Minimising stormwater runoff

Development usually increases stormwater runoff because of an increase in impermeable surfaces (eg roofs, roads, car parks, or compacted soil). If existing natural or artificial drainage is overloaded, stormwater flows overland.

Flooding can occur when:

• Stormwater volumes exceed that which the system was designed for
• Drains, grates or other structures in the flow path (drainage or waterway) become blocked
• Overland flow paths are blocked by solid fences, sheds, unsuitable planting or even permanent buildings

Your responsibilities

As landowner, it is your responsibility to manage stormwater that falls naturally onto your property, and to maintain the private drainage and watercourses on your land.  How you manage stormwater on your property can affect your neighbours and others downstream.

Landowners with buildings on higher land are obliged to construct a drainage system that will dispose of stormwater to an appropriate outfall. The Building Code requires all buildings and site work to be constructed in a way that protects people and other property from the adverse effects of stormwater.

Your stormwater may soak into the ground, flow across the ground downhill, or discharge to the public stormwater system or into a watercourse.

Lower land must receive stormwater which falls naturally from higher land.  However, this does not permit the owners of the higher land to cause damage by land use, work or structures that alter how stormwater would naturally have flowed.

If a problem on neighbouring land is causing flooding or other problems on your property, talk to your neighbour about it.

Our responsibilities

If the drain is part of our stormwater network (see our water services map) and there are issues with it, please let us know as soon as you can (477 4000).

To reduce stormwater runoff

Use permeable paving on your property

Stormwater runoff can be reduced by using permeable paving for driveways, footpaths and parking areas instead of hard, impervious paving such as asphalt or concrete. Permeable or porous surfaces include:

• concrete-grass paving (the runoff is slowed by open paving blocks that allow water to pass through gaps, where grass is usually planted)
• porous concrete/asphalt
• open-jointed paving over gravel (paving blocks on a sand base with open joints can slow runoff but they are not as permeable concrete/grass paving)

Permeable surfaces are best suited to flatter areas (with a slope of 1:10 (5°) or less), with low traffic volumes and low speed traffic.

Use the water that drains off your roof

Collect and store rainwater for gardening, toilet flushing or other uses. This also reduces reliance on the town water supply.  You may need a building consent or a resource consent to install a rainwater tank.

Introduce native plants to your garden

Lawns aren't always effective at absorbing and retaining water, especially during heavy rains. Native plants tend to develop more extensive root systems that take in and hold water better than a lawn.

Create a rain garden

A rain garden is planted in a slight depression in the ground that collects water and allows it to permeate the soil gradually. Rain gardens are usually planted at the base of a slope or anywhere where water flows naturally or can be directed. Water-loving plants and a base of permeable soil enhanced with fertile loam and a topcoat of mulch allow the rain garden to absorb even large amounts of water quickly.

Install berms and vegetated swales

A berm is a slightly raised area, while a swale is a wide, shallow drainage channel. They are typically planted with grass or other plants can and are used to slow and direct runoff.  Swales also provide a means of infiltration into the subsoil.

Enviroschools

The Enviroschools Programme in Dunedin information in the related information section can support schools in implementing good practice for stormwater management in the school community.

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