#3 - Erosion and Sediment Controls

We have all driven by a construction site and have seen hundreds of meters of silt fence, rock check dams, straw wattles and perhaps bales of hay installed everywhere.

"Wow! That owner, the engineers and contractors are taking environmental protection seriously."

But is that true? I would argue that much of what you see as you drive by these sites are more show than go. Let's start with what erosion and sediment controls (ESC) are supposed to do.

ESC are best management practices and structures designed to reduce loss of material from a work site and prevent its migration primarily into aquatic habitats. The loss prevention aspect is secondary to the prevention of deleterious substances from impacting an aquatic ecosystem. However, a focused approach on preventing erosion from occurring almost completely solves the sedimentation problem.

Erosion control is simply the process of reducing the transfer of energy from wind, rain, and human interaction to the soil. Intuitively, it makes sense that if there is no energy available to erode soil, no erosion can occur. If there is no erosion, there is no potential for sedimentation.

Best management practices such as keeping the disturbed area as small as possible, leaving as much undisturbed vegetation on site as possible until it is necessary to disturb, use of temporary road mats (swamp matting), low slope profiles, diversion drainage systems, and covering of disturbed areas with geotextiles, woven mats or even tarps all are designed to reduce and dissapate energy transfer to the soils.

Sedimentation controls also are designed to reduce energy but specifically, the flow velocity of water. Faster flow means more and larger sediment particles can be transported. Sediment controls are largely structures that create semi-permeable dams. As flowing water encounters a dam, it begins to back up, and the velocity behind the dam is reduced. Sediment begins to drop out of suspension. For these controls of work effectively, they need to resemble a dam that can slow the flow of water.

Let us return to the construction site we are passing through (review that first picture again). That long line of silt fencing in the grass parallel to the disturbed area...what exactly is its purpose? Where are the "sediment particles" coming from? How is that line of fencing supposed to dam water? Lastly, where is the closest aquatic ecosystem that the sedimentation controls are protecting from the release of deleterious substances?

Alberta Transportation's 2011 Erosion and Sediment Control Manual's Best Management Practice #1 makes it very clear that silt fencing should be constructed in either a "J" or a "Smile" configuration and that long installations should be avoided. The example below shows how a properly functioning silt fence creates a low velocity backwater pond that allows sediment to fall out of suspension in the water column (there are still a lot of improvements to this ESC plan that could be made, but it does show how a silt fence works...).

This leads me to ask the question. If the company designing your erosion and sediment control plan fails to fully understand AT's BMP #1, what else is wrong? Or maybe, just maybe, it is just a show.