Treat coarse sediment

Coarse and medium sized sediments can be lost from agricultural production systems via surface water run-off. Sediments can increase turbidity, alter habitat and smother plants and animals including corals, wetlands and marine environments. Treatment systems to remove sediment from run-off use physical processes to settle out coarse and medium sized sediments (generally larger than 125μm, i.e. sand[3]). These treatment systems need to be designed to slow the water velocity sufficiently to trap the target sediment type. Fine sediments such as silts, clays and colloidal material (less than 125μm[3]) need to be removed through different processes, such as enhanced sedimentation, adsorption and filtration[4]. See fine sediments, nutrients and other toxicants for more information.

Quick facts

The trapping efficiency of dams,

such as the Burdekin Falls dam (1,860,000 ML), is about 60% for suspended sediments, and 95% for coarser sediment (>30 ųm)[2].

Which

treatment systems can be used?

• High efficiency sediment basin
• Sediment basins
• Vegetated buffers and swales
• Vegetated drains
• Treatment wetlands

Treatment systems for agriculture – Treat coarse sediment

Select from the tabs below

• Overview
• Understand system
• BMP
• Treat coarse sediment
• Treat nutrients, toxicants

The soil type within the catchment will influence the type and size of sediment requiring removal and the treatment system will need to be designed and sized accordingly (Table 1). A coarse sand (1000μm) has a settling velocity of 0.1 metres per second (m/s) whereas a very fine sand (125μm) has a settling velocity of 0.011m/s[1]. Therefore, to remove a very fine sand requires slower water velocities and a longer detention time which typically requires a larger structure.

Usually a simple, open, pond-like structure (e.g. sediment basin) is adequate to reduce the velocity sufficiently to enable coarse and medium sediments to settle out, provided it is sized appropriately for the site conditions when considering catchment area, soil types, local rainfall and hydrology. Dense vegetation, such as grasses and reeds, can also act to slow water velocity and physically trap coarse sediments and other pollutants, such as leaf litter and rubbish (plastics etc.).

*NB: Assuming a sediment basin is part of the wetland design

Table 1 on the main treatment system page provides more details on main treatment processes, type of flow treated and land uses treatment systems have been primarily used in.

Additional information

Minnesota Pollution Control Agency—Processes for removing pollutants from stormwater runoff

Melbourne Water—Constructed wetlands

Wetland Management Handbook

Wetland Technical Design Guidelines

References

1. ^ Pilgrim, D (2001), 'Australian Rainfall and Runoff: a guide to flood estimation', Institution of Engineers Australia Barton, vol. 1, Institution of Engineers Australia Barton, ACT.
2. ^ Tomkins, K (2013), Estimated sediment infilling rates for dams in northern Australia based on a review of previous literature. A technical report to the Australian Government from the CSIRO Flinders and Gilbert Agricultural Resource Assessment, part of the North Queensland Irrigated Agriculture Strategy.. [online], CSIRO Water for a Healthy Country and Sustainable Agriculture flagships, Australia.. Available at: https://publications.csiro.au/rpr/download?pid=csiro:EP137367&dsid=DS5.
3. ^ a b Wentworth (1922), 'A Scale of Grade and Class Terms for Clastic Sediments', The Journal of Geology, vol. 30, no. 5, pp. 377-392.
4. ^ Wong, T, Fletcher, T, Duncan, H, Coleman, J & Jenkins, G (2002), 'A model for urban stormwater improvement conceptualization', Global Solutions for Urban Drainage, pp. 8-13.

Last updated: 10 June 2022