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West Coast Hazard Project

TR 2008/14
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Report: TR 2008/14

Author: Tonkin & Taylor Ltd

Abstract

Tonkin and Taylor Limited (T&T) have been commissioned by Environment Waikato to undertake an assessment of coastal hazard information along the west coast of their region. The west coast region is an area where there is little knowledge of coastal processes and associated hazards. Therefore this assessment consisted of collating available information on coastal hazards and prioritising areas in need of further investigation.

A summary of the four district council plans (Franklin, Waikato, Otorohanga and Waitomo) revealed some differences in the way each council treated coastal hazards, even though they occupy similar coastal environments. The disparity is likely to be related to the differing current land use and zoning regimes within the coastal areas of each council and associated lack of information on coastal hazards.

Four major hazard processes (soft shore dynamics, cliff erosion/instability, inundation and mobile dune sand) were assessed along six distinct shorelines/environments. The predominant west coast environments are along the open coast and within the three main harbours (Raglan, Aotea and Kawhia). The open coast has been broken down into north of Raglan Harbour, Raglan to Aotea Harbour and south of Kawhia Harbour areas.

Investigations into existing knowledge on coastal hazards along the west coast show varying amounts of information. While some site specific areas have a good to high amount of detail and information, on a regional scale there is very little information.

Based on information from the four district councils, priority areas for further information on coastal hazards have been identified and they include:

  • Port Waikato
  • Raglan Township coastline (inner harbour)
    • The low-lying sandy area from Riria Kereopa Memorial Drive to Kopua Point is the critical area. There have been development plans for the Maori land in this area, which is Pa Zone.
    • The unprotected cliffs in Raglan township.
    • The protected/seawall areas around Raglan. Many seawalls are old and there will be calls for them to be replaced.
  • Kawhia Harbour
    • Kawhia township
    • Te Waitere
  • Aotea Harbour
  • Marakopa

Recommendations for future information gathering include the use of LiDAR information to better assess soft shore dynamics, coastal inundation and cliff instability. Analysis of water levels (particularly storm surge levels and mean level of the sea) along the coast and within harbours, video analysis of open coast sediment fluxes and identification of geological failures should be approached within an integrated methodology.

West Coast Hazard Project [PDF, 1.1 MB]

Contents
1 Introduction 1
2 Current planning environment 1
2.1 New Zealand Coastal Policy Statement 1
2.2 Waikato Regional Coastal Plan 2
2.3 Approach of adjacent regional councils 3
2.3.1 Auckland Regional Council 3
2.3.2 Taranaki Regional Council 4
2.4 District council objectives and policies 4
2.5 District council rules 5
2.5.1 Franklin District Council (FDC) 5
2.5.2 Otorohanga District Council (OtDC) 6
2.5.3 Waikato District Council (WkDC) 6
2.5.4 Waitomo District Council (WtD) 7
3 Hazard processes explained 7
3.1 Soft shore dynamics 7
3.2 Cliff erosion and instability 8
3.3 Coastal inundation 8
3.3.1 Coastal inundation within estuaries 9
3.4 Mobile wind blown sands 10
4 Geographic review of existing information 10
4.1 West Coast environmental setting 10
4.2 Open coast North Raglan 11
4.2.1 Soft shore dynamics 11
4.2.2 Coastal cliff instability 13
4.2.3 Wind erosion and mobile dune sands 13
4.2.4 Coastal inundation 14
4.3 Raglan Harbour 14
4.3.1 Soft shore dynamics 14
4.3.2 Coastal cliff instability 18
4.3.3 Coastal inundation 18
4.3.4 Wind erosion and mobile sand 18
4.4 Open coast Raglan to Aotea 19
4.4.1 Soft shore dynamics 19
4.4.2 Coastal cliff instability 20
4.4.3 Coastal inundation 20
4.4.4 Wind erosion and mobile sand 21
4.5 Aotea Harbour 21
4.5.1 Soft shore dynamics 21
4.5.2 Coastal cliff instability 23
4.5.3 Coastal inundation 23
4.5.4 Wind erosion and mobile sand 23
4.6 Kawhia 24
4.6.1 Soft shore dynamics 24
4.6.2 Coastal cliff instability 25
4.6.3 Coastal flooding 26
4.6.4 Wind erosion and mobile sand 26
4.7 Open coast South Kawhia 26
4.7.1 Soft shore dynamics 26
4.7.2 Cliff instability 32
4.7.3 Coastal inundation 32
4.7.4 Mobile sand and wind erosion 33
5 Priority areas for hazards assessments 33
6 Future information recommendations 34
6.1 LiDAR 34
6.1.1 Soft shore dynamics 35
6.1.2 Coastal inundation 35
6.1.3 Cliff instability 35
6.2 Water level analysis 35
6.2.1 Elevation datum 36
6.3 Video analysis (Cam Era) 36
6.4 Geology failure mechanisms 36
6.5 Integrated approach 37
7 Applicability 37
8 References 38
Appendix A: West Coast Areas
Appendix B: Tables