Sediment pollution is an ever present issue in construction sites and urban areas. As a result, traps that control the release of silt and pollutants into water courses can play a key role in protecting the environment from pollutants that can endanger wildlife and damage stormwater systems.
Sediment runoff
Sediment is the presence of silt, soil, sand, mud and clay in water bodies, often carried into water courses by runoff in areas where land has been eroded.
In the countryside sediment runoff is typically caused by soil erosion from weather events as well as deforestation, mining or agriculture. These practices disrupt the earth and can cause huge volumes of mud and silt to flood surrounding watercourses.
In urban areas sediment often comes from construction sites, as well as run off from roads and hard surfacing.
Sediment from construction sites and urban areas often contains more than just soil or clay; heavy metals, microplastics and petrol all pass into shared drainage systems and watercourses.
Environmental impact of sediment pollution
Controlling sediment is crucial as sediment can damage both ecosystems and infrastructure:
- Sediment increases the turbidity (cloudiness) of watercourses, this disrupts the lifecycles of aquatic animals and plant life. Silt and soil block sunlight and prevent photosynthesis, affecting plant growth and by extension food sources and habitats for animals.
- Sediment can smother insects and fish eggs, destroy spawning grounds as well as the clog gills of fish and amphibians.
- Sediment carries toxins such as heavy metals, oils and microplastics into shared watercourses, affecting the health of animals as well as the quality of drinking water in local aquifers.
- Excessive nitrogen from agriculture in runoff can cause algal blooms – where algae feed on the high nutrient levels, eventually starving waterbodies of oxygen and harming other aquatic life.
- Increased flood risk – sediment clogs and reduces the capacity of drainage channels and stormwater pipes, making floods more likely.
Types of sediment control – passive vs active
Passive sediment control
Basins, silt fences and vegetative buffers rely on gravity, filtration, and sedimentation to capture and remove sediment.
Sediment is removed by settling or by trapping in vegetative and geotextile barriers. These systems are low tech, affordable and easily implemented, but may struggle with high flow rates or removal of fine particles.
Sediment basins
- Deep basins, where moving water is collected and slowed, heavy suspended particles like silt and soil settle at the bottom before clean water is allowed to flow out.
- Basins may be temporary (built to serve a construction project) or permanent for long lasting sediment control.
Catch basin inserts
- Filtration devices used in drainage systems to remove sediment from stormwater runoff.
- They can use filtration, absorption or even biochar to remove pollutants like metals and oil.
Silt fences
- Silt fences are temporary barriers made of permeable geotextiles, often used along embankments and around construction sites.
- The permeable geotextile fabric captures silt and sediment, while still allowing clean water to pass through.
Silt curtains
- Vertical barriers used in water to contain sediment and particles released during underwater construction or excavation, helping to protect marine environments.
- Typically made from impermeable geotextile fabric with flotation devices that keep the upper edge above the waterline, preventing overflow.
Permanent silt traps
- Installed in urban drainage systems, creating a space where sediments can settle and be removed from the water, protecting downstream infrastructure from clogging.
Interceptors
- Devices for urban and industrial drainage systems, designed to capture and separate pollutants before they reach main watercourses.
- In these systems, water flows into a tank where heavy particles settle at the bottom, lighter pollutants such as oil rise to the top, and clean water exits through pipes positioned at the mid-level of the tank.
River mats
- Netted mats are layered and secured to the bottom of a watercourse, so water is allowed to flow over them, trapping sediment as it settles on the riverbed.
Vegetative buffers
- Engineered natural systems for sediment and pollutant filtration that use plants like grasses, shrubs and trees.
- They can be used to re-establish native species in areas disrupted by construction and can provide valuable habitats for wildlife.
Active sediment control systems
Active systems use engineered processes – pumps, vortexes, or chemicals to remove sediment from water.
Chemical flocculants aggregate particles, while pumps and vortexes aid sedimentation.
Active systems can be complex, requiring technical expertise for maintenance but can handle high sediment loads and fine particles effectively.
They are often used in sensitive sites where powerful treatment is needed to meet environmental requirements.
Clarifiers
Clarifiers are specialised treatment tanks designed to separate solid particles from water.
- Clarifiers use sedimentation and often chemical flocculation (use of chemicals that cause solid particles to bind together) to remove fine solids.
- They can be used alongside silt traps, where coarse particles are first removed by the silt trap and then finer solids by the clarifier.
- In urban settings catch basin inserts filter debris from road runoff before an underground clarifier chamber removes fine particles, protecting downstream rivers and wetlands.
Hydrodynamic separators
These stormwater treatment systems use vortex flow patterns to separate pollutants and sediment from runoff.
- Particles are removed by circulating water, causing heavy particles to settle and separate.
- Installed below ground inside storm drain networks and are used as pre treatment systems to prevent pollutants passing into watercourses.-
Innovations and future trends
Sediment control integration with the IoT for smart cities
- By using sensors to detect pressure changes and light scattering, sediment levels can be monitored. This allows predictive maintenance as well as trend analysis, helping track ecological shifts in water courses.
- Active filtration systems can be combined with sensors and automation, triggering flocculant release or filtration based on real time data.
- Solar powered sensors and low energy communication tools allow IoT integration even in off grid and hard to reach locations.
Why silt and sediment control should be key in any sustainable development
Sediment control has wide ranging benefits, protecting water quality and local biodiversity as well as improving urban flood resilience.
Passive systems like basins and vegetative buffers, as well as active interventions clarifiers and hydrodynamic separators can all play a role in protecting water courses, drainage systems and habitats – therefore making sediment control pivotal in any sustainable development.
Read more about silt and sediment control:
Preventing silt pollution whilst working in rivers
The impact of stormwater – sediment
Read more from Foundations blog about best practice design for the built environment: