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| ! Recommended depth (with underdrain pipe) | | ! Recommended depth (with underdrain pipe) |
| ! Recommended depth (no underdrain pipe) | | ! Recommended depth (no underdrain pipe) |
− | ! Typical void ratio (''V<sub>r</sub>'') | + | ! Typical void ratio (''V<sub>R</sub>'') |
| |- | | |- |
− | | Ponding depth | + | | Ponding (''d<sub>p</sub>'') |
| | 300 mm | | | 300 mm |
| | See below | | | See below |
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| | | | | |
| * 0.7 for wood based | | * 0.7 for wood based |
− | * 0.4 for aggregates | + | * 0.4 for [[stone]] |
| |- | | |- |
− | | [[Bioretention media]] | + | | [[Bioretention: Filter media|filter media]] (''d<sub>m</sub>'') |
| | colspan="2" | | | | colspan="2" | |
| * 300 mm to support turf grass (and accept only rainwater/roof runoff) | | * 300 mm to support turf grass (and accept only rainwater/roof runoff) |
| * 600 mm to support flowering [[perennials]] and decorative [[grasses]] | | * 600 mm to support flowering [[perennials]] and decorative [[grasses]] |
| * 1000 mm to support [[trees]] | | * 1000 mm to support [[trees]] |
− | | 0.3 | + | | |
| + | *[[Bioretention media storage| 0.4]] for sandy mix |
| + | * 0.35 for a more loamy mix. |
| |- | | |- |
− | | [[choker gravel|Choker course]] | + | | [[choker layer|Choker course]] |
| | colspan="2" |100 mm | | | colspan="2" |100 mm |
| | 0.4 typical | | | 0.4 typical |
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| | 0.4 | | | 0.4 |
| |- | | |- |
− | | Storage reservoir | + | | Storage reservoir (''d<sub>s</sub>'') |
| | See below | | | See below |
| | See below | | | See below |
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| ==Calculate the maximum overall depth== | | ==Calculate the maximum overall depth== |
| *Step 1: Determine what the planting needs are and assign appropriate depth of media, using the table above. | | *Step 1: Determine what the planting needs are and assign appropriate depth of media, using the table above. |
− | *Step 2: Select an underdrain pipe diameter (typically 100 - 200 mm), assign this as an 'embedding' depth. | + | *Step 2: Select an underdrain pipe diameter (typically 100 - 200 mm), assign this as an 'embedding' depth. *Note that this component does not apply if a downstream riser is being used to control an extended saturation zone. |
− | *Step 3: Calculate the maximum possible storage reservoir depth beneath the pipe (''d<sub>s</sub>''): | + | *Step 3: Calculate the maximum possible storage reservoir depth beneath the pipe (''d<sub>s, max</sub>'', mm): |
− | <math>d_{s}=f'\times38.4</math> | + | <math>d_{s, max}=f'\times t</math> |
| {{Plainlist|1=Where: | | {{Plainlist|1=Where: |
− | *''f''' = Design infiltration rate in mm/hr, and | + | *''f''' = Design infiltration rate (mm/hr), and |
− | *38.4 comes from multiplying desired drainage time of 96 hours by void ratio of 0.4}} | + | *''t'' = [[Drainage time]] (hrs). Check local regulations for drainage time requirements.}} |
| | | |
| ===Additional step for system without underdrain=== | | ===Additional step for system without underdrain=== |
− | *Step 4: Determine maximum permissible ponding depth (''d<sub>p</sub>''): | + | * Step 4: Determine maximum permissible ponding depth (''d<sub>p, max</sub>''): |
− | <math>d_{p}=f'\times19.2</math> | + | <math>d_{p, max}=f'\times48</math> |
