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Diffuse sediment in Waikato waterways - sources, practices for reduction and policy options

TR 2012/02
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Report: TR 2012/02

Author: H Ritchie

Abstract

This report presents information on land use practices giving rise to sediment loss to waterways. The emphasis is on diffuse sources of sediment, rather than consented activities.

Sediment loss is of concern in waterways due to:

  • water quality impacts on in-stream aquatic ecosystems
  • water quality impacts on human uses and values of the water
  • downstream sedimentation, flow changes, and flooding which affect both people and the aquatic ecology.

There are also on-site issues when soil erosion occurs such as diminished productive capacity, infrastructure damage from slips and landscape effects.

This report reviews published information on sediment sources, and on the effectiveness, economic and practical impacts of land management practices to reduce sediment in waterways. Wider issues such as the water yield effects or social impacts
of large-scale afforestation are beyond the scope of this study.

Land use and sediment were reviewed in an earlier report on hill country resource management issues for Waikato Regional Council (Ritchie 2000). MAF has since published a comprehensive review of hill country erosion, information resources and social learning about this issue (Basher et al. 2008).

The main findings of those earlier reports relating to sediment loss are included in this current review, supplemented by
recent research.

Diffuse sediment in Waikato waterways - sources, practices for reduction and policy options [PDF, 630 KB]

Contents
  Executive summary 3
1 Introduction 5
2 Factors determining sediment loss 5
2.1 Sediment losses under different land uses 9
3 Sediment sources within a catchment 14
3.1 Critical source areas 19
4 Practices to reduce sediment loss 19
4.1 Forest or tree cover 19
4.1.1 Economic drivers and other considerations for forest cover 22
4.2 Soil conservation plantings, gully and earthflow stabilisation 23
4.2.1 Economic drivers and other considerations for soil conservation plantings 24
4.3 Riparian management 25
4.3.1 Stock exclusion 25
4.3.2 Buffer or filter effect 25
4.3.3 Riparian planting for bank stabilisation 27
4.3.4 Economic drivers and other considerations for riparian management practices 29
4.4 Bank protection, in-stream and channel works 32
4.5 Forestry practices 33
4.5.1 Economic drivers and other considerations for forestry practices 34
4.6 Pasture, soil and grazing managment to control run-off 34
4.6.1 Economic drivers and other considerations for pasture, soil and grazing management 35
4.7 Track design and management 37
4.8 Silt traps, drains and wetlands for sediment deposition 38
4.9 Crop managment 39
5 What can be achieved through catchment-scale work? 40
5.1 Waitomo Landcare 40
5.2 Whatawhata catchment project 40
 5.3 The "best dairying catchments" experience 41
 5.4 Modelling for the Waikato River catchment 42
6 Policy options 43
6.1 Education and voluntary adoption 44
6.2 Financial incentives 45
6.3 Farm plans and catchment management 46
6.4 Regulation 48
6.5 Relationship of national climate change policy and regional soil conservation policy 49
7 Summary and conclusions 50
7.1 What are the main drivers of sediment loss? 50
7.2 How site-dependent are the expected gains from mitigation practices? 51
7.3 How much sediment reduction can be achieved by different mitigation practices? 51
7.4 Will riparian work make a difference? 52
7.5 Do grazing practices matter? 52
7.6 Where can education, incentives and regulation be most effective? 53
7.7 What effect will carbon trading have on farmers' decisions? 53
7.8 Is it feasible to achieve contact recreation clarity standards? 54
8 References 56