Straw wattles, also known as fiber rolls or sediment logs, are cylindrical structures used in land management to protect disturbed soil from water erosion. They are a standard best management practice on construction sites and in land restoration projects, particularly on slopes and near waterways. A wattle consists of a mesh tube tightly packed with fibrous material, which forms a semi-permeable barrier against water flow. These logs slow down runoff and trap suspended soil particles before they cause environmental damage.
Composition and Structure of Wattles
A straw wattle’s physical makeup is designed to be environmentally conscious. The core filling material is compressed straw, often sourced from wheat or rice, and sometimes certified as weed-free to prevent the introduction of invasive species. Some variations may use other natural fibers, such as coconut coir or excelsior (shredded wood fibers), which provide similar filtering and structural properties. The density of this compressed filling determines how efficiently the log can slow water and filter sediment.
The fibrous core is contained within an outer netting that maintains the cylindrical shape of the wattle. This netting is made from materials like biodegradable jute, burlap, or photodegradable plastic polymers. Standard wattles measure 8 to 12 inches in diameter and 10 to 25 feet in length, weighing 35 to 50 pounds on average. The use of biodegradable materials ensures that the wattle can be left in place after the site has stabilized, allowing it to decompose naturally without removal.
Primary Function in Erosion and Sediment Control
The straw wattle has a dual role in managing both erosion and sediment runoff. When placed along the contour of a slope, the wattle intercepts surface water, which reduces the velocity of sheet flow. By slowing the water’s momentum, the wattle decreases its ability to detach and transport soil particles, preventing the formation of rills and gullies. This action shortens the effective length of the slope, stabilizing disturbed areas.
The second function is to act as a physical filter, trapping suspended sediment and debris carried by the runoff. The tightly packed straw allows water to pass through slowly, while the fibrous matrix captures fine soil particles and other pollutants. This filtration helps to protect nearby storm drains and waterways from becoming clogged or contaminated with sediment. By collecting soil, the wattle also encourages water to infiltrate into the ground, which increases soil moisture and promotes the establishment of permanent vegetation. Wattles are applied in areas with exposed soil, such as new construction sites, post-fire landscapes, and steep road embankments.
Proper Installation and Placement
Effective sediment control relies on correct placement and secure anchoring. Installation begins by excavating a shallow trench, 2 to 4 inches deep, along the contour line of the slope. This trenching ensures that the wattle is firmly “seated” in the soil, preventing runoff from flowing underneath the log and undermining its function. The soil excavated from the trench is then compacted against the uphill side of the wattle to create a tight seal against the surface.
The wattle must be secured to the ground using wooden stakes or rebar driven through the center of the cylinder. Stakes should be driven deep into the soil at regular intervals, every four feet, with an additional stake placed near each end. For continuous coverage across a long slope, adjacent wattles should be tightly abutted or overlapped by about a foot, with the ends turned slightly uphill to prevent water from flowing around the sides. The vertical spacing between rows depends on the steepness of the slope, requiring closer placement to manage water flow effectively.