Natural Environment

The natural environment is a huge part of the appeal of this area. The coastline, saltmarshes, open spaces and reserves are highly valued by Ngāi Tahu, residents and visitors.

However, the factors that make this location so special also highlight some of its challenges:

  • It is a narrow strip of land surrounded by water on three sides.
  • It is, and always has been, a dynamic environment subject to changing coastal processes. Over the years there have been major changes to the size, shape and location of the mouth of the Avon Heathcote Estuary/Ihutai and the south end of the Spit.
  • Most of the area is low lying, with ground levels less than 2 metres above sea level.
  • Groundwater is shallow and influenced by the tides.
  • The monthly high tides pose the largest risk of coastal flooding.
  • The earthquakes caused significant changes to the natural environment and the area is still adjusting.



Physical features of the area

The Regeneration Strategy project area borders the open coast of Te Kaikai a Waro/Pegasus Bay on the east; the mouth of the Ōtākaro/Avon River (north of Craddock Street) and the Estuary of the Ōpāwaho/Heathcote River and Ōtākaro/Avon Rivers/Ihutai (“the Estuary/Ihutai”) to the west; and the mouth of the Estuary/Ihutai to the south.

In 2015 the location of the river mouth was adjusted in regional council maps and descriptions to the north of the Regeneration Strategy project area, adjacent to Evans Ave. This reflects the change in estuary dynamics as a result of subsidence following the earthquakes. For this reason, the western coastline of the Regeneration Strategy project area is referred to as the estuary edge throughout this document.

Most of the Regeneration Strategy project area is relatively low lying with ground levels less than 2 metres above mean sea level. Sand dunes, which range in height from 5 to 9 metres, act as a buffer to the ocean on the open coast and south end of the Spit. On the west side of the dunes the landscape gradually slopes towards the Estuary/Ihutai.

The land is made up of sand overlying fine grained silts, peats and clays. The sands on the Estuary/Ihutai side are loose, finer and less dense than those closer to the dunes.

The deeper soils and sediments are made up of coarse-grained gravels, and fine-grained silts which form a multi-layered aquifer system. The water table sits within the uppermost sediments and is typically less than 10m deep across the Canterbury Plains, becoming shallower near the rivers and the coast. Within most of the Regeneration Strategy project area the water table is less than 2 metres below the ground surface. Groundwater generally becomes shallower towards the Estuary/Ihutai and the south end of the Spit, but varies with rainfall, daily tides, and drainage. Groundwater depth and quality is influenced by sea water.


Why this is important

The physical features of this area are important and they are part of its appeal. They also highlight some of the challenges of the location, including that:

  • It is a narrow strip of land surrounded by water on three sides.
  • Most of the area is low lying.
  • The land is made up of fine grained, loose soils.
  • Groundwater is shallow and influenced by the tides.


What we don’t know

While some generalised statements can be made about the physical features of the land across the whole Regeneration Strategy project area, there are likely to be localised differences at an individual property level.



Dynamic coastal processes

The coastal sand dunes and the Southshore Spit were formed by, and continue to be affected by 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 that changes over time, depending on a combination of the movement of water, the physical features of the coast, and the availability of sediment to be deposited.

The open coast, on the eastern side of the Regeneration Strategy project area, is currently accreting. In recent years the dunes have also been increasing in volume and building seawards, mainly due to dune management and planting efforts. While the current cycle of growth is stabilising the dunes, there is a risk of localised erosion from storms, or a cycle of erosion in the longer term if the supply of sediment from the Waimakariri River changes.

On the east coast the fortnightly influence of the sun on the tide is weaker than other places, and the monthly influence of the moon has a greater impact on the heights of the tide. This results in one dominant high tide a month (perigean tide) rather than two fortnightly spring high tides. Coastal flooding via the Estuary/Ihutai usually occurs when an extreme coastal storm or other weather event coincides with these monthly perigean tide.

The tide behaves differently in the shallow water of the Estuary/Ihutai from how it behaves on the open coast. It takes more time for the tide to move over the mouth of the Estuary/Ihutai and sand bars, into shallow water and up the rivers than it does to move up the open coast. As a result, the heights and timing of the tide varies at different sites in the Estuary/Ihutai and the 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 500mm higher.

Tides also influence groundwater levels near the coast, causing daily and monthly fluctuations.

Extreme sea levels 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. Read more



Water quality

The Ōtākaro/Avon River drains through a large part of Christchurch City before flowing into the Estuary/Ihutai, which is also fed by the Ōpāwaho/Heathcote River. Both rivers originate from groundwater springs, meaning their water quality is dependent on both the groundwater quality in the springs and the quality of discharges, stormwater and runoff which feed into them.


