14.7 Erosion & Sediment Control BMPs, Basins & Site Stabilization

1. Best Management Practices (BMPs): Perimeter & Inlet Protection

Temporary Erosion and Sediment Control (ES&C) BMPs are physically categorized into Erosion Controls (source controls that prevent soil detachment) and Sediment Controls (structural mechanisms that trap detached soil particles before they leave the site).

+-------------------------------------------------------------------------+
|                    STANDARD TRENCHED SILT FENCE DETAIL                  |
+-------------------------------------------------------------------------+
|                 Woven Geotextile Fabric (36" height)                    |
|                 ====================================                    |
|                 |                                  |                    |
|  Sheet Flow     |                                  |   Hardwood Stakes  |
|  =============> |                                  |   (2"x2" driven 16"|
|  (Max 0.25 ac   |                                  |    into ground on  |
|   per 100')     |                                  |    DOWNSLOPE side) |
|                 |                                  |         |          |
|   Ponded Water  |                                  |         |          |
|   & Sediment    |                                  |         |          |
|   ~~~~~~~~~~~~~ |                                  |         |          |
|   |           | |                                  |         |          |
|   FG =========+=+==================================+=========+=======   |
|               | |                                  |                    |
|               | |  Trench: 6" deep x 6" wide       |                    |
|               +-+  Backfilled and compacted        |                    |
|                 +-- with native earth/gravel       |                    |
+-------------------------------------------------------------------------+

Silt Fences (Filter Fabric Fence)

• Operational Principle: A temporary linear sediment barrier composed of synthetic woven geotextile fabric attached to hardwood stakes or steel posts. Silt fences do NOT act primarily as a filtration sieve; they act as a temporary detention dam that ponds runoff, allowing sediment to settle out via gravitational sedimentation.
• Absolute Physical Limits:
  • Sheet Flow Only: Silt fences must never be placed across concentrated drainage channels, gullies, ditches, streams, or pipe outfalls. Concentrated flows will instantly overtop or tear the fabric away.
  • Contributing Drainage Area: Maximum allowable drainage area is 0.25 acres per 100 linear feet of fence.
  • Maximum Slope Length Behind Fence: 100 feet for a 2:1 slope; 150 feet for a 3:1 slope; 200 feet for a 4:1 slope.
• Installation Specifications:
  • Geotextile fabric must be trenched a minimum of 6 inches deep and 6 inches wide into the earth in an "L" or "J" hook configuration, and the trench must be thoroughly backfilled with soil or crushed stone and compacted.
  • Wooden stakes ($2\times 2\text{ inch}$ hardwood) or steel T-posts must be driven a minimum of 16 inches into undisturbed ground on the downslope (support) side of the fabric, spaced no further than 6 to 8 feet apart.
  • The ends of the silt fence must be flared uphill ("J-hooks" or "smiles") to prevent bypass flow around the wings.

Compost Filter Berms & Filter Socks

• Physical Properties: Three-dimensional tubular mesh sleeves (typically 8, 12, 18, or 24 inches in diameter) packed with coarse composted organic wood mulch.
• Advantages Over Silt Fence: Does not require ground trenching (eliminating soil disturbance in rocky ground or over tree roots); heavier weight maintains continuous contact with uneven ground; can be walked or driven over by machinery; can be left in place permanently by slitting the photodegradable netting and incorporating the organic compost into the topsoil.

Storm Drain Inlet Protection

Inlet protection keeps sediment out of the closed storm sewer system during construction:

• Drop Inlets (Excavated / Sump): Protected using a wooden frame wrapped in geotextile fabric with a gravel-jacketed stone base, or block-and-gravel barriers (concrete masonry units laid on their sides with wire mesh and clean crushed stone against the open cores).
• Curb-Opening Inlets: Protected using prefabricated weighted sediment sausages, gravel bags, or under-grate filter bags equipped with overflow bypass slots to prevent hazardous street ponding.

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2. Sediment Traps & Basins: Sizing and Detailing

Where concentrated site runoff accumulates, sediment must be captured in an engineered impoundment before leaving the property.

• Sediment Trap: A small temporary impoundment formed by an earthen embankment or riprap stone berm serving a contributing drainage area of less than 5.0 acres. Typically maintained for a design lifespan of under 18 months.
• Sediment Basin: A larger engineered impoundment with a formal pipe riser or skimmer outlet serving contributing drainage areas of 5.0 acres up to 100 acres.

