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Opitonui Stream Suspended Sediment Analysis

TR 2005/45

Report: TR05/45

Authors: Michelle Wild, Murray Hicks (NIWA)


Over recent years, there have been concerns that suspended sediment (SS) from Opitonui Stream has impacted on the water quality of the Whangapoua Estuary. As a result, since 1991 Environment Waikato have sampled suspended sediment in the Opitonui Stream downstream of the Opitonui and Awaroa stream confluence to monitor the changes in sediment supply upstream of the confluence. The National Institute of Water and Atmospheric Research (NIWA) have analysed the data to assess whether any significant change in suspended loads and concentrations in the Opitonui catchment has occurred since monitoring commenced; determine whether forest harvesting has contributed to the change; and expand any findings to other sub-catchments in the Whangapoua catchment to estimate relative suspended sediment contributions from these streams. Also, NIWA advise on future monitoring for the Opitonui Stream and Whangapoua catchment.

Opitonui Stream Suspended Sediment Analysis [PDF, 1.1 MB]

Executive Summary i
1 Introduction 1
2 Literature review 2
2.1 Pre-1991 (before exotic-forest logging commenced) 2
2.2 Post 1991 - Catchment monitoring and hydrology studies 3
2.2.1 NIWA catchment monitoring 3
2.2.2 Hydrology of landuse changes in catchment 5
2.3 Post 1991 - Whangapoua Harbour monitoring 6
2.4 March 1995 rainstorm studies 8
2.4.1 Impacts on Whangapoua Forest landscape and streams 10
2.4.2 Impacts on the sediments, flora and fauna of Whangapoua Harbour 11
2.5 Other documents relating to forest harvesting 12
3 Suspended sediment monitoring program 14
3.1 Catchment summary 14
3.2 Available data 15
4 Analysis 16
4.1 Inspection of data 16
4.2 Sediment rating relationship 17
4.3 Hysteresis relationships between suspended sediment concentration and water discharge 21
4.4 Event sediment yields 22
4.5 Long-term variability in event and annual sediment yields 26
4.6 Comparison with Phillips et al. (2005) study 32
5 Conclusions on monitoring at Opitonui Site 33
6 Discussion on utility of snapshot sampling 35
7 Recommendations 37
8 Acknowledgements 38
9 Glossary 38
10 References 39