Beach erosion and flooding

Coastal erosion and flooding occurs in the Auckland region, and can cause significant problems if people build too close to the coast. Beaches move constantly, and naturally oscillate between periods of erosion and accretion (onshore collection of beach sediment). Sometimes these fluctuations reach extremes. Extreme fluctuations are only ever a problem when humans develop buildings or infrastructure too close to the coast. Coastal hazards occur when dynamic coastal movements interact with static human resources that we have placed along the coast. Two of the most common coastal hazards in the Auckland region are:

• coastal erosion
• coastal flooding.

Coastal Erosion

The natural coastline processes of erosion and accretion can also be altered by human actions or human modification of the coastal environment. The amount and movement of sediment along the beach can be modified when humans place structures, extract sediment or undertake other works. This has been known to inhibit coastal accretion and enhance coastal erosion.

Coastal flooding

Inundation (coastal flooding) of buildings, roads and properties is exacerbated when the beach is eroded as there is less protection offered by dunes and accreted beach sediment.

Coastal flooding is commonly associated with severe storm events, and the occurrence of storm surge. Storm surges are produced by the combination of a change in sea level caused by low barometric pressure (storm conditions), and wind action. Storm surges are most dangerous when they combine with high tides. The Firth of Thames coastline is particularly prone to storm surges. Storm surges also:

• allow increased penetration of storm waves
• cause increased shoreline erosion.

Storm surges have been known to:

• flood homes and businesses
• force people to evacuate their homes
• impact land transport
• cause significant economic loss.

For example, a storm surge reaching only 2.5 m above mean sea level along the east coast of the Firth of Thames in July 1995 inundated residential homes, damaged roads and caused an estimated $3-4 million worth of damage.

Historical Events

1978 Cyclone

A large cyclone in 1978 caused significant storm surge, and consequence coastal flooding and widespread erosion at Omaha, East Coast of the Hauraki Gulf. Prior to this storm, significant development occurred at the northern end of Omaha beach. As part of this development, much of the dune closest to the sea was flattened and lowered. These human modifications enhanced the amount of damage caused by this storm. Some of the observed effects include the destruction of a wooden sea wall and wide spread erosion.

 

 

1996 Cyclone

1996 Cyclone

Another large cyclone in 1996 caused flooding in the Upper Waitemata Harbour.