FIGURE 2 (B1 B8)
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Technical Specifications B1 B8
Erosion and Sedimentation Control Measures - Silt Fencing (B1)
A. Design Criteria
1. No formal design is required for many small projects and for minor incidental applications.
2. Silt fences shall have an expected usable life of six months. They are applicable aroundperimeters and stockpiles, and at temporary locations where continuous construction changesthe earth contour and runoff characteristics.
3. Silt fences have limited applicability to situations in which only sheet or overland flows areexpected. They normally cannot filter the volumes of water generated by channel flows, andmany fabrics do not have sufficient structural strength to support the weight of water pondedbehind the fence line.
1. Synthetic filter fabric shall be a pervious sheet of propylene, nylon, polyester, or polyethyleneyarn. It shall contain ultraviolet ray inhibitors and stabilizers to provide a minimum of 6 monthsof expected usable construction life at a temperature range of 0 degrees F, to 120 degrees F.
2. The stakes for a silt fence shall be 1 x 2 inches (2.5 x 5 cm) wood (preferred), or equivalentmetal with a minimum length of 3 feet (90 cm).
3. Wire fence reinforcement for silt fences using standard-strength filter cloth shall be a minimumof 36 inches (90 cm) in height, and shall be a minimum of 14 gauge, and shall have a maximummesh spacing of 6 inches (15 cm).
1. The height of a silt fence shall not exceed 36 inches (90 cm). Higher fences may impoundvolumes of water sufficient to cause failure of the structure.
2. The filter fabric shall be purchased in a continuous roll cut to the length of the barrier to avoidthe use of joints. When joints are necessary, filter cloth shall be spliced together.
3. Posts shall be spaced a maximum of 10 feet (3 m) apart at the barrier location and drivensecurely into the ground a minimum of 12 inches (30 cm). When extra-strength fabric is usedwithout the wire support fence, post spacing shall not exceed 6 feet (1.8 m).
4. A trench shall be excavated approximately 4 inches (10 cm) wide and 4 inches (10 cm) deepalong the line of posts and upslope from the barrier.
5. When standard-strength filter fabric is used, a wire mesh support fence shall be fastenedsecurely to the upslope side of the posts using heavy-duty wire staples at least 1 inch (25 mm)
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long, tie wires, or hog rings. The wire shall extend into the trench a minimum of 2 inches (5 cm) and shall not extend more than 36 inches (90 cm) above the original ground surface.
6. The standard-strength filter fabric shall be stapled or wired to the fence, and 8 inches (20 cm) ofthe fabric shall be extended into the trench. The fabric shall not extend more than 36 inches (90cm) above the original ground surface.
7. When extra-strength filter fabric and closer post spacing are used, the wire mesh support fencemay be eliminated. In this case, the filter fabric is stapled or wired directly to the posts with allother provisions of item #6 applying.
8. When attaching 2 silt fences together, place the end post of the second fence inside the endpost of the first fence. Rotate both posts at least 180 degrees in a clockwise direction to createa tight seal with the filter fabric. Drive both posts into the ground and bury the flap.
9. The trench shall be backfilled and the soil compacted over the filter fabric.
10. The most effective application consists of a double row of silt fences spaced a minimum of 3 feetapart, so that if the first row collapses it will not fall on the second row. Wire or synthetic meshmay be used to reinforce the first row.
11. When used to control sediments from a steep slope, silt fences should be placed away from thetoe of slope for increased holding capacity.
12. Silt fences shall be removed when they have served their useful purpose, but not before theupslope area has been permanently stabilized.
1. Silt fences shall be inspected within 24 hours after each inch rainfall event and at least once a week. Any required repairs shall be made immediately.
2. Should the fabric on a silt fence decompose or become inefficient before the end of the expected usable life and the barrier is still necessary, the fabric shall be replaced promptly.
3. Sediment deposits should be removed when deposits reach approximately one-half the height of the barrier.
4. Any sediment deposits remaining in place after the silt fence is no longer required shall be dressed to conform with the existing grade, prepared, and seeded.