What we know

Water quality within the Estuary/Ihutai is ‘poor’ at the mouths of the Ōpāwaho/Heathcote River and Ōtākaro/Avon River, and ‘good’ close to the Estuary/Ihutai mouth and on the open coast.

Since March 2010, Christchurch City’s treated wastewater has been discharged via a three kilometre ocean outfall, instead of into the Estuary/Ihutai. This contributed to an improvement in the water quality in the Estuary/Ihutai.

The earthquakes had an impact on the levels of bacteria in the Estuary/Ihutai and in the lower reaches of the rivers, largely caused by untreated wastewater discharges from damaged infrastructure following the earthquakes. Once major infrastructure works were completed, levels recovered to pre-earthquake concentrations. They continue to be monitored by Environment Canterbury and the Council.


Why this is important

Changes in the water quality of the Estuary/Ihutai can have an impact on ecosystems, as has been seen with historic discharges, and recent earthquakes.



Ecology

The Regeneration Strategy project area has three main habitats:

  • Coastal sand dunes.
  • Intertidal areas of saltmarsh along sections of the Ōtākaro/Avon River and the edge of the Estuary/Ihutai, which are nationally and internationally significant bird habitat.
  • Areas of open space and reserves which are a mix of exotic plants, open grassed areas and native regeneration planting.

These habitats provide an important buffer between the natural and built environment on both the open coast and the Estuary/Ihutai edge.

These ecosystems are highly sensitive to changes in salinity, water level, moisture, sedimentation, the concentration of contaminants, temperature, and sunlight. Some areas, such as the lower Ōtākaro/Avon River, have seen an increase in bird populations and the richness of species over the last 30 years. In other areas, where there has been subsidence, existing plants are becoming increasingly stressed by the change in salinity and water levels. Read more.


Changes to the natural environment because of the earthquakes

The 2010-2011 Canterbury earthquakes and, to a lesser extent, the 2016 Kaikōura-Hurunui earthquake, caused the bed of the Estuary/Ihutai to tilt, lifting in the south and subsiding in the north east, particularly around the mouth of the Ōtākaro/Avon River.

Much of the westside of South New Brighton, north of Bridge Street dropped between 0.2-1.0m, while some parts of Southshore experienced some uplift.

Depending on the location, changes in ground levels may have increased or decreased susceptibility to natural hazards such as shallow groundwater or flooding, and for the lower areas, exacerbated the effects of climate change. These changes have also affected the Estuary/Ihutai and it is not yet clear what the longer-term implications will be. Read more.

The natural environment is a huge part of the appeal of this area. The coastline, saltmarshes, open spaces and reserves are highly valued by Ngāi Tahu, residents and visitors.

However, the factors that make this location so special also highlight some of its challenges:

  • It is a narrow strip of land surrounded by water on three sides.
  • It is, and always has been, a dynamic environment subject to changing coastal processes. Over the years there have been major changes to the size, shape and location of the mouth of the Avon Heathcote Estuary/Ihutai and the south end of the Spit.
  • Most of the area is low lying, with ground levels less than 2 metres above sea level.
  • Groundwater is shallow and influenced by the tides.
  • The monthly high tides pose the largest risk of coastal flooding.
  • The earthquakes caused significant changes to the natural environment and the area is still adjusting.



Physical features of the area

The Regeneration Strategy project area borders the open coast of Te Kaikai a Waro/Pegasus Bay on the east; the mouth of the Ōtākaro/Avon River (north of Craddock Street) and the Estuary of the Ōpāwaho/Heathcote River and Ōtākaro/Avon Rivers/Ihutai (“the Estuary/Ihutai”) to the west; and the mouth of the Estuary/Ihutai to the south.

In 2015 the location of the river mouth was adjusted in regional council maps and descriptions to the north of the Regeneration Strategy project area, adjacent to Evans Ave. This reflects the change in estuary dynamics as a result of subsidence following the earthquakes. For this reason, the western coastline of the Regeneration Strategy project area is referred to as the estuary edge throughout this document.

Most of the Regeneration Strategy project area is relatively low lying with ground levels less than 2 metres above mean sea level. Sand dunes, which range in height from 5 to 9 metres, act as a buffer to the ocean on the open coast and south end of the Spit. On the west side of the dunes the landscape gradually slopes towards the Estuary/Ihutai.

The land is made up of sand overlying fine grained silts, peats and clays. The sands on the Estuary/Ihutai side are loose, finer and less dense than those closer to the dunes.

The deeper soils and sediments are made up of coarse-grained gravels, and fine-grained silts which form a multi-layered aquifer system. The water table sits within the uppermost sediments and is typically less than 10m deep across the Canterbury Plains, becoming shallower near the rivers and the coast. Within most of the Regeneration Strategy project area the water table is less than 2 metres below the ground surface. Groundwater generally becomes shallower towards the Estuary/Ihutai and the south end of the Spit, but varies with rainfall, daily tides, and drainage. Groundwater depth and quality is influenced by sea water.


