Sediment Loading Study
Introduction
Top of the South Wood Council (TOTSWC) is leading five forestry innovation projects funded by Te Hoiere Project. In Focus is the first in our series to examine these more in depth, beginning with the sediment loading study.
Who is involved?
Top of the South Wood Council, Te Hoiere Project, forest owners Marberry Estate Ltd, forest managers M&R Forestland Management Ltd, NIWA, Sediment Monitoring Expert Murray Hicks who is helping with the study and Jeremy Patterson-Green, Green Forest Management, who assisted with the initial identification of the study site.
Where is it?
Marberry Forest on Kenningtons Road near Okaramio. This 200ha block is 27 years old and is a first-rotation forest which means there has been no previous harvesting or earthworks activity. Its harvest is due to start at the end of 2025.
How long is the study for?
It started in June 2024 and will continue for 12 months to create a baseline data set.
What is the study’s aim?
The aim of the project is to measure the suspended sediment load from the Marberry catchment while it is under full forest cover prior to any harvest or earthworks taking place. The goal is to gain a better understanding of what influence land disturbance activities have on sediment load in a typical Te Hoiere forest.
Why this site?
The monitoring site required a permanently flowing waterway in a catchment with mostly exotic forest cover and due to be harvested in the next couple of years. The site chosen was the only waterway that met the study requirements.
It is in a waterway that is typical of those forestry companies work near, located in a small steep upper catchment tributary stream with legacy-planting (trees planted right up to the waterway with no buffers).
The soil is also highly erodible as commonly found in the Northbank, Kaituna and Pelorus catchments and the Marlborough Sounds.
How is the sediment load measured?
Sediment load is calculated as the product of water discharge and the mass concentration of suspended sediment (SSC).
Since directly measuring suspended sediment is not possible, turbidity (the clarity of the water) serves as a useful indicator using a turbidity sensor. This study focuses on measuring turbidity, SSC and sediment composition to gain insights into sediment load.
A relationship between turbidity and SSC is developed by collecting field samples, using an automated sampler, and analysing them in the lab for SSC by weighing the sediment captured on filter paper. Total SSC is then analysed to work out the mud/sand and organic/mineral content to better understand and characterize the suspended sediment coming from the forest.
A weir is used as it provides a stable stream bed and a consistent pool of water into which the monitoring sensors can be installed.
When is the data captured?
Water level and turbidity are measured continuously at 5-minute intervals using a data logger and the data is sent every 10 minutes to NIWA.
The SSC samples are triggered to be taken during specific rain events with sampling timed to capture the increasing flow expected in the stream. The autosampler collects 1-litre sample volumes every 30 minutes.
Since the weir was put in, four rain events have been successfully captured with a good-sized rain event at the end of October.
What happens next?
TOTSWC is keen to continue the study while the trees in the catchment are harvested so that this data can be compared to the baseline study. This will be dependent on funding.
TOTSWC’s Angela Mackenzie says:
A usual commercial forestry rotation is approximately 27 years and for most of that time very little land disturbance takes place.
Gaining an understanding of sediment load in a local context, prior to heightened land activity such as earthworks and harvesting, provides a valuable insight into forestry in the region. It will give a baseline for comparison when land disturbance is increased at harvesting and then declines again as the next rotation is established and grows.
This project has the potential to provide insights and learnings into land disturbance and management both within primary industries and beyond.
