Install the Skimmer and Let it Simmer! Water Quality Control During Construction

Figure 1. An alluvial fan visualizes how sediment is transported by a river and then deposited at the river mouth when the flow slows down, changing the terrain and how water flows over the surface, finding a new low point (WorldHistory.org).

​Sediment and erosion control is always a challenge on construction sites when soil is exposed to the wind and rain. As rain falls on the exposed soil, it loosens it up and begins to wash away as fine-grained sediment. Sediment is the top pollutant impacting Ohio’s streams. It clouds the water column, choking out aquatic life, and changes stream morphology by settling out in areas of slow flow. Alluvial fans are a great large-scale visualization of this (Fig. 1). As the water slows down, it drops the sediment out which raises the ground level. Water will seek a new path of least resistance. Hopefully not towards someone’s house! Sediment can change stream morphology too. Deep pools within a stream provide cool-water habitat for fish and these can be filled in as sediment is deposited. What once was a great swimming hole is now too shallow. One way we can reduce sediment discharge to Ohio’s lakes and streams is by installing sediment settling basins on construction sites. These are required by the Construction General Permit as the stormwater must be treated before being discharged from the site.

​A sediment basin is a temporary settling pond that filters runoff and releases it at a controlled rate. They can be used for sites where the drainage area is 100 acres or less. Any project that increases the impervious surface is required to have a permanent, post-construction stormwater basin, so it makes sense to install the sediment basin on the low point of a site, where it will be converted to its post-construction design at the end of the project. They should be installed as a first step to the mass grading process, so they can be functional and ready to accept sediment-laden runoff once the earth is disturbed. Since sediment basins trap sediment, the sediment will need to be removed before project completion. This ensures that the basin has the proper water quality storage capacity when it gets converted to the post-construction stormwater design at the end of the project. All these considerations should guide the design process.

Figure 2. A sediment basin should be stabilized as soon as possible to prevent bank erosion and reduce sediment deposition in the bottom of the basin, decreasing capacity of the water quality volume. Straw matting is recommended for the slopes.

​Ohio EPA’s Rainwater and Land Development Manual (https://epa.ohio.gov/divisions-and-offices/surface-water/guides-manuals/rainwater-and-land-development) is the guiding document for stormwater practices used during development. Chapter 6 covers sediment basin design. Figure 3 illustrates the various components of the design criteria, which includes pool design, embankment design, dewatering design, and spillway design. A floating skimmer device (Fig. 4) is required throughout the duration of the project to drain the water from the top of the water column, where the sediment is less concentrated. So, install the skimmer and let it simmer!

Figure 3. A sediment basin will have a sediment storage zone, a dewatering zone and a skimmer device attached to the outlet structure to dewater from the top of the water column (Ohio EPA).

The Ohio EPA has created a sediment basin compliance tool, which we require to be completed for all projects. This tool is helpful to determine the sizing of your basin by inputting numbers relating to total disturbed area and total drainage area draining to the basin. The dewatering zone shall be a minimum of 1800 cubic feet per acre of drainage while the sediment storage zone shall be 1000 cubic yards per disturbed acre that drains to the basin. After inputting the drainage area and disturbed drainage area, it will calculate the required sediment and dewatering zone volumes as well as the orifice size and skimmer size. There is also a Water Quality Volume tool very similar to this tool that guides the post-construction stormwater design. Our office is happy to provide these tools to you upon request.
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Stabilization is not just important for upland controls, but the basin itself should be smooth graded and stabilized as a first step to the mass grading process. That way the basin can be in place before any upslope disturbance occurs to accept sediment-laden stormwater. Towards the end of the project, when the entire site is stabilized, the sediment basin needs to be converted to its post-construction stormwater design. This usually entails removing the skimmer device and installing an orifice plate to achieve the proper water quality orifice size for post-construction treatment of stormwater. As-Builts need to be completed and submitted to the Engineer of Record for the municipality.

Figure 4. A Faircloth skimmer is one of a few different skimmers available on the market. Proper installation is critical as it must be able to rise and fall with the water level, and be sealed tight where it connect to the outlet structure to prevent bypassing (Ohio EPA).

​Sediment basins should not solely be relied upon as the only sediment and erosion control measures on a site. Ohio EPA has found that they are only about 50-80% effective at removing sediment. The finer silt particles remain suspended in the water column and are too small to filter out. That’s why a whole toolbox of SECM’s should be used on a site.  The best management practice (BMP) is to prevent erosion and sediment transport in the first place by stabilizing areas at final grade or areas that lie dormant for longer than 14 days. Ohio EPA has found that erosion control BMPs like stabilization are 90-98% effective. So, it takes multiple tools to properly handle stormwater runoff on a site. Sediment basins are a crucial part of the stormwater solution by satisfying both water quantity and water quality requirements. And don’t forget, install the skimmer and let it simmer!

For assistance, please call our office at 513.695.1337

​Article Written by Justin Bedocs