Dynamic coastal processes

The Spit was formed by, and continues to be affected by, dynamic processes of waves, wind, sea level, and currents which change the shape of the coastal environment by adding (accreting), removing (eroding) or transferring sediments.


These processes are part of a natural cycle and will change over time, depending on a combination of the movement of water (hydrodynamic processes); the physical features of the coast (geomorphology); and the availability of sediment to be deposited (sediment budget).


Left to themselves, sandspits naturally change over time. They constantly move and change shape, especially at estuary mouths, as the sea interacts with loose sediment (sand) rather than firm land.


What we know

Sediment supply
Sediment, primarily from the Waimakariri River, has been transported south along Te Kaikai a Waro/Pegasus Bay and deposited along the Christchurch coast to form the dunes and the Spit.
The open coast, on the eastern side of the Regeneration Strategy project area, is currently accreting, building sediment on the beach. This is largely because of the sediment continuing to discharge from the Waimakariri River. The dunes have also been increasing in volume and building seawards (accreting), mainly due to dune management and planting efforts to stabilise the dunes. Despite this pattern of growth, the beach and dunes can experience short term periods of erosion because of storms.
The south end of the Spit is a dynamic and constantly changing area because of the combination of effects from the open coast and the Estuary/Ihutai channel processes. This creates fluctuations in the size, shape and location of the Estuary/Ihutai mouth and end of the Spit.


Between 1940 and 1949 the Estuary/Ihutai position shifted to a more northwards position. This and several large coastal storms caused significant erosion of the end of the spit. Following this period of erosion there was a subsequent period of growth when the previously eroded foredunes were reformed. Some of these reformed foredunes were developed for residential purposes. Here's a map that shows the shows the range and scale of physical changes to the Spit between 1941 and 2011.


Just inside the Estuary/Ihutai mouth the channel is more stable, but the Estuary/Ihutai bed is constantly changing. Even small changes to the river or tidal currents can cause the loose, fine silty sands of the Estuary/Ihutai bed to move.


Tides
Tides are primarily caused by the gravitational attraction of the sun and moon which result in ocean long waves interacting with the sea floor to produce a rise and fall in sea levels (tides).


The sea level we see at the coast is made up of a combination of tides, storm effects (such as storm surges), wave effects, and medium and longer-term changes in sea level due to climatic and/or geological processes, and is constantly changing. We have a good understanding of the tides and their influences in Christchurch. Our tides are typical of the east coast of New Zealand where the fortnightly influence of the sun on the tide is weaker than other places, and the monthly influence of the moon has a much more dominant impact on the heights of the tides. This means that on the Christchurch coast we experience a more dominant high tide period once a month (perigean tide) rather than two fortnightly spring high tide periods. Flooding usually occurs when an extreme coastal storm or other weather event coincides with these monthly perigean tide.


The tide in the shallow water of the Estuary/Ihutai behaves differently from the tide on the open coast. It takes more time for the tide to move through the mouth of the Estuary/Ihutai and over its sand bars, into shallow water and up the rivers than it does to move up the open coast, and effects like friction distort the shape of the tidal wave. As a result, the heights and timing of the tides differ at different sites in the Estuary/Ihutai and rivers. For example, high tide at the Ferrymead Bridge is 53 minutes later and 100mm higher than at Sumner. Low tide is nearly three hours later and 700mm higher .

Groundwater fluctuations i n response to tidal patterns.

Prepared by Tonkin & Taylor on behalf of Christchurch City Council, (2018).

Tides influence groundwater levels near the coast, causing daily and monthly fluctuations and also affects the groundwater levels adjacent to the lower reaches of the rivers and near the river mouths (for example, the area North of Bridge Street).


Wave dynamics and water levels
The open coast on the eastern side of the Regeneration Strategy project area is affected by deep water waves from distant offshore storms and locally generated wind waves. In Te Kaikai a Waro/Pegasus Bay prevailing waves approach from the north-east (particularly in summer), east and south-east (particularly in winter). Banks Peninsula shelters this part of the coast from the worst of the storms that come from the south .


On both the open coast and in the Estuary/Ihutai, water level plays an important role in shaping and changing the shoreline. Water level at the coast is a component of the tide, storm surge , and wave run-up. Wave run-up is a combination of the elevated mean water level during the breaking process (called the ‘wave set-up’), and the maximum level the waves reach on the beach (called the ‘wave swash’) .

Wave components impacting water levels (Tonkin & Taylor Ltd, 2017).


