Design of Traction Sand Traps

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Design of Traction Sand Traps
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Traction Sand Traps
Traction Sand Trap

• Description
• Applications and Siting Criteria
• Factors affecting preliminary design
• Workshop exercise

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General Description
Traction Sand Trap

• Device that temporarily detains runoff to allow
  traction sand or abrasives to settle out.

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Types of Traction Sand Traps
Traction Sand Trap

• Vault
• An underground concrete structure designed with a
  sedimentation chamber and peak flow diversion.
• Modified CMP Riser
• An underground vertical CMP that captures
  traction sand below the invert of the outlet pipe
  (the outlet pipe invert is above the invert of
  the vertical CMP)

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Types of Traction Sand Traps (continued)
Traction Sand Trap

• Infiltration Basin Forebay
• See Appendix B (Section B.3 of the PPDG).
  Forebays are designed to capture sediment prior
  to discharging into the basin. Forebays may be
  sized to accommodate additional traction sand
  loading.
• Detention Basin
• See Appendix B (Section B.4 of the PPDG).
  Detention basins may be sized to store additional
  traction sand loading. Preferred method of
  capturing Traction Sand.

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Vault
Traction Sand Trap

• Below grade structure placed in-line or off-line
  in storm drain systems to capture Traction Sand.
• Vault has sedimentation chamber to slow flow
  velocities and settle sand.
• Sites with large traction sand volumes and not
  limited by space are most suitable for detention
  basins, not vaults.

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Outlet from louvered pipe
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Corrugated Metal Pipe
Traction Sand Trap

• Below grade structure that uses commercially
  available corrugated metal pipe (CMP) placed
  vertically with an outflow pipe offset from the
  invert of the trap to capture sand.
• Sites with small traction sand volumes and/or
  limited space are most suitable for modified CMP
  traps.

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Modified CMP

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Modified Corrugated Metal Pipe
Traction Sand Trap

This is a double-barreled traction sand trap with
asphalt dikes.
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Infiltration Basin Forebay
Traction Sand Trap

• Infiltration basins may be designed with a
  sedimentation forebay to settle and store the
  anticipated traction sand volume.
• Sedimentation forebay is sized to retain the
  traction sand volume and the sediment load
  associated with site conditions (sediment loads
  not associated with winter activities).

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Infiltration Basin Forebay
Traction Sand Trap
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Detention Basin
Traction Sand Trap

• Detention basins may be designed with additional
  storage volume for the anticipated traction sand.
• Detention basins should be designed per Appendix
  B of the PPDG.

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Detention Basin
Traction Sand Trap
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Traction Sand Trap - Siting and Design Criteria
Traction Sand Trap

• Sites where traction sand or abrasives are
  commonly ( gt twice per year) applied to the
  roadway.
• Detention Basins are considered first.
• Consult District NPDES Storm Water Coordinator to
  ensure that the BMP is not classified and
  regulated as a underground injection well.
• Locate device so water is not introduced above
  the roadway subgrade in case of blockage.

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Preliminary Design Factors
Traction Sand Trap

• Design for anticipated sand recovery volume.
• Sufficient volume to store the settled sand
  through the winter and avoid scour.
• Sufficient hydraulic head for gravity flow.
• Inlet and outlet arrangement to minimize short
  circuiting of the flow.
• Invert depth no greater than 3 meters if cleaned
  by vactor trucks.

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Preliminary Design Factors (CMP type)
Traction Sand Trap

• Weep holes to allow proper drainage and prevent
  standing water.
• Invert of the sand trap 1 to 2 m above
  groundwater.
• Maintenance space and/or access ramps for large
  equipment to clean out traps.
• Flow velocities within the trap should be
  minimized (0.6 m/s is typically used) to promote
  particle settling and prevent scour.

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Preliminary Design Factors (CMP type)
Traction Sand Trap

• If the design storage volume cannot be met with
  one sand trap device then additional storage can
  be obtained by
• adding more traps in-line
• increasing frequency of clean-out
• Using basin or vault-type traction sand trap
• discussing other options with District
  Maintenance and Storm Water Coordinator.

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Sand Volume (All types)
PPDG Page B-21

• V (S)(R)(L)(E)/F
• Where
• V Total volume of traction sand that must be
  stored (m3).
• S Estimated volume of sand applied (m3/yr).
• R Reduction factor to account for sand
  recovered by roadway sweeping.
• L Factor to account for other miscellaneous
  losses/accumulations.
• E Estimated recovery efficiency.
• F Number of times the trap will be cleaned
  (times/yr).

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Estimating the Volume of Traction Sand
PPDG Page B-21

• S Estimate of the volume of sand applied
  (m3/yr)
• Typical application rates range from 47
  m3/lane/km/yr for area with average application
  rates to 95 m3/lane/km/yr for areas with high
  application rates.
• Consult District Maintenance.

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Estimating Volume
PPDG Page B-21

• Factors that may affect traction sand application
  volume may include
• Exposure Roadways on north facing slopes
  generally require more traction sand than similar
  south facing slopes. The surrounding vegetation
  may also significantly affect exposure.
• Roadway grade Steeper grades generally receive
  more traction sand.
• Other sources of similar material Adjacent cut
  slopes and other non-paved tributary areas may
  contribute similar-sized sediment or other debris
  that will be retained in the trap.

