The silt fence--causes of failure

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The silt fence is one of the most effective BMPs at construction sites.  It acts as a dam along the perimeter of a site.  Water pools behind the dam, while sediment settle out.

However, in the year that I've been inspecting construction sites, I've seen a number of failures.  These failures can be serious, because the silt fence is usually the last line of defense.  Large amounts of impounded water and sediment can be suddenly released, causing a large sediment spill.

Inspectors should carefully inspect sediment fences, because they are so important, and because of ineffective installation, which is common.

When the fence fails, all that built-up water and sediment is released--causing a substantial "sediment spill."

The fence can fail because

• Too much runoff is concentrated at one place along the fence,
• The stakes holding it are too weak,
• The runoff makes and end run around the fence, or
• Gaps in the fence allow leaks.

Too much runoff in one place

Many construction sites have a shallow swale that concentrates the runoff and directs it towards one part of the perimeter.  The fence at that point can receive a too large volume of runoff.

A shallow depression leads toward the center of this fence, causing potential overload.  Note that the right stake is damaged.  Click photo to enlarge.

The lowest point on the perimeter can be barely perceptible.  Still, the fence all along the lower side of the property will channel the runoff towards the one lowest point, where water will build up, causing failure.  To prevent such concentration of water, the fence should be "scalloped."

Shallow channels or check dams should be used to redirect the runoff within the construction area, so that it's directed to several places along the perimeter, rather than just one.

The stakes are too weak (or damaged)

Specifications for the sediment fence call for stakes of a certain size.  This may be adequate for a fence that doesn't pool much water.  But where a lot of water backs up, the stakes will break.

Stakes on both sides failed, when large volumes were dammed behind the fences.

The best defence is to use larger stakes to begin with, or to inspect the fence after a storm, and beef up the stakes where it's clear that water has pooled.

One of the best ways to ensure the fence doesn't fail, due to too much runoff or weak stakes, is to have TWO rows of fences.

The water makes an end run

This is an obvious flaw, easily found by inspection.  One easy way to fix it is to use a silt sock to stop any water slipping around the end.

Gaps

One of the more common gaps is between a building (or wall) and the silt fence.  These can be plugged with wadded fabric or a silt sock.

Additional photo examples

This fence served too large an area--and most runoff was concentrated on one low spot.

Same problem.  When the runoff was concentrated on one spot, stakes failed, and the fence was overtopped.

This low corner of the lot is likely to receive much of the runoff.  Just one stake at the corner will carry much of the load.  The corner should be rounded, and additional stakes added.

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Links to more info on these sites and photos:

Photos #1 and #4:  Ellis Stone Construction Co. at Wildcat Mt. State Park

Photos # 2 and #6: Tri-North in Monona, WI.

Photos #3 and #5:  Rawson Contractors on Old Middleton Rd.(Madison, WI), street reconstruction and sediment ponds.  Rawson has had a lot of problems with erosion control   To find them, put "Rawson" into the search box, upper left.