For some years now, you and I have seen long ribbons of black or orange sheeting surrounding construction sites in the mid-valley. I guessed these low fences were intended to prevent dirt and water from escaping, and wondered if they did any good. Now that I've finally looked them up, it's apparent to me that in many cases stringing all that "geotextile fabric" is a waste of money and time.
What we are looking on construction sites are "sediment fences," recommended as a best management practice by the Oregon Department of Environmental Quality and evidently -- from their ubiquity -- required by some code. As the term implies, the sheeting is supposed to control sediment from construction sites lest it escape and dirty the waters of the state.
From a DEQ manual: "Sediment fencing causes heavy soil particles to be retained both through a filtering operation and through the creation of a small settling basin upslope of the fence through restriction and retardation of the runoff flow velocity. The main benefit of sediment fencing is slowing the runoff, which causes the heavier particles in the runoff to settle out."
Sounds reasonable -- except in places where as a result of the flat topography there is likely to be no runoff even if it rains a lot. A year or two ago, the gas company built many miles of pipeline through the valley, and sediment fencing lined both sides of the construction corridor the entire way, which looked especially pointless where the work was done in perfectly level grass-seed fields.
The fences are supposed to be set in trenches 6 inches deeep, but I'm not sure that is always done. Many of these barriers don't look to me as though they could slow any flowing water.
Though they may be unnecessary, they still cost money. In a comparison of erosion control measures, the Environmental Protection Agency says 400 feet of 3-foot fencing of this type on a 6-month construction project would average $3,200-$3,800 for materials, installation, repairs, removal and disposal. Lowe's sells a 100-foot roll of 2-foot sediment fencing for $22.47, according to its website.
So, compared to the cost of construction, the price of sediment fencing is negligible. But if the fencing doesn't do anything, it's still a waste. (hh)