Sediment basins

Sediment basins — Planning and design

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• Overview
• Key considerations
• Planning and design
• Construction and operation
• Links and references

Site selection

Sediment basins can be located throughout the farm. Small basins can be used within drainage lines, while larger basins should be used where water flows converge and prior to water entering other treatment systems or natural wetlands or waterways. Site assessment will determine whether it is better to use a series of smaller basins or a single, larger basin.

Sediment basins are not ideally suited to sites with:

• shallow bedrock as the required basin depth can rarely be reached and the structures may be unstable during high flows
• high water tables as the basins will fill with water, thereby reducing capacity for sedimentation/settling
• acid sulfate soils
• sandy soils or soils with sand lenses, due to potential for losses to groundwater
• regular flooding or regular large flows, where sediments would be resuspended and washed out of the basin
• dispersive soils (unless lining with geo-textile fabric is undertaken)[1].

These site characteristics don’t preclude the use of sediment basins, but they may require additional design considerations and have cost implications.

Flat catchments can also pose a problem when trying to convey flows to a central sediment basin. A drainage network leading to a central point could assist in this instance.

Sizing

The hydrology and area of the sub-catchment (which determine water flow at the site) and type of soil to be trapped needs to be understood when calculating the size of a sediment basin.

A sediment basin needs to be sized to have a sufficient detention time, i.e. the time taken for sediment to settle out of the water column. The detention time depends partly on the soils in the catchment and therefore the type and size of sediment being transported in run-off and entering the sediment basin. Sands will settle quickly but fine clays may take days (and are generally impractical to trap in sediment basins)[2]. Sediment basins are ideally designed to capture medium to larger sized sediments (typically 125μm and larger), such as fine sands. The settling velocity of the soil particles is used to calculate the size of the sediment basin required to settle out the target sediment (see Table 1). The volume of sediment to be stored at the bottom of the sediment basin must also be considered and this will be determined by the amount of sediment being generated in the sub-catchment and the frequency of desilting.

There are different options for calculating the size of a sediment basin. The first option (equation 1) below is a simple way to estimate the surface area of a sediment basin based on the size of the sediment and its setting velocity, by dividing the flow rate (Q) by the settling velocity (Vs) of the target sediment[1]. Table 1 lists the typical settling velocities under 'ideal conditions' (i.e. velocity in standing water).

This will help land managers quickly determine the amount of land required for a sediment basin and where it can fit within the farm.

1. Equation 1

   Calculate sediment basin surface area (from the Wetland Management Handbook)[1]

   

   Where:

   A = surface area of the sediment basin (m²)

   Q = flow rate (m³/sec)

   V_s = sediment settling velocity (m/sec) see Table 1

   Table 1 Settling velocities (V_s) under ideal conditions. Source: Pilgrim 2001 in DEEDI (2011) Wetland Management Handbook

   Classification of particle size	Particle diameter (μm) (microns)	Settling velocity (m/s) Vs
   Very course sand	2000	0.2
   Coarse sand	1000	0.1
   Medium sand	500	0.053
   Fine sand	250	0.026
   Very fine sand	125	0.011
   Coarse silt	62	0.0026
   Medium silt	31	0.0066
   Fine silt	16	0.0018
   Very fine silt	8	0.0004
   Clay	4	0.00011

Other options consider more parameters such as turbulence, short-circuiting and depth to calculate the size required to achieve a certain level of treatment performance. They have been developed for various land uses, such as urban, utilities or aquaculture with different performance objectives and site characteristics. These options can be compared to determine which is most suited to a particular site and objectives:

• The Water By Design (2017) Wetland Technical Design Guidelines[4] method (page 47) uses flow rate and sediment settling velocity, similar to equation 1. and includes a turbulence/short-circuiting parameter (n) and depth (d). This ensures that turbulence/short-circuiting (related to hydraulic efficiency) and depth is considered when calculating the size of the sediment basin, as these factors will influence detention time and storage capacity[4].

• The International Erosion Control Association Australasia (2018) sediment basin factsheet describes a variety of ways to calculate sediment basin volume including the sediment storage volume[3].
• The Global Seafood website outlines how to design settlement ponds for aquaculture wastewater.

