FAQs

What are tides?

Tides are the periodic variations in the surface water level of the oceans, harbours and inlets. Tides are the result of the gravitational attraction of the sun and the moon on the earth. Because of the physical processes which occur to produce the tidal system, there are two high tides and two low tides each day.

What are King Tides?

The term “King Tide” is a common term for the highest tides that occur over the course of the year. These especially high tides eventuate when a new or full moon occurs at the same time as the moon is at its closest to the earth (in its perigee). Learn more about King Tides here.

What are some other measures and terms relating to high tide events?

There are a variety of different measures and heights used to describe the range of tides that occur over the tidal cycle. Some of the more common measures are provided below.

• Spring Tides are exceptionally high tides that occur during or just after a full or new moon. A common measure associated with spring tides is Mean High Water Springs (MHWS), which represents the highest level that spring tides reach on the average over a period of time. Auckland Council used a MHWS-10, the level equaled or exceeded by the highest 10% of all tides, for development of the new indicative coastline for the region as shown in the proposed Auckland Unitary Plan.
• Mean High Water Perigean Springs (MHWPS) represents approximately to highest 2% of all tides in the Auckland region and is the measure we are using to indicate the occurrence of a King Tide event.
• Neap Tides are tides where the difference between high and low tide is the least – they have the smallest tidal range. Like spring tides, a common measure of neap tides is Mean High Water Neaps (MHWN) – which represents the average level that neap tides reach over a period of time.
• Highest Astronomical Tide (HAT) is a measure of the maximum high tide in a 100 year prediction excluding any change in mean sea level over that time, including sea level rise.

Are there tidal differences between our East and West Coasts?

Yes, the tides vary considerably from place to place around NZ. The tidal range (difference between high and low tide) reaches 1-2 metres on the East Coast and 3.5 – 4 metres on the West Coast. High tide and low tide also occur at different times on Auckland’s East and West Coasts. This is because the tide has to travel from the Waitamata Harbour up and around Northland to reach the Manukau Harbour – a journey that takes about 3½ hours.

You can find out more information on New Zealand tides from the LINZ website

What causes sea levels to rise?

Sea levels are rising as a result of climate change. Increasing temperatures are causing the seas to expand and changes in glacier volumes and land-based ice sheets are contributing more water to the world’s oceans.

How much are sea levels rising by?

Measurements show that over the past 100 years the global average sea level has risen by around 19 cm and the rate of rise has accelerated in the last 20 years. Locally, sea levels have risen around 16 cm over the past 100 years. Measurements taken from the Port of Auckland tide gauge show an average sea level rise of around 1.6 mm per year since 1899.

How much are sea levels expected to continue to rise?

Projections from the 5th assesment report released by the Intergovernmental Panel on Climate Change (IPCC) suggest that by 2100 global sea levels will rise by 26 cm – 98 cm and by 1 -3 metres or more by 2300.

What are the potential impacts of sea level rise?

Sea level rise will change the nature of our coastal environment. These changes may impact our homes, buildings, property, infrastructure and wildlife habitats. The following impacts could potentially occur as result of higher water levels:

• Periodic or permanent inundation (flooding) of low-lying areas.
• Increase in the frequency and severity of flooding during large storms and high tides.
• Increase in the rate and occurrence of coastal erosion and shoreline recession.
• Loss of coastal wetlands and other coastal ecosystems.
• Contamination of soil and groundwater from salt water intrusion.

What can we do about sea level rise?

Unfortunately, sea level rise is inevitable and can’t be stopped. Because expanding oceans are a reality, it is imperative that we plan and prepare for the coming impacts. This is where the King Tides Auckland photo initiative comes in.

With your help the images we collect of King Tides that occur today, will enable us to understand how we can protect our coastal areas for the future. You can also find out more about how you can personally plan for change now and useful information on what to do in the event of coastal inundation by reading the Auckland Civil Defence Emergency Management resource on Coastal Inundation.

It may also be possible to slow down the rate of rising seas by reducing our global greenhouse gas emissions.

What is climate change?

Climate change is a longterm shift in the state of the climate, identified by significant changes in temperature, wind, precipitation and other indicators. The mean surface temperature of the Earth is warming and the latest IPCC report concludes that it is extremely likely (95 percent confidence in science speak) that the changing climate is due to greenhouse gas emissions and other human activities.

