12.1 Perimeter & Sheet-Flow Sediment Barriers: Silt Fence, Fiber Rolls, Straw Bales, and Filter Dikes

Hydraulic Principles of Perimeter and Sheet-Flow Sediment Barriers

While erosion controls protect the soil surface from raindrop impact and detachment, sediment control best management practices (BMPs) serve as the second line of defense by capturing detached soil particles before stormwater runoff exits the construction site or enters receiving waters. Perimeter and linear slope barriers are engineered strictly to intercept shallow overland sheet flow—diffused surface runoff typically less than $1\text{ to }2\text{ inches}$ deep—and convert kinetic flow energy into temporary quiescent ponding so suspended sand and coarse silt particles settle by gravity.

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Silt Fence the approved practice detail

A silt fence is a temporary linear sediment barrier consisting of woven or non-woven permeable geotextile fabric stretched across vertical support posts with its lower edge entrenched and mechanically compacted into the ground. It is the most widely deployed perimeter control on construction sites—and simultaneously the most frequently misinstalled BMP encountered by CESSWI inspectors.

Drainage Area Limits and Prohibited Locations

Because silt fence relies on low-velocity sheet-flow ponding and has limited structural shear resistance, regulatory specifications strictly limit its contributing drainage area:

• Maximum Drainage Area: No more than $\le 0.25\text{ acre per }100\text{ linear feet}$ of silt fence.
• Maximum Upslope Sheet-Flow Length: Up to $100\text{ feet}$ on flat or gentle slopes ($< 2\%$), decreasing to $50\text{ feet}$ on moderate slopes ($2\%\text{ to }10\%$) and $20\text{ to }25\text{ feet}$ on steep slopes ($> 10\%$). Beyond $100\text{ feet}$, overland sheet flow naturally coalesces into concentrated rill and gully flow.
• Prohibited Installations: NEVER install silt fence across concentrated-flow ditches, swales, channels, intermittent streams, waterways, or pipe/culvert outlets. Concentrated flow exerts excessive hydrostatic head that rapidly overtops, undercuts, or tears down the fabric posts, releasing both the current storm's runoff and all previously trapped sediment downstream.

Mandatory Installation Specifications

When inspecting a silt fence installation, verify every structural element from the subgrade trench to the terminal ends:

1. Placement Along a Flat Elevation Contour: The main body of the silt fence must follow a single flat elevation contour. If a silt fence is angled across contours (running downhill along a property boundary), intercepted runoff flows laterally along the upslope face of the fabric like a gutter, concentrating at the lowest elevation until it scours underneath or blows out the fence.
2. Terminal Upslope J-Hooks ("Smiles"): Turn the last $6\text{ to }10\text{ feet}$ of both ends of the fence upslope at a $45^\circ\text{ to }90^\circ$ angle across the contours. These J-hooks tie the barrier into higher ground so ponded water is contained in a cradle and cannot bypass (flank) around the ends.
3. Upslope Trench Excavation and Backfill Compaction: Excavate a minimum $6\text{-inch deep by }6\text{-inch wide}$ ($6\text{ in.} \times 6\text{ in.}$) trench along the upslope side of the post line (or use a specialized static-slicing plow that inserts the fabric $8\text{ to }12\text{ inches}$ vertically into the soil before wheel-rolling). Lay $12\text{ inches}$ of geotextile fabric down the vertical trench wall and flat across the bottom ($6\text{ in.}$ down plus $6\text{ in.}$ horizontal toe flap), backfill with native soil or gravel, and mechanically compact the backfill until the fabric cannot be pulled out by hand. Uncompacted loose dirt backfill is the number-one cause of silt fence undercutting (piping) during the first storm.
4. Downslope Post Orientation and Spacing: Drive wooden stakes ($2\text{ in.} \times 2\text{ in.}$ hardwood) or steel studded T-posts ($\ge 1.25\text{ to }1.33\text{ lb/ft}$) at least $18\text{ to }24\text{ inches}$ into undisturbed ground on the DOWNSLOPE side of the fabric. Placing posts on the downslope side ensures that hydrostatic water and sediment pressure pushes the fabric against the supporting posts rather than pushing the fabric away and ripping staples or zip-ties loose. Space posts a maximum of $6\text{ to }8\text{ feet}$ apart for standard woven geotextile, or $3\text{ to }5\text{ feet}$ apart in high-hazard areas and wire-backed/super-silt fence installations.
5. Joining Two Continuous Runs (Rolled-Stake Joint): Never splice two lengths of silt fence by simply overlapping the fabric flat between posts—hydrostatic pressure forces water and mud right through the unsealed seam. Instead, wrap both fabric ends around their respective end stakes, place the two stakes side by side, twist/rotate them together $180^\circ\text{ to }360^\circ$ (a rolled-stake or twist joint), and drive them into the trench as a single locked unit (or construct two overlapping J-hooks).

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Fiber Rolls / Wattles (Section 12.4.1) and Compost Filter Socks

Straw, Coir, and Excelsior Fiber Rolls (Wattles)

Fiber rolls (wattles) are prefabricated cylindrical tubes ($8\text{ to }20\text{ inches}$ in diameter and $10\text{ to }25\text{ feet}$ long) stuffed with compressed rice/wheat straw, coconut coir fiber, or curled wood excelsior encased in photodegradable plastic or biodegradable jute/coir netting. They are installed along the contour at regular intervals ($10\text{ to }25\text{ feet}$ apart, closer on steeper grades) across cut-and-fill slopes to shorten uninterrupted slope length ($LS$ factor in RUSLE), slow sheet flow before it forms rills, and capture coarse sediment on the slope face.

