Coastal Erosion Hazard Shoreline Projection This information consists of two parts which should be read together: • maps indicating projected future shorelines based on a coastal erosion risk assessment (attached) • the following explanation of the coastal erosion risk assessment(s) relating to this property.
Background The Resource Management Act 1991 requires all councils to assess coastal erosion hazard risks. Between 2005 and 2012, the Council carried out an assessment of these risks to Kāpiti. In 2010, during the assessment, central government released the New Zealand Coastal Policy Statement (NZCPS) which includes a policy (no. 24) requiring councils to adopt a 100-year time horizon in planning management of their coastlines. The NZCPS can be viewed at http://www.doc.govt.nz/publications/conservation/marine-and-coastal/new-zealand-coastal-policystatement/new-zealand-coastal-policy-statement-2010/. Detailed reports relating to the Council’s assessment can be viewed at http://www.kapiticoast.govt.nz/District-Planning/Coastal-Hazards/ or in hard copy at Council Libraries and Service Centres. Note 1: Assessment of the coastal erosion risk is carried out as part of the District Plan Review as required by legislation and the New Zealand Coastal Policy Statement. Various factors which contribute to the assessment, such as the climate change and coastal processes science, may continue to be updated. This may, in turn, lead to updates to this coastal erosion risk information over time. Note 2: The New Zealand Coastal Policy Statement requires the adoption of a precautionary approach in relation to the coast (Policy 3). Consequently, the risk assessment is based on a worst case scenario. Note 3: This erosion assessment has been carried out at a detailed local level but property owners have the option of commissioning more detailed site-specific assessments which may further refine the projected shorelines. Note 4: The risk assessment does not take account of privately owned protective structures (e.g. seawalls) on the basis that the Council has no ability to ensure current or future owners maintain those structures so they continue to provide their current levels of protection. The shoreline is defined as the most landward extent of active processes on a natural beach. That landward extent is identified by the edge of coastal vegetation. Active processes include erosion caused by storms so the shoreline will be further inland than the regular high tide. The total assessment considers erosion risks at 50 and 100 years for: • open coasts • river/stream mouths (inlets).
Potential shorelines have been established for 50 and 100 years based on: • the continuation of current publicly managed controls (public sea walls, stop banks and measures to manage river/stream mouths) • the absence of any publicly-managed controls.
Assessment methodology The erosion hazard risk assessment, which generated the projected shorelines shown in these maps, uses an empirically-based approach which identifies the projected future shoreline as a result of coastal hazard processes by summing several components. In particular, these components consist of: 1) longer-term historical shoreline change(LT) which is derived by statistical analysis of up to 135 years of data (depending on location) 2) shorter–term shoreline change (ST) which is also defined with respect to records of past shorelines 3) retreat associated with anticipated acceleration in sea-level rise from global warming (SLR), which is derived via a shoreline adjustment model that used the substantial beach profile data-set held by the KCDC 4) retreat of a dune scarp (DS) (formed by undercut by storm waves) to achieve a stable slope, which is based on a slope stability model that used the KCDC’s three-dimensional LIDAR (Light Detecting and Ranging) data 5) a combined uncertainty (CU) term which provides an acceptable safety margin. The methodology for the open coast projected shorelines is: Projected Shoreline = LT + ST + SLR + DS +CU The methodology for the river and stream mouth (inlet) area boundaries is: Inlet area boundary line = IM – (LT + SLR + DS + CU) where IM = inlet migration. The remaining terms are as defined above and the negative sign refers to adjustment in a landward direction. The inlet area boundary line has an additional factor of inlet migration based on historic shoreline data. Inlet migration refers to the tendency for river and stream mouths to migrate up and down the coast over time if unmanaged. Note 1: no value for accretion of the shoreline has been included in these equations. It is acknowledged that the shoreline north of the Waikanae River mouth and, to a lesser degree, at the northern end of Paraparaumu has been undergoing long term accretion with shorter periods of erosion. However, current understanding of the causes of this is very imperfect. There are no tested and reliable models or analyses which could be used with confidence to assess whether this process will continue or reverse under the impact of sea level rise and increased storm intensity. As a consequence, and in line with accepted best practice and the required precautionary approach, the risk assessment has considered this process but has not assigned a numerical value to it in the methodology. Note 2: upper level estimates of sea level rise over the next century, which may exceed the 0.9 m figure used in the projections and therefore lead to more erosion than estimated have not been
considered in this analysis. Sea level rise has only been considered over the next century; best present information suggests sea level rise will continue for several centuries. Note 3: A precautionary approach to existing uncertainties has been adopted as required by the NZCPS but has not been unduly precautionary. Ongoing monitoring and research will be conducted and the precautionary assumptions can be adjusted in future reviews if this is warranted. In addition to using the highest quality raw data that was available, the assessment used the most recent developments in image processing, data abstraction and statistical analysis, thereby ensuring robust output.
