Sediment Basins Part 2 – Types, Sediment trap erosion control & Silt traps

Sediment basins are temporary pond structures that are used to remove coarse sediments from stormwater by reducing flow velocities to the settling velocity of the target sediment size. In this part two of GRT’s sediment basins articles, we reiterate the purpose of sediment basins:

• slow runoff velocity.
• retain coarse sediments from runoff.
• protecting downstream elements from sediment overload.
• provide storage for minor flood attenuation.
• removal of low levels of hydrocarbons.

The article evaluates sediment basins, sediment trap erosion control, silt traps, compares sediment basin vs sediment trap and wraps up with desilting and rehabilitation of sediment basins. 

What are the different types of sediment basins?

There are two types of sediment basins namely the ‘dry’ sediment basin and the ‘wet’ sediment basin. There are further classified according to the type of soils, in which dry sediment basins are Type C for coarse-grained soil. The different types of wet sediment basins include Type F for fine-grained soils and Type D for dispersive soils. 

Here are 6 fast facts about dry sediment basins:

• continuous flow and free draining in nature.
• area that fills up with water is just an extended detention.
• sufficient detention time is key based on depth and area.
• there is filtration of minor flows through aggregate or geotextile.
• critical design parameter is surface area.
• water commences discharge as soon as it enters the basin.

Here are 6 fast facts about wet sediment basins: 

• plug flow in nature.
• use depends on type of outlet system adopted.
• designed to retain sediment-laden water for extended periods.
• allows for adequate time for gravitational settlement of fines.
• operations may be assisted by chemical flocculants.
• only drained after suitable water quality is achieved.

What is sediment trap erosion control?

Sediment trap erosion control is a best management practice used for smaller disturbed areas of less than five acres to intercept sediment-laden runoff for a sufficient period to allow majority of the sediment to settle before being released from the site. These are the conditions that need to be met for effective sediment traps:

• must be installed early in the project.
• prerequisite before the site clearing phase.
• consider sediment trap size and location based on natural drainage of site.
• locate areas of potential sediment runoff.
• determine the likely pathway for water draining from site.
• place sediment traps between sediment source/site perimeter/runoff receiving body.
• place sediment trap as closes as possible to source of sediment runoff.

What are the advantages and disadvantages of sediment basins?

Sediment basins require enough space and fitting topography for their construction but there are advantages and disadvantages of sediment basins. 

The advantages of sediment basins are as follows:

• Greatly improves the quality of runoff released.
• Removes suspended sediment on a large-scale.
• Can be permanent structure for future detention.
• Helps improve long-term water quality enhancement.

The disadvantages and limitations of sediment basins:

• Large in both area and volume.
• Use is topography dependent.
• Careful design is key for large storm events.
• Cannot be located within live streams.
• May require protective fencing.
• Type of soil can be limiting.
• Water table can be limiting.

What is a silt trap?

Silt traps deter suspended sediment from eroding with water runoff and are used for the treatment of wastewater from commercial discharge. This is how silt traps work: 

• slow the influent.
• sediment settles at the bottom of silt trap
• allow contaminated water full of silt/contaminants to settle.
• discharge into the sewer system.
• collect and remove surface water.
• desilting to increase capacity of silt trap.

Sediment trap vs sediment basin.

Both sediment traps and basins function by slowing and detention of site runoff, allowing soil particles to settle at the bottom before discharge. Sediment trap and basins have similarities in:

• embankments.
• controlling flow rate.
• inspection and maintenance.
• controlling volume of runoff from a site.
• construction prior disturbance of upslope areas.

Sediment trap vs sediment basin are assessed by evaluating the following differences:

• size – sediment basins serve larger areas than sediment traps.
• detention time – sediment basins retain runoff for long periods.
• capacity – sediment basins have larger capacity than sediment traps.
• placing – basins are a single structure, but traps can be built as series of traps.
• siting – basins have larger footprint so should be sited where runoff converges.

Desilting and sediment disposal of sediment basins.

The process of desilting of sediment basins takes place in two ways. The first method is using long-reach excavation equipment operating from the sides of the basin. The second method is by allowing machinery access into the basin. If excavation equipment needs to enter directly into the basin, then it is better to design the access ramp so that trucks can be brought to the edge of the basin, rather than trying to transport the sediment to trucks located at the top of the embankment. The ideal access ramp should have a maximum 6:1 slope. If the sediment is removed from the site, then a suitable sediment drying area should be made available adjacent to the sediment basin, or at least somewhere within the sediment basin’s catchment area. There are several sediment disposal methods. Trapped sediment can be mixed with on-site soils and buried or removed from the site. If sediment is removed from the site, then it should be de-watered prior to disposal. De-watering must occur within the catchment area of the sediment basin. 

How do you decommission and rehabilitate sediment basins?

Details on the required decommissioning and rehabilitation of the sediment basin must be included in the erosion and sediment control plan compliance documentation used in obtaining the permit from the regulatory authorities. The decommission and rehabilitation process involves:

• Waiting until all up-slope site stabilization measures are implemented and working.
• Conversion of the sediment basin into a site stormwater treatment network.
• Conversion into detention/retention basin.
• Conversion into bioretention system, wetland or pollution containment system.
• Implementation temporary sediment control measures downslope of the basin.
• Removal of all water and sediment.
• Disposal of any materials in a manner that will not create erosion/pollution.

In conclusion.

Are you looking to construct, maintain, rehabilitate or decommission your, sediment basin? sediment trap? silt trap? Speak to Daniel Grundy and hear how GRT can assist you in getting the most out of your sediment control plan. Water quality from your sediment basin/trap and silt trap is our responsibility and allow us to implement innovative solutions whilst your site work progresses without any disturbances. Time is money, GRT values your time. 

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References:

Sediment control practices – Sediment traps and basins. Retrieved 20/09/21.

Sedimentation Basins. Retrieved 20/09/21. 

Sediment basins. Retrieved 20/09/21. 

Sediment Basins. Retrieved 20/09/21. 

Sediment basins. Retrieved 20/09/21.