Why this is important
Mean High Water Springs (MHWS) is calculated differently in Christchurch. MHWS is used to describe the boundary between land and sea . It is an important planning boundary and separates areas of responsibilities between the Christchurch City Council and Environment Canterbury. MHWS was previously measured at 10.40m above the Christchurch Drainage Datum (CDD) for Christchurch , but has recently been recalculated at 10.30m above CDD . A drainage datum is a reference point used for surveying land and water levels which is typically set below lowest tide levels (see Figure 8). Any future modelling and investigations will use this updated level.

Relationship between datum and MHWS (Land and Information New Zealand).


Modelling in Christchurch reflects the local dynamic processes
The Coastal Hazard Assessment for Christchurch and Banks Peninsula 2017[1] estimates areas potentially at risk from coastal inundation and erosion. It uses the best currently available data on water levels and sediment budgets. Tidal fluctuations in groundwater and river levels are incorporated into modelling for shallow groundwater assessments and flood risks.



What we don’t know

Long-term sediment supply
While the current cycle of accretion is stabilising the dunes, there is a risk of a cycle of erosion in the longer-term if the supply or transport of sediment from the Waimakariri River changes. There are a number of reasons why the supply of sediment could change, including climate change effects on how river sediment is transported; wind, wave and rainfall patterns; earthquakes; water abstraction from the Waimakariri River and aquifers; and gravel extraction in the lower reaches of the Waimakiriri River .
As part of the Christchurch City Council’s Land Drainage Recovery Programme, a study on the sediment budget for southern Te Kaikai a Waro/Pegasus Bay is underway. The results of the study will help the Christchurch City Council to understand how future changes in sediment supply may affect coastal processes such as erosion, accretion and inundation on the open coast and within the Estuary/Ihutai.

Whether statistics of extreme sea levels need to be recalculated
Extreme sea levels during storms are currently defined as those that exceed 10.81m above the Christchurch Drainage Datum (at Bridge Street). Since 2010 there have been five separate extreme sea level events - four of them since June 2017. This may mean the levels currently used for extreme sea levels in the Estuary/Ihutai are being underestimated and higher sea levels may occur more often. Christchurch City Council and Environment Canterbury are currently doing more work to review and, if necessary, update the extreme sea level statistics.


The Spit was formed by, and continues to be affected by, dynamic processes of waves, wind, sea level, and currents which change the shape of the coastal environment by adding (accreting), removing (eroding) or transferring sediments.


These processes are part of a natural cycle and will change over time, depending on a combination of the movement of water (hydrodynamic processes); the physical features of the coast (geomorphology); and the availability of sediment to be deposited (sediment budget).


Left to themselves, sandspits naturally change over time. They constantly move and change shape, especially at estuary mouths, as the sea interacts with loose sediment (sand) rather than firm land.


What we know

Sediment supply
Sediment, primarily from the Waimakariri River, has been transported south along Te Kaikai a Waro/Pegasus Bay and deposited along the Christchurch coast to form the dunes and the Spit.
The open coast, on the eastern side of the Regeneration Strategy project area, is currently accreting, building sediment on the beach. This is largely because of the sediment continuing to discharge from the Waimakariri River. The dunes have also been increasing in volume and building seawards (accreting), mainly due to dune management and planting efforts to stabilise the dunes. Despite this pattern of growth, the beach and dunes can experience short term periods of erosion because of storms.
The south end of the Spit is a dynamic and constantly changing area because of the combination of effects from the open coast and the Estuary/Ihutai channel processes. This creates fluctuations in the size, shape and location of the Estuary/Ihutai mouth and end of the Spit.


Between 1940 and 1949 the Estuary/Ihutai position shifted to a more northwards position. This and several large coastal storms caused significant erosion of the end of the spit. Following this period of erosion there was a subsequent period of growth when the previously eroded foredunes were reformed. Some of these reformed foredunes were developed for residential purposes. Here's a map that shows the shows the range and scale of physical changes to the Spit between 1941 and 2011.


Just inside the Estuary/Ihutai mouth the channel is more stable, but the Estuary/Ihutai bed is constantly changing. Even small changes to the river or tidal currents can cause the loose, fine silty sands of the Estuary/Ihutai bed to move.


Tides
Tides are primarily caused by the gravitational attraction of the sun and moon which result in ocean long waves interacting with the sea floor to produce a rise and fall in sea levels (tides).


