Woven mesh netting (1 ft diameter by 6 to 20 ft in length, 0.3 m diameter by 1.8 m to 6.1 m in length) filled with straw or hay and sometimes seed mixes, used to trap sediment and promote infiltration.
Purpose: Straw wattles are permeable barriers used to detain surface runoff long enough to reduce flow velocity. Their main purpose is to break up slope length. They have also been used in small drainages or on side slopes for detaining small amounts of fine suspended sediment.
Relative Effectiveness: Excellent-33% Good-33% Fair-33% Poor-0% (Replies = 3) The effectiveness rating of straw wattles ranges from “excellent” to “fair” depending on the circumstances in which they were used and the quality of the installation. Comments within one Region on straw wattle effectiveness ranged from being an “excellent” treatment at a reasonable cost and still functioning after 2 years, to that of exhibiting pronounced undercutting immediately on the downhill side. Visual monitoring has noted that straw wattles usually remain in place and often fill with soil material on the uphill side. Where that happens, good seed germination occurs. Straw wattles have been placed onto specific sites and randomly located on slopes. Some monitoring observations have noted that there does not appear to be a difference in overall vegetative recovery between contour felled log areas and straw wattle treatment areas. Overall effectiveness can be affected by break-down of the wattles and release of built-up sediment onto the rest of the slope or into drainages.
Implementation and Environmental Factors: Correct installation of straw wattles is crucial to their effectiveness. They are labor intensive because they need to have good ground contact and anchoring. Wattles can be anchored to the ground by trenching and backfilling or staking. An effective anchoring technique is to use “U” shaped 1/8 in (3 mm) re-bar. Re-bar can hold wattles to the ground without trenching and is less likely to break than wood stakes in shallow soils. Straw wattles can work well on slopes greater than 40 percent but they are difficult to carry and hard to install on steep terrain. Spotting the wattles with helicopters can solve some of this problem.
Other Factors: The cost of straw wattle installation is about one half that of contour-felled logs.
Straw Wattle Implementation
Straw wattle dams are used as contour felled logs, only where there aren't any trees. The idea is to place the straw wattles along the contour of the slope to stop overland flow, add roughness, and minimize erosion.
Straw wattles (straw worms, bio-logs, straw noodles) come in different diameters and are made of different materials. The length is more standard - 25 feet. Diameters range from 8 -12". The outside tube can be made of jute, nylon, or photo degradable materials. Wattles are generally stuffed with straw, rice or wheat. Each wattle weighs 35 pounds (of course this depends on the diameter - this is an average).
SITE SELECTION CRITERIA
|Slopes can be up to 70 percent. However, manufacturer recommendations vary and some maximums are to 50 percent.|
|Soils can be shallow, but not less than 8 inches.|
CONSTRUCTION AND EQUIPMENT
Purchase the product - The first task is finding them. Supplies are low during sever fire seasons.
Getting wattles to the site - Getting them to the site was not a problem. A helicopter can sling them to the site, or they can trucked in if road access available. Carrying them for any distance was a problem. Each wattle weighs approximately 35 pounds, the weight isn't the problem. It takes coordination and team work.
Basic installation - Installation of the wattles is basic. First, smooth out a shallow depression for the wattle to lay into. Second, using 1" x 1" stakes (not the wimpy ones that come with the wattles), drive a stake through the wattle and into the ground, so the stake is at least 6 inches in the ground and about two inches above the wattle. The wattle will flatten out over time, so you don't need it any higher. Put 5 stakes in each wattle.
Design - Wattles should be placed in a checkerboard pattern with a 15' horizontal spacing and a 30' vertical spacing (manufacturers). These recommendations can be altered. Consider how contour felled logs would be placed across the slope, and then add a few more wattles.
Safety - There are few safety concerns with installing straw wattles. Basic tool safety with polaski, shovels, and field conditions cover it.
PERSONNEL AND TRAINING
Crews used for this treatment can be volunteers and Correctional Institute crews. Work could be contracted out, anyone can wattle. Depending on time into the site, 10-20 acres per day is reasonable for a five person crew.
COST AND COST CONSIDERATIONS
Cost vary for $500 to $1000 or more per acre depending on the density of the wattles and labor and transportation costs.
Straw wattles are as effective as hand trenching, and when you considering effectiveness duration, straw wattles last much longer.