Design of Gross Solids Removal Devices

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Design of Gross Solids Removal Devices
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Gross Solids Removal Devices

• Description
• Target Pollutants
• Appropriate Applications and Siting Criteria
• Water Quality Flow
• Factors Affecting Preliminary Design
• Checklist T-1 Questions
• Workshop Exercises on BMP Design

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Description
PPDG Page B-25

• Gross solids removal devices (GSRDs) are
  structures that remove litter and solids 5 mm
  (0.25 inch nominal) and larger from the
  stormwater runoff using various screening
  technologies
• 5 mm requirement of Los Angeles River watershed
  trash TMDL.
• Two types of GSRDs are approved for use by
  Caltrans the linear radial device and the
  inclined screen device

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Linear Radial Device
Gross Solids Removal Devices
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Gross Solids Removal Devices
Inclined Screen Device
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Target Pollutants
Gross Solids Removal Devices
PPDG Table 2-2, Pg 2-7
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Applications and Siting Criteria
PPDG Page B-25

• Receiving water bodies are 303(d) listed for
  gross solids (litter/trash)
• Where recommended by Maintenance if trash
  persistently affects the storm drain system
• Where TMDLs require gross solids removal
• Require sufficient space and access for
  maintenance

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Applications and Siting Criteria
PPDG Page B-25

• Linear Radial Device
• Requires low hydraulic head to operate
• Suited for narrow and flat rights-of-way with
  limited space
• Inclined Screen
• Requires about 1.5 m (5 ft) of hydraulic head
• Well suited for fill sections

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Design Storm
Gross Solids Removal Devices

• HDM Section Design Storm That particular
  storm which contributes runoff which the drainage
  facilities are designed to handle. This storm is
  selected for design on the basis of its
  probability of exceedance or average recurrence
  interval.
• The design Storm is typically one with a return
  period of 25 years for Roadway Drainage (HDM
  Table 831.3).
• Consult District Hydraulics for determination of
  the design storm and peak flow rates.

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Factors Affecting Preliminary Design
PPDG Page B-29

• Sized to accommodate gross pollutants storage for
  a given maintenance period (typically one year)
• Litter and debris accumulation data need to be
  available to properly size the devices for the
  tributary area or use 0.7 m3/ha/yr as default
• Incorporate overflow or bypass
• Retrofits should be sized greater than or equal
  to the design flow of the pipe run

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Linear Radial Device
Gross Solids Removal Devices

• Utilizes modular well casings with 5 mm
  (0.25-inch nominal) louvers
• Contained in a concrete vault
• Flows pass radially through the louvers
• Litter and solids are trapped in casing
• Smooth bottom conveys litter to the end of the
  device

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Gross Solids Removal Devices
Linear Radial Device
Views of Linear Radial Device during Dry Weather
and Wet Weather
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Gross Solids Removal Devices
Linear Radial Device
Access for Cleaning
View of top of device with hatch removed
View of Door at End of Device
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Linear Radial Device (contd)
Gross Solids Removal Devices

• Pilot tested for 1 slope
• Flatter slopes may work but not tested
• Configuration to incorporate overflow/bypass if
  the unit becomes plugged
• Can be placed in an open channel or vault

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Linear Radial Device (contd)
Gross Solids Removal Devices

• First meter is non-louvered (blank) with an open
  top for influent bypass, if clogged . Bypass
  capacity Q25
• Louvered sections have access doors for cleaning
  with Vactor truck or other equipment
• The material shall be 316 stainless steel.

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Access to Linear Radial Device
Gross Solids Removal Devices
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Litter Accumulation in Linear Radial Device
Gross Solids Removal Devices
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Linear Radial (Preliminary Sizing)
Assume Volume of Litter VL 0.7 m3 / hectare
unless site-specific quantities are
available. Volume of Linear Radial Device VD
2 VL Area Length of device L VD / AD
where AD Cross-sectional area of device. Casing
material is available in 1.52 meter modules.
Round-up (L) to nearest multiple of 1.52
meters. Inside length of Vault to be L
Diameter 1.2 meters. Inside width of Vault to
be Diameter 0.9 m 1 m separation between
units (if more than one parallel row)
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Preliminary Sizing (Diameter)

• Testing on the Linear Radial GSRD has not been
  completed. At this time, estimate the diameter
  of the louvered pipe to be equal to the diameter
  of the storm drain system into which the GSRD is
  being inserted.
• Louvered pipe diameter goes up to 600 mm.
  Slotted pipes are available in 750 mm and 900 mm
  diameters.

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Exercise 1

• Given
• Tributary area 2.0 hectares
• There are no records on historic litter volumes
• Allowable footprint is 10 meters long x 3 meters
  wide
• Linear Radial Device using 750 mm slotted pipe
• Find
• Volume of Litter
• Volume of Device
• Footprint of Device

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Exercise 1
Solution 1) Volume of Litter Tributary area
2.0 hectares Since no site-specific quantities
are available, assume Volume of Litter VL 0.7
m3 / hectare 2 hectares Volume of Litter 1.4
m3 2) Volume of Device VD 2 VL 2.8 m3 3)
Footprint of Device Slotted Pipe Length VD /
Cross Sectional Area Length 2.8 m3 /
3.14(0.752)/4 6.34 meters Requires 6.34 /
1.52 5 sections _at_ 1.52 meters long
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Exercise 1 (Solution)
Length of Vault Width of influent channel 1.0
m Interior wall thickness 2 0.15 m 0.3
m Device Length 5 1.52 7.6 m Allowance for
Door Swing Diameter 0.75 m Total Length of
Vault 9.65 m Since total length of vault is lt
10.0 m footprint length, use only one row of
slotted pipe.
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Exercise 1 (Solution)
Width of Vault Interior wall thickness 2 0.15
m 0.30 m Device Width 1 0.75 m 0.75
m Allowance for Access 2 0.30 m 0.60 m Total
Length of Vault 1.65 m Since total width of
vault is lt 3.0 m footprint width, this
configuration will work.
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Inclined Screen Device
PPDG Page B-27

• Storm water flows over the weir and falls through
  inclined bar rack
• Wedge-wire screen, 5-mm maximum spacing between
  the bars
• Flow passes through the screen and exits via the
  discharge pipe
• Trough distributes influent over Inclined Screen
• Storm water pushes captured litter toward the
  litter storage area

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Gross Solids Removal Devices
Inclined Screen Device
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Inclined Screen Device

• Pilot studies with
• Parabolic wedge-wire screen, 3-mm openings
• Inclined at 60 degrees
• 90-degree elbow configuration used to minimize
  footprint
• Worked effectively
• Other configurations may work but have not been
  pilot tested

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Inclined Screen Device

• Other site-specific screen configurations can be
  used
• Gross solids storage area
• Sloped
• Drain to prevent standing water
• Overflow/bypass above litter storage area
• Design storage for up to one year
• Cover with load-bearing grating, if necessary
• Other screen sizes up to 5 mm (0.25 inch) may be
  available in the future

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If you ever have to incorporate a GSRD within a
project, immediately contact your District NPDES
Coordinator. The PE should establish the
estimated volume of litter, volume of device, and
footprint area. Final design will be
accomplished by a consultant.