Statutory Sizing Standard: 3,600 Cubic Feet per Acre

Under EPA CGP and state regulatory standards, temporary sediment basins and traps must provide a minimum total storage volume of 3,600 cubic feet per disturbed contributing drainage acre:

$\text{Total Required Storage Volume } (V_{total}) = A_{disturbed} \times 3,600\text{ cu ft/acre}$

Storage Allocation (Wet vs. Dry Storage)

The standard 3,600 cu ft/acre volume is split equally into two distinct functional zones:

1. Wet Sediment Storage Zone ($1,800\text{ cu ft/acre}$): The lower permanent settling pool situated below the lowest dewatering orifice. Provides quiescent settling for coarse sand and silt bedload. Must be cleanout-dredged when sediment fills $50%$ of this volume ($900\text{ cu ft/acre}$).
2. Dry Settling / Detention Storage Zone ($1,800\text{ cu ft/acre}$): The upper temporary flood pool situated between the cleanout elevation and the crest of the emergency spillway. Detains peak runoff waves and provides extended settling time.

+-------------------------------------------------------------------------+
|                    SEDIMENT BASIN STORAGE ALLOCATION                    |
+-------------------------------------------------------------------------+
|                     Emergency Overflow Spillway                         |
|                     ===========================                         |
|                     |                         |                         |
|  DRY DETENTION      |   Dry Storage Volume    |  Dewatering Skimmer     |
|  STORAGE ZONE       |   (1,800 cu ft/acre)    |  (Draws clean surface   |
|  (Temporary Pool)   |                         |   water over 48-72 hrs) |
|  ~~~~~~~~~~~~~~~~~> +-------------------------+====\                    |
|                     |   Cleanout Elevation    |     \                   |
|  WET SEDIMENT       |   (50% of wet volume)   |      \   Outlet Barrel  |
|  STORAGE ZONE       | - - - - - - - - - - - - |       +===============> |
|  (Permanent Pool)   |   Wet Sediment Storage  |                         |
|                     |   (1,800 cu ft/acre)    |                         |
|  Basin Floor =====> +=========================+                         |
+-------------------------------------------------------------------------+

Surface Dewatering Skimmers vs. Perforated Risers

• Historic Perforated Risers: Vertical CMP pipes drilled with hundreds of small holes and wrapped in geotextile and gravel. Prone to severe clogging with fine silt; draws murky water from the bottom and middle of the water column.
• Modern Surface Skimmers (e.g., Faircloth Skimmers): Floating mechanical dewatering arms equipped with a buoyant orifice that rises and falls with the water surface. Surface skimmers draw only the cleanest, clearest top 6 inches of water from the pond surface, discharging water over a calibrated 48- to 72-hour dewatering period while keeping settled bottom sludge completely undisturbed.

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3. Site Stabilization, Washouts & Inspection Protocols

Stabilization Practices & Deadlines

• Temporary Seeding & Mulch: Fast-growing temporary cover (annual ryegrass, winter wheat, sudangrass) applied to bare areas that will sit inactive during construction. Under EPA CGP rules, temporary stabilization must be initiated immediately whenever grading operations have permanently or temporarily ceased on any portion of the site and will remain inactive for 14 or more calendar days (reduced to 7 days in sensitive or impaired watersheds).
• Straw Mulch with Tackifier: Clean agricultural straw spread at a minimum rate of 2.0 tons per acre ($90\text{ to }100\text{ lbs per }1,000\text{ sq ft}$) to cover $80%\text{ to }90%$ of bare soil, secured with liquid asphaltic or plant-based polymer tackifier.
• Erosion Control Blankets (ECBs): Biodegradable straw, excelsior wood, or coconut coir blankets encased in photodegradable plastic or jute netting. Anchored to smoothed slopes using metal staples or biodegradable pins driven flush at 1.0 to 2.0-foot intervals.
• Bonded Fiber Matrix (BFM): A hydraulically sprayed slurry consisting of thermally refined wood fibers, cross-linked water-resistant tackifiers, and bonding polymers. BFM dries to form a continuous, flexible, water-resistant crust that protects $2:1$ slopes without netting.

Construction Tracking Controls & Concrete Washouts

• Stabilized Construction Entrance (Tracking Pad):
  • Located at every vehicle access point connecting the construction site to paved public highways.
  • Composed of clean, coarse, angular crushed aggregate conforming to AASHTO #1 or #2 stone ($2.0\text{ to }3.0\text{ inch}$ stone size).
  • Dimensions: Minimum length of 50 feet (recommended 70 to 100 ft), minimum width of 24 feet (or full width of egress), and minimum thickness of 6 inches.
  • Must be underlain by a heavy Class 1 nonwoven geotextile filter fabric to prevent the heavy rock from sinking into the soft subgrade.
  • Knocks caked mud and clay out of truck tire treads before vehicles enter public roadways.
• Concrete Washout Facilities:
  • Unhardened concrete slurry has an extremely high caustic pH ($11.0\text{ to }12.0$), toxic to aquatic life and groundwater.
  • Washouts must be constructed as a watertight, lined excavation pit or pre-manufactured leak-proof container.
  • Minimum setback: Must be located at least 50 feet away from storm drain inlets, swales, open watercourses, wetlands, and sensitive buffer zones.
  • Zero liquid discharge to the ground or storm system is permitted; wash water must evaporate or be vacuumed and hauled off-site for disposal.

