Ōpāwaho/Heathcote River bank stabilisation - stage 1

The inspections identified a number of areas where the banks are unstable and eroding. While slumping and other instabilities have been a feature of the river for a long time, the earthquakes have accelerated this process and caused more widespread instability than would usually be present. A number of areas were identified as high priority, requiring work within the next 5 years to maintain waterway capacity or prevent further bank failure.

Funding has been obtained to address these high priority areas. The work will proceed in two stages. This first stage is currently being consulted on. The second stage are those areas which may be affected by low stop banks if these are selected to manage frequent flooding. As further approval and consultation is required for the low stop banks, work on the second stage will not take place until later in 2018.

Stage 1: What is proposed

The stabilisation works need to ensure that the drainage function of the river is improved (through additional capacity and preventing further slumping), while also ensuring that cultural, recreational, ecological, landscape and heritage values are taken into consideration. Therefore, wherever possible, waterway area has been increased by widening the channel. This enables bank stabilisation through a ‘naturalised’ bank profile (rather than hard engineering structures), and increases the capacity of the river. The work will be in keeping with the Mid Heathcote/Ōpāwaho Linear Park Masterplan, which was approved in 2009.

Different types of bank instability require different methods to address the issue. The different designs to be used along the waterway are described in detail in pages 12 to 20, and these designs are then referenced on each plan. To see what is in your area you will need to refer first to the plan, and then look for the detailed description and cross-section of each design type.

There are a number of common features throughout most
designs:

• Preserving trees where possible
• A key trigger of bank instability is erosion and undercutting of the bank toe. To address this instability, a rock edge is proposed in most places
• Crevasses and undercutting, while being bad for bank stability, provide habitat for aquatic life. To provide a partial replacement of this habitat, small pipes are proposed within the rock edge, along with planting above the rock edge.
• Slumping caused by over steepened banks is generally addressed by cutting back the banks to achieve a stable profile. This increases the waterway area of the river, improving the flow.
• Plantings along the river edge protect the bank to prevent further slumping.
• Where space is constrained then additional stability works are required, either through a second layer of rocks as a low wall, or in a few limited areas by constructing gabions.

In some areas it is proposed to narrow the road and remove some bankside carparks. This allows drainage capacity
to be increased, setting aside space for footpaths and still considering the other community values.

Impacts on trees and landscape planting

Trees provide significant landscape and ecological values to the waterway, and many of the existing trees are providing benefits to bank stability through their roots. The intention is to retain and protect as many healthy and structurally sound trees as possible and increase the existing tree cover within the area, while maintaining stability of the river banks and not impeding water conveyance.

The potential effects of the bank stabilisation works on trees has been quantified as much as possible during the design phase of the project. As part of this, a tree survey has been undertaken to confirm the condition of existing trees and inform the project design.

Amendments to the design may be required due to unforeseen circumstances, especially during construction, which may result in additional tree removals or changes to the proposed tree planting being required.

For new tree planting, final species selection will be dependent on potential variations in site conditions, and availability of tree stock.

The key design principles are:

• In the majority of locations, where a tree is providing good bank stability features, the tree will be retained.
• Where trees are to be retained, minimising the potential effects of the proposed works on existing trees will be allowed for during construction.
• Where there is severe erosion or undercutting, rock edge protection will be placed around the existing root mass of trees that are retained.
• In areas where excavation is required for cutting back the bank and scalloping to the river banks, adequate setback distances will be provided to avoid damage to the root systems of trees that are to be retained.
• Wherever possible tree removals will be limited to unhealthy and structurally unsound trees, and the project will provide opportunities to remove and replace trees in poor condition.
• In some locations, existing site constraints and the extent of works required will result in the removal of healthy and structurally sound trees.
• Relocating young, healthy and structurally sound trees will occur where possible.

