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Designing Effective Barrier Installations
Floating turbidity barriers are effective in preventing silt and sediment migration into water courses. However certain rules must be followed in order to make the installation successful. Three of the most critical rules are:

  1. The silt and sediment must be contained in the retention area long enough to allow it to settle.
  2. The preceeding rule implies that the silt and sediment particle sizes will actually settle in the time alloted by the construction conditions. The table below shows the time required for particles of various sizes to settle one foot in calm water at 68 degrees F.
  3. The current in the containment area must not be so great that the particles are allowed to migrate beyond the barrier before settlement can occur.
Particle Size (mm)
Description
Time to Settle 1 Ft.
.002
Coarse Clay
24 hours
.006
Fine Silt
3 hours
.02
Medium Silt
14 minutes
.06
Coarse Silt
2 minutes
.2
Fine Sand
8 seconds
.6
Medium Sand
1 second*
*medium sand is not covered by Stokes Law - value is assumed

    
If the above conditions are met and the barrier is maintained in its proper position, it will function as intended. Maintaining the barrier in position will involve anchorage as described in other sections of this site. The figure below shows a typical cross-section through a construction site where the disturbed area is likely to cause silt and sediment migration into the waterway. To determine the best distance to place the barrier from the work site, do the following:
  1. Select the sediment particle size or description that is relavent to the site.
  2. Enter values for the flow velocity and the barrier depth.
  3. Click the Length button and the optimum position of the barrier will be displayed.
  4. This data must be regarded as only a quide since precise measurements cannot be derived from the general information on which the result is based.
Note that the flow rate has a significant effect on size of the settlement area.
.002mm- Coarse Clay
.006mm- Fine Silt
.020mm- Medium Silt
.060mm- Coarse Silt
.200mm- Fine Sand
.600mm- Medium Sand
Flow(ft/sec)
Depth(ft)