Why this is important

The physical features of this area are important and they are part of its appeal. They also highlight some of the challenges of the location, including that:

  • It is a narrow strip of land surrounded by water on three sides.
  • Most of the area is low lying.
  • The land is made up of fine grained, loose soils.
  • Groundwater is shallow and influenced by the tides.


What we don’t know

While some generalised statements can be made about the physical features of the land across the whole Regeneration Strategy project area, there are likely to be localised differences at an individual property level.



Dynamic coastal processes

The coastal sand dunes and the Southshore Spit were formed by, and continue to be affected by 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 that changes over time, depending on a combination of the movement of water, the physical features of the coast, and the availability of sediment to be deposited.

The open coast, on the eastern side of the Regeneration Strategy project area, is currently accreting. In recent years the dunes have also been increasing in volume and building seawards, mainly due to dune management and planting efforts. While the current cycle of growth is stabilising the dunes, there is a risk of localised erosion from storms, or a cycle of erosion in the longer term if the supply of sediment from the Waimakariri River changes.

On the east coast the fortnightly influence of the sun on the tide is weaker than other places, and the monthly influence of the moon has a greater impact on the heights of the tide. This results in one dominant high tide a month (perigean tide) rather than two fortnightly spring high tides. Coastal flooding via the Estuary/Ihutai usually occurs when an extreme coastal storm or other weather event coincides with these monthly perigean tide.

The tide behaves differently in the shallow water of the Estuary/Ihutai from how it behaves on the open coast. It takes more time for the tide to move over the mouth of the Estuary/Ihutai and sand bars, into shallow water and up the rivers than it does to move up the open coast. As a result, the heights and timing of the tide varies at different sites in the Estuary/Ihutai and the 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 500mm higher.

Tides also influence groundwater levels near the coast, causing daily and monthly fluctuations.

Extreme sea levels 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. Read more



Water quality

The Ōtākaro/Avon River drains through a large part of Christchurch City before flowing into the Estuary/Ihutai, which is also fed by the Ōpāwaho/Heathcote River. Both rivers originate from groundwater springs, meaning their water quality is dependent on both the groundwater quality in the springs and the quality of discharges, stormwater and runoff which feed into them.


What we know

Water quality within the Estuary/Ihutai is ‘poor’ at the mouths of the Ōpāwaho/Heathcote River and Ōtākaro/Avon River, and ‘good’ close to the Estuary/Ihutai mouth and on the open coast.

Since March 2010, Christchurch City’s treated wastewater has been discharged via a three kilometre ocean outfall, instead of into the Estuary/Ihutai. This contributed to an improvement in the water quality in the Estuary/Ihutai.

The earthquakes had an impact on the levels of bacteria in the Estuary/Ihutai and in the lower reaches of the rivers, largely caused by untreated wastewater discharges from damaged infrastructure following the earthquakes. Once major infrastructure works were completed, levels recovered to pre-earthquake concentrations. They continue to be monitored by Environment Canterbury and the Council.


Why this is important

Changes in the water quality of the Estuary/Ihutai can have an impact on ecosystems, as has been seen with historic discharges, and recent earthquakes.



Ecology

The Regeneration Strategy project area has three main habitats:

  • Coastal sand dunes.
  • Intertidal areas of saltmarsh along sections of the Ōtākaro/Avon River and the edge of the Estuary/Ihutai, which are nationally and internationally significant bird habitat.
  • Areas of open space and reserves which are a mix of exotic plants, open grassed areas and native regeneration planting.

These habitats provide an important buffer between the natural and built environment on both the open coast and the Estuary/Ihutai edge.

These ecosystems are highly sensitive to changes in salinity, water level, moisture, sedimentation, the concentration of contaminants, temperature, and sunlight. Some areas, such as the lower Ōtākaro/Avon River, have seen an increase in bird populations and the richness of species over the last 30 years. In other areas, where there has been subsidence, existing plants are becoming increasingly stressed by the change in salinity and water levels. Read more.


Changes to the natural environment because of the earthquakes

The 2010-2011 Canterbury earthquakes and, to a lesser extent, the 2016 Kaikōura-Hurunui earthquake, caused the bed of the Estuary/Ihutai to tilt, lifting in the south and subsiding in the north east, particularly around the mouth of the Ōtākaro/Avon River.

Much of the westside of South New Brighton, north of Bridge Street dropped between 0.2-1.0m, while some parts of Southshore experienced some uplift.

Depending on the location, changes in ground levels may have increased or decreased susceptibility to natural hazards such as shallow groundwater or flooding, and for the lower areas, exacerbated the effects of climate change. These changes have also affected the Estuary/Ihutai and it is not yet clear what the longer-term implications will be. Read more.