The sea level we see at the coast is made up of a combination of tides, storm effects (such as storm surges), wave effects, and medium and longer-term changes in sea level due to climatic and/or geological processes, and is constantly changing. We have a good understanding of the tides and their influences in Christchurch. Our tides are typical of the east coast of New Zealand where the fortnightly influence of the sun on the tide is weaker than other places, and the monthly influence of the moon has a much more dominant impact on the heights of the tides. This means that on the Christchurch coast we experience a more dominant high tide period once a month (perigean tide) rather than two fortnightly spring high tide periods. Flooding usually occurs when an extreme coastal storm or other weather event coincides with these monthly perigean tide.


The tide in the shallow water of the Estuary/Ihutai behaves differently from the tide on the open coast. It takes more time for the tide to move through the mouth of the Estuary/Ihutai and over its sand bars, into shallow water and up the rivers than it does to move up the open coast, and effects like friction distort the shape of the tidal wave. As a result, the heights and timing of the tides differ at different sites in the Estuary/Ihutai and rivers. For example, high tide at the Ferrymead Bridge is 53 minutes later and 100mm higher than at Sumner. Low tide is nearly three hours later and 700mm higher .

Groundwater fluctuations i n response to tidal patterns.

Prepared by Tonkin & Taylor on behalf of Christchurch City Council, (2018).

Tides influence groundwater levels near the coast, causing daily and monthly fluctuations and also affects the groundwater levels adjacent to the lower reaches of the rivers and near the river mouths (for example, the area North of Bridge Street).


Wave dynamics and water levels
The open coast on the eastern side of the Regeneration Strategy project area is affected by deep water waves from distant offshore storms and locally generated wind waves. In Te Kaikai a Waro/Pegasus Bay prevailing waves approach from the north-east (particularly in summer), east and south-east (particularly in winter). Banks Peninsula shelters this part of the coast from the worst of the storms that come from the south .


On both the open coast and in the Estuary/Ihutai, water level plays an important role in shaping and changing the shoreline. Water level at the coast is a component of the tide, storm surge , and wave run-up. Wave run-up is a combination of the elevated mean water level during the breaking process (called the ‘wave set-up’), and the maximum level the waves reach on the beach (called the ‘wave swash’) .

Wave components impacting water levels (Tonkin & Taylor Ltd, 2017).


Why this is important
Mean High Water Springs (MHWS) is calculated differently in Christchurch. MHWS is used to describe the boundary between land and sea . It is an important planning boundary and separates areas of responsibilities between the Christchurch City Council and Environment Canterbury. MHWS was previously measured at 10.40m above the Christchurch Drainage Datum (CDD) for Christchurch , but has recently been recalculated at 10.30m above CDD . A drainage datum is a reference point used for surveying land and water levels which is typically set below lowest tide levels (see Figure 8). Any future modelling and investigations will use this updated level.

Relationship between datum and MHWS (Land and Information New Zealand).


Modelling in Christchurch reflects the local dynamic processes
The Coastal Hazard Assessment for Christchurch and Banks Peninsula 2017[1] estimates areas potentially at risk from coastal inundation and erosion. It uses the best currently available data on water levels and sediment budgets. Tidal fluctuations in groundwater and river levels are incorporated into modelling for shallow groundwater assessments and flood risks.



What we don’t know

Long-term sediment supply
While the current cycle of accretion is stabilising the dunes, there is a risk of a cycle of erosion in the longer-term if the supply or transport of sediment from the Waimakariri River changes. There are a number of reasons why the supply of sediment could change, including climate change effects on how river sediment is transported; wind, wave and rainfall patterns; earthquakes; water abstraction from the Waimakariri River and aquifers; and gravel extraction in the lower reaches of the Waimakiriri River .
As part of the Christchurch City Council’s Land Drainage Recovery Programme, a study on the sediment budget for southern Te Kaikai a Waro/Pegasus Bay is underway. The results of the study will help the Christchurch City Council to understand how future changes in sediment supply may affect coastal processes such as erosion, accretion and inundation on the open coast and within the Estuary/Ihutai.

Whether statistics of extreme sea levels need to be recalculated
Extreme sea levels during storms are currently defined as those that exceed 10.81m above the Christchurch Drainage Datum (at Bridge Street). Since 2010 there have been five separate extreme sea level events - four of them since June 2017. This may mean the levels currently used for extreme sea levels in the Estuary/Ihutai are being underestimated and higher sea levels may occur more often. Christchurch City Council and Environment Canterbury are currently doing more work to review and, if necessary, update the extreme sea level statistics.