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Coastal erosion in the inner harbour

Coastal erosion is the loss of land due to coastal processes such as waves and tidal currents wearing away that land suddenly or overtime. Within the inner harbour this can happen in two ways, the slow erosion of shorelines, or cliff/bank slips due to slope instability.

Erosion is influenced by the effects of climate change, such as sea level rise, changes in weather patterns and the increasing intensity of rainfall. A further explanation of harbour erosion and the factors that can influence it is available by viewing the infographic below:

Inner harbour erosion infographic (615kb pdf)

Climate change and sea level rise

Recent Natural Hazard mapping for the Tauranga Harbour

We engaged Tonkin and Taylor (T&T) to undertake a Tauranga Harbour wide assessment of the erosion potential of the shoreline and cliff faces. This research considered how these areas would erode over time dependent on the extent of sea level rise.

In our study we took a range of sea level rise scenarios into account. These scenarios are based on the recently released Guideline on Coastal Hazards and Climate Change from the Ministry for the Environment. This document outlines a recommended adaptive planning assessment that Councils can use in the consideration of sea level rise and when/how to use differing sea level rise scenarios in natural hazard planning.

Guidance on Coastal Hazards and Climate Change - Ministry for the Environment

Analysis of this study is based on mapping two things:

  1. areas of current slope instability and erosion, and
  2. future erosion and instability hazards based upon sea level rise scenarios. These are mapped as likelihoods (or probabilities) of the cliff or shoreline regressing landward due to slope instability/erosion and the projected sea level rise.

The results are a range of potential erosion hazard distances, varying from ‘likely’ to ‘very unlikely’ when considering current slope instability or erosion, timeframe and sea level rise scenarios out to the year 2130.

Coastal hazards and risk assessment - Tonkin+Taylor (4mb pdf) Appendix A - Site assessments

Erosion Mapping Viewer

This viewer gives you the ability to select a specific property, and find both the current land instability and erosion risks as well as choose a sea level rise scenario and see what the predicted erosion hazard could be over time.

There are a number of variables that influence erosion risks for a specific property, over time, including:

  • Slope
  • Level of (pre)existing erosion at that point of time
  • Height of sea level rise
  • Storm event
  • Soil characteristics
  • Existing sea protection structures

It is important to note that different properties are exposed to different levels of probability that the hazard will in fact occur, including the likelihood of each projected sea level rise scenario occurring. The reports and modelling results apply to wider areas and may be superseded by site specific assessments by qualified professionals who use improved or more detailed data than presented in this study or by the presence of erosion protection structures.

After opening the viewer, click on the magnifying glass symbol at the top right, then enter the address for the property you would like to see details for.

Erosion Mapping Viewer

2015 High Level Coastal Erosion Study

In 2015 a joint study between Tauranga City Council, Western Bay of Plenty District Council and the Bay of Plenty Regional Council was run. Part of a wider hazard research program, it looked at erosion processes on inner harbour landforms (cliffs/shores) and estimated erosion rates. Included in the study are parts of Matua, Hairini, Maungatapu and Whareroa. The completed study produced high level erosion risk maps in order to provide an understanding of financial risk to properties in the above areas from coastal erosion. The maps show this risk as either low, medium or high over two timeframes, 20 years (2035) and 100 years (2115).

Tauranga Inner Harbour Erosion Report

Frequently asked questions

The Tauranga harbour erosion hazard maps identify land surrounding the City’s inner harbour that is potentially at risk of instability and erosion by the sea. The maps are a result of modelling that has been completed by a partnership between Tauranga City Council, Western Bay of Plenty District Council and Bay of Plenty Regional Council.

The maps show current slope instability, the potential extent of harbour erosion areas over a range of sea level rise (SLR) scenarios and time periods as far as the year 2130. The maps that indicate potential future areas at risk are based on sea level rise scenarios as recommended for use by the Ministry for the Environment.

The harbour erosion maps provide an illustration of a range of land instability and erosion distances for both cliff and low-lying shorelines around Tauranga Harbour. The erosion hazard areas are defined using a probabilistic approach (based on the likelihood of certain circumstances happening).

Erosion hazard distances have been mapped for various timeframes and possible sea level rise scenarios out to 2130.

