Flooding in the Wellington region

	Flooding in Riverside Drive Feb 2004

Floods can occur along rivers, coasts or near stormwater networks in urban areas.  River flooding generally occurs when high rainfall causes a river channel to become completely full and overflow onto nearby land.

Rivers have natural floodplains to accommodate flooding. However, people have modified and developed these flat, fertile areas by clearing forests and building and living there. As a result, people and property may be at risk when a nearby river floods.

When a heavy rain event occurs, rivers can burst their banks and water can enter homes and businesses. Widespread damage can happen to buildings, infrastructure and networks and lives can be lost. There can also be major damage to the river environment.

Runoff from roads and buildings and sewage overflow into stormwater systems can pollute streams, rivers and marine environments. The effects of flooding can be made worse by activities such as draining wetlands, river straightening, and deforestation.

Causes of flooding

Floods occur when torrential rain falls in a rivers catchment, or the area of land which drains into a river system. A storm or heavy rainfall event in almost any part of a catchment could result in a large volume of water flowing down through streams and rivers. This accumulation of all of this water could result in a large flood.

Extreme high tides, storm surge or tsunami can also cause flooding of low-lying coastal areas. Torrential rain in urban areas can sometimes overwhelm stormwater systems, causing surface flooding.

What’s at risk in a flood?

	Bank erosion by house

All floods can be dangerous to people and property, from the stream that runs past your house, to a big river that has many homes and businesses built alongside it.

Floodwaters are highly dangerous. Fast flows can sweep people off their feet, move vehicles and damage homes and other buildings. Deeper, slow-moving waters or ponding can be equally as dangerous.

A large flood could break through or overtop stopbanks on any of our major floodplains and cover extensive areas in water.

A flood may last a few hours or many days. Floodwaters can cause physical damage and disruption to property and services. Electricity, water supply, sewerage and telecommunications could be put out of action for days.