Sediment, including soils, mud, clay, silt and sand, is the single main pollutant generated at construction sites and largely arises from the erosion of exposed soils by surface water runoff.
Good surface water management during construction is essential in preventing sediment pollution and minimising risk of prosecution, with the potential for large financial penalties and reputational damage.
The adoption of a surface water plan including appropriate erosion and silt controls provides many benefits:
- reduce downtime costs and risk of delays to construction works from poor or wet ground conditions
- save space and land acquisition costs by reducing reliance on settlement ponds
- improve relationships with the client, regulator and neighbours
- reduce complaints, disputes and the potential for compensation claims
- enable quicker, cheaper and better land reinstatement
- maintain and improve reputation within the industry
Why control silt?
Good surface water management is essential for the protection of sensitive environmental receptors such as aquatic life in rivers, streams and lakes. Groundwaters, drinking water aquifers, abstractions for industrial and domestic processes as well as wastewater treatment facilities also require protection.
Sediment pollution can cause significant damage that may result in criminal prosecution, prison sentences and fines of up to £3 million.
How can silt reach receptors?
Sediments can be mobilised from a number areas on site:
- roads and drainage ditches
- excavations and dewatering areas
- wheel washing facilities
- surface soil stripping
- river crossings
- material storage areas and stockpiles
Once sediments are mobilised they will travel the path of least resistance, often resulting in silty water escaping site. Key pathways include:
- bore holes
- land drains
- ditches and streams
- overland flow
- surface water and foul drains
What are the main principles of silt control?
Controlling a pollution event at source i.e. preventing it from happening in the first place will nearly always be less expensive then having to manage a pollution event.
A comprehensive understanding of site drainage is a key element in planning how to prevent clean water from entering site as well as how to manage silty water produced on site.
It is best practice to retain vegetation cover, minimise soil stripping and establish new vegetation on bare ground at the earliest opportunity. Erosion of soils can be caused by wind but is normally attributed to rainfall. Sheet flows cause slips, gulleys and rills to form whilst drainage channels can give rise to scouring of bed and banks, all factors that can increase the mobilisation of material.
Identifying key areas on site that require protection is another important aspect of planning, these include:
- haul roads and site compounds
- entrances to drains
- at river crossings
- in drainage channels
- in lagoons
- on slopes
Settlement units and lagoons are commonly used for treating muddy water on site but they often can’t treat the smallest sediment particles. Boosting the treatment power of lagoons using Water Lynx™ anionic flocculants, Silt Net, Treated Geo-Jute or Pipe Reactor can be a key consideration, especially when managing the downstream impact of clayey soils.
Planning silt control interventions is relatively quick and painless. Slowing the flow of silty water naturally aids deposition, where sediment can be trapped and removed whilst allowing water time to infiltrate. frog environmental provide a range of products that are inexpensive and simple to deploy, all designed to help our clients control silt on site:
- Silt Mat – captures sediment and prevents resuspension
- Silt Wattle – reduces flow on sloping ground and in channels & ditches
- Floc Mat – traps sediment using a mild anionic flocculant
- Silt Net – for placement in lagoons to aid sediment deposition
- Treated Geo-Jute – for polishing water and lining channels
- Rock Check – reduces and distributes water flow in channels
- Water Lynx™ anionic flocculant – used to support silt and water separation
- Pipe Reactor – mobile water treatment unit
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