Design of Infiltration and Detention Basins

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Design of Infiltration and Detention Basins
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Infiltration and Detention Basins

• Description
• Appropriate Applications and Siting Criteria
• Water Quality Volume
• Factors Affecting Preliminary Design
• Workshop Exercises on BMP Design

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Description
Infiltration Basins

• An infiltration basin is a device designed to
  remove pollutants from surface discharges by
  capturing the Water Quality Volume (WQV) and
  infiltrating it directly to the soil rather than
  discharging it to receiving waters.

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Typical Infiltration Basin Layout
Infiltration Basins
PPDG Figure B-1, Page B-3
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Infiltration Basins
Infiltration Basins
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Siting Criteria
PPDG Table B-2, Page B-4

• gt 3 m to seasonally high water table
• Soil infiltration rate ? 1.3 centimeters per hour
  
• Infiltration Rate gt 6.4 cm/hr, contact RWQCB
• Clay content lt 30
• Silt and Clay combined lt 40
• Site should not be located in area containing
  fractured rock within 3 meters of basin invert.
• Infiltrated water is unlikely to affect the
  stability of downgradient structures, slopes, or
  embankments
• Minimum WQV for basin must be gt 123 m3

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Pre-Screening for the Infiltration Basins
Infiltration Basins
PPDG Figure B-2, Page B-6
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Water Quality Volume
Infiltration Basins

• The Water Quality Volume (WQV) is the amount of
  runoff that enters the BMP and must be treated
  during the water quality storm event. Use Basin
  Sizer or consult with District Hydraulics to
  determine water quality storm depth. See PPDG
  Page 2-16.
• The WQV is the water quality storm depth x the
  area tributary to the BMP x the weighted average
  runoff coefficient for that area

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Minimum Footprint Area for Infiltration (PPDG
Page B-8)

• Aest 100SFWQV/(Kestt)
• Aest Estimated invert area (m2)
• SF Safety factor of 2.0
• WQV Water Quality Volume (m3)
• Kest Estimated Infiltration Rate (cm/h)
• T drawdown time, 48 hours

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Preliminary Design Factors
Infiltration Basins

• When designing a BMP, it is critical to remember
  that the BMP must not negatively impact drainage
  of the roadway.
• Consult with District Hydraulics to ensure that
  the design will not compromise roadway drainage.
• It is Caltrans policy to minimize runoff from
  pervious areas to Treatment BMPs.

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Preliminary Design Factors
Infiltration Basins

• Provide Maintenance access road around basin and
  provide ramp to basin invert
• Provide scour protection at inlet
• Use a factor of safety of two (2) to size the
  invert area of the basin
• Provide an Emergency/maintenance gravity drain,
  if practical
• Use 13 side slope ratios or flatter
• Provide minimum 0.3 meters of Freeboard
• Size to capture the Water Quality Volume, minimum
  volume should be 123 m3

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Infiltration Basin Design Approach

• Determine if location meets siting requirements
  listed in Appendix B.
• Calculate tributary area.
• Determine Water Quality Volume.
• Calculate minimum footprint area using Equation
  1, Page B-8.
• Calculate actual footprint area accounting for
  natural terrain, desired basin depth, side
  slopes.

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Description
Detention Basins

• A detention basin temporarily detains runoff
  under quiescent conditions, allowing sediment and
  particulates to settle out before the runoff is
  discharged.
• A detention basin includes a water outlet
  structure to provide controlled discharge from
  the basin to a surface water.

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Typical Detention Basin
Detention Basins
PPDG Figure B-4, Page B-15
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Detention Basins
Water Quality Outlet
Emergency Outlet
Basin Inlet
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Appropriate Applications and Siting Criteria
Detention Basins

• If conditions are not suitable for an
  infiltration basin, then a detention basin should
  be considered.
• Determine that water stored in the basin does not
  cause an objectionable backwater condition in the
  storm drain system.
• Determine that seasonally high groundwater will
  be no higher than the invert elevation of the
  basin.