Extensive tree planting and landscaping is proposed as part of this project, with the intention of improving the landscape and ecology values of the area in the long term. The new plantings have been designed in alignment with the MidHeathcote River/Opawaho Linear Park Masterplan (2009) and Council’s Streamside Planting Guide, with the following aims:

• Implementation of plantings that are suitable to the local environments, especially waterway conditions.
• For all lower bank planting, native species have been selected suitable to the local environment, for low maintenance, good root stability, to enhance the ecology of the river and to minimise potential effects on the waterway capacity and flow.
• Native trees will be planted in groups and as specimen trees where appropriate, and exotic tree species will be planted as specimen trees to complement the existing trees within the area.
• Significant areas of grassing on the banks will occur where the battered slopes are gradual enough in order to maintain access and views of the waterway.

The approximate number of trees to be removed, relocated and new trees to be planted include the following:

Arboricultural report - 1 November 2017 [PDF, 5.4 MB]

T1 is the most preferred bank profile, but can only be applied where there is sufficient space to cut the bank back to provide a more stable slope and also increase the waterway area. It features:

• Rock edge to protect against bank toe erosion andundercutting
• Cutting back the bank increases the waterway area (thedotted line below represents the original bank line)
• Small pipes laid into the rocks to provide habitat for aquatic life
• Strip of riparian planting overhanging the rock edge to provide shading and habitat for aquatic life. Plant selection is based on both the ability to handle the wet environment, but also they must be low in height and flatten during floods to reduce barriers to flow
• The gentler slope of the bank allows for a grass bank to be maintained

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T2 is similar to T1, but due to various constraints (such as the road, or to preserve space for other assets or recreational use) the batter is steeper and so needs to be planted. It also features:

• Rock edge to protect against bank toe erosion and undercutting
• Battering increases the waterway area (the dotted line below represents the original bank line)
• Small pipes laid into the rocks to provide habitat for aquatic life
• Strip of riparian planting overhanging the rock edge to provide shading and habitat for aquatic life. Plant selection is based on both the ability to handle the wet environment, but also they must be low in height and flatten during floods to reduce barriers to flow
• Planting of batter is needed to improve stability and for ease of maintenance

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T3 is used where space constraints mean that a steeper bank is needed. It includes a small rock wall above a lower bench. This allows the waterway area to be increased (to improve flows) and the profile to be varied to match the space available. It features:

• Rock edge to protect against bank toe erosion and undercutting
• Increased waterway area (the dotted line below represents the original bank line)
• Small pipes laid into the rocks to provide habitat for aquatic life
• Strip of riparian planting overhanging the rock edge to provide shading and habitat for aquatic life. Plant selection is based on both the ability to handle the wet environment, but also they must be low in height and flatten during floods to reduce barriers to flow
• Planting of batter is needed to improve stability and for ease of maintenance

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E1 is used to preserve an existing mature tree in good health, or where the bank is stable enough currently and does not require cutting back. This type is used to minimise the trees needing removal, but includes succession planting of a new tree further away from the bank to facilitate future bank stabilisation works when stability or the existing tree deteriorates. It also includes:

• Rock edge around existing tree roots to protect against bank toe erosion and undercutting
• Small pipes laid into the rocks to provide habitat for aquatic life
• Riparian planting overhanging the rock edge to provide shading and habitat for aquatic life, and planting of the bank to improve stability
• Preserves the existing canopy which shades the river and reduces weed growth
• Planting selected to improve stability and reduce amount of excavation required within tree root zone and to be appropriate for position under a tree canopy

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The following types of bank treatment are specifically designed for each location and so a typical cross-section is not able to be provided for each site. This includes gabion walls (structured rock filled baskets) and the treatment around bridge abutments. 

A general description is provided of each below.

Gabion Type - G1 and G2

Gabion walls are proposed where there are severe space limitations due to existing assets (e.g. roads) or very steep and high waterway banks. They are also used where there are deep slope failures that extend below the river bed.

