Clear Water Thoughts / To Silt Fence or Not

There are a lot of ways of installing silt fence, the problem is that more often than not, it's the wrong way. Ok, so what's so hard about installing silt fence properly. There are a few key things to understand and remember to do it right. The first is that silt fence, as shown in the drawing below is designed to be installed in a trench, typically 6" deep and 6" wide so that the fabric forms an L shape in the trench. The second point is that the back fill over the silt fence in the trench must be compacted in order to secure the fabric properly. The silt fence posts must be located on the downhill side of the fabric, so that the fabric when under water pressure from runoff does not pull away from the stakes securing the fabric. Finally the joining of each length of silt fence (100') must not only overlap, but actually spin the end posts 1 full turn of of the fabric prior to pounding in the stakes.

There are several tricks to installing silt fence properly, and I think one of the best is to trench down 12" and backfill 6", this technique reduces the number of broken stakes considerably.
The second trick is to stretch the fabric straight and level between the stakes, and reinforce the attachment of fabric and stake with a zip tie.

Remember the next time you see blue sky under the silt fence, it's in wrong and will do nothing to minimize sediment flow. Silt Fence is also not designed to be placed in a direct flow of water, e.g. accross a stream, dry or otherwise, because it will blow out quickly. On the other hand, it is excellent when installed a foot or so out from the toe of the slope. The other key element for silt fence to be effective is maintenance. Silt will settle on the uphill side of the silt fence, and will bulge out. It is critical that these sediment materials be removed prior to the next rain event. Finally silt fence can be used in combination with other BMP's, for example a gravel bag berm on the uphill side of the fence may be needed to help control and minimze the water pressure against the actual fence. Remember the idea behind these BMP's is to slow the water down long enough so that the sediment has an opportunity to drop out and settle.

What's in your bag?

Over the last few years I have become aware that most people feel that a sand bag is a sand bag, is a sand bag. But there is a difference, especially if we are talking about compliance with the NPDES permit and specifically using the right type of bag for the job.

Sand Bags, i.e. bags filled with "sand", have a specific, and limited application to either become part of a dam or to divert storm water around a working area (run-on control).

Gravel Bags, specifically bags filled with birds eye (5/16") gravel, are the bag of choice in most jurisdictions because they allow water to slow down as it passes through and therefor allows the sediment to drop out (settle) behind the bag. The advantage is that it reduces the amount storm water de-watering required, as it allows water passing out of the site to be sediment free.

Rock Bags, typically 3/4" Rock is specified and often required by jurisdictions with especially fine silts which might clog regular gravel bags and therefor become ineffective except as a dam.

So, why is the wrong bag in the wrong application on the job, it isn't cost, because the pricing is usually the same, or relatively close, no, the reason is a lack of understanding, either by the inspector, the contractor or both. Do the bag suppliers know the difference? They should, or you may as well use dirt bags...Oh, that's right, some people still do.

Linear BMP,s Gravel Bags

So now you've had time to consider Worst Practices, but what about Best Practices? What are Best Practices? Best Management Practices, BMP's are a combination of materials and procedures to minimize sediment and other pollutants from entering the storm drain system. These BMP's can be used singly or in combination with other BMP's. The following is a simple description of basic linear BMP's used to protect areas outside of the construction zone. The first consists of a Gravel Bag Berm, which typically consists of a double high line of gravel bags placed along the perimeter of a project. The objective of this Bag Berm is to slow rain related run-off long enough that the sediments settle or dropout behind the bags, and that only Clear Water runs out of a site. The same objective is achieved using check dams and gravel bag chevrons, on sloped areas with added benefit that the run-off is also slowed down sufficiently to reduce water velocity and therefore the effects of erosion on the slope surface.

As you can see from the pic on the lower right, it is far from a perfect science, but if implemented properly can go a long way to mitigating contaminated run-off.

Also keep in mind that the new EPA standards will be adopted by the revised California NPDES Permit, which in turn means your TMDLs will be much more tightly monitored.

Clear Water Thoughts Tuesday March 24th 2009

Recently I stopped in at a local electronics store and noticed that the adjoining construction site was not protecting the inlets and perimeter properly. I find it interesting that in this day and age, in Orange County that I would see such a poor implementation of basic BMP's. It's one thing to have properly installed BMP's fail when you have a sudden heavy rain, it's another to choose not to properly implement and install them in the first place. The problem is pretty basic, either because of ignorance or neglect this contractor looks like they don't care enough to protect the perimeter and the storm drain system. You say, so what a little dirt goes down the storm drain, along with other pollutants, but do you realize that this dirty water flows straight into the ocean with no treatment. The storm drain system is designed to take away rain water, not construction contaminated rain water. As it stands now, cities, counties, the state and therefor we the people get to pay for the clean up. It costs millions of dollars a year to maintain the storm drain system, a significant part of the clean up is from trash discarded in the street, but a significant portion of the sediments and other chemicals can be traced back to construction. If you know of any particularly nasty sights in Southern California, please share the info here so we can get on them to stop mucking up our world.

Worst Practices 03/23/09

As you may well know the Clean Water Act, and specifically the states NPDES Permit are the guiding documents for Storm Water issues within the state (Ca). I was recently in Orange County Ca, and saw a great example of worst practices, as opposed to best practices. Being a certified erosion & sediment control person, I was absolutely amazed to see the complete lack of understanding of basic controls, and proper implementation. There appeared to be little or no regard for either the environment, the regulations, the construction companies reputation as a polluter or the trade craft of erosion & sediment control. I realize that budgets are beyond tight, but I also realize that failure to implement properly, the most basic of BMP's, Best Management Practices, violates us all. It allows pollutants, sediment and other contaminants to flow into our storm drains and straight out to the ocean. As you probably know storm drains, unlike sewers, have no treatement and flow straight into the streams and rivers and ultimately to the ocean.