HYDRO20750 - Laserfiche WebLink

Home Browse Search

When geotextflea mats are applied over seed and mulch, they are very effective in stopping erosion. Because the cost is very high, they are used only where immediate ground cover is needed; large areas cannot be covered by this method. Geotextilea are particu- larly effective for steep upper slopes where other materials (seed, mulch alone) will blow off. Material must be anchored securely to remain effective, espe- cially along streambanks. Implementatron andEnvironmental Factors An expe- rienced crew is needed to ensure that good contact is eatabliehedbetween thefabricmaterial and theground, and that the fabric is securely anchored. Fabric mat- ting is difficult to apply on rocky ground. Plastic netting on some geotextfles material can trap small rodents and birds. Jute netting does not provide com- plete ground cover but it has not been reported to trap animals. The complete cover provided by some geoteatilea can reduce native plant establishment. Silt Fences Purpose: Silt fences are installed to trap sediment in swalea, small ephemeral drainages, or alonghillalopes where other methods cannot be used. They provide temporary sediment storage. Silt fences are also in- stalled to monitor sediment movement as part of effectiveness monitoring. RelativeF.tlectlveness.Excellent 38%Good-02%Fair -0% Poor-0% (Replies = 8) Silt fences were considered "good" or °excellent" by interviewees (table 16). Moat respondents felt they worked well in ephemeral channels, but not all. The size of the watershed above the fence may be impor- tant, and silt fences cannot handle debris flows or heavy sediment loads. They work better on gentler slopes, such as swalea. Silt fences can be installed on rocky elopes where log erosion barriers would not achieve good ground contact. Sealing the bottom of the fence to the ground well is critical to effectiveness and seems to work beat if a trench is dug behind the fence to trap sediment. Silt fences also effectively catch small rocks and ravel on slopes above buildings. Implementation andEnvironmentalFactors Aa noted above, silt fences must be anchored and sealed to the ground to be effective. Sandbags can be used as an- chors. Burying the bottom of the fence in a trench is also useful. Rockiness of the soil affects how well the toe ofthe fence can be buried. When used in ephemeral channels, silt fences must be cleaned out or they can fail and release the stored sediment all at once. They are useful for monitoring sediment movement, and can last several years before failing. Sand. Soil or Gravel Baea Purpose: Sand, soil or gravel bags are used in small channels or on hillslopea to trap sediment and inter- rupt water flow. Relative effectiveness. No interviewees rated this treatment. Comments indicate that bags are useful in ephemeral channels or on slopes, where they are placed in stag- gered rows like contour felling in areas where there are no trees. Rows of bags break water flow and promoteinfiltration. Theystoreaedimenttemporarily, then break down and release it. Implementaton and Environmental Factors When bags are used in channels, installation sites must be selected by an experienced person. They are not appro- priatefor use in V-shaped channels. Installation of soil bags is labor-intensive, but they can be a relatively cheap treatment if volunteer labor is used. The bags are easy for volunteers to fill and install. Channel Treatments Channel treatments are implemented to modify sediment and water movement in ephemeral or emall- order channels, and to prevent flooding and debris torrents that may affect downstream values at risk. Straw Bale Check dame Purpose. Straw bale check dams are used to prevent or reduce sediment inputs into perennial streams during the first winter or rainy season following a wildfire. Straw bales function by decreasing water velocity and detaining sediment-laden surface runoff long enough for coarser sediments to deposit behind check dame. The decreased watervelocity also reduces downcutting in ephemeral channels. Relative Effectiveness Excellent-30% Good-30% Fair- 309o Poor-10% (Replies = 10) Straw bale check dams were judged to cover the range from "good" to °poor" effectiveness. They often fill in the first few storms, so their effectiveness can dimin- iahrapidly. However, channel gradients can be easily stabilized, and sediment is stored and released at a afoweror diminished rate. They appear to work well in front of culverts, and in semi-arid environments re- quire little maintenance. Structural survival rates of 90 percent have been reported after 1 year with 76 to 100 percent sediment storage, and 95 percent survival after rainfall of 2.4 in hr-1(60 mm hr-1) fora 10-min duration. However, a common negative comment was that straw bale check dams tend to blow out in large storms. Failure can occur if the dame are poorly USDA Forest Service Gen. Tech. Rep. RMRS-GTR-63. 2000 79