These include:

• Gabion wall to provide geotechnical stability
• Planting above the wall to improve landscape value, and create a natural barrier to the wall for safety. Plant selection based on plants suitable for the river bank environment, with planting beds of 1-2m width
• These are not the preferred treatment due to greater impacts on landscape and ecology. As such the use of these treatments has been minimised as far as possible and measures will be put in place to address impacts (planting, habitat creation).

Bridge Abutment – BA

• These are location specific treatments around bridge abutments (the base of a bridge). These are to prevent further undercutting of bridges while maintaining the waterway area.

Image of Gabion Wall

• Bridge abutment works to protect footbridge, including proposed lower canopy bank planting.

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• Where mature trees are stabilising the banks, Edge Type E1 treatment to be implemented.
• Where slumping is observed and space is available, use of Bank Type T1 or Bank Type T2 treatments will be implemented.
• One existing damaged tree opposite 187 Ashgrove Terrace will be removed.
• Seven new specimen trees are proposed in this section of Ashgrove Terrace. Two smaller growing native trees are proposed in the low canopy planting outside 167 and 187 Ashgrove Terrace.
• No parking proposed on the riverbank side between 167 Ashgrove Terrace and 187 Ashgrove Terrace. This will improve safety for vehicles and reduce risks to bank stability.
• New proposed lower canopy bank planting to provide ecological and landscape benefits that offset rock edge works on both bank.

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• Where mature trees are stabilising the banks, Edge Type E1 treatment to be used.
• Where severe slumping observed and space is available, use of Bank Type T1 or Bank Type T2 treatments will be used.
• Edge Type E1 treatment works to be used on the outside bend of the opposite bank to address instability.
• Removal of two trees opposite 211 Ashgrove Terrace will be required for the bank stabilisation works.
• Four new specimen trees proposed in this section of Ashgrove Terrace. Two smaller growing native trees are proposed in the low canopy planting outside 193 and 211 Ashgrove Terrace.
• New lower canopy bank planting to provide ecological and landscape benefits that offset rock edge works on both banks.

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• Where mature trees are stabilising the banks, Edge Type E1 treatment will be used.
• Due to the severity of slumping around existing trees and risks to the waterway and other assets, Bank Type T1 is the preferred treatment for stability. This will result in the removal of four trees between 327 and 335 Ashgrove Terrace.
• Six new specimen trees are proposed in this section.
• New lower canopy bank planting on Ashgrove Terrace bank is proposed.
• Existing road corridor between 327 and 333 Ashgrove Terrace will be narrowed. Two lane road width can be maintained with parking removed on both sides.

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• Instability on Hunter Terrace bank observed, however no assets are currently at risk so works are not a high priority for this project.
• Critical instability observed on Waimea Terrace. The bank will be flattened to a shallower batter by realigning the waterway closer to Hunter Terrace (closed road).
• Bank Type T1 treatment on Hunter Terrace side of the bank will be used. The bank will be flattened further to provide additional waterway area, and create variation in the waterway profile.
• The existing waterway bed/base width and grade will be maintained to protect the riffles and ecological habitat in the area.
• Realignment of the waterway into Hunter Terrace requires the removal of four trees.
• Two new specimen trees are proposed to be planted opposite this section outside 25 Waimea Terrace.
• New lower canopy bank planting on Waimea Terrace is proposed.
• Removal of the willow trunk outside 11 Waimea Terrace.

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• Critical instability observed on Waimea Terrace. The bank will be flattened by realigning the waterway closer to Hunter Terrace (closed road).
• Bank Type T1 treatment on Hunter Terrace side of the bank will be used. The bank will be flattened further to provide additional waterway area, and create variation in the waterway profile.
• The existing waterway bed/base width and grade will be maintained to protect the riffles and ecological habitat in the area.
• In this section the realignment of the waterway into Hunter Terrace requires the removal of three trees.
• One tree outside 43 Waimea Terrace requires removal where bank stabilisation works are required.
• Two new specimen trees are proposed to be planted in this section of Waimea Terrace.
• New lower canopy bank planting on Waimea Terrace bank is proposed.

