Erosion

The process of erosion involves dislodging of soil particles, transport of the suspended particles, then sedimentation. Photo credits: (Gregg, 2009), (Hitson, 2022), (Glenn, 2024).

Overview

Runoff moves dislodged soil from upland areas to lower elevations (TRCA, 2020).

Erosion is the movement of earthen materials from one area to another due to forces such as wind, water, and ice. This process occurs slowly under natural conditions. However, erosion is often accelerated by anthropogenic activities, such as construction, agriculture, and deforestation (AAFC, 2025). This accelerated rate of erosion can compromise the effectiveness of LID systems and impact watershed health.

Managing the risk associated with erosion is one of the primary roles of Conservation Authorities under the Conservation Authorities Act (TRCA, 2025b). This Act allows Conservation Authorities to establish and undertake initiatives on private and public land to help achieve its objectives and can include :

• monitoring of areas affected by flooding, erosion, and or slope instability;
• study and investigation of the watershed; and
• remediation of erosion and/or slope stability hazards

One way Conservation Authorities manage the risk of erosion is by recommending the use of Erosion and Sediment Controls (ESC) during construction projects. An ESC plan may be required for projects within a Regulated Area. For more information, contact TRCA's Erosion Risk Management department.

TRCA Regulated Area 2024 (TRCA, 2025a)

Impacts of erosion

Clay sediment accumulated on top of the mulch layer of a bioretention facility resulting from improper erosion and sediment controls (MECP, 2021).

Erosion releases sediment-laden runoff and airborne dust, which can have a range of environmental and infrastructure-related impacts. These include:

• Reducing the effectiveness of LID features by clogging filters or inlets and limiting infiltration capacity.

• Increasing reservoir maintenance needs, such as dredging to remove accumulated sediment, due to reduced water volume capacity (Bledsoe, 2002).

• Degrading aquatic habitats by compromising fish spawning grounds and increasing suspended sediment that is detrimental to aquatic organisms (Kemp et al., 2011).

• Transporting pollutants, such as heavy metals and nutrients bound to sediment, into receiving water bodies (Baker et al., 2006).

• Increasing sediment deposition on roadways and in stormwater management systems (McDonough et al., 2017).

• Impairing air quality, especially through wind-blown dust (Tian et al., 2021).

ESC practices are essential for minimizing the impacts of erosion during construction. While these measures help reduce sediment-laden runoff and airborne dust, it's important to recognize that they do not eliminate all suspended sediment (MECP, 2021). To maximize effectiveness, ESC plans should be properly designed, implemented, and maintained throughout the construction process. Visit the ESC wiki to learn more about best practices for controlling erosion and protecting downstream environments.

Types of erosion

Types of erosion (CISEC, 2011).

Runoff is the primary cause of erosion in urban areas (Ferreira et al., 2021) , where impervious surfaces and vegetation removal increase the volume and speed of surface water. As runoff flows over the land, it detaches and transports soil particles. Wind and gravity can also contribute to erosion, particularly in dry or steeply sloped areas.

Types of erosion include (Government of Alberta, 2021; STEP, 2019):

• Raindrop (splash) erosion: Raindrop impact dislodges soil particles.
• Sheet erosion: Thin, even layers of soil are removed when water flows over the surface.
• Rill erosion: Small channels form on slopes as runoff concentrates and cuts into the soil.
• Gully erosion: Larger channels form when rills enlarge or combine.
• Channel erosion: Erosion of stream or ditch banks and beds.
• Wind erosion: Dry, bare soils moved by wind.
• Gravitational erosion: Mass movement of soil down slope due to gravity.

Managing the different types of erosion requires a 2-step ESC approach during construction (MECP, 2021):

1. Eliminate or reduce erosion
2. Control sediment releases

Signs of erosion

• 

  Rills in the landscape (USDA, 2025)

• 

  Undermined roads or buildings (NIWA, 2011).

• 

  Accumulated sediment (Valor Environmental, 2023).

