Erosion and Sediment Control Tip No. 23




 

It is often amazing how terms tend to “catch on” over time.  Take for instance the term “Silt Fence”.  While it does resemble a fence, it really has very little to do with silt.  From what we know today about a Silt Fence, it should really be called a “Geotextile Coarse Particle Sediment Barrier Impoundment Fence”.  This is a mouthful, so I guess Silt Fence is OK.

 

Here's what we know about Silt Fences:

  • They should be installed in a manner to pond runoff, either on the contour, in a j-hook, or a c-configuration.
  • Ponding water allows for coarse particles to settle from suspension.
  • Clays and fine silts tend to stay in suspension a very long time.
  • Silt Fences alone do not really reduce turbidity.
  • The design runoff volume should be less than the storage volume available behind the fence.
  • The runoff from a construction site will cause the geotextile to “blind” or clog and drastically reduce the flow-thru over time.
  • Less undermining occurs with an off-set installation.
  • If long term ponding is an issue, use a controlled dewatering device.
  • Plan for an excess rainfall event that needs to safely bypass over the top of the fence.
  • If properly designed and installed, they can be very effective as a ditch check practice.

 

While GCPSBIFs (Silt Fences) have their place on the construction site, don’t use them to fool the unknowing public that effective erosion and sediment control is being performed on the site.  Effective erosion and sediment control occurs upstream of the Silt Fence.

 

I’ll get off my soapbox now.

 

 

Perry L Oakes, PE

Erosion and Sediment Control Program Coordinator

AL Soil and Water Conservation Committee

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