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Silt Fence vs. Straw Wattle: An Examination of Stormwater Runoff Filtration Capabilities

When managing stormwater runoff, particularly on construction sites or other disturbed landscapes, choosing the appropriate erosion control method is critical. In the field of stormwater management, two prominent techniques are used: silt fencing and straw wattle. These methods are commonly employed to filter and slow down runoff, preventing erosion and the transportation of sediment into natural water bodies. In this article, we will compare silt fences and straw wattles, primarily focusing on their stormwater runoff filtration capabilities.

What are Silt Fences and Straw Wattles?

Before we delve into the comparison, let's clarify what these methods are.

A silt fence is a temporary sediment control device used on construction sites to protect water quality in nearby streams, rivers, lakes and seas from sediment in stormwater runoff. It's often made of a geotextile fabric that's stretched across supporting stakes. Its primary purpose is to detain runoff long enough to allow the sediment to settle out, thus acting as a barrier and protecting waterways from contamination.

On the other hand, a straw wattle is an erosion and sediment control device made up of straw, wood shavings or other similar materials bound into a tubular roll. Straw wattles are usually placed across the slope or contour of the land to break up slope length and slow water flow, reducing erosion and filtering sediment from runoff.

Filtration Capabilities: Silt Fence vs. Straw Wattle

When it comes to the stormwater runoff filtration capabilities of silt fences and straw wattles, both have unique strengths and weaknesses.

Silt Fence

Silt fences are considered effective at capturing and detaining sediment-laden stormwater, allowing the sediment to settle out before the water seeps under the fence. They can handle high flow volumes and are best used on sites with slopes, around the perimeter of a project, or downstream of any exposed soils.

However, silt fences are not designed to filter out all types of pollutants. For instance, they're less effective at removing dissolved pollutants or very fine sediment particles. Moreover, a silt fence requires regular maintenance and inspection, especially after rain events. The sediment build-up behind the fence must be removed when it reaches one-third to one-half the height of the fence to maintain its effectiveness.

Straw Wattle

Straw wattles, conversely, are best used on gentle to moderate slopes and flat areas where runoff flow rates are low to moderate. They serve to slow the flow of water, spreading it across a larger area, which helps to decrease erosion and improve infiltration.

While straw wattles may not have the high volume capacity of a silt fence, they can be highly effective in filtering out sediment, including fine particles, depending on the filler material used. Straw wattles, however, might not be as effective at removing other types of pollutants, like dissolved chemicals.

The maintenance of straw wattles is generally easier than that of silt fences. They need to be inspected regularly, especially after significant rain events, but the sediment can be left in place to become part of the landscape after the wattles decompose.

Choosing the Right Erosion Control Method

Selecting between a silt fence and straw wattle largely depends on the specific conditions and requirements of your project site.

If the primary concern is sediment and the site has steeper slopes or high flow volumes, a silt fence might be the better choice. Conversely, for lower slopes with a need for filtering fine sediments and promoting infiltration, straw wattles could be the more appropriate option.

In Utah, where we have relatively low levels of precipitation, both silt fences and straw wattles provide about the same level of sediment reduction. However, because Utah is a mountainous state, the determination of when to use which type of control really comes down to the steepness of any slopes on site. At Accena Group, we typically use straw wattle where possible because it is more cost effective, easier to install correctly, and easier to maintain.

Remember, both methods are not exclusive and can be used together to complement each other's capabilities. And importantly, these solutions should be part of a broader, comprehensive erosion and sediment control plan.

In conclusion, silt fences and straw wattles both offer valuable tools in the arsenal for controlling erosion and sediment transport in stormwater runoff. Understanding their unique filtration capacities can help project managers make more informed decisions, thereby better protecting our vital water resources.

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