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Erosion and Sedimentation Control Measures - Turbidity Curtain (B2)
A. Design Criteria
1. Turbidity curtains should extend the entire depth of the watercourse whenever it is not subjectto tidal action and/or significant wind and wave forces. This prevents silt-laden water fromescaping under the barrier, scouring and resuspending additional sediments.
2. In situations with tidal and/or wind and wave action, the curtain should never be so long as totouch the bottom. There should be a minimum of 1 foot (30 cm) gap between the weighted lowerend of the skirt and the bottom at mean low water. The movement of the lower skirt over thebottom due to tidal reverses or wind and wave action on the flotation system may fan and stirsediments already settled out.
3. In situations with tidal and/or wind and wave action, it is seldom practice to extend a turbiditycurtain lower than 10 to 12 feet (3 to 4 m) below the surface, even in deep water. Curtains thatare installed deeper than this will be subject to very large loads with consequent strain on curtainmaterials and the mooring system. In addition, a curtain installed in this manner can billow uptoward the surface under the pressure of moving water, resulting in an effective depth that issignificantly less than the skirt depth.
4. Turbidity curtains should be located parallel to the direction of flow of a moving body of water.They should not be placed across the main flow of a significant body of moving water.
5. When sizing the length of the floating curtain, allow an additional 10 to 20% variance in thestraight-line measurements. This will allow for measuring errors, make installation easier, andreduce stress from potential wave action during high winds.
6. An attempt should be made to avoid an excessive number of joints in the curtain; a minimumcontinuous span of 50 feet (15 m) between joints is a good rule of thumb.
7. To maintain stability, a maximum span of 100 feet (30 m) between anchor or stake locations isalso a good rule of thumb.
8. The ends of the curtain, both floating upper and weighted lower, should extend well up into theshoreline, especially if high-water conditions are expected. The ends should be secured firmlyto the shoreline to fully enclose the area where sediment may enter the water.
9. When there is a specific need to extend the curtain to the bottom of the water-course in tidal ormoving water conditions, a heavy, woven, pervious filter fabric may be substituted for thenormally recommended impervious geotextile. This creates a flow-through medium thatsignificantly reduces the pressure on the curtain and helps to keep it in the same relative locationand shape during the rise and fall of tidal waters.
10. The number of spacing of external anchors may vary depending on current velocities ad potentialwind and wave action; the manufacturers recommendation should always be followed.
11. Be certain that the type, location, and installation of the barrier are as shown on the approvedplan and permit. Additional permits may be required in navigable waterways, especially whenthe barrier creates an obstruction.
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1. Barriers should be a bright color (yellow or international orange are recommended) that willattract the attention of nearby boaters.
2. The curtain fabric must meet the minimum requirements.
3. Seams in the fabric shall be either vulcanized welded or sewn, and shall develop the full strengthof the fabric.
4. Flotation devices shall be flexible, buoyant units contained in an individual flotation sleeve orcollar attached to the curtain. Buoyancy provided by the floatation units shall be sufficient tosupport the weight of the curtain and maintain a freeboard of at least 3 inches (8 cm) above thewater surface level.
5. Load lines must be fabricated into the bottom of all floating turbidity curtains. Types II and IIImust have load lines also fabricated into the top of the fabric. The top load line shall consist ofwoven webbing or vinyl-sheathed steel cable and shall have a break-strength in excess of10,000 pounds. The supplemental (bottom) load-line shall consist of a chain incorporated intothe bottom hem of the curtain in a vertical position. Additional anchorage shall be provided asnecessary. The load lines shall have suitable connecting devices that develop the full breakingstrength for connecting to load lines in adjacent sections.
6. External anchors may consist of 2 x 4 inch (5 x 10 cm) or 2- inch (6 cm) minimum diameterwooden stakes, or 1.33 pounds per linear foot steel posts when Type I installation is used; withType II and III installations, bottom anchors are used.
7. Bottom anchors must be sufficient to hold the curtain in the same position relative to the bottomof the watercourse without interfering with the action of the curtain. The anchor may dig into thebottom (grappling hook, plow or fluke-type) or may be weighted m