PPDG Table B-4, Page B-20
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Water Quality Volume
Detention Basins

• - Detention Basins are sized to capture the Water
  Quality Volume.
• - Detention Basin Volumes are determined in
  exactly the same manner as Infiltration Basins.
• Minimum Detention Basin volume is 123 cubic
  meters.

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Preliminary Design Factors
Detention Basins

• The BMP must not negatively impact drainage of
  the roadway.
• Consult with Caltrans Hydraulics to ensure that
  the design will not compromise roadway drainage.
• It is Caltrans policy to minimize runoff from
  pervious areas to Treatment BMPs.

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Preliminary Design Factors
Detention Basins

• Maximum water level should not cause groundwater
  to occur under the roadway within 0.2 m (0.7 ft)
  of the roadway subgrade
• Provide a Maintenance access road around basin
  and provide a ramp to basin invert

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Preliminary Design Factors
Detention Basins

• Provide an Upstream diversion if possible.
• Provide a downstream overflow structure / flood
  control outlet (mandatory)
• Discharge through a water quality outlet with
  debris screen (or equivalent)
• Flows should enter at low velocity. Use scour
  protection on inflow, outfall and spillway if
  necessary.
• Use 13 side slope ratios or flatter
• Provide minimum 0.3 meters of Freeboard

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Basin Dimensions
Detention Basins
F Freeboard (0.3m minimum)
L
L (2Zy)
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• The minimum Detention Basin Length to Width (LW)
  ratio is 21 (at WQV surface, not at invert).

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Workshop Exercise 1 (15 minutes)

• Required
• For an Infiltration basin, calculate WQV, minimum
  footprint, overall dimensions.
• Given
• Tributary Area 2.0 hectares (Cave 0.95)
• Water Quality Storm Depth 2.5 cm
• Infiltration Rate 5.0 cm per hour
• Assume
• Vertical Side Slopes
• Square Basin Shape
• Seasonal High Groundwater at 5.0 meters below
  Original Grade (O.G.)

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Workshop Exercise 1 Calculations
Infiltration Basins

• Dimensions of Basin
• Step 1 Determine WQV
• WQV
• 2.0 hectares (10,000 m2 / hectare) 2.5 cm
  (1 m/100 cm) 0.95
• WQV 475 cubic meters

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Minimum Footprint
Infiltration Basins

• Aest 100SFWQV/(Kestt)
• Aest 1002475 / (5.048)
• Aest 396 square meters
• For sake of calculations, use
• Area 400 square meters

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Basin Dimensions
Infiltration Basins

• WQV 475 cubic meters
• Minimum Area 400 square meters
• Depth WQV / Area 1.12 m
• Groundwater is at 5.0 m, minimum separation is
  3.0 meters, therefore maximum depth must be less
  than
• 5.0 - 3.0 2.0 m
• Design Basin as 20 m x 20 m x 1.2 m deep, volume
  480 cubic meters. Remember to add 0.3 meters
  of freeboard. Total basin depth becomes 1.2 m
  0.3 m 1.5 meters.

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Workshop Exercise 2 (15 minutes)

• Required
• For a Detention basin, calculate WQV, available
  depth, overall dimensions.
• Given
• Tributary Area 3.0 hectares (Cave 0.85)
• Water Quality Storm Depth 3.0 cm
• Assume
• Vertical Side Slopes
• Rectangular Basin Shape, Length 2 Width
• Groundwater at 4.5 meters below O.G.

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Workshop Exercise Calculations
Detention Basins

• Dimensions of Basin
• Step 1 Determine WQV
• WQV
• 3.0 hectares (10,000 m2 / hectare) 3.0 cm
  (1 m/100 cm) 0.85
• WQV 765 cubic meters

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Calculate Available Depth
Detention Basins

• WQV 765 cubic meters
• Groundwater is at 4.5 m, minimum separation is
  3.0 meters, therefore maximum depth must be less
  than
• 4.5 - 3.0 1.5 m
• Minimum freeboard is 0.3 meters.
• Use Detention Basin Depth 1.2 meters

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Basin Dimensions
Detention Basins