• Trenching Requirement: Excavate a shallow, concave trench $2\text{ to }4\text{ inches}$ deep (roughly $1/3$ of the roll diameter) along the contour and press the fiber roll firmly into the soil cradle. Never lay a straw wattle flat on top of ungraded, cloddy, or hard-packed ground without trenching—runoff will immediately pipe underneath the lightweight roll and erode a channel beneath it.
• Staking and Anchoring: Drive wooden stakes ($1\text{ in.} \times 1\text{ in.}$ or $2\text{ in.} \times 2\text{ in.}$) through the center of the fiber roll—or in a crossed A-frame pattern straddling the roll without puncturing the netting where specified—every $3\text{ to }4\text{ feet}$ along its length and at each end. Drive stakes at least $12\text{ to }18\text{ inches}$ into the soil, leaving only $2\text{ to }3\text{ inches}$ of stake projecting above the wattle so buoyant straw rolls cannot float upward off the stakes during heavy runoff.
• Joints and Terminal Ends: Butt adjacent roll ends tightly together (or overlap them by $12\text{ inches}$ in a common trench) so no gap exists between sections, and angle the terminal ends upslope into miniature J-hooks.

Compost Filter Socks

Compost filter socks consist of a heavy-duty woven mesh tube ($8\text{ in.}, 12\text{ in.}, 18\text{ in.},\text{ or }24\text{ in.}$ diameter) pneumatically filled on site or pre-filled with coarse, screened, composted wood chips and bark media. Unlike lightweight straw wattles, compost filter socks offer major structural and ecological advantages:

• Intimate Ground Contact Without Trenching: Because wet compost media is dense, heavy, and flexible, the sock conforms tightly to uneven micro-topography without requiring a $2\text{- to }4\text{-inch}$ trench.
• Hard-Surface and Sensitive Root Zone Deployment: Compost filter socks can be installed directly over asphalt/concrete pavement, exposed bedrock, frozen ground, and sensitive tree Critical Root Zones (CRZs) where trenching would sever tree roots or is physically impossible. On soil, stakes are driven along the downslope toe or through the sock every $5\text{ to }10\text{ feet}$; on pavement, socks are weighted or anchored without ground penetration.
• Three-Dimensional Biofiltration: While ponding sheet flow to settle coarse particles, the coarse wood-chip matrix also maintains higher hydraulic flow-through rates than blinded woven silt fence and adsorbs dissolved metals, nutrients, and trace petroleum hydrocarbons.

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Straw Bale Barriers (Section 12.4.10)

Historically used as temporary perimeter barriers, straw bale barriers are now prohibited or restricted by the U.S. EPA and many state DOT/MS4 manuals because they decompose rapidly (service life $< 3\text{ months}$), shrink as they dry to open wide gaps between bales, become buoyant in ponded water, and suffer chronic undercutting.

Where local permits still allow straw bales for short-term sheet-flow containment, an inspector must verify five strict installation rules:

1. Contour Placement: Place bales in a single continuous row along a flat contour with ends turned upslope.
2. Binding Orientation ("Strings on the Sides"): Place every bale on its edge so the baling wire or poly/sisal twine runs HORIZONTALLY around the vertical sides of the bale—never touching the bottom soil trench. When bindings rest on wet soil, wire corrodes and twine rots within weeks, causing the bale to burst apart.
3. 4-Inch Trenching and Compaction: Excavate a trench at least $4\text{ inches}$ deep and equal to the bale width, set the bales in the trench, and backfill and compact the upslope excavated soil tightly against the upslope face of the bales up to ground level.
4. Double Staking (Angled First Stake): Drive two (2) wooden stakes ($2\text{ in.} \times 2\text{ in.}$) or steel rebar pins through each bale at least $18\text{ to }24\text{ inches}$ into the underlying soil. Drive the first stake of each new bale at an angle toward the previously laid bale to draw and wedge the two bales tightly together.
5. Chinking Between Bales: Tightly stuff (chink) loose straw into every vertical joint between butted bales to eliminate concentrated flow-through gaps.

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Triangular Filter Fabric Dikes (Section 12.4.12)

A triangular filter fabric dike is a reusable, prefabricated structural barrier formed from $6\text{-gauge}$ $6\text{ in.} \times 6\text{ in.}$ welded wire mesh bent into a rigid $60^\circ$ equilateral triangle ($18\text{ inches}$ high with a $30\text{-inch}$ base in $10\text{- to }20\text{-foot}$ modular panels) wrapped in non-woven or woven geotextile fabric with an integral $12\text{-inch}$ upslope fabric skirt (apron).

• Primary Applications: Specifically engineered for exposed bedrock, rocky outcrops, frozen ground, concrete/asphalt pavement, utility trenches, and bridge decks where excavating a $6\text{-inch}$ silt fence trench is impossible.
• Installation & Sealing: Panels are placed along the contour with overlapping geotextile sleeves at joints and terminal ends turned upslope. To prevent undercutting, the $12\text{-inch}$ upslope fabric skirt must be sealed continuously to the surface using $6\text{ inches}$ of compacted soil or washed gravel, a continuous row of interlocking gravel bags, or landscape staples ($6\text{-in.}$ pins on $2\text{-ft}$ centers) in soil.

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Comparison of Perimeter and Sheet-Flow Sediment Barriers