• 

  Exposed soil and roots (Clackamas Soil and Water Conservation District, 2017)

Erosion potential

Established groundcover helps to decrease erosion potential (TRCA, 2020).

The potential for soil erosion is influenced by several factors, including (STEP, 2019):

• Rainfall characteristics: Droplet size, intensity, frequency, duration
• Climate: Soil temperatures, types of native vegetation, time of year
• Topography: Slope length and steepness
• Soil type: Soil texture, structure, permeability, organic matter content
• Ground cover: Type and quality/areal density of cover

Erosion potential of soil types and ground covers

Soil Type	Ground Cover	Erosion Potential
Well graded gravel Poorly graded gravel Sand Loamy sand Heavy clay Clay Sandy clay Silty clay Sandy clay loam Silty clay loam Sandy loam Silty sand Loam Silt loam Silt	Densely vegetated areas Sodded/established vegetated areas Soil sealant and rolled erosion controls Hydroseeded/hydromulch areas prior to growth Established temporary crop covered/vegetated lands Seeded lands prior to growth Sparsely vegetated lands Bare lands (exposed soil)	Least Most

Erosion and LID

Stepped bioswales that incorporate strategic rock and vegetation placement can help decrease the volume and speed of runoff, providing erosion control.

Big Bluestem (left) and Little Bluestem (right) are native ground cover ideal for stabilizing soils and have demonstrated good performance in projects designed, installed and monitored by STEP (Grow Me Instead, 2020).

LID can help to minimize conditions that lead to erosion by:

• Reducing runoff volume: Practices like infiltration trenches and permeable pavements allow water to soak into the ground, reducing runoff volume. LID practices also often promote evaporation and evapotranspiration.
• Reducing runoff speed: LID features like vegetated swales help slow the speed of water by increasing surface roughness. This reduces the velocity of runoff that would otherwise erode soil surfaces.
• Stabilizing soils: Vegetated components (e.g., filter strips) anchor the soil with plant roots, protecting surfaces from erosion.
• Applying Erosion and Sediment Control during construction: Slowing runoff, trapping sediment, and protecting exposed soils reduces the amount of sediment released during construction. It is especially important to apply erosion and sediment controls during LID construction, since sediment build-up in infiltration-based LID BMPs can result in malfunction or failure (MECP, 2021).
• Controlling flow paths: Controlling the path of water helps to prevent the formation of concentrated flow paths that can intensify erosion, such as rills or gullies.

The map below highlights areas in Newmarket, Ontario with low, medium, and high stormwater runoff rates based on land use. The red areas, which have the highest runoff rates, are locations where erosion is also likely to be more severe. These zones should be prioritized for future LID projects to reduce runoff volume and limit erosion.

Click on the map to enlarge it (Town of Newmarket, 2024).

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

Toronto and Region Conservation Authority

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

The drainage area of a river.

An area of land that drains into a river or a lake. The boundary of a watershed is based on the elevation (natural contours) of a landscape.

1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

Sustainable Technologies Evaluation Program

A long narrow trench or furrow dug in the ground, as for irrigation, drainage, or a boundary line.

Defined as those plants (including grasses, herbaceous species, shrubs, vines and trees) that have historically existed within a particular area. Native plants have co-evolved with the local ecosystems and natural processes.

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

Mineral particles which are smaller than 2 mm, and which are free of appreciable quantities of clay and silt. Coarse sand usually designates sand grains with particle size between 0.2 and 0.02 mm.

1. A mineral soil separate consisting of particles less than 0.002 millimeter in equivalent diameter. 2. A soil texture class. 3. (Engineering) A fine-grained soil (more than 50 percent passing the No. 200 Sieve) that has a high plasticity index in relation to the liquid limit. (Unified Soil Classification System).

Soil or media particles smaller than sand and larger than clay (3 to 60 m)

Includes the protection of soil from dislocation by water, wind or other agents.

Abiotic transfer of water vapour to the atmosphere.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

A body of water smaller than a lake, often artificially formed.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

A biological community, including humans and their natural environment.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).