Design of Biofiltration Swales and Strips

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Design of Biofiltration Swales and Strips
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Biofiltration Swales and Strips
Biofiltration

• Description
• Appropriate Applications and Siting Criteria
• Factors Affecting Preliminary Design
• Water Quality Flow Rate
• Workshop Exercises of BMP Design

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Description
Biofiltration

• Swales are conveyances (typically trapezoidal
  channels) where the flow passes through
  vegetation at some specified depth
• Strips are broad vegetated surfaces that receive
  and discharge flow in relatively thin sheets
  (i.e., overland sheet flow)

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Biofiltration Swale
Biofiltration
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Biofiltration Strip
Biofiltration
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BioStrip - Schematic
Biofiltration
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Appropriate Applications and Siting Criteria
Biofiltration

• All sites downstream of impervious surfaces where
  practicable, if
• Site conditions/climate allows vegetation
  establishment
• Flow velocities that do not cause scour
• Consistent with other Caltrans policies
  (e.g., Highway Design Manual)

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Factors Affecting Preliminary Design - Strips
Biofiltration

• Maximize treatment
• long as possible (perpendicular to flow)
• flat as site will allow
• No minimum length
• Max. width (in direction of flow) 100 m
• Free of gullies and rills
• Vegetation mix appropriate for climate and
  locationDeveloped by District Landscape Architect

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Factors Affecting Preliminary Design - Swales
Biofiltration

• Provide adequate hydraulic function for
• high flow routing
• scour protection
• Tributary area is typically less than four
  hectares

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Factors Affecting Preliminary Design - Swales
Biofiltration

• Vegetation mix appropriate for climate and
  locationDeveloped by District Landscape
  Architect.
• Use herbaceous vegetation (grasses and forbs) for
  filtration characteristics

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Checklist T-1, Part 2 Questions
Biofiltration
PPDG Page E-28,
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Water Quality FlowPPDG Page 2-17
Biofiltration

• Water Quality Flow (WQF) - Flow rates that
  must be treated by the Biofiltration BMPs

• WQF Rainfall intensities established
  cooperatively by Caltrans, SWRCB, and RWQCBs

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Rainfall Intensity forWater Quality Flow RateQ
0.28CI(mm/hr)A(sq. kilometers)
Biofiltration
PPDG Section 2.4.2.2., Page 2-17
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Design Criteria
Biofiltration

• BioStrips and BioSwales must meet the
  following
• Caltrans Highway Design Manual (HDM) requirements
  for velocity
• Specific BioSwale requirements for Hydraulic
  Resident Time (HRT), Depth and Velocity at WQF

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Highway Design Manual Criteria
Biofiltration

• Evaluate capacity of swale using 25-year design
  storm (Q25)
• Assume Mannings Coefficient (n) 0.05
• For intermittent flow, maintain velocity lt 1.2
  m/s
• Width at invert 1.2 m lt b lt 4.0 m
• Side slopes 14 or flatter

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Rational Equation

• Q 0.28 C i A
• Where
• Q Peak flow (m3 / sec)
• 0.28 Conversion Factor
• C Runoff Coefficient
• i Rainfall Intensity (mm / hr)
• A Tributary Area (km2)
• Note HDM Section 819 requires a C(f) muliplier
  for Q25 and larger. For Q25, C(f) 1.1

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Specific BioSwale Requirements

• HRT / (DV) gt 0.22
• Where
• HRT Hydraulic Resident Time measured in minutes
• (minimum HRT is 5 minutes)
• D Depth of flow at WQF in millimeters
• (maximum depth 150 millimeters)
• V Velocity at WQF, measured in meters per
  second
• (maximum velocity is 0.3 m/s)

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Mannings Coefficient for BioSwales
Biofiltration

• Evaluation at WQF
• Manning's n 0.20 for routinely mowed swales
• Manning's n 0.24 for infrequently mowed swales
  (shallow flows)
• Caltrans Highway Design Manual
• Manning's n is a function of the vegetation
  density, surface roughness, channel irregularity,
  and channel alignment. For Q25, the n value
  should be 0.050 per Table 864.3A.

