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The potential for debris flows from Karaka stream at Thames, Coromandel

Report: TR06/17
Authors: M.J. McSaveney and R.D. Beetham - Institute of Geological and Nuclear Sciences Limited (GNS Science)


As part of the resource consent process for the new development proposed at Thames Hospital, Environment Waikato were concerned, given the recent Matata event, about the potential for debris flows to occur within the Karaka Stream catchment that could affect the hospital building and adjacent properties.

To address these concerns, it was agreed with the Waikato District Health Board and the Thames Coromandel District Council that a report be commissioned from Geological and Nuclear Sciences Limited (GNS) to investigate the hazard further. You can download a full copy of this report below.

The Potential for Debris Flows from Karaka Stream at Thames, Coromandel
(789 kb, 112 seconds to download, 56k modem)

Table of Contents

Executive summary III
1.0 Introduction 1
1.1 Purpose and scope of the report 3
1.2 The brief 3
1.3 Terminology used 4
1.4 Debris flows and lahars 10
1.5 Relevant terms in New Zealand statutes 10
1.6 Relevant New Zealand standards for structures (AS/NZS 1170.0:2002) 12
2.0 Assessment of catchments and processes contributing to debris flows 13
2.1 Geology 13
2.1.1 Mantling deposits around Thames 13
2.1.2 Kuaotuna formation 13
2.1.3 Waiwawa formation 13
2.1.4 Fans and stream deposits 16
2.1.5 Anthropic deposits 16
2.1.6 Source of the boulders 16
2.1.7 Faulting in the Karaka Stream catchment 16
2.1.8 Regional faulting and earthquakes 16
2.2 Geomorphology of Karaka Stream 17
2.3 Other streams at Thames 18
2.4 Effects of geology on headwater slope stability 19
2.5 Storm rainfalls 19
2.6 Land cover and effect of vegetation on stability and erosion potential 19
2.7 Sediment transport processes active during big storms 20
2.8 Debris flows caused by damming of streams 21
2.9 Channel erosion 22
2.10 Previous debris flows, and debris-flow recurrence interval at Thames 22
3.0 Quantity of solid material and debris likely to be delivered from Karaka catchment 25
4.0 Likely response to future hydrological events 26
4.1 In the immediate term 26
4.2 In the short term (up to 10 years) 26
4.3 In the long term (up to 1,000 years and beyond) 26
5.0 Areas at risk from debris flows and debris floods from Karaka Stream 27
6.0 Options to mitigate risk and minimise the area affected by future debris flows 30
6.1 Regional and local warning systems 31
6.2 Mitigation options at Karaka Stream 32
7.0 Conclusions 34
8.0 Recommendations 35
9.0 Acknowledgments 36
10.0 References 37