Open coasts where public management measures are in place in 2012 The following information refers to projected shorelines where public management measures are in place in 2012 and those measures continue for some time. These are labelled as ‘managed’ shorelines on the map(s).
Fifty-year time frame The 50-year managed lines on the maps represent the projected shoreline if existing public sea walls and other protective structures are maintained in their current configuration for the next 50 years. It is assumed that: • where some parts of the public sea walls or other protective structures fail, they are quickly repaired • within 50 years, very heavy storms or other coastal processes cause the entire public sea wall(s) or other protective structures to fail and be repaired • no new hard protection structures are built • existing structures are not made higher or extended • the sea level rises by 0.3 m. Note 1: Where part of a sea wall or other protective structure fails, some local coastal erosion could occur before repairs are carried out, particularly if further storms follow in quick succession. This means the shoreline could gradually retreat in parts. Note 2: Should a future Council decide not to (or be unable to) maintain hard protective structures, the projected shorelines associated with the unmanaged scenarios would apply. Note 3: It is the Council’s current policy, as noted in the Long Term Plan 2012, to maintain current protective structures as long as is practicable but not to replace them when they fail.
One hundred-year time frame There is no 100 year ‘managed’ shoreline for open coasts because: • current sea walls are assumed to fail in the 50 to 100 year time frame • hard protective structures would have to be built progressively higher and stronger to protect the shoreline from erosion. This is not considered to be practicable and the risk of failure would still remain • in line with the New Zealand Coastal Policy Statement, Council has signalled in its Long Term Plan 2012 that it intends to move away from hard protective structures on the coast once current structures fail. Consequently, there is only an ‘unmanaged’ 100 year projected shoreline for all open coasts.
Open coasts where no public management measures are in place in 2012 The following information refers to projected shorelines where there are no public management measures in place in 2012. These are labelled ‘unmanaged’ shorelines on the map(s). Note: In line with the New Zealand Coastal Policy Statement and the Greater Wellington Regional Council’s (GWRC) Regional Policy Statement, it is Council’s policy not to introduce new hard protection structures on the coast.
Fifty-year time frame The open coast north of Paraparaumu is unmanaged at present (i.e. no publicly owned hard protective structures are in place), as are parts of the open coast south of Paraparaumu. The unmanaged 50 year projected shoreline assumes that: • the unmanaged scenario continues • the sea level rises by 0.3 m.
One hundred-year time frame The 100 year unmanaged shoreline projection assumes that: • the unmanaged scenario continues • the sea level rises by 0.9 m.
Stream/river mouths (inlets) Currently, GWRC straightens the channel at the mouths of the Ōtaki and Waikanae Rivers and the Waitohu, Mangaone and Waimeha Streams, and the Hadfield Drain. It also has engineered structures such as stop banks in some locations which it maintains. Kāpiti Coast District Council has received confirmation from the Regional Council that it has no plans to change this practice in the foreseeable future (subject to final development of the Natural Resources Plan, any consenting decisions and any unanticipated significant natural disaster). Should this management be discontinued, properties in the shaded areas on the accompanying map(s) could be affected by coastal erosion within 50 to 100 years. Additionally, Kāpiti Coast District Council manages some other stream mouths on an as-needed basis. There are budgets in its Long Term Plan 2012 to continue this as-needed management of those stream and drain mouths not managed by GWRC for the next 20 years and there is no intention to change that practice. However, this management of stream mouths is governed by the GWRC Regional Plan which is currently being reviewed. This review may result in changes to KCDC operations if required by the Regional Plan.
What do the maps show? The accompanying map(s) show 50 and 100 year projected shorelines based on a continuing managed scenario. Behind these lines is a shaded area which indicates an area which could be at risk of coastal erosion over the next 50 to100 years if the management regimes of GWRC and this Council are discontinued and the sea level rises 0.3 m over 50 years and 0.9 m over 100 years.
Scenarios represented by the projected shorelines Two scenarios were considered for the risk assessment: one where this management regime continues and one where it does not. The second scenario was considered because there can be no absolute guarantee that GWRC or any other council will continue the current management regime of inlets on the 50 or 100 year time scales. The two projected shorelines shown are underpinned by the following assumptions: Fifty-year time frame - managed The managed 50 year projected shoreline assumes that: • the councils’ management regimes continue • the sea level rises by 0.3 m. One hundred-year time frame - managed The 100 year managed shoreline projection assumes that: • the councils’ management regimes continue • the sea level rises by 0.9 m.