Treat coarse and medium sediments
Coarse and medium sized sediments (i.e. generally larger than 125μm, i.e. sand) can be lost from agricultural production systems and enter waterways and wetlands. Sediments can increase turbidity, smother vegetation and animals including corals, alter habitat and kill sensitive plants and animals in waterways, wetlands and marine environments. To improve agricultural runoff water quality there are treatment systems that use physical processes to settle out coarse and medium sized sediments. These treatment systems need to be designed to slow the water velocity sufficiently to trap the target sediment type.
Treatment systems for coarse sediment removal work by intercepting surface runoff and slowing water velocity. By reducing velocities sufficiently, coarse and medium sediments (generally sediments larger than 125μm, i.e. sand) are removed through the physical process of sedimentation. Fine sediments such as silts, clays and colloidal material (less than 125μm) need to be removed through different processes, such as enhanced sedimentation, adsorption and filtration. See fine sediments, nutrients and other toxicants for more information.
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. Therefore, to remove a very fine sand requires slower water velocities, a longer detention time and often a larger structure. Structures for removing coarse and medium sediments need to be sized according to the size and type of sediment being generated in the catchment area of the contributing catchment and size of the rainfall events to be treated (i.e. the runoff volumes to be captured and treated).
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 e.g. catchment area and hydrology, rainfall regime, type and amount of sediment to be removed. 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.).
Sediments will build up over time, so regular maintenance to remove the accumulated sediment is required to retain the trapping capacity and minimise the risk of resuspension and export of sediments previously trapped in the system.
In addition to the standard disclaimer located at the bottom of the page, please note the content presented is based on published knowledge of treatment systems. Many of the treatment systems described have not been trialled in different regions or land uses in Queensland. The information will be updated as new trials are conducted and monitored. If you have any additional information on treatment systems or suggestions for additional technologies please contact us using the feedback link at the bottom of this page.
Last updated: 3 October 2018
This page should be cited as:
Department of Environment and Science, Queensland (2018) Treat coarse and medium sediments, WetlandInfo website, accessed 24 September 2020. Available at: https://wetlandinfo.des.qld.gov.au/wetlands/management/treatment-systems/for-agriculture/trap-coarse-pollutants/