quantities, directly on to planting beds. The type of soil in a garden will affect drainage: heavy soils (clays and silts) tend to cause more problems than free-draining types (sands, gravels and sandy loams).

Water flows quickly down steeep sites, seeking a low point and, eventually, an underground pipe, open ditch or stream. Pay close attention to water moving over bare soil or sparsely vegetated surfaces where it will can erode land and form gullies. Contained landscapes need an overflow to prevent water from gather in the lowest points and forming an unintentional bog.

Consider hiring a contractor to help you position your drains for maximum advantage, or for doing the job in its entirety. Any time you dig into the ground to create ponds, alter slopes or install drains you risk hitting underground services, such as water and gas pipes, electrical cables or existing drains and sewers. Contractors are skilled at excavating sites and addressing the differences between what plans show should be down there, and what really is. If you decide to install the drainage yourself, take your time to identify problems and consider hiring a specialist surveyor just to help you excavate.

Where waterlogging is not severe, excess surface water can be directed into a drainage ditch or pond. If your water table - that is, the point where your underground water pressure is kept in check by gravity - is high, you will need to install an underground drainage system. This project is not for the average DIY-er; it really will benefit from being performed by a specialist contractor (image 1).

Groundwater may be a problem, but it is also an opportunity. A naturally high water table or butyl-lined bog garden can make an ideal place for growing a range of beautiful moisture-loving plants (image 2).

All rainwater falling on this garden will eventually find its way into the ground or to the pond, which is located at the lowest point. An overflow may be needed to channel any excess water into an underground drain or gutter (image 3).

Recycling Water

Water is increasingly expensive and natural rainfall is scarce in some areas. Careful planning allows many gardens to be self-sufficient in water, as rain can be collected in barrels and storage tanks. This reduces the use of local water and your bills, ensures that the water is free from added chemicals and reduces pressure on over-burdened public drainage systems. Embrace your environmentalism, but take a word of caution: Common household wastewater can be used on trees, shrubs, plants and lawns if properly treated, but is not suitable - and can be, in fact, dangerous - for edible crops.

This recycled barrel holds enough rainwater to cover a short period of dry weather, and makes an attractive addition to the overall appearance of the garden (image 1).

An informal pool can be used to capture excess water and will serve as a perfect habitat for wetland and aquatic plants and animals (image 2).

Design Considerations

If your garden is on a slope, you'll need to create flat, usable surfaces. Often this requires construction work so, when drawing up plans, consider budget and time constraints, the overall size and shape of the proposed spaces and possible access for earth-moving machines. More complex solutions may be required for steeper sites and slopes that are less stable, or where especially large level areas are required.

Wood is the best material for making flat platforms or walkways, with the least disturbance to the slope. Decking is especially useful for areas inaccessible by heavy equipment, when slopes are too steep to change and on softer, rolling surfaces around wetlands. However, wood is short-lived compared to other hard material solutions, and will need to be replaced sooner (image 1).

Use small-scale terracing to make horizontal planting beds on a slope. A series of retaining walls, set one above the other, provide structure, then soil is cut away from the slope for backfilling. Work can be done by hand or with a mechanical digger. Consult professional designers and engineers to terrace larger areas, where the implications of sound structure are much greater (image 2).

Scape meandering land into gentle slopes or flatter areas. While work of this kind has less structural concerns than decking and terracing, creating gentle slopes does raise a few concerns. Your cost can go up if your design generates excess soil or base material that has to be hauled away, or requires that you bring in more soil or base material. Also, any changes you make will destroy existing vegetation and cannot be carried out beneath the canopies of trees that you want to retain (image 3).

Excerpted from Garden Design

©Dorling Kindersley Limited 2009

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