Difference between revisions of "Pretreatment features"
Jenny Hill (talk | contribs) m (→Design) |
Dean Young (talk | contribs) |
||
(6 intermediate revisions by one other user not shown) | |||
Line 10: | Line 10: | ||
:<math>V_{f}=A_{c}\times R\times L_{o} \times F_{c}</math> | :<math>V_{f}=A_{c}\times R\times L_{o} \times F_{c}</math> | ||
{{plainlist| 1=Where: | {{plainlist| 1=Where: | ||
*''A<sub>c</sub>'' = The area of the catchment ( | *''A<sub>c</sub>'' = The area of the catchment (ha), | ||
*''R'' = The capture efficiency (default to 0.8), | *''R'' = The capture efficiency (default to 0.8), | ||
*''L<sub>o</sub>'' = The sediment loading rate (m³/ha/yr), and | *''L<sub>o</sub>'' = The sediment loading rate (m³/ha/yr), and | ||
Line 31: | Line 31: | ||
===Example calculation=== | ===Example calculation=== | ||
A parking lot catchment of 1.7 ha is being routed through a small forebay into a bioretention cell. The design flow rate is 0. | A parking lot catchment of 1.7 ha is being routed through a small forebay into a bioretention cell. The design flow rate is 0.07 m³/s. The system should be designed to require cleaning no more often than once per year. | ||
The volume is calculated as: | The volume is calculated as: | ||
:<math>V_{f}=1.7\times 0.8\times 0.6\times 1=0.816\ m^{3}</math> | :<math>V_{f}=1.7\times 0.8\times 0.6\times 1=0.816\ m^{3}</math> | ||
The area required to keep the maximum head of water within the forebay to 0.15 m is calculated as: | The area required to keep the maximum head of water within the forebay to 0.15 m is calculated as: | ||
:<math>A_{f}=\frac{0.816}{0. | :<math>A_{f}=\frac{0.816}{0.15}=5.44\ m^{2}</math> | ||
The area required to settle the 1 mm particles is calculated as: | The area required to settle the 1 mm particles is calculated as: | ||
:<math>A_{f}=120\times 0. | :<math>A_{f}=120\times 0.07 = 8.4\ m^{2}</math> | ||
So to meet the target particle removal, the forebay will be | So to meet the target particle removal, the forebay will be 8.4 m² in area. This gives the storage volume of 1.26 m³, which can be returned to the initial equation to determine the minimum cleaning frequency as: | ||
:<math> | :<math>F_{c}=\frac{1.26}{1.7\times 0.8\times 0.6}=1.5\ years</math> | ||
==Gallery== | ==Gallery== | ||
Line 46: | Line 46: | ||
==Proprietary products== | ==Proprietary products== | ||
As with underground [[pretreatment]] there are an increasing number of proprietary products available. Some of those listed below are currently only available overseas. | As with underground [[pretreatment]] there are an increasing number of proprietary products available. Some of those listed below are currently only available overseas. | ||
*[http://www.rainguardian.biz/ | *[http://www.rainguardian.biz/ Rain Guardian] | ||
*[https://oldcastleprecast.com/oldcastle_product/4-out-swalegard-pre-filter-grassy-swale/101168/ Swalegard] | *[https://oldcastleprecast.com/oldcastle_product/4-out-swalegard-pre-filter-grassy-swale/101168/ Swalegard] | ||
---- | ---- | ||
[[Category:Pretreatment]] | [[Category:Pretreatment]] |
Latest revision as of 02:16, 15 July 2020
![](/images/thumb/f/ff/Forebays.png/300px-Forebays.png)
Small concrete forebays are a pretreatment option for open inlets such as curb cuts. Energy of the incoming flow is dissipated, causing suspended particles to drop out of the water. These accumulated particles/sediment can then be easily swept or vacuumed during routine maintenance and doesn't end up clogging downstream filter media or material. A well designed feature will distribute the flow, reducing erosion around the inlet. One effective way of achieving this is by surrounding the pad with some form of level spreader on all sides. The level spreading could be a sharp crested weir in metal or concrete, or be soft edged with irregular landscaping stone.
For BMPs serving up to a two hectare catchment area, a significant forebay feature may not be required. However, for larger catchments or those with heavy sediment loading a properly sized feature will help prevent clogging of the filter media and ease maintenance requirements of bioretention facilities.
Design[edit]
1. The required volume for a pretreatment feature (Vf, m³) serving catchments up to five hectares may be calculated as:
Where:
- Ac = The area of the catchment (ha),
- R = The capture efficiency (default to 0.8),
- Lo = The sediment loading rate (m³/ha/yr), and
- Fc = The cleanout frequency (yrs)
Sediment loading rates from impervious surfaces studied by STEP were between 0.3 - 0.6 m³/ha/yr [1]. In Brisbane a value of 0.6 m³/ha/yr is the default used to size small forebays [2].
2. a) The area of a feature (Af) with 80% capture efficiency of particles ≥ 1 mm may be estimated[2] as:
Where:
- Q = Design flow rate (m³/s),
2. b) To size a feature for a maximum depth (df, m), where df must be ≤0.3 m and is recommended to be between 0.1 - 0.2 m:
It is recommended that both sizing calculations be made and the pretreatment feature be designed to meet both targets. In some cases the additional storage required to meet both targets will reduce the expected frequency of maintenance. See below.
Example calculation[edit]
A parking lot catchment of 1.7 ha is being routed through a small forebay into a bioretention cell. The design flow rate is 0.07 m³/s. The system should be designed to require cleaning no more often than once per year. The volume is calculated as:
The area required to keep the maximum head of water within the forebay to 0.15 m is calculated as:
The area required to settle the 1 mm particles is calculated as:
So to meet the target particle removal, the forebay will be 8.4 m² in area. This gives the storage volume of 1.26 m³, which can be returned to the initial equation to determine the minimum cleaning frequency as:
Gallery[edit]
Solid splash pad preventing erosion from the flow from the inlet. Image credit Dylan Passmore
Forebay connected to drainage area in the roadway with a curb cut, overflow visible in the centre of the feature, level spreading is encouraged with the rock check dams, Milwaukee, WI, Photo credit: Aaron Volkening
Rounded 'river rock' and a series of check dams slow water from this inlet.
This forebay has a rock splash pad to slow water down before it reaches a bioswale.
Proprietary products[edit]
As with underground pretreatment there are an increasing number of proprietary products available. Some of those listed below are currently only available overseas.
- ↑ Goncalves, C., & Van Seters, T. (2012). Characterization of Particle Size Distributions of Runoff from High Impervious Urban Catchments in the Greater Toronto Area. Retrieved from https://sustainabletechnologies.ca/app/uploads/2013/03/PSD-2012-final.pdf
- ↑ 2.0 2.1 Healthy Waterways Ltd. (2014). Bioretention Technical Design Guidelines. Retrieved from https://hlw.org.au/u/lib/mob/20150715140823_de4e60ebc5526e263/wbd_2014_bioretentiontdg_mq_online.pdf