Stormwater

Acidic and alkaline

pH describes on a scale of 0-14 the concentration of hydrogen ions in a solution. A solution with a pH of 1 has 10 times more than one with a pH of 2. The more hydrogen ions, the lower the pH. When the pH is below 7, the solution is acidic, and when the solution is above 7 it is basic (alkaline). Lemon juice has a pH of about 4 - it is very acidic. Rain water is slightly acidic at about 6; pure water is neutral at 7 and sea water is slightly alkaline at 8. Concrete has a pH of over 12.5 - it is very alkaline.

Aquifer

A layer of permeable rock that can both hold water and let water move through it. Auckland's basalt rocks are wonderful aquifers, holding plenty of water (see groundwater).

Catchment

Area contributing flow to a point on a drainage system.

Catchment management plan

Plan for dealing with the runoff generated in a catchment (normally to meet specified water quantity and quality objectives).

Catchpit

Small chamber incorporating a sediment trap that runoff flows through before entering a reticulated stormwater system (also termed cesspit).

Combined sewer

Across most of Auckland the stormwater network is separate from the wastewater network, but in some older parts of the city, stormwater and wastewater go into the same system, called the combined system. In dry weather its contents go to the Mangere wastewater treatment plant before being discharged into the harbour, but in very wet weather, the high volumes of stormwater can cause the combined system to overflow into streams and harbours.

Detention of stormwater

Temporarily detaining runoff on a site before discharging it to the reticulated or natural system (also see retention).

Ecology

The science of the mutual relationships between living things and their environment.

Estuary

Tidal mouth of a river or inner area of a sheltered harbour where fresh and sea waters mix.

Flood plain

The area that a streams covers when it floods - as stream naturally do.

Ground water

Water in the ground that has percolated down from the surface to form an underground water body that is contained in the pores in soil, subsoil and rocks (see aquifer).

Impermeable / impervious surfaces

Surfaces like roofs, roads, paths, car parks and paved yards are impermeable to water - it can't get through them and must run off. The more impermeable area on a piece of land, the more stormwater will run off it instead of soaking into the ground. This is why cities have stormwater pipes to carry the water away to prevent surface flooding.

Infill and intensification

Auckland City can't grow out - so most growth will be in existing built-up areas, with infill development (additional houses built on the same piece of land) and more intensive development, with medium or high-rise units. This means increased density in some parts of the city - that is more people living and working closer together.

Infiltration

The passage of water through soil to reach groundwater.

Low impact design

Design approach for site development that protects and incorporates natural site features into erosion and sediment control and stormwater management plans.

On-site stormwater device or management measure

Built or natural ways to use, detain and/or retain runoff on a site. These ways can include raintanks for water use; natural depressions in the ground or stormwater planters to detain water (delay its entry into the natural or reticulated stormwater system); or rain gardens or soakpits to keep (retain) stormwater by infiltrating it into the soil on your property (also see detention and retention).

Outfall

The end of a stormwater pipe or network where water leaves the artificial stormwater system and enters the natural environment, at a stream, lake or beach, or at a treatment device like a wetland or pond. It is important to protect streams and beaches from erosion by stormwater at outfalls.

Overland flow path

A route taken by flood runoff not able to be contained in the reticulated or natural stormwater conveyance system. Before runoff enters a stream or the stormwater system or if there is too much rainfall for the stormwater system to carry, the water will begin to flow across the ground in what is known as an overland flow path. We all have a role to play in keeping these natural overland flow paths free of obstruction, in order to prevent flooding of buildings and other downstream property.

Permeable / pervious

Natural ground covers like trees, shrubs, grass and soil are permeable to water - it can get through them and soak away into the ground. This helps to reduce flooding.

Porous

Full of small pores - tiny holes - that water can move through.

Retention

Temporarily retaining runoff for disposal on site by infiltration.

Reticulation

Network - for example of water supply, stormwater or wastewater pipes. Streams also form natural networks.

Runoff

The flow of rainwater across the ground or an artificial surface generated by rain falling on it. See stormwater.

Sewer

Pipe for carrying away wastewater (water from sinks, toilets, showers, baths and basins inside buildings). Click here to find out more about Auckland's wastewater treatment plant at Mangere.

Soakage pit

Specially built pit in the ground to collect stormwater from a site and allow it to soak away naturally into the soil. Runoff from impervious surfaces is directed into the pit, which contains stone materials of varying sizes that trap silt and contaminants in order to protect water quality in the aquifers.

Stormwater

Rain that has fallen and has begun to flow across the surface as stormwater runoff.

Wastewater

Used water from sinks, toilets, showers, baths and basins inside buildings.

Water sensitive urban design

Low impact development (as defined above), with an added emphasis on sustainable vegetation practices and low-level of water usage.