That Little Black Temp Fence Mat'l...

What’s the point of putting a 2-ft high, black plastic fence around a construction site? I have even heard the law requres it! What the heck can it possibly do? What a crock! - Jinx

Apparently, it’s called a [silt fence.]( wpc/sed_ero_controlhandbook/sf.pdf) The purpose is to allow sediment from storm or flood water to settle rather than getting washed into streams or sewers. Why exactly this is desirable, I don’t know.

Excavation of foundations exposes subsoils such as clays and silts; runoff washing this off site can cause various problems; if the silt washes into a drain, it can cause a blockage and if it runs into a pond or river, it can cause significant damage to the ecosystem there. I can’t find it now, but I remember reading an article a while back about (I think)a fish farm that suffered severe loss of stocks due to runoff contaminated with yellow clay from a nearby construction site.

You want to keep runoff out of natural bodies of water because the material blocks sunlight and inhibits the growth of underwater plants. It also reduces the amount of oxygen in the water and, if enough material is present, can silt the stream so badly that flow is completely blocked.

You also don’t want to lose the dirt for the sake of the site. After any building project, the ground will be regraded. Any rain during construction will have eroded the ground, but all the soil and sediment that gathered up near barriers will all be part of the regrading.

These barriers also help prevent light to modest wind erosion.

I should also mention something here that is probably irrelevant, but is interesting nonetheless; a while ago, I kept seeing small areas of motorway verge fenced off with a low barrier made from timber and clear polythene; I wrote to the highways agency to enquire about them and was told that they were reptile barriers; after they were erected, a herpetologist would visit and remove any wild reptiles from within the cordoned area, to avoid harming them once the planned construction work began.

Huh. And here I thought they were to keep small dogs out of the construction site…

Storm Water Pollution Prevention is part of the U.S.'s Clean Water Act, passed in 1972 and amended in 1977:

<snip>Growing public awareness and concern for controlling water pollution led to enactment of the Federal Water Pollution Control Act <snip>

<snip>The Act established the basic structure for regulating discharges of pollutants into the waters of the United States.<snip>

<snip>The Act made it unlawful for any person to discharge any pollutant from a point source into navigable waters, unless a permit was obtained under its provisions<snip>

For the construction industry, that means drawing up a Storm Water Pollution Prevention Plan (SWPPP) detailing the measures to be taken to reduce or eliminate the discharge of sediment from the job site. One of the most commonly used measures is the silt fence.

Local building permit agencies are responsible for enforcement. They can slap you with a fine or even issue a “stop work” order if you do not have your erosion controls up.