Te Awahōhonu

Te Awahōhonu Forest Trust, owns and operates two large scale sheep and beef farms and a 5780ha commercial forest. The Trustees are responsible for the management and protection of the Trust assets on behalf of approximately 1,500 owners.  The Trust appointed Interpine to comprehensively assess the effects of Cyclone Gabrielle. This interactive Story Map presents our findings, and shows how Matāurangi Māori can be enhanced with modern analysis’ 

‘Te mohoi ki te whenua’ (the return to the land)

The goals of this project were to:

• conduct a comprehensive damage assessment following Cyclone Gabrielle that caused unprecedented flooding and damage across parts of the North Island in early 2023;
• develop an interactive web interface that allows users to visualise the before-and-after impacts of the cyclone using satellite imagery and Lidar data;
• provide a geospatial risk assessment for potential soil migration in the event of future severe weather events.

The assessment covers the entirety of Te Awahōhonu, including native forests, planted forests, and the steep hill-country farming area, highlighting the areas that have been severely affected. The before and after cloud free storm imagery that could be sourced were captured on 4 Dec 2022 and 14 March 2023 and includes Cyclones Hale and Gabrielle.

 ^{Click on the EXPAND button in the top right corner of the map to view the map on full screen. Click on the EXIT full screen mode button to get back to this view. Zoom in and out of the map using the PLUS or MINUS buttons in the bottom right corner of the map. SLIDE the bar to compare the impact of Cyclone Gabrielle.} 

Pre-Cyclone Imagery Post-Cyclone Imagery

Interpine conducted change detection analysis to assess the damage caused after Cyclone Gabrielle to the Te Awahōhonu Trust lands. For the analysis, Interpine acquired satellite imagery from pre-cyclone (4th December 2022) and post-cyclone (14th March 2023) Gabrielle, and used these images to identify and quantify changes to land cover.

The dataset resulting from the change detection analysis was examined and refined to quantify the degree of soil movement. Additionally, a comprehensive desktop assessment was done to establish whether any of the observed damages were from windthrow trees, and if any roads were blocked or damaged.

Damaged areas not automatically captured by the satellite imagery analysis were added by Interpine’s analysts, using geospatial tools to produce accurate results. A variety of factors were considered, including the influence of slope and aspect (compass direction the downhill slope faces).

The analysis identified 226 hectares of affected area. A field survey followed, to validate the findings and provide ground-level images.

between 4 Dec 2022 and 14 March 2023

Only the areas which have been damaged by severe weather storms between 4th December 2022 and 14th March 2023 (post-cyclone Gabrielle) have been captured.

 ^{Click on the PLAY button in the map tour below to view the photos from the field survey. Click in the view and drag your mouse from side to side to see the 360° view. Once the map tour has opened, click on the MAP LOCATION button in the left bottom corner to see the map of where these photos were taken. Click on the location points (small orange points) on the map to take you back to the photos taken at that location.} 

A Risk Analysis was conducted to predict possible areas prone to soil migration. Detailed Elevation models, Slope, Soil type, Soil depth, Soil Drainage, Vegetation Type and Aspect (direction the downhill slope faces) were integrated into the spatial analysis.

Soil gradients were calculated and classified into 3 classes.

• gentle slopes 0-25°,
• moderate slopes 25-35° and
• steep slopes > 35°.

Steep slopes reduce the stability of soil and increases the risk of soil slips. Consequently, these areas are more LIKELY to experience soil migration, erosion or landslides. This risk analysis takes both moderate and steep slopes into account, considering the role of slope and aspect.

Through a weighted sum technique, we integrated Soil drainage, Soil depth, Soil type, Vegetation type, Slope and Aspect datasets to create a comprehensive map highlighting areas at high risk of erosion. The consideration of Aspect helps assess the influence of the slope's orientation on erosion susceptibility, providing a view of the landscape's vulnerability. This predictive model aids in identifying priority areas for targeted erosion control measures, supporting sustainable land management practices.

• This Trust area shows promise for Emissions Trading Scheme (ETS) inclusion, offering an opportunity for additional revenue through carbon credits. We recommend conducting an ETS eligibility assessment to confirm its viability.
• Also recommended is a risk calculation which combines the probability of a landslide with the severity of its consequences.
• We recommend a change detection analysis on pre-cyclone Gabrielle damage to calculate the damage from historic events to get an overall idea of loss within the trust block.
• We propose a land feasibility management assessment, focused on the planning, implementation, and monitoring of forestry practices and land management techniques that reduce the risk of landslides in the forestry and grassland areas.
• Effective risk management in slip prone areas should start with thorough hazard identification, focusing on landslide causes, historical occurrences, and geological features. Consider implementing risk reduction strategies such as reforestation, erosion control measures, and emergency response plans.
• Explore a financial model focused on cost avoidance.