Maintenance and Certified Inspection Protocols

Under the federal EPA Construction General Permit, site compliance inspections must be conducted by a certified SWPPP inspector at one of two authorized schedules:

1. Weekly Schedule: At least once every 7 calendar days; OR
2. Bi-Weekly and Post-Storm Schedule: At least once every 14 calendar days AND within 24 hours of the end of a precipitation event reaching 0.25 inches or greater (many state permits specify $0.50\text{ inches}$).

Inspection Compliance Protocol: Every inspection must be documented in a written report detailing: date, time, certified inspector qualifications, weather conditions, active phase of construction, condition of all BMPs, non-conforming items, corrective action directives, and dated photographic logs. When a deficiency is identified (e.g., undermined silt fence or filled tracking pad), corrective maintenance must be initiated immediately and completed within 24 to 48 hours. All SWPPP records, permits, and inspection logs must be retained on-site and archived for at least 3 years following the filing of the Notice of Termination.

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4. Real-World Case Scenario: Developing an ES&C Strategy for a 12-Acre Site

Scenario: A landscape architect is authoring the SWPPP and erosion control plan for a 12.0-acre hillside residential parcel with an average slope of $8%$. The native soil is a fine silt loam ($K = 0.42$, highly erodible). Phase 1 mass grading will disturb 8.0 acres simultaneously.

Strategic ES&C Implementation:

1. Calculate Temporary Sediment Basin Capacity: $V_{total} = 8.0\text{ acres} \times 3,600\text{ cu ft/acre} = \mathbf{28,800\text{ cubic feet}}$
   • Wet storage requirement: $28,800 / 2 = 14,400\text{ cu ft}$.
   • Dry storage requirement: $14,400\text{ cu ft}$.
   • Sizing a Faircloth surface skimmer to draw down the upper 14,400 cu ft over 48 hours.
2. Perimeter Controls:
   • Specify 1,200 linear feet of trenched woven silt fence along the downslope property boundary.
   • Verify that no single 100-foot segment receives drainage from more than 0.25 acres ($1,200' / 100' \times 0.25 = 3.0\text{ acres}$ max contributing area; the site is graded with diversion dikes to ensure contributing sheet flow is evenly distributed).
3. Tracking and Egress:
   • Specify a stabilized entrance pad: 70 ft length, 24 ft width, 8 inches thick of AASHTO #1 stone (3-inch aggregate) over nonwoven geotextile.
4. Stockpile & Slope Stabilization:
   • Topsoil storage stockpiles must be surrounded by compost filter socks and temporary-seeded with annual ryegrass within 7 days of completion.
   • Graded cut/fill slopes at $3:1$ must receive straw erosion control blankets (ECBs) stapled at 1.5-foot centers immediately upon reaching rough grade.
5. Establish Inspection Protocol:
   • Certified inspections scheduled every 7 calendar days and within 24 hours of any $0.25\text{-inch}$ rainfall event.

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5. Exam Traps & Pitfalls

1. Silt Fence Across Concentrated Channels: Specifying a silt fence across a swale, ravine, ditch, or creek. Silt fence is strictly engineered for shallow sheet flow. Concentrated flows will wash beneath or blow out the fabric, resulting in immediate regulatory fines.
2. Exceeding Silt Fence Drainage Area Limits: Connecting more than 0.25 acres per 100 linear feet of silt fence. Overloading causes hydraulic ponding that collapses hardwood stakes.
3. Premature Notice of Termination (NOT) Filing: Filing a Notice of Termination the day construction ends or right after hydroseeding is sprayed. The permit remains legally active until 70% uniform perennial native vegetative cover is fully established across all disturbed areas.
4. Concrete Washouts Near Storm Inlets: Placing a concrete washout within 50 feet of a storm sewer inlet or open drainage channel. Concrete slurry runoff carries a caustic pH of 12, constituting an illegal chemical discharge.
5. The 3,600 cu ft/acre Basin Sizing Trap: Forgetting the statutory 3,600 cubic feet per disturbed acre formula, or forgetting that it is divided equally into $1,800\text{ cu ft}$ of wet permanent storage and $1,800\text{ cu ft}$ of dry settling volume.