Most construction projects involve earth-moving activities, which expose soil to high erosion rates. This could lead to heavy sediment loading into nearby streams. Studies have shown that erosion rates on active construction sites are 10 to 100 times those of farm fields, and are even worse relative to stable vegetated areas. Single storms might erode more soil than a whole year of erosion on an agricultural field. Fortunately, sediment control measures usually are required, including a sediment basin near the lowest elevation point to capture sediment in runoff prior to discharge from the site. Simply direct all the runoff into the basin and problem solved, right? Well, that depends on how the basin is designed and maintained, of course.
Research has found a wide variation in sediment capture rates under field and controlled conditions. When sediment basins were first introduced to construction site management, the concept was to have them release water relatively quickly to avoid overtopping during heavy storm events. The typical outlet for the basins consisted of a porous dam constructed of large rocks covered with a layer of gravel. Investigations into the sediment capture rates for these basins showed that only about half of the sediment in the runoff was being trapped, resulting in considerable sediment released from these sites. Most would agree that this is unacceptable as a best management practice.
One problem in that design was that the porous dam released water too fast to allow sufficient settling. In addition, an outlet that releases water from the entire water column is going to release the dirtiest water—near the bottom, where sediment is settling—as well as the cleaner water near the surface. A solution to this problem would be to remove the basin water from near the surface only—but how?
The first product to achieve this goal was the Faircloth skimmer, which floats on the surface with an outlet pipe connected to a flexible hose attached to a pipe through a solid dam. This dewaters the basin slowly, allowing it to fill and giving the sediment time to settle. When storm flows exceed the storage/drainage capacity, water is released through a lined spillway, which, in effect, also is a surface outlet. There are now a number of skimmer devices on the market, and in states with recently updated Construction General Permits from the U. S. Environmental Protection Agency, surface outlets are required.
With the outlet problem solved, the other problem in any basin designed for settling is turbulence and preferential flow or short-circuiting. Runoff on a construction site typically is collected in one or more ditches and routed into the basin. When this concentrated flow hits the basin, it flows directly toward the opposite side, bypassing much of the basin volume and creating tremendous turbulence.
Water treatment engineers have long been using solid baffles to intercept the incoming flow and redirect it either side-to-side or over-and-under to create a longer flow path and greater retention time. However, we discovered that using porous baffles across the entire basin was more effective in both forcing the flow across the entire basin width and reducing turbulence. With these, we also found increased sediment capture compared to either no baffles or traditional solid baffles. The design used by the North Carolina Department of Transportation, incorporating coir baffles (700 g m-2), can be found at http://tinyurl.com/zrqsvv6.
When the surface outlet and porous baffle features are added to a sediment basin, how much sediment do you capture? We measured this on active construction sites and found that more than 90% of the incoming sediment was being captured. Mark that an “A” on your environmental impact exam.