Tips and tricks: There are different methods for calculating sediment basins and each method will produce different results. It is worth using more than one method to compare results and determine which aligns best to the objectives and budget.

Design

• Stabilised inlet structure reinforced with rock to dissipate flow and prevent scour.
• Settling/sedimentation basin approximately 1.5-2m deep to limit stratification, maximise sediment accumulation and reduce the frequency of maintenance[4].
• Overflow/high flow bypass weir or spillway to direct high flows away from the sediment basin and downstream treatment systems. The spillway should be designed to minimise the risk of scour of the basin. Spillways can be armoured with suitably sized rock and using geo-textile to prevent erosion (for more information see Design of rock chutes for gully stabilisation). Where a bypass is not possible upstream of the sediment basin, safe conveyance of large storm events must be factored into the design.
• Batter slope between 1:3-1:5 (vertical:horizontal)[1].
• Inlet and outlet structures located at opposite ends of the sediment basin, with flow spread out or baffles incorporated within the basin to reduce short-circuiting and increase hydraulic efficiency[4]. The key principle being that elongated basins are generally more effective in capturing and storing sediment.
• When used as a pre-treatment for other treatment systems, such as a treatment wetland, there needs to be a connection to the treatment wetland to regulate flows, allowing only the prescribed flow rate (design flow) to enter the treatment wetland with the remainder bypassing the treatment wetland. This outlet structure can be a pipe or weir.
• Installing rocks on the bottom of the sediment basin to define the base can make it easier to identify when the right depth has been reached during maintenance desilting[1], but it can be cost prohibitive in larger basins.
• An access pad for desilting with an excavator.

Specific design adaptations for tropical Queensland

Aquatic weeds and introduced pasture grasses can be a significant issue in water bodies in tropical Queensland. Although aquatic weeds will have minimal impact on the sediment trapping capacity of the sediment basin they can get flushed out and block or damage downstream infrastructure or treatment systems. Design adaptations to minimise the risk of aquatic weeds is to have dry sediment basins  or vegetated sediment basins.

Dry Sediment Basins

Unlike typical sediment basins, dry basins do not have a permanent pool. The basin fills up with water during a rain or irrigation event and then drains away. The height that water fills to is called the ‘extended detention’ level. The depth and area of extended detention is critical when designing dry sediment basins to ensure that there is sufficient detention time to allow sediments to drop out of the water column before the system drains (Figure 3).

The available extended detention depth and hence sediment storage capacity is reduced as sediment accumulates. To ensure the basin is free draining, a 1.5m deep sediment basin requires at least that same vertical difference between the invert (bottom) of the upstream drain and the water level in the downstream receiving waterbody. Where a dry basin has no gravity outfall, the basin may need to be emptied by pump after events. Another way to keep a sediment basin dry, is to reuse the water on-farm.

Vegetated Sediment Basins

Vegetated sediment basins ensure that the inundated areas are already occupied by desirable vegetation (typically reeds and sedges) to limit the growth of weeds.

The design of a vegetated sediment basin should incorporate:

• a first compartment, sized for capturing coarse particles which should be easily accessible for regular maintenance, and
• a deeper vegetated area to maximise sediment storage volume and avoid open water.

Although vegetated basins look like a treatment wetland, they are smaller and deeper and water tends to flow through them faster. Therefore, they don’t have the detention time required for effective removal of nutrients and pesticides.

References

1. ^ a b c d e Department of Employment, EDI (2011), Wetland Management Handbook: Farm Management Systems (FMS) guidelines for managing wetlands in intensive agriculture.. [online], Queensland Wetlands Program, Brisbane. Available at: https://wetlandinfo.des.qld.gov.au/resources/static/pdf/resources/reports/fms/fms_025_handbook_web.pdf.
2. ^ Guillou, M (2013), Water storage and sedimentation basins: concept and sizing, Agriculture and Agri-food, Canada, Quebec.
3. ^ International Erosion Control Association Australasia (2016), Sediment basin factsheet. [online], International Erosion Control Association Australasia. Available at: https://www.austieca.com.au/documents/item/698.
4. ^ a b c d Water by Design (2017), Wetland Technical Design Guideline. [online], Water by Design, Brisbane. Available at: https://waterbydesign.com.au/.

Last updated: 5 November 2022