• WQV 765 cubic meters
• Initial Design Depth 1.2 m
• Area WQV / Depth 765/1.2 638 m2
• Minimum Flow Path Length / Width ratio is 21
  (L/W).
• Therefore, Area L W (2W W) 2W2
• 638 2W2, Therefore W2 319, and W 17.9
• Let W 18.0 m, L 36.0 m, Depth 1.20 m
• Then Volume 778 cubic meters

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Basin Dimensions
Detention Basins
F Freeboard (0.3m minimum)
L
L (2Zy)
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Emergency Overflow / Bypass
Overflow / Bypass

• All basins must be equipped with either an
  upstream Bypass (off-line) or an Emergency
  Overflow (on-line). Bypass structures are
  preferred, since they prevent large flows from
  scouring out the basins.
• Most basins use weirs as the overflow/bypass
  structure.

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Emergency Overflow / Bypass
Overflow / Bypass

• Broad Crested Weir Calculations
• Flow Over Weir Q (C)(L)(H1.5) rearrange
  terms L (Q)/(C)(H1.5)
• Q Design Storm (Q100 or Q25)
• C Weir coefficient
• L Length of weir (perpendicular to flow) Note
  Minimum L is 1.0 meter
• H is the difference between the basin elevation
  at full depth minus the weir elevation (set at
  water surface at WQV)
• Note See HEC No. 22, Table 8-1 for C values

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Workshop Exercise 3 (15 minutes)

• Required
• For an Infiltration basin, calculate Emergency
  Overflow weir length using a broad crested weir.
• Given
• Q100 0.4 cubic meters per second
• H 0.3 meters
• (Note for this exercise, H Minimum freeboard)
• C 1.50

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Exercise 3 Solution
Overflow Weir

• Weir Calculations
• Weir Length L (Q)/(C)(H1.5)
• Solve for L (Q100)/ (C)(H1.5)
  (0.4)/(1.50)(0.31.5) 1.63 meters (Use L
  1.75 meters)
• Since 1.75 m gt Minimum weir length of 1.0 meters,
  use L 1.75 meters

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Detention Basin Water Quality Outlet Design
Detention Basins

• Refer to PPDG, Appendix B, Page B-18

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Detention Basin Water Quality Outlet Design
Detention Basins
 
PPDG Page B-17
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Water Quality Outlet DesignPage B-18 PPDG
Detention Basins
For metric units, single row of orifices a
(2106)(A)(H-Ho)0.5 / 3600CT(2g)0.5 Where
a area of drainage orifices (square mm) A
Surface area of the basin at mid elevation (sq.
meters) C orifice coefficient commonly 0.4 to
0.6 T Basin Drawdown Time (normally 40 to 48
hours) g Acceleration due to gravity (9.81
m/sec2) H Elevation of water surface at WQV
(m) Ho Elevation of basin invert (m)
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Water Quality Outlet Design
Detention Basins
For metric units, multiple rows of orifices at
(2106)(A)(hmax) / 3600CT(2g (hmax -
hcentroid))0.5 Where at total area of
drainage orifices (square mm) A Surface area
of the basin at mid elevation (sq. meters) C
orifice coefficient commonly 0.4 to 0.6 T
Basin Drawdown Time (normally 40 to 48 hours) g
Acceleration due to gravity (9.81 m/sec2) hmax
Height from lowest orifice to WQV elevation
(m) hcentroid Height from lowest orifice to
centroid of orifice configuration (m)
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Exercise 4 (15 minutes)
Detention Basins
Calculate orifice configuration (single row)
Find Total orifice area (a), Number of holes,
and orifice Diameter Given A 450 sq.
meters at mid elevation C 0.6 T 48 hours g
9.81 m/sec2 H 101.25 m Ho 100.00 m
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Exercise 4 (Outlet Design)Page B-18 PPDG
Detention Basins
For metric units, single row of orifices a
(2106)(A)(H-Ho)0.5 / 3600CT(2g)0.5 Where
a (2106)(450)(101.25-100.00)0.5
__________________________________________________
___________________ 36000.648(29.81)0.5
a 2,191 square mm Use three orifices, 120
degrees apart. Then, each orifice has an area of
a/3, so each orifice area 730 mm2 Therefore,
orifice diameter (4730)(3.14)0.5 30 mm
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Questions