Flooding

Flooding is the submergence of water over normally dry land and is New Zealand’s most commonly occurring natural hazard. Every year floods cause significant damage to property and threaten people’s lives.  Moving water can be very destructive as it has power to pick up trees, cars or built structures and carry them in its path. Flash floods pose significant dangers as they occur with little or no warning. A flash flood recorded in 1992 near Roxburgh in Central Otago was caused 80 mm of rain that fell in the space of 45 minutes. In 2017, 40 mm of rain fell in 3 hours. 

Flooding is common in areas such as river floodplains and low lying areas near the coast. Floods in river floodplains are usually caused by excess rain, causing rivers and streams to overflow their banks, but also may occur from landslides or dam failures. ⁴⁷¹⁸ Excessive rain may cause lakes to overflow. When flood water recedes the aftermath is often a blanket of mud and debris left in its wake, and clean up costs can be substantial.

 Flooding in coastal areas is usually associated with coastal storm tide inundation. ⁴⁷¹⁹ This occurs during storms where wind and atmospheric pressure changes cause high winds and temporary sea level rises to force breaking waves across adjacent land and waterways. High tides can magnify these effects.

Coastal wetlands can assist in providing protection to coastal communities from storm surges. ⁴⁷²⁰ Depending on the type of storm and typography, sufficiently dense and wide vegetation such as mangroves may reduce storm surge water levels that reach land by slowing the flow of water and reducing surface waves.

An effect of climate change is expected to be increases in heavy rainfall resulting in more flooding in prone areas. Sea level rise will increase coastal erosion and inundation around  many parts of New Zealand’s coast by increasing the severity of coastal floods and storm surge impacts. ⁴⁷²¹

Last updated at 11:01AM on February 8, 2018