Analysis of this study is based on mapping both areas of slope instability and future erosion based upon sea level rise scenarios.  This is mapped as likelihoods (or probabilities) of the cliff or shoreline regressing due to slope instability and the projected sea level rise. The outcome is a range of potential erosion hazard distances, varying from ‘likely’ to ‘very unlikely’ when considering current slope instability, and sea level rise scenarios out to 2130. 

The information from the Inner Harbour Erosion study will be used in the following council processes:

  • Future land-use planning, to aid in planning for and understanding the effects of natural hazards and sea level rise on existing land-uses, potential subdivisions and growth management.
  • Current and future infrastructure planning, in the consideration of upgrades to existing infrastructure, provision of infrastructure and understanding the long term effects on the operation and performance of infrastructure.
  • Building consent processes, and applications for new building consents.
  • Subdivision consent processes, and applications for new subdivisions.
  • Land information memoranda.
     

All new or altered buildings which fall wholly or partly within the erosion data set of the 2130 timeframe, P5% scenario with 1.25m of sea level rise should be applied when assessing building consents and subject to an assessment under sections 71 through 73 of the BA04.

For any application for a new subdivision, council is required to assess the risk of the proposal. This includes the likelihood of a natural hazard event occurring, as well as the potential consequences. All new subdivisions which fall wholly or partly within the erosion data set of the 2130 timeframe, P5% scenario with 0.8m, 1.25m and 1.6m of sea level rise should be subject to an assessment of risk under section 106(1A)(b) and (c) of the RMA.  

The Council has an obligation to make hazard information it holds available to the public under the Local Government Official Information and Meetings Act 1987. It will be made clear on any Land Information Memorandum (LIM) requested for a property within a hazard area that Council holds information in respect of coastal erosion for the property which has been derived from a harbour hazard study.

It is important to note that the likelihood of a hazard occurring is different for every property, including the likelihood of each sea level rise scenario. The reports and modelling results apply to wider areas and may be superseded by site specific assessments by qualified professionals who use improved or more detailed data than presented in this study or by the presence of erosion protection structures.

The council is required to make this information available in LIM’s. However, we cannot advise you about any effect this information may have on property value or insurance. We recommend you seek professional advice from a property valuation or insurance expert about any concerns you may have.

While protection structures (like retaining walls) can mitigate the impacts of coastal erosion of the land contained, they don’t address the erosion of the shoreline surrounding the structure. Structures therefore require continuous maintenance and renewal to be effective over a long term. For the purpose of this erosion hazard study, all protection structures (except specific structures around the Port and critical roading infrastructure), have been ignored. 

The justification for this approach is:

  • The quality and design standards of protection structures is unknown and variable.
  • The limited lifespan of protection structures and the potential they are not renewed in future.
  • Potential of events larger than that which the structure is designed for (e.g. a once in a 100-year storm).
     

Council plans to communicate natural hazard information in a range of ways: 

  • Letters to landowners.
  • Website updates.
  • Property files.
  • Land Information Memoranda (LIMs).
  • Opportunities to e-mail, phone or meet with council staff. 
     

Now that we have this information, it allows us to revisit our approach to addressing the potential impacts of inner harbour erosion considering possible future sea level rise. This will include reviewing our coastal structure policy (i.e. how council involves itself in sea walls), consideration of regulatory provisions to manage future development within these risk zones and how to best adapt to changes in hazard information in the future.

Mapping out a process to consider, change current practices and build a long term strategy to managing sea level rise, along with its effect on natural hazards will take time. The council will start working on developing a progressive strategy, which will consider options of defending, adapting and/or retreating from areas long term. We have started this process by an initial understanding of the potential effects, and by sharing this information with the public.

Tonkin and Taylor Appendix A  - Site assessments

Bethlehem (1.6mb pdf)
Matapihi (1.7mb pdf)
Matua (1.9mb pdf)
Maungatapu (2.1mb pdf)
Otumoetai (1.8mb pdf)
Pilot Bay (1mb pdf)
Tauranga East (2.1mb pdf)
Tauranga Wast (1.7mb pdf)
Te Maunga (1.6mb pdf)
Waikareao (2mb pdf)
Waimapu (1.7mb pdf)
Waitao (1.7mb pdf)
Welcome Bay (1.6mb pdf)
Whareroa (1.9mb pdf)


Last Reviewed: 09/08/2019
 

 
 

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