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Mangrove Habitat Expansion in the Southern Firth of Thames: Sedimentation Processes and Coastal Hazards Mitigation

TR 2008/13
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Report: TR 2008/13

Author: National Institute of Water and Atmospheric Research Ltd (NIWA)

Abstract

Grey mangrove (Avicennia marina subsp australasica) or Manawa has rapidly colonised intertidal areas of the southern Firth of Thames during the last 50 years or so. Today, mangrove habitat occupies some 7 km2 of former intertidal flat between the Piako and Waitakaruru Rivers and 11 km2 in the southern firth as a whole (section 1). Grey-mangrove seedlings can colonise intertidal areas down to about mean sea level (MSL), where they are submerged for < six hours per tide. Mangrove-habitat expansion has occurred in many North Island estuaries in recent decades as sediments delivered by rivers has built intertidal habitat suitable for mangrove colonisation.

The Firth of Thames is an 800 km2 meso-tidal estuarine embayment. Currents, sea level and waves within the firth are strongly linked to oceanographic and meteorological processes occurring within the wider Hauraki Gulf (section 2). The low-lying coast of the southern firth is potentially exposed to erosion and inundation due to sea and swell waves, storm surge and tsunami.

Environment Waikato commissioned NIWA to undertake a study of mangrove-habitat expansion and sedimentation processes in the southern firth. The main objectives of the study are to:

  • reconstruct the historical sequence mangrove-habitat expansion based on historical aerial photography (1944-2005) and dated sediment cores
  • quantify sediment accumulation rates (SAR) and changes in SAR resulting from mangrove colonisation and habitat expansion
  • determine the role of mangrove habitat in mitigating coastal erosion and inundation hazards in the southern firth.

Please note, due to the large file size of this document, it has been split into two sections for ease of download.

Mangrove-Habitat Expansion in the Southern Firth of Thames: Sedimentation Processes and Coastal-Hazards Mitigation (Chapters 1-4) [PDF, 1.8 MB]

Mangrove-Habitat Expansion in the Southern Firth of Thames: Sedimentation Processes and Coastal-Hazards Mitigation (Chapters 5-10) [PDF, 1020 KB]

Contents
Executive summary iv
1 Introduction 1
1.1 Firth of Thames - mangrove-habitat expansion 3
1.2 Role of mangroves in coastal-hazards mitigation 3
1.3 Study objectives 4
1.4 Report outline 4
2 Study area and hazards 5
2.1 Physical setting 5
2.2 History 5
2.3 Hydrodynamic setting 6
2.3.1 Depths and mean level of the sea 6
2.3.2 Tides, currents and waves 6
2.4 Sedimentary setting 7
2.5 Tectonic setting and tsunami hazard 8
2.6 Storm-tide hazards 10
3 Methodology 14
3.1 Mangrove-habitat expansion 14
3.2 Shore-normal elevation profiles 14
3.3 Sedimentation 15
3.3.1 Sediment dating 17
3.3.2 X-radiographs 18
3.3.3 Sediment bulk density 19
3.3.4 Particle-size analysis 19
3.4 Tides and storm surge 20
4 Results 22
4.1 Mangrove-habitat expansion in the southern firth 22
4.2 Tides, storm surge and large-scale geomorphology 28
4.3 Sedimentation 31
4.3.1 How might mangrove-habitat expansion affect sedimentation? 31
4.3.2 Sediment-core physical parameters 32
4.3.3 Site LC-3 (old-growth forest) 33
4.3.4 Site LC-4 (old-growth forest) 36
4.3.5 Site LC-5 (old-growth forest) 37
4.3.6 Site LC-6 (old-growth forest) 40
4.3.7 Site LC-7 (recent mangrove forest) 42
4.3.8 Site LC-8 (recent mangrove forest) 44
4.3.9 Site LC-9 (recent mangrove forest) 45
4.3.10 Site LC-10 (recent mangrove forest) 46
4.3.11 Site LC-11 (2005 forest fringe) 49
4.3.12 Site LC-12 (mudflat) 51
4.3.13 Pollen profiles 52
5 Synthesis 54
5.1 Overview 54
5.2 Mangrove-habitat expansion 54
5.3 Sedimentation processes 57
5.3.1 Particle-size changes 57
5.3.2 Sedimentation rates 59
5.3.3 210Pb Budget and implications for fine-sediment fate 61
5.3.4 Pollen chronology 63
5.4 Role of storms in mangrove-forest development 64
5.5 Mangrove colonisation: triggers, controls and future habitat expansion 70
5.6 Large-scale morphological evolution 72
5.7 Coastal-hazards mitigation by mangroves 77
5.7.1 Coastal erosion 77
5.7.2 Coastal inundation 78
5.8 Future fate of the mangrove forest 85
6 Summary 87
7 Acknowledgements 93
8 References 94
9 Appendix one: dating of estuarine sediments 103
9.1 Overview 103
9.2 137Cs dating 103
9.3 210Pb dating 104
9.3.1 Pollen dating 106
9.4 Sediment accumulation rates (SAR) 109
9.5 SAR from 210Pb dating 110
9.6 Sediment mixing 111
10 Appendix two: pollen profiles 112