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• On Waimea Terrace near Tuscany Place, no works proposed where large mature trees are providing significant stability benefits, and no critical erosion is observed. This maintains the existing ecology and landscape amenity the trees and minor undercutting provides.
• Instability on Hunter Terrace bank observed, however no assets are currently at risk so works are not a high priority for his project.
• Where mature trees are stabilising the banks, Edge Type E1 treatment will be used.
• Bridge abutment works will be implemented to protect the footbridge.
• Where slumping observed and space is available, use of Bank Type T1 treatment will be used.
• Removal of one tree opposite the BMX track on Hunter Terrace due to structural deterioration.
• Two additional trees on HunterTerrace near the footbridge require removal. These trees are exhibiting decline/decay.
• Two new specimen trees and four new smaller growing native trees are proposed on Hunter Terrace within the vicinity of the tree removals.
• New specimen tree proposed to be planted on the bank outside 61Waimea Terrace.
• New lower canopy bank planting proposed through this section of Waimea Terrace and three smaller sections of bank planting on Hunter Terrace.

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• Existing retaining wall within the Sloan Terrace bank requires repairs at the base, the wall, and the road above the wall.
• Construction of new gabion walls will be used to address stability and will require the removal of ten existing native trees on this section of the bank.
• Three new smaller growing native trees are proposed in addition to new lower canopy bank planting.
• Works to address bank instability on the left bank (Waimea Terrace) are included in Stage 2 of this project.

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• Edge Type E1 treatment will be used on Centaurus Road where the bank is well vegetated and no slumping is observed.
• Where there is observed deep slope slip failure, Gabion Type G2 treatment will be implemented to address risk to Centaurus Road.
• The gabion treatment requires the removal of four trees. Two additional silver birch trees will be removed opposite 42 and 44 Centaurus Road due to their poor condition.
• Within this section of the riverbank, ten new smaller growing native trees are proposed. 
• New lower canopy bank planting on Centaurus Road bank is also proposed.
• Works to address bank instability on the left bank (Waimea Terrace) are included in Stage 2 of this project.

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• Waterway to be realigned upstream of the Bowenvale Bridge using Bank Type T1 treatment. 
• The realignment will provide additional bank area on the Waimea Terrace side to be used for stabilisation in Stage 2. The realignment through cutting back the bank will address the observed erosion and slumping on the Centaurus Road bank.
• Through the Bowenvale Bridge, rock edge treatment will be used to stabilise the banks.
• Gabion Type G2 treatment will be used where deep slope slip failure has been observed to address risk to Centaurus Road.
• Edge Type E1 treatment will also be used on Centaurus Road where it is well vegetated, large mature trees are stabilising the banks, and no slumping is observed.
• Within this section of the riverbank, three new smaller growing native trees are proposed. 
• New lower canopy bank planting on Centaurus Road bank is also proposed.
• No works proposed where large mature trees are providing stability benefits, and no critical erosion is observed. This maintains the existing ecology and landscape amenity the tree and minor undercutting provides.
• Works to address bank instability on the left bank (Eastern Terrace) are included in Stage 2 of this project.

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• No works proposed where there is no critical erosion or slumping is observed, and large mature trees or vegetation is providing stability benefits. This maintains the existing ecology and landscape amenity the tree and minor undercutting provides.
• Edge Type E1 treatment will be used where some undercutting/erosion is observed opposite Major Aitken Drive.
• Works to address bank instability on the left bank (Eastern Terrace) are included in Stage 2 of this project.

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• No works proposed where there is no critical erosion or slumping is observed, and large mature trees or vegetation is providing stability benefits. This maintains the existing ecology and landscape amenity the tree and minor undercutting provides.
• Edge Type E1 treatment will be used where some undercutting /erosion is observed on both sides of the river opposite 81/83 Eastern Terrace.
• New specimen tree proposed opposite 83 Eastern Terrace.
• New lower canopy bank planting on both sides of the river where works will be undertaken is also proposed.

