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Surface water management

Rain gardens and rainwater tanks are two ways landscapers can design environments that cope well with heavy rainfall. Read on to find out how BRANZ recommends professionals install both features

Rain gardens (and drainage swales) are designed to collect and filter surface water from adjoining hard surfaces such as roofs, roads, driveways and parking areas so as to minimise negative impacts of excessive run-off from these surfaces.

Other terms to describe such areas include mini-wetland, stormwater garden, water quality garden, stormwater marsh, backyard wetland, low swale, wetland biofilter or bioretention pond.

These all describe ways of dealing with rainfall in a sustainable and economical way. In the process of intercepting run-off, nutrients are used by plants in the rain garden that otherwise would be collected into streams or other water bodies.

Rain gardens are constructed as strategically located shallow depressions (rather like a dry pond or stream), lined with stones or gravels placed over a geotextile membrane and then planted usually with native plants that might be found in local stream beds. The object is to collect and filter stormwater run-off for slow release into the ground immediately beneath. An overflow system must be provided to direct larger-than-normal volumes of water via the normal flow path to existing stormwater drainage.

Gravitational pull

The site for a rain garden must be selected to enable gravity to provide both inflow and outflow of run-off. The inflow could be water diverted from a roof via a downpipe, for example, with excess water during heavy rain able to overflow to a natural watercourse or to a stormwater drain collection point.

Free-draining soils (comprising 50-90% sand) allow the water collected in the rain garden to permeate directly to the ground beneath. Less permeable existing soils may have to be dug out and replaced. Typically, rain gardens are up to 900mm deep, with a ponding depth of 100-200mm. The area of a rain garden should equate to roughly 4-8% of the source catchment area. A well-designed rain garden should completely drain in a 24-hour period following rainfall.Plant selections should tolerate both short periods of ponding and dry periods and groundcover plants should be placed at a density of about 4-10 plants per m2.

Maintenance will include watering during establishment and periodic weeding. The inflows and outflows should also be checked to ensure proper functioning of the system.

Rainwater tanks

Tanks are available in many shapes and sizes. Some are designed to be installed underground, others above ground. There are rigid, semi-rigid and even flexible bladder tanks. Some low, flat tanks are designed to fit under floors and other upright narrow tanks are specifically designed to fit in the narrow side yard under the eaves. A local plumber's merchant can assist with selection of the tank for a particular situation.

Full water tanks are extremely heavy (1m3 of water weighs a tonne) and so structural supports and restraint under earthquake loading must be carefully considered.

Debris should be deflected before it enters the tank. Contaminants and bird droppings that collect on a roof surface will be flushed by rainfall into a collection tank unless a first-flush diverter is fitted. These devices ensure that the first amount of water off a roof will not go into the tank. Some territorial authorities require the use of these devices.

Where water is collected for household potable use, tanks may be 25,000 litres or even larger. The Ministry of Health's Drinking-Water Standard for New Zealand 2005 (updated 2018) requires drinking water to be filtered to remove particulates and treated to remove contaminants. This can be achieved by passing water through two screening filters (one of 20 microns and another of one micron) and then passing the water through a UV light process filter. The new regulator Taumata Arowai is developing new drinking water standards which come into effect on 14 November 2022. A building consent may be required for installation if the tank is larger than 20,000 litres (on a rural property or larger than 500 litres on a platform) .*

A pump may be needed to provide adequate pressure, and pump location, power supply and controllers must be considered. The tank will need to be drained out and cleaned every few years, or the water can be chemically treated periodically to remove organic build-up.

*Consult your local T.A. to be sure of exemptions that may apply.

The information in this article was first published in BRANZ’s Landscape Construction book, Second Edition and is reproduced with permission.

The book covers the design, building and planning rules as they apply to landscape construction and provides guidance on design and construction to ensure good practices are followed. It can be purchased at


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