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Tuesday, November 17, 2015

The Most Overlooked Green Site Practice

As design engineers I believe that our tendency is to focus on post construction stormwater controls and best management practices. This is natural and to some degree warranted- after all we are usually hired to design a finished product and how it gets constructed is often left to the contractor. Couple that with the fact that the insurance companies and attorneys are always advising design professionals that "we are not responsible, in any way, for the means, methods, sequence, procedures, techniques, scheduling of construction" and it is understandable that the focus is on post construction stormwater management. However, because of that I think Erosion Prevention and Sediment Controls (EPSC) during construction are often overlooked as a green site design and planning technique. Give your erosion prevention and sediment controls a little love!



It might not be sexy but EPSCs or lack thereof can have a tremendous environmental impact. According to the EPA a typical, unmanaged construction site can lose approximately 35-45 tons or soil per acre in one year. By comparison, forest or farm land will lose 1 ton or less per acre over the same time period. Left unmanaged that soil can travel downstream and clog natural and man-made waterways, affect water supplies, damage aquatic life, and otherwise adversely impact adjacent property owners and waterways. In addition, bare soil can increase runoff velocities resulting in further erosion on site and downstream, increase runoff volumes causing flooding and reducing groundwater recharge and increase water temperatures negatively impacting aquatic species. Beyond that, if you ever want to upset the neighbor of one of your construction sites try dumping some dirty water or sediment on their property - it will definitely get their attention!

If you are applying LID (Low Impact Development) techniques to your site, it becomes even more critical to pay attention to the erosion and sediment controls. I have experienced this on some of the first LID projects that I designed and I saw otherwise good LID designs perform poorly because the contractor did not properly install and maintain erosion controls. With traditional stormwater controls much of the flow is directed to catch basins and pipes where it is easy to manage sediment and control flows. However, when the post construction design relies on vegetative practices such as swales and raingardens it is critical to prevent erosion rather than just control sediment. And it is equally important to make sure that infiltration practices don't become clogged and compacted during construction which can negatively impact their post construction performance. Many times these items are overlooked because LID techniques are new to the design and construction team.

So what can you do? Of course it is very site specific and you may have little or no control over construction phase activities, but there are some very basic tenants that you can apply that will do wonders for your erosion and sediment controls during construction.
  1. Disturb as little area as possible - the most basic thing that you can do to reduce soil erosion and sediment loss is to limit the amount of area that you disturb. You can do this by getting involved early in the planning process and working with the architect or planner to locate the building and infrastructure and being mindful of the natural slopes and general topography.
  2. Phase your grading operations - the common practice on most sites, especially smaller ones, is for the excavator to come in at the begging of the project, strip all of the topsoil in disturbed areas and grade the whole site to sub-grade. Much or all of the site is then left disturbed for the life of the construction project. By phasing the grading operations you can limit the amount of time the land is disturbed and therefore limit the amount of time that it can erode.
  3. Reduce slope gradients and lengths - steep and long slopes erode exponentially more than shorter flatter slopes. Try to keep slopes to 4:1 or less and less than 40' long. If slopes are steeper and/or longer they should be terraced, furrowed, serrated or stepped.
  4. Establish vegetation on disturbed areas - the best way to limit erosion and control sediment is to get vegetation established. A good stand of grass will beat silt fence any day. Here is a case where the color green really means something - a brown construction site will not perform as well as a green one!Unfortunately, the vegetation is often put off until the completion of the project rather than using it during construction to control erosion.
  5. Maintenance, maintenance, maintenance - don't wait for erosion controls to fail or for the local of state authorities to come calling before cleaning and maintaining erosion controls. Its always easier to maintain than it is to fix it once it gets out of hand.
The five techniques above are really just a sampling of what can be done to help prevent erosion and sediment loss, but I think that the key points are to be mindful of it in the design phase, stay on top of it and establish vegetation early. Traditionally the focus has been more on sediment control, yet it should be on erosion prevention. Doing that will limit environmental impact and improve the post construction performance of storm water controls.
http://www.thegreencivilengineer.com/