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Eastern Terrace

• Where mature trees are stabilising the banks, Edge Type E1 treatment will be used.
• Where slumping observed and space is available, Bank Type T1 treatment will be used.
• The battering and rock walls require the removal of five trees on Eastern Terrace. An additional tree outside 117 Eastern Terrace will also be removed due to its poor condition.
• It is proposed to plant 15 new smaller growing native trees.
• One tree outside 118 Malcolm Avenue will be relocated.
• A new specimen tree is proposed on the bank at the end of Malcolm Avenue.
• New lower canopy bank planting is also proposed.

Palatine Terrace

• Where significant slumping is observed and the bank height is relatively high (Palatine Terrace), Gabion Type G1 treatment will be used. Space is limited in these areas, and the area in the corner of Palatine Terrace means road narrowing or battering is not feasible for safety reasons.
• Gabion Type G1 treatment will be used to address the bridge abutment stability, and also in front of the Orion transformer kiosk.
• Where space is restricted due to the existing footpath and kerb on Palatine Terrace, Bank Type T3 treatment will be used. This assists in addressing the height of Palatine Terrace and tying in to the existing footpath.
• To implement the bank stabilisation works on Palatine Terrace, 13 trees require removal and one tree to be relocated.
• It is proposed to plant 22 new smaller growing native trees.
• New lower canopy bank planting is also proposed.

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Eastern Terrace

• Where mature trees are stabilising the banks, Edge Type E1 treatment will be used.
• Where slumping is observed and space is available, Bank Type T1 treatment will be used.
• Three trees require removal due to the bank stabilisation works and their poor condition. These are located outside 119 Malcolm Avenue, 96 Martin Avenue, and 149 Eastern Terrace.
• Six new specimen trees and three smaller growing native trees are proposed.
• New lower canopy bank planting is also proposed.

Palatine Terrace

• Where mature trees are stabilising the banks, Edge Type E1 treatment will be implemented.
• Where space is restricted due to the existing footpath on Palatine Terrace, Bank Type T3 treatment will be used. This assists in addressing the height of Palatine Terrace and tying in to the existing footpath.
• Gabion Type G1 treatment will be used to address instability in front of the water supply well and pump. 
• One tree opposite 30 Palatine Terrace requires removal for bank stabilisation works.
• Three trees will be relocated from outside 24, 28 and 30 Palatine Terrace.
• Thirteen new smaller growing native trees are proposed.
• New lower canopy bank planting is also proposed.

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Eastern Terrace

• Where mature trees are stabilising the banks, Edge Type E1 treatment will be used.
• Where slumping is observed and space available, Bank Type T1 treatment will be used.
• Eastern Terrace north of Sandwich Road is already narrow, and used by Beckenham School and park users (including buses). As space is limited in this section, and slumping and damage to the existing rock wall is observed, Gabion Type G1 treatment will be used.
• One tree requires removal outside Beckenham School for bank stabilisation works.
• Four new specimen trees and two smaller growing native trees are proposed.
• New lower canopy bank planting is also proposed.

Palatine Terrace

• Where mature trees are stabilising the banks, Edge Type E1 treatment will be used.
• Where slumping is observed and space available, Bank Type T1 treatment or Bank Type T2 treatment will be used.
• Four trees require removal and one tree relocated due to the bank stabilisation works and their current condition.
• Three new specimen trees and ten smaller growing native trees are proposed.
• New lower canopy bank planting is also proposed.

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• Where mature trees are stabilising the banks or no slumping is observed, Edge Type E1 treatment will be used.
• Instability on Eastern Terrace bank observed, however no assets are currently at risk so works not a high priority for this project.
• New lower canopy bank planting on Palatine Terrace bank is also proposed.
• Works to address bank instability north of the pedestrian bridge are included in Stage 2 of this project.

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