Regardless of your personal position on the matter climate change is happening now, our planet is getting warmer and this will have a wide range of consequences for Earth and its inhabitants.

What is the relationship between sea level rise and climate change?

Increasing global temperatures are driving two major processes that cause sea levels to rise:

• Thermal expansion – the oceans are expanding as they get warmer.
• Melting of glaciers and ice-sheets – higher temperatures are affecting the world’s mountain glaciers and the ice sheets at the Poles.

What is the relationship between King Tides and climate change?

King Tides are a normal part of the tidal cycle and are not caused by climate change. However, the higher sea levels that occur during King Tides allow us to visualise how climate change might impact our coastlines. One of the purposes of the King Tides AKL photo initiative is to raise awareness of sea level rise and its consequences. Photographing the effects of King Tides on the built and natural environment in our coastal areas provides a unique opportunity to glimpse into the future and witness how sea level rise could affect us.

Coastal inundation is the flooding of low-lying land in coastal areas by raised ocean waters. Coastal inundation can be caused by severe weather events, large waves and high tides.  Coastal inundation is more likely to occur when high tides, storm surge and/or large waves occur at the same time.

NIWA produce an annual Red Alert tide calendar help us keep an eye on potentially adverse weather conditions like low barometric pressure or strong onshore winds from storms, elevated river levels and rough seas that could combine with the high tide to increase the risk of coastal inundation. The calendar is available here.

How can coastal inundation affect us?

Coastal inundation can have significant effects on coastal communities, potentially causing serious flooding to properties, reserves and infrastructure.  Some of the potential impacts include:

• Transport routes can be disrupted through surface flooding.
• Storm-water and drainage networks can be overwhelmed due to reductions in discharge capacity.
• Associated large waves can cause beach erosion to  coastal properties and reserves as well as damage to coastal structures.

What are the impacts of sea level rise on coastal inundation?

Sea level rise could magnify the impacts of storms by raising the water level that storm surges affect. There will be a greater chance of inundation during storms, irrespective of any changes in the frequency or magnitude of storm surges, in storminess or wave conditions.  Areas that currently experience coastal inundation are more likely to experience it in the future and areas that are currently unaffected may become susceptible.

What are the impacts of climate change on coastal inundation?

Several impacts arising from climate change will heighten the risk and influence the extent of coastal inundation:

• Changes in the severity and frequency of storms will increase the occurrence and magnitude of flooding due to storm surges.
• Changes to wave conditions and weather patterns, increased rainfall and sea level rise will also increase the risk and effects of coastal flooding.

What should I do in the event of coastal inundation?

To find out what to do in the event of coastal inundation or if a warning is issued check the Auckland Civil Defence Emergency Management website.

What is storm surge?

A storm surge is a rise in sea level that occurs as a result of the wind and atmospheric pressure changes associated with storm events. During a storm surge, high winds and waves force water ashore which can increase the risk of coastal inundation particularly if coinciding with a higher than normal tide.

What factors can affect storm surge?

The likelihood of storm surge and flooding occurring during severe weather is increased with high tides and high water levels. The topography and physical characteristics of an area, such as low-lying land, can also determine if storm surge will result in coastal inundation.

What are storm tides?

Storm tide is the total water level rise due to the combination of storm surge and the astronomical tide at the time.

You can learn more about storm surge here.

What is coastal erosion?

Erosion is a natural process that results in the loss of beach and dune sediments (fragments) and the wearing away of coastal land as a result of waves and ocean currents.

What factors can affect coastal erosion?

Coastal areas where the land is comprised of softer materials tend to have higher rates of erosion than hardier rocky areas. Coastlines that have been modified by farmland, construction and other land-clearing activities have an increased risk of erosion. Increasing sea levels and storms associated with climate change will intensify the rate and occurrence of coastal erosion.

What are the consequences of erosion?

Erosion can damage or completely remove coastal wildlife habitats and ecosystems. The loss of dunes, rocks, wetlands and other buffer zones increases the risk and magnitude of coastal inundation and flooding. Houses, buildings, roads and other infrastructure built along the coastline can be severely comprised, damaged, or lost to the sea. Pollution from sediment and other land run off due to erosion is dangerous to marine wildlife.