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As civil engineers and surveyors, there are many aspects of our daily jobs that are second nature to us. But we hear common questions about our field often from the public. In this “Did You Know” blog series, we’ll answer some of these questions.
On any transportation or site engineering project, erosion control is always an element we factor in and plan for. While permits are required on projects that involve more than one acre of disturbance, erosion control is important on projects of all sizes.
With erosion control, we are trying to keep the sediment and dirt in place while reducing run-off to maintain clean water. It’s especially important for projects in the Birmingham area since water flows to the Cahaba River, which is a major water source for the City of Birmingham Water Works. Eventually the public will be drinking this water. Clean water is also very important for wildlife.
When we see mud running into creeks or through ditches, it’s an indicator that erosion control has not been handled properly. So what are some of the ways we can handle it the right way.
Wattles – You’ve probably seen these big rolls of hay in ditches on the side of the road or around construction sites. As water flows through the wattles, it traps dirt and sediment and allows the clean water to run through.
Silt fencing. These are common at construction sites as well. This black fabric is attached to wooden stakes and is similar to wattles in how they function to filter sediment as it runs off from the site.
Grass. Ever notice beautiful green grass that has grown around a construction site in what seems like just a few days? Once construction is complete, contractors are required to have vegetation growing within 14 days to help with erosion control. In the case of grass, they add fast growing seeds or sod that will quickly establish roots to help hold dirt in place.
Sediment traps or basins. On site projects, it’s common to see sediment traps or basins that involve digging out a pond area where turbid water can be trapped. Sediment traps or basins are used primarily during major grading phases of projects. They allow water to pond so that sediment can settle to the bottom of the basin and the cleanest water, located at the top of the pond, is allowed to drain. Once grading is complete these basins are typically replaced by other erosion control measures.
Innovative products. While the above measures are very effective, there are times when out of the box thinking is also helpful, such as soil treatments or flocculation. These techniques may include applying a sticky substance to “glue” soil in place to prevent soil particles from being washed away. Another technique could include application of flocculation blocks into the flow stream of turbid storm water, which causes the sediment to settle out more quickly before the water leaves the construction site.
Many times, these measures are used in conjunction with each other to provide the greatest possible erosion control. It’s also not always a one-time process. Measures may be taken initially, but they have to be maintained over the life of the project. If wattles get clogged or silt fences are knocked down, those things have to be fixed in order for erosion to be controlled properly.