| {{Plainlist|1=Where: | | {{Plainlist|1=Where: |
− | *''f''' = Design infiltration rate in mm/hr, and | + | *''f''' = Design infiltration rate (mm/hr), and |
− | *19.2 comes from multiplying desired drainage time of 48 hours by void ratio of 0.4. Note that conceptually the drainage of the ponded area is limited by ex-filtration at the base of the practice.}} | + | *48 = Drainage time of the ponding (hrs) |
| + | *Note that conceptually the drainage of the ponded area is limited by ex-filtration at the base of the practice.}} |
| * Step 5: Sum total depth of bioretention, and compare to available space above water table and bedrock. Adjust if necessary. | | * Step 5: Sum total depth of bioretention, and compare to available space above water table and bedrock. Adjust if necessary. |
| | | |
| ==Calculate the remaining dimensions== | | ==Calculate the remaining dimensions== |
| * Step 6: Multiply the depth of each separate component by the void ratio and then sum the total to find the 1 dimensional storage (in mm). | | * Step 6: Multiply the depth of each separate component by the void ratio and then sum the total to find the 1 dimensional storage (in mm). |
− | * Step 7: Calculate the required total storage (m<sup>3</sup>): | + | * Step 7: Calculate the required total storage (S<sub>T</sub>, m<sup>3</sup>): |
− | <math>Storage=RVC_T\times A_c\times C\times 0.1</math> | + | <math>S_{T}=RVC_T\times A_c\times 10</math> |
| {{Plainlist|1=Where: | | {{Plainlist|1=Where: |
| *''RVC<sub>T</sub>'' is the Runoff volume control target (mm), | | *''RVC<sub>T</sub>'' is the Runoff volume control target (mm), |
− | *''A<sub>c</sub>'' is the catchment area (Ha), | + | *''A<sub>c</sub>'' is the catchment area (Ha), and |
− | *''C'' is the runoff coefficient of the catchment area, and
| + | * 10 is the units correction between m<sup>3</sup> and mm.Ha.}} |
− | * 0.1 is the units correction between m<sup>3</sup> and mm.Ha.}} | |
| * Step 8. Divide required storage (m<sup>3</sup>) by the 1 dimensional storage (in m) to find the required footprint area (''A<sub>p</sub>'') for the bioretention in m<sup>2</sup>. | | * Step 8. Divide required storage (m<sup>3</sup>) by the 1 dimensional storage (in m) to find the required footprint area (''A<sub>p</sub>'') for the bioretention in m<sup>2</sup>. |
− | * Step 9. Calculate the peak flow rate (''Q<sub>p</sub>'', in L/s) through the filter media: | + | * Step 9. Calculate the peak [[flow through perforated pipe|flow rate through the perforated pipe]], |
− | <math>Q_{p} = A_{p}\times K_{sat}\times 3.6 \times 10^{-3}</math>
| + | * Step 10. Calculate the peak [[flow through media|flow rate through the filter media]], |
− | {{Plainlist|1=Where:
| + | * Step 11. Determine if downstream [[flow control]] is required to achieve hydrologic objectives. |
− | *''K<sub>sat</sub>'' is the saturated hydraulic conductivity of the filter media (mm/hr), and
| |
− | *''A<sub>p</sub>'' is the area of the practice (m<sup>2</sup>).}} | |
− | ----
| |
| | | |
| ==Additional calculations== | | ==Additional calculations== |
− | ===Drainage time (3D)==
| |
− | [[file:Hydraulic radius.png|thumb|Two practice areas of 9 m<sup>2</sup>.<br>
| |
− | P = 12 m (left), P = 20 m (right)]]
| |
− | <poem>
| |
− | Narrow, linear cells drain faster than round or blocky footprint geometries.
| |
− | Divide the area of the practice (''A<sub>p</sub>'') by the perimeter (''P'').
