Environmentally friendly options - reducing run-off

In North Shore City we enjoy a generous average rainfall of 1250mm per year. This means managing stormwater on public and private property is important. We are working to manage stormwater better and prevent flooding and pollution. You can help by landscaping your property with stormwater management in mind.

Gardens and porous paving allow water to be absorbed by the ground. This reduces flooding and filters out contaminants before stormwater flows on to our streams and beaches.

The runoff from hard surfaces is the predominant cause of harm to the environment in urban areas. For this reason awe encourage you to keep 50 per cent of your site as grass, gardens or permeable paving.

What is porous paving?

Porous paving allows water to be absorbed through it to the ground beneath. Permeable surfaces have three main components:

1. Surface materials
Allow some water penetration in or through the material to the ground below.

2. The underlying construction (sub-base layer)
A free draining sub base layer such as gravel or scoria must be used underneath the surface materials. You can check the suitability of your local subsoil with an engineer. Sand, crushed stone or recycled aggregate may need to be added and sub surface drains installed. A gradient or fall away from traffic areas may also be part of a successful drainage solution.

3. The supporting soil (sub-grade layer)
The permeability of this layer is critical to a successful permeable paving solution, especially on heavy clay soils. Heavy clay sub-grade layers act as a water barrier and can prevent the proper function of a permeable surface treatment.

Reduce run-off from your property

Different surfaces have different levels of permeability. Aim for a surface with low percentage runoff or high soakage.

(Source: NZ Building Code Clause E1 Surface Water, Table 2)

The less permeable a material, the faster surface water will run off. High run-off contributes significantly to downstream flooding and pollution. Impacts can be seen at local streams and beaches after storm events.

When bush covered areas are replaced by grass, the stormwater runoff is twenty times greater. When grass is replaced by urban surfaces, the stormwater runoff is approximately five times greater again. This cumulative effect has significant downstream impacts

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Types of permeable paving:

Porous/ semi- porous pavers -traditionally concrete and plastic based products in the shape of traditional pavers. Small particles allow more water penetration than normal concrete products. Suitable for all traffic situations

Concrete turf block - segmented concrete paving systems with large voids to enable grass to grow within the block. Used to allow higher trafficability of grassed areas e.g. carpark areas

Plastic grass paving - a plastic interlocking paving system with voids to allow grass growth/ gravel. Invisible once installed and covered with grass or gravel. Also used to allow higher trafficability of grassed/gravel areas e.g. carpark areas

Sand paving - short polyethylene fibres mixed with sand to create a heavy load bearing capacity and greater shock absorption. The depth of the sand varies depending on the load bearing capacity required. Grass surfacing provides a natural cover instead of hard surface to be used for access routes, car parking, racetracks, sportsfields, golf courses.

Other porous alternatives and design ideas:

Gravel chip - is completely permeable and can be used in some circumstances instead of hard surfacing. Suitable for driveways and parking areas.

Two formed concrete strips - can be used instead of a large concrete slab driveway, and allows for a greater degree of landscaped/lawn areas for extra permeability.

Atlantis system - a subsurface system combining plastic drainage cells and strip filter drains. Incorporating; retention tanks for use of stored water, detention tanks for slow release to the stormwater system (to prevent downstream flooding) and percolation tanks for recharging the groundwater.

French drains - gravel filled soakage pits used at the base of slopes and overflow areas. Allow a gradual discharge of water.

Drainage swales - landscaped swales or "ditches" act as small water detention areas, allowing gradual absorption of surface water.

Hard paving with permeable material - impermeable paving blocks with wide joints filled with a permeable material e.g. gravel.

Remember it also helps to:

• Use existing natural drainage patterns on your site wherever possible 
• Use timber decking instead of hard paving to allow water penetration 
• Reduce building setback to shorten driveway 
• Reduce driveway width 
• Sweep surfaces instead of hosing down 
• Wash cars on the lawn, not the road 
• Use rainwater tanks to collect rainwater for non-drinking use

All designs should incorporate good edge landscaping to soak up water. Ask your architect to consider stormwater friendly solutions.

Porous and semi-porous pavers are unsuitable in areas used by heavy vehicles, areas with gradients steeper than 1:8, and large, flat expanses with compacted clay sub-base.

For construction sites, vehicle access should be temporarily formed with base course. The pavers should only be laid after completion of construction activity to avoid damage by trucks and heavy vehicles.

What you can do

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