Hillslope erosion

What is hillslope erosion?

Hillslope erosion is a natural process that involves the detachment and movement of soil and rock fragments down a slope. The eroded material is then transported to other locations by water, wind or gravity.

Hillslope erosion is the major form of water erosion. It includes sheet and rill erosion, where water flows over the soil surface and creates small channels.

Hillslope erosion has been identified as a significant emerging threat in New South Wales. We need to measure and monitor erosion rates to protect and conserve our lands and biodiversity.

Why is hillslope erosion a problem?

Hillslope erosion negatively affects soil quality, agricultural productivity, water quality and biodiversity. It can also damage infrastructure and cause economic losses. Some of the impacts of hillslope erosion are:

• Loss of topsoil: the most fertile layer of soil is removed, reducing land productivity and soil health.
• Sedimentation: eroded material can accumulate in rivers and other water bodies, affecting water quality and ecosystems.
• Infrastructure damage: roads, buildings and other structures can be undermined by erosion.
• Economic losses: reduced agricultural yields and increased infrastructure repair and sediment management costs.

In areas where the rate of soil loss is greater than that of soil formation, it is critical to develop efficient erosion control measures to sustain soil condition and capability.

What causes hillslope erosion?

Hillslope erosion is a complex process influenced by a combination of factors, including weathering, climate, slope steepness and vegetation cover. Understanding these factors is essential for predicting erosion rates and implementing effective soil conservation practices.

Some of the primary causes of hillslope erosion are:

• Weathering: This is the breakdown of rocks and minerals at the earth's surface through physical, chemical and biological processes. Weathering plays a crucial role in hillslope erosion because it makes the soil and rock more susceptible to detachment and movement.
• Climatic conditions: Climate significantly affects weathering rates and the intensity and frequency of erosive events, such as heavy rainfall. Changes in climate, such as alterations in rainfall patterns, can lead to changes in hillslope erosion rates.
• Slope steepness: The steepness of a slope is directly related to the potential for erosion. Steeper slopes have a greater gravitational force acting on them, which can lead to increased rates of soil movement and a higher likelihood of landslides or other mass-wasting events.
• Vegetation cover: Vegetation cover is a critical factor in controlling hillslope erosion. Vegetation can protect the soil surface from the impact of raindrops, which can dislodge soil particles and initiate erosion. Plant roots also help bind and stabilise the soil, reducing the likelihood of soil movement. Areas with less vegetation cover are more susceptible to erosion because there is less protection for the soil surface and the binding effect of roots is diminished.
• Land clearing: Human activities can accelerate the erosion process. For example, deforestation, overgrazing and improper land use can remove the vegetation cover that protects the soil and reduce soil structure and organic matter.

How can we prevent hillslope erosion?

Several strategies can be employed to prevent hillslope erosion. These include:

• Vegetation plantings: Plant roots help to bind and stabilise the soil. Planting native species can also enhance biodiversity and ecosystem services.
• Terracing: Creating terraces on slopes breaks up the slope length and reduces the speed of water runoff. Terraces can also improve water infiltration and soil moisture.
• Cover crops: Planting cover crops can protect the soil from rain impact and reduce runoff. Cover crops can also increase soil organic matter and fertility.
• Reduced tillage: Minimising soil disturbance helps maintain soil structure and prevent erosion. Reduced tillage can also improve soil health and water retention.
• Drainage control: Proper drainage systems can help manage water flow and reduce erosion potential. Drainage control can also prevent waterlogging and salinity problems.

Implementing these methods requires a comprehensive understanding of the local environment, soil characteristics and the specific causes of erosion. It is also important to consider the ecological impacts of any erosion control measures to ensure they contribute positively to the local environment and biodiversity.

Collaborative efforts between researchers, practitioners and the community are crucial for successfully mitigating hillslope erosion.

What are we doing to research hillslope erosion?

In New South Wales, the Science Division of the Department of Climate Change, Energy, the Environment and Water undertakes continual research into hillslope erosion.

We work with organisations like the National Parks and Wildlife Service, WaterNSW, the Environment Protection Agency, the Department of Primary Industries and Regional Development, and the Soil Conservation Service. We provide event-based erosion modelling to help with post-fire assessment and water management decisions.

Several research studies on hillslope erosion in New South Wales provide valuable insights into the causes, effects and management of this environmental challenge. Some of these are:

• Climate change impacts on rainfall erosivity and hillslope erosion (PDF 5.5MB): This study models and assesses the impacts of climate change on rainfall erosivity and hillslope erosion risk in southeast Australia based on New South Wales and Australian Regional Climate Modelling (NARCliM) projections. It includes detailed predictions of mean annual hillslope erosion values and their future changes across New South Wales.
• Fire and erosion risk: Rapid assessment of hillslope erosion risk after the 2019–20 wildfires and storm events in the Sydney Drinking Water Catchment.
• Modelled hillslope erosion over New South Wales: This dataset provides a comprehensive overview of the state and trends of hillslope erosion across New South Wales.
• Results from the Hillslope Erosion Modelling: Data packages of Annual and Monthly Hillslope Erosion across New South Wales are available via the NSW SEED Portal.
• Sediment and nutrient transport: This project estimates sediment and nutrient transport associated with water and wind erosion across New South Wales.
• Soil erosion and rainfall erosivity research: Published by the NSW Government, this research summarises how soil erosion and erosivity are expected to increase due to climate change. It provides resources and data to help analysts and decision makers understand and plan for these changes.