Straw Wattle Dams
Small structure in zero or first order channels made of rocks, logs, or straw wattles designed to stabilize the channel gradient and store a small amount of sediment.
Purpose: Straw wattle dams work on the same principal as straw bale check dams. They trap sediment on side slopes and in the upper ends of ephemeral drain-ages by reducing channel gradient. Straw wattles are easy to place in contact with the soil and provide a low risk barrier to soil movement.
Relative Effectiveness: Excellent-33% Good-67% Fair-0% Poor-0% (Replies = 3). The limited number of interviewees that rated this treatment scored straw wattle dams as “excellent” or “good” in terms of controlling movement of sediment in channels. In one instance, only 10 of 3,300 wattles failed during the first storm after installation. Another reported an 80 percent first-storm survival rate, and excellent channel energy dissipation and trapping of sediment.
Implementation and Environmental Factors: Like any other channel treatment, good plans, designs, and experienced crews go a long ways to ensure successful implementation. Straw wattles work best on first order ephemeral channels with slopes less than 45 percent gradient. They can be easily placed by relatively untrained crews since they conform to the soil surface very well. This is a distinct advantage over rigid barriers like logs. Placement of straw wattle check dams is easiest on loamy sand soils that can be readily excavated. The closer together straw wattles are placed in steep terrain the more effective they are in detaining sediment. “U” shaped re-bar is very effective in keeping straw wattles fastened down but is another factor to consider in the logistics plans for this type of ESR project. Shallow or rocky soils can cause problems with re-bar usage, but hard pans can be penetrated by driving the rebar. Straw wattle dams are a good alternative in burned areas where logs are absent, poorly shaped, or scarce. Wattles can be used quite effectively in combination with straw bale check dams. They also can be easily prepositioned by helicopters.
Other Factors: Straw wattles are relatively cheap to buy. They can be disturbed by grazing animals, decompose, and catch fire. Although the wattle netting is photodegradable, there are concerns that it persists long enough to pose hazards for small animals. Supply is a major problem, particularly for a large project. There are concerns among some users about the cost effectiveness of straw wattle dams since the material and labor costs are quite high.
Straw Wattle Dams 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.
Straw Wattle Dam Implementation
|To prevent sediment from entering perennial streams during the first winter following a wildfire.|
|To trap and slowly meter sediment release through the system.|
|To decrease water velocity.|
|General area where straw bale check dams can be effective
|Specific individual dam site locations
CONSTRUCTION AND EQUIPMENT
Placement of the dam is critical to the success of the structure. This straw wattle dam is placed only in an upper swale where there is little or no channel definition. As in the straw bale check dam, the wattle is placed in a horseshoe shape. See diagram.
Smooth the ground where the structure will be placed making sure that all the ash is cleared. This insures that the wattle seats properly to the ground, lessening the potential for water to flow under the structure.
Anchor the wattle to the ground using 3/8 inch rebar, 12 inch wooden stakes or 9 inch geotech fabric staples. Ground conditions usually dictate which anchoring type will work best.
An energy dissipater is not needed with this structure.
ADVANTAGES OF STRAW WATTLE DAMS
Effective at trapping sediment and slowing the velocity of the water.
Straw wattle will decompose over time allowing regrowth of native vegetation that will help to stabilize the site.
Straw wattles are sterile and seedless, eliminating the introduction of non-native vegetation.
Low cost, a single person can usually construct up to 6 a day.
DISADVANTAGES OF STRAW WATTLE DAMS
Bulky and unwieldy, they are difficult to truck and hand transport and therefore often unpractical for inaccessible sites.
ESTIMATED COST TO BUILD A 20' STRAW WATTLE DAM
|Materials and Labor||Cost|
|1 straw wattle 20 feet long||$45.00|
|Transportation of wattle to site||$ 5.00|
|Anchoring (wooden stake, rebar of fencing staple)||$ 7.00|
This estimated cost may vary with any changes in any of the above items. All figures that were used to achieve this cost estimation were derived from the price list of materials and labor used in the Fork Fire Burned Area Emergency Rehabilitation project 1996.