Project - Erosion - Marlborough District Council

Debris Flow Analysis

The Marlborough District Council holds responsibilities under section 6 of the Resource Management Act 1991 to manage significant risks from natural hazards as a matter of national importance. Council commissioned the University of Canterbury and Scion Research in July 2021 to provide analysis of risks around debris flows in forestry. The scope of this work has since widened with the 2021 and 2022 storm events to encompass all of North Marlborough. The purpose of the Debris Flow Analysis is to provide a detailed map of debris flow and debris flood susceptibility to further Marlborough District Council’s understanding of landslide hazards within the defined study area of North Marlborough.

Debris flows are intense sediment-laden flows that occur in steep catchments and can be triggered by heavy rainfall causing slope failures which are especially hazardous in the steep terrain of North Marlborough.

The result of this project is intended to inform landowners or prospective land developers on the potential susceptibility of upslope land to failure and production of debris flows or floods. Note that the data layer is not intended as a definitive regulatory tool, nor is it intended for use on property LIMs. It is provided for information and to prompt further geotechnical investigation where indicated. The work is currently undergoing a verification process and is due for completion by the end of 2023.

GNS storm reporting (multi-variate analysis)

Two recent significant storms Marlborough experienced in July 2021 and August 2022 triggered widespread and damaging landslides which left multiple roads and houses impacted.

With the assistance of the GNS Landslide response team, the two major storm events resulted in multiple technical outcomes. Over 7500 landslides were mapped across the province.

GNS have provided two reports for Marlborough for the two major weather events in 2021 and 2022. These cover landslide distribution, field assessments of property damage and landslide characteristics. These reports show that even small catchments in Marlborough can generate damaging debris flows and have provided us with the likely locations where major landslides will occur again.

The Phase II report has indicated that further work is required to better understand landslide dynamics in the region. This work is currently being considered and is likely to include re-training of the GNS Rainfall Induced Landslides model, further multivariate analysis of the 2021 and 2022 storm data and development of a landslide risk assessment for North Marlborough.

Phase II: Remote mapping of landslides triggered by the July 2021 and August 2022 Marlborough storms, and selected field investigations of landslide impact – GNS Science Report July 2023

River Geomorphology

Under the Te Hoiere Project Marlborough District Council has received funding from Ministry for the Environment to help address sediment and flooding issues in the Te Hoiere catchment. the first step in this work is the help of Jon Tunnicliffe from The University of Auckland to provide Geomorphic and Connectivity Maps for the Te Hoiere/Pelorus, Rai, Ronga, Opouri, Tunakino, Wakamarino and Kaituna Rivers.

This work is intended to:

Improve spatial and temporal understanding of the sources and sinks of sediment within the catchment
Identify important sources of streambank erosion, both historic sources and where sediment may come from in the future.
Map sediment connectivity (hillslope to river) along major river corridors
Provide a set of Geographic Information Systems (GIS) layers that can be used to help prioritise restoration action and funding allocation for sediment mitigation works

The first step in this work has been largely completed with some refinements currently underway.

Image 1: Light pink are hillside sediment deposits. Mid-pink colour indicates fan deposits from Alfred Stream. Dark pink indicates older terrace deposits. Greens are alluvial plains with darker hues being younger deposits. Black lines indicate old features from previous river movements.

Image 2: Imagery of Rai Valley. Black lines are drawn from a LiDAR Relative Elevation Model (REM). Dark blue is river height and red is high above the river. Old river meandering can be seen in the coloured traces delineated in the REM.

Image 3: Imagery of Rai Valley showing modelled future river meanders. From this modelling it seems likely that the two large meander bends will continue to push outward and eventually the river will avulse to a new course or reoccupy former channels.

The next stage in this work is engagement with the community around the use of river management. River management is a complex practice that often requires significant public and private expenditure, strategic whole of catchment planning and community buy-in. Council plans to address this in the coming year.

Nature Based Solutions Flood Management

The rivers and sub-catchments of the Te Hoiere/Pelorus catchment are regularly subject to major flooding events, most recently in 2021 and 2022 resulting in disruption to roading links (SH6), significant damage to property and community isolation requiring large-scale emergency responses.

The River Geomorphology Study carried out by Jon Tunnicliffe in the Te Hoiere catchment is an example of valuable, science-led information being generated from a coordinated and integrated approach. The goal of the geomorphic modelling was to better understand river migration dynamics and inform a flood management plan. In addition to the river management process outlined above, Council has agreed to participate in a national-level science project seeking to inform Nature-based solutions for river management. All councils around the country are participating in this work coordinated by the Ministry for the Environment. In the Te Hoiere catchments aNature Based Solutions feasibility study will undertake whole-of catchment rainfall-runoff and hydrodynamic inundation modelling, based on a series of actual rainfall events and projected climate change scenarios. The modelling scenarios tested would include:

Construction of runoff attenuation features (RAFs) or water storage areas (such as detention bunds, cross-slope swales
High-flow storage
Wetlands
Leaky barriers
Targeted afforestation or native restoration
Increasing agro-forestry
Reconnection of rivers to paleochannels and increased wetland restoration

The aim of the work is to understand if it is possible to ‘flatten the hydrograph’ or reduce storm peak flows in the rivers. This is likely to reduce erosive power in the rivers but will require coordinated action by landowners across the catchment in addition to conventional river management.

The Nature Based Solutions approach has been adopted in large scale in Europe and America but is less well understood in a New Zealand context. The national-scale of the project is seeking to rapidly improve our understanding of this approach in New Zealand catchments and farming systems. This work is set to be completed April 2025.