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Estimating Volume
PPDG Page B-22

• R Reduction factor to account for sand removed
  by roadway sweeping.
• Estimate a value between 1.0 (no roadway
  sweeping) and 0.6 (aggressive winter roadway
  sweeping).
• Consult with Maintenance.

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Estimating Volume
PPDG Page B-22

• L Factor to account for other miscellaneous
  losses or accumulations.
• Accounts for sand that has been carried into or
  out of the tributary area by various means such
  as wind, snow clearing equipment, or tracking by
  vehicles.
• Estimate an appropriate value in the range of 0.8
  (high losses from known sources such as snow
  blowers) to 1.2 (high accumulation from known
  sources).
• Use a factor of 1.0 for no losses/accumulations.

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Estimating Volume
PPDG Page B-22

• E Estimated recovery efficiency
• Accounts for traction sand that passes through
  the sand trap without settling out.
• Not all the sand can be recovered because of
  particle size limitations, settling
  inefficiencies, turbulent flow conditions, and
  other factors.
• Until empirical information is obtained from
  pilot studies, a value of 1.0 should be used.

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Estimating Volume
PPDG Page B-22

• F Number of times trap will be cleaned per
  year.
• Typically F 1.0, as most basins are cleaned
  once per year (normally in the summer).
• If obtaining the required storage volume is
  difficult, it may be possible to implement
  mid-season cleaning.
• Consult District Maintenance staff to make sure
  mid-season cleaning is practicable and feasible.
• Mid-winter cleaning will also likely affect trap
  design
• Maintenance equipment must access the trap under
  wet or snowy conditions.
• Cleaning equipment and trap manhole covers or
  lids must be operable during cold conditions.

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Exercise 1
Traction Sand Trap

• Given
• Lies on a north-facing slope
• No records of sand application amounts available
• Heavy snow area (Sand applied gt 15 times/year)
• Moderately steep slopes
• Two-lane highway
• Snow is plowed towards cut slope
• Is part of an aggressive roadway sweeping program
  
• Treatment area length of 0.4 km
• Assume detention and infiltration basins are not
  feasible

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Exercise 1
Traction Sand Trap

• Find
• Estimate the total volume of traction sand
  applied (S).
• Determine the total volume of traction sand that
  should be stored (V).
• Which sand trap type is most appropriate to store
  the traction sand volume?
• Determine total volume of traction sand to be
  stored (V) if snow blowers were used?

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Traction Sand Trap
Exercise 1 (Solution)

• Estimate the amount of sand applied
• North facing slope, moderate roadway slopes, and
  greater than 15 applications per year should
  yield an high application rate 95
  m3/lane/km/yr.
• Two-lane highway, therefore annual load estimate
  (S) 2 lanes x 95 m3/lane/km/yr 190 m3/km/yr.
• The sand trap will treat 0.4 kilometers of
  roadway, therefore the annual volume applied is
  190 m3/km/yr x 0.4 km S 76 m3/yr.

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Traction Sand Trap
Exercise 1 (Solution)

• The site is aggressively swept throughout the
  winter, therefore a reduction factor (R) 0.6
  should be applied.
• Snow plows push snow to the inside shoulder which
  accumulates traction sand as well as side slope
  sediment. The total volume estimate should then
  increase due to the additional sediment load (L
  1.2)
• Use recovery efficiency (E) 1.0
• Assume trap is cleaned once a year (F 1.0)

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Traction Sand Trap
Exercise 1 (Solution)

• The equation for calculating the volume of
  traction sand storage is
• V (S)(R)(L)(E)/F
• V (76)(0.6)(1.2)(1.0)/1 54.7 m3
• Design Sand Trap for 55 m 3

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Traction Sand Trap
Exercise 1 (Solution)

• Which traction sand trap is most appropriate to
  store 55 cubic meters of sand?
• Calculate the volume of one 900 mm CMP riser
  inlet with a 450 mm outlet pipe

Total available depth (assume there is minimal
distance from inlet to crown of the outflow
pipe) (3 m - 0.15 m - 0.45 m) 2.4 m available
depth Area of the 914 mm pipe (0.914m/2)2 x P
0.66 m2 Volume of one CMP riser 0.66 m2 x
2.4 m 1.6 m3 Number of 914 mm CMP risers
needed for traction sand storage 55 m3 /1.6 m3
34.4 or 35 risers in-line
3m
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Traction Sand Trap
Exercise 1 (Solution)

• Find size of sediment chamber within a vault
• Assume a depth of 2.5 m (Note maximum 3.0 m)
• Assume a width of 2 m
• Required vault length volume / (Width x Depth)
  55/(2.5 x 2) 11 m
• Length may exceed site constraints, therefore
  increase the width to 3 m
• Required vault length 55/(2.5x3) 7.33 m
• A detention basin or vault-type traction sand
  trap is most appropriate for this site due to the
  excessive number of CMP risers needed for
  sediment storage

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Traction Sand Trap
Exercise 1 (Solution)

• If the maintenance crews used snow blowers then
  the L factor would decrease from 1.2 to 0.8.
• The new traction sand volume is
• V (SxRxLxE)/F (76x0.6x0.8x1.0)/1.0 36.5 m3
  or 37 m3

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Questions