A straw wattle is a common, temporary, linear device used for managing both sediment and erosion on disturbed land, particularly on slopes. These tubular structures are recognized as a Best Management Practice (BMP) within construction, landscaping, and land restoration projects. They are specifically designed to reduce the impact of water runoff and stabilize soil until permanent vegetation can be established. Their application helps to protect nearby water bodies from excess sediment.
Defining the Straw Wattle and Its Components
A straw wattle is essentially a tube-shaped device that is densely packed with natural, fibrous materials. The most common filling is weed-free agricultural straw, such as wheat or rice straw, though some variants use rice hulls, coconut coir, or wood fibers. This core material is tightly compressed and encased in a durable, flexible netting.
The outer netting is often made from a synthetic polypropylene material that is photodegradable, or from biodegradable materials like jute, coir, or burlap. Wattles are typically manufactured in standard dimensions, ranging from 8 to 12 inches in diameter and 10 to 25 feet in length. The dense packing of the organic material, often weighing around 2 to 3 pounds per linear foot, provides structural integrity and effectiveness.
How Wattles Control Erosion and Sediment
Wattles employ a dual mechanism of action to protect disturbed soil, simultaneously controlling erosion and managing sediment. Their physical presence acts as a barrier that forces stormwater runoff to slow down as it moves across a slope. This reduction in water velocity, known as flow control, significantly diminishes the water’s kinetic energy and its capacity to detach and transport soil particles, which is the definition of erosion control.
As the water’s speed decreases, the wattle also functions as a filter, allowing water to pass through while trapping suspended soil particles behind the barrier. This process, known as sediment control, causes the heavier, detached sediment to settle out on the uphill side of the wattle. By interrupting the length of a slope, the wattles promote “sheet flow,” which is a thin, non-erosive layer of water movement, and encourage water infiltration into the underlying soil. The accumulated sediment and trapped moisture also create a favorable microenvironment for the germination of new vegetation, aiding in long-term stabilization.
Proper Installation and Placement
Effective wattle performance depends entirely on proper installation, which begins with strategic placement across the site. Wattles must be positioned perpendicular to the direction of water flow and parallel to the contour of the slope to ensure they intercept the maximum amount of runoff. Correct placement also involves digging a shallow trench, typically 2 to 5 inches deep, along the contour line to “seat” the wattle.
The wattle is then firmly placed into this trench, ensuring it makes full contact with the soil to prevent water from flowing underneath, a process known as “undercutting”. The excavated soil should be compacted against the uphill side of the wattle to secure it further in the trench. Wattles are anchored using wooden stakes, which are driven through the center of the wattle and into the ground at regular intervals, often every four feet, with a stake placed near each end.
When joining multiple wattles to cover a long contour, the ends should be tightly butted together or slightly overlapped to prevent gaps where concentrated water flow could pass through. Stake length requirements, generally 18 to 24 inches, and the necessary spacing between wattles depend on the steepness of the slope and the soil type, with steeper slopes requiring closer spacing to maintain effectiveness. The stakes are driven so that a small portion, about two to three inches, remains above the wattle, providing a secure anchor against the force of runoff.