| |
− | </poem>
| |
| | | |
− | ==Step 3, depth of bowl,''d<sub>b</sub>''== | + | ===Calculating infiltration practice drainage in 1 dimension=== |
− | [[File:Sizing bio2.png|thumb|The duration of surface ponding is usually determined by the emptying of the facility below.]] | + | This spreadsheet compares drainage in a single dimension under zero head conditions, mean head conditions and falling head conditions. It provides a more conservative measurement of the drainage time for the purposes of groundwater mounding (where a shorter drainage time causes a greater impact). |
− | <poem>
| + | |
− | The 'bowl' area is simply the depressed elevation of the surface compared to the surrounding areas; a space to permit temporary ponding.
| + | {{Clickable button|[[Media:Darcy drainage.xlsx|Download drainage time calculator(.xlsx)]]}} |
− | The depth of the bowl would not typically exceed 0.5 m due to safety concerns.
| + | |
− | Some jurisdictions or particular sites may demand a lower maximum depth of ponding. It is worth bearing in mind that in most cases this ponded water will only occur once or twice per year.
| + | ===Drainage time (3D)<ref>Woods Ballard, B., S. Wilson, H. Udale-Clarke, S. Illman, T. Scott, R. Ahsley, and R. Kellagher. 2016. The SuDS Manual. 5th ed. CIRIA, London.</ref>=== |
− | The ponded water within the bowl should drain within 48 hrs. This addresses public concerns about [[mosquitoes]] and reduces the probability of losing vegetation due to saturated growing conditions.
| + | [[file:Hydraulic radius.png|thumb|Two practice areas of 9 m<sup>2</sup>.<br> P = 12 m (left), P = 20 m (right)]] |
− | The drainage of the bowl is not usually constrained by the surface infiltration (filter media should drain > 25 mm/hr). The infiltration of water from the bowl is determined by the drainage of the filter media and reservoir into the soils.
| + | In some situations, it may be desirable to reduce the size of the bioretention required, by accounting for rapid drainage. |
− | To estimate the time (''t'') to fully drain the facility: | + | Typically, this is only worth exploring over sandy soils with rapid infiltration. |
− | :<math>t=\frac{V_{R}A_{p}}{f'P}ln\left [ \frac{\left (d+ \frac{A_{p}}{P} \right )}{\left(\frac{A_{p}}{P}\right)}\right]</math> | + | |
− | </poem> | + | Note that narrow, linear bioretention features (or [[bioswales]]) drain faster than round or blocky footprint geometries. |
| + | |
| + | *Begin the drainage time calculation by dividing the area of the practice (''A<sub>p</sub>'') by the perimeter (''P''). |
| + | *To estimate the time (''t'') to fully drain the facility: |
| + | :<math>t=\frac{V_{R}A_{p}}{f'P}ln\left [ \frac{\left (d_{T}+ \frac{A_{p}}{P} \right )}{\left(\frac{A_{p}}{P}\right)}\right]</math> |
| + | {{Plainlist|1=Where: |
| + | *''V<sub>R</sub>'' is the void ratio of the media, |
| + | *''A<sub>p</sub>'' is the area of the practice (m<sup>2</sup>), |
| + | *''f''' is the design infiltration rate (mm/hr), |
| + | *''P'' is the perimeter of the practice (m), and |
| + | *''d<sub>T</sub>'' is the total depth of the practice, including the ponding zone (m).}} |
| | | |
− | ==Groundwater mounding== | + | ===Groundwater mounding=== |
| <poem> | | <poem> |
| When you wish to model the extent of groundwater mounding beneath an infiltration facility. This tool uses Hantush's derivation (1967). | | When you wish to model the extent of groundwater mounding beneath an infiltration facility. This tool uses Hantush's derivation (1967). |
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| Note that this is a minor adaptation (metric units and formatting) from the original tool, written and [https://pubs.usgs.gov/sir/2010/5102/ hosted by the USGS]. | | Note that this is a minor adaptation (metric units and formatting) from the original tool, written and [https://pubs.usgs.gov/sir/2010/5102/ hosted by the USGS]. |
| </poem> | | </poem> |
| + | ---- |