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Peat subsidence near drains in the Waikato region


Report: TR05/40

Authors: Neil Fitzgerald, Malcolm McLeod (Landcare Research), Sarah Basheer (Environment Waikato)


The majority of the once extensive peat wetlands in the Waikato have been drained and converted to agriculture and horticulture, a process that inevitably leads to peat shrinkage and surface subsidence. Subsidence occurs due to mineralisation of the organic material and consolidation, and is most pronounced close to deep drains where the drainage effect is greatest.

Obtaining information on subsidence rates and the extent of subsidence from drains is important for future land-use management and the development of mitigation strategies to reduce subsidence rates and CO2 emissions from peat mineralisation, and to protect sensitive wetlands set aside for nature conservation.

During February and March 2005, Landcare Research, Hamilton, measured land surface profiles of farmed peat soils in the Waikato Region and modelled the relationship between peat subsidence, drain depth, and distance from drain, for Environment Waikato (Waikato Regional Council). The objective was to model peat subsidence in relation to drains of various depths so that the lateral influence of drain depth on peat subsidence can be estimated, as well as the amount of subsidence at any distance from the drain.

Peat Subsidence Near Drains in the Waikato Region [PDF, 712 KB]

Acknowledgments i
Summary v
1 Introduction 7
2 Background 7
3 Objective 7
4 Methods 7
1.4 Statistical evaluation 5
5 Results 8
5.1 Model development 9
5.2 Correlation of the model with measured subsidence profiles 13
5.3 Drain depth estimation 22
6 Conclusions 23
References 25
Appendix I: Transect grid references 27
Appendix II: Transect location maps 28
Appendix III: Subsidence profiles and regression curves of 21 transect sections 32