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  Services » Publications » Technical reports - by year » tr200536

Whaingaroa (Raglan) Harbour: Sedimentation and the effects of historical catchment landcover changes

Report: TR05/36
Authors: A Swales, R Ovenden, R Budd, J Hawken (NIWA), M S McGlone (Landcare Research Ltd, Lincoln), N Hermanspahn, M J Okey (National Radiation Laboratory, Christchurch)

Abstract

Whaingaroa (Raglan) Harbour is an important coastal resource for the Waikato Region. The community has expressed concerns regarding perceived environmental degradation of the harbour and its catchment.

Environment Waikato commissioned NIWA to collect and analyse sediment cores from Whaingaroa Harbour. The specific objectives of the study were to:

  1. quantify long-term sediment accumulation rates (SAR) under undisturbed native forest land cover before human settlement
  2. quantify changes in SAR associated with large-scale catchment deforestation (~1880-1925 AD) and subsequent conversion to pasture
  3. determine differences in SAR and sediment profiles in the Waingaro and Waitetuna arms of the harbour.

Results from radio-isotope and pollen analysis indicate that Whaingaroa Harbour had largely filled with sediment prior to human settlement. Consequently, all but the upper two metres of the present day sediment column was deposited before 6000 years BP and thousands of years before the arrival of Maori some 700 years ago.

The effects of large-scale catchment deforestation (1890-1920s), conversion to pasture and plantation forestry on sedimentation have occurred very differently in the Waingaro and Waitetuna arms of the harbour. Results from the Waitetuna arm indicate that pre-human sediment accumulation rates have increased threefold following deforestation since 1890. In contrast, results from the larger Waingaro arm indicate that long-term sedimentation has not occurred in this part of the harbour for at least the last 150 years and probably much longer.

Whaingaroa (Raglan) Harbour : Sedimentation and the Effects of Historical Catchment Landcover Changes
(1092 kb, 156 seconds to download, 56k modem)

Table of Contents

Executive summary iv
1 Introduction 1
1.1 Study background 1
1.2 Limitations of present knowledge 2
1.3 Study objectives 2
1.4 Catchment and harbour characteristics 3
2 Dating of recent estuarine sediments 9
2.1 Summary 9
2.1.1 137Cs dating 9
2.1.2 210Pb dating 11
2.1.3 Pollen dating 13
2.1.4 Radiocarbon dating 17
2.2 Sediment accumulation rates (SAR) 17
2.2.1 SAR from 210Pb dating 18
3 Methods 20
3.1 Sediment cores 20
3.2 Particle size 22
3.3 Bulk density 22
3.4 Pollen 23
3.5 Radioisotopes 23
4 Results 25
4.1 Overview 25
4.2 Core 1A: Waingaro Landing 26
4.3 Core 2B: Waingaro (intertidal flat) 29
4.3 Core 2B: Waingaro (intertidal flat) 29
4.4 Core 6A: Waingaro 32
4.5 Core 10B: Okete Bay 34
4.6 Core 12B: Waitetuna 38
5 Sedimentation in Whaingaroa Harbour 42
5.1 Harbour creation - shore platforms 42
5.2 Sedimentation processes 43
5.3 Study limitations 47
5.4 Harbour evolution - controls and changes 48
5.5 What does the future hold? - final comments 49
6 Acknowledgements 52
7 References 53
8 Appendix I: 210Pb Inventories and mean annual supply rates 59
9 Appendix II: Radiocarbon dating 60
10 Appendix III: Analytical results of Radioisotope analyses 61
10.1 Cores 1A, 2B & 6A Radioisotope results 61
10.2 Cores 10B & 12B Radioisotope results 65
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