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Waikato suspended sediment indicators: State and trend

TR201443

Report: TR 2014/43

Author: J Hoyle (NIWA)

About this report

Waikato Regional Council  monitors suspended sediment at various sites across the region to assess the impact of land use on water quality and estimate sediment loads from the monitored catchments.

This report provides Waikato Regional Council with up-to-date regional indicators of state and trend for suspended sediment based on 24 monitored catchments in the Waikato region.

State is reported by mean annual suspended sediment yield. NIWA last calculated sediment yields for 23 catchments in the Waikato region in 2011. This report provides an update of those calculations using flow and suspended sediment records updated through to December 2013 and the rating relationships developed by NIWA in 2011. An additional catchment has been added in this study (Ohinemuri at Karangahake),

Trends are reported for all 24 catchments. For the nine catchments where updated suspended sediment data were available, these trends have been updated to the end of 2013. Trends for the remaining 15 catchments are based on analysis of data from the 2011 study.

Four catchments showed a statistically significant increase in suspended sediment yields over time. These sites were: Matahuru Stream at Myjers, Waitomo at Aranui Caves Bridge, Wharekawa at Adams Farm Bridge and Mangatutu Stream at Walker Road Bridge. Four catchments showed a statistically significant decrease in sediment yields over time. These sites were: Waipa at Otewa, Waihou at Okauia, Piako at Paeroa Tahuna Road Bridge and Waitoa at Mellon Road Bridge. The remaining catchments showed no statistically significant trend over time.

These trends may reflect changes in land use, but can also be influenced by sediment supply variations stemming from large storms. We note that all the sites that are showing increasing trends over time are those with up-to-date monitoring, indicating that Waikato Regional Council are focusing their monitoring efforts on the appropriate catchments.

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Waikato suspended sediment indicators: State and trend (2mb)

Contents
  Executive summary 
1 Introduction
1.1 Background and purpose
1.2 Objectives
1.3 Catchment locations
1.4 Data availability for sediment yield analysis
1.5 Catchment characteristics
2 Analysis methods
2.1 The sediment concentration rating approach
2.2 The storm event sediment yield rating approach
2.3 Testing for time trends
3 Results
3.1 State of suspended sediment yields in Waikato region
3.2 Trends in suspended sediment yields in Waikato region
4 Conclusions and recommend further work
5 References
  Appendix A: Rating relationships
  Appendix B: Residual plots showing trends with discharge