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Highway Design Manual Criteria

• Establish peak flow (Q25) that channel must
  convey (HDM Index 821.3, Table 831.3)
• If possible, select cross-section that has
  flattened side slopes and curved transition to
  channel bottom (HDM Index 861.4)
• Permissible flow velocities for flexible channel
  linings (Table 873.3I)
• Intermittent Flow (Vegetation) 1.2 m/s
• Open channel flow equations (HDM Index 864.3)

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Bioswale Schematic
Biofiltration

• Where
• b bottom width (m)
• z slope section factor
• y water depth (m)

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Mannings Equation
Biofiltration

• Q 1 A R2/3 S1/2
• n
• Where
• n Mannings Number
• n 0.05, for Peak Flow.
• n 0.24, for Water Quality Flow.

Cross Sectional Area (A) (b zy)y Wetted
Perimeter (P) b 2(y2y2z2)0.5 Hydraulic
Radius (R) A/P S Longitudinal Slope (m/m)
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Workshop Exercise 1
Biofiltration

• Given
• Biofiltration swale with trapezoidal channel
• iWQF 0.5 cm/hr
• 25-year storm ipeak 10.0 cm/hr
• Tributary Area 1.5 hectare
• Paved Surface C 0.90
• Find
• Q25 and WQF in m3 / sec

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Example 1 - Solution
Biofiltration

• Calculate flow rates Q 0.28 C i A
• Q25 (1.1) 0.28 0.9 (100 mm/hr)(0.015 km2)
• Q25-yr 0.42 m3/s
• For the Water Quality Flow
• QWQF 0.28 0.9 (5 mm/hr) (0.015 km2)
• QWQF 0.02 m3/s
• Note HDM Section 819 requires a C(f) muliplier
  for Q25 and larger.

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Workshop Exercise 2
Biofiltration

• Given Biofiltration swale
• Bottom width (b) 1.25 meter
• 14 (VH) side slopes (i.e. Z 4)
• Longitudinal Slope of 2 (i.e. S 0.02)
• Swale length (L) 100 meters
• Q25 0.42 m3 / sec, WQF 0.02 m3 / sec
• n 0.050 for Peak Flow, n 0.24, for WQF
• Find
• Depth of flow and Velocity at Q25 and WQF using
  spreadsheets.

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Exercise 2
Biofiltration

• Using Mannings equation, calculate the depth y
• Q 1 A R2/3 S1/2
• n
• Where
• n Mannings Number
• n 0.05, for Peak Flow.
• n 0.24, for Water Quality Flow.

Cross Sectional Area (A) (b zy)y Wetted
Perimeter (P) b 2(y2y2z2)0.5 Hydraulic
Radius (R) A/P
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Sample Spreadsheet for Mannings Equation
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Sample Spreadsheet for Mannings Equation
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Exercise 2 - Solution
Biofiltration

• Note Use a spreadsheet to determine the
  following values

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Workshop Exercise 3
Biofiltration

• Given
• Same Biofiltration swale, length 100 meters
• Find Compare Velocity and Depth at Q25 WQF
  versus min/max values. Calculate HRT.
• Reminder Check HRT / (DV) gt 0.22

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Exercise 3
Biofiltration

• First determine if the maximum permissible HDM
  Chapter 800 velocity criterion has been met (at
  Q25).
• V25 0.83 m/s
• Since 0.83 m/s lt 1.2 m/s (allowable velocity of
  intermittent flow), design is adequate.

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Exercise 3
Biofiltration

• Determine if the maximum permissible WQF velocity
  criteria has been met
• VWQF 0.11 m/s
• Since 0.11 m/s lt 0.3 m/s (allowable velocity for
  WQF), design is adequate for velocity at WQF.

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Exercise 3
Biofiltration

• Determine if the maximum permissible WQF depth
  criterion has been met
• DepthWQF 105 mm
• Since 105 mm lt 150 mm (allowable depth for WQF),
  design is adequate for depth of flow at WQF.

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Exercise 3
Biofiltration

• Determine Hydraulic Residence Time (HRT)
• Time Length/Velocity
• Given Length 100 m
• V 0.11 m/s
• Therefore
• T 100/0.11 909 seconds 15 minutes
• Since 15 minutes gt 5 minutes minimum HRT required
  (for WQF flow), the design passes this criteria.

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Exercise 3 Summary
Biofiltration

• Summary of proposed Bioswale
• WQF depth 105 mm lt 150 mm Passed
• WQF velocity 0.11 m/s lt 0.30 m/s Passed
• WQF HRT 15 min gt 5 min Passed
• HRT / (DV) 